Understanding AM/FM Radio Stations

[Imparcticle]

I have come to understand the meaning of the terms "am" and "fm" as different kinds of radio emissions. Sadly, I have not understood the meaning of the numbers used to identify certain radio stations. If someone could tell me, it would be fantastic. I've been dying to know.

 

[chroot]

Both kinds of radio signals use a base signal called a "carrier" to encode information. The carrier itself is nothing too special -- it's just a simple sine wave of a given frequency. The radio station Live 105.3 in San Francisco, for example, broadcasts with a carrier frequency of 105.3 MHz.

If the radio station were broadcasting nothing but silence, it'd be broadcasting a perfect 105.3 MHz sine wave.

The carrier by itself contains no information -- it's just a sine wave. If you want to convey information, you have to modulate the carrier with the music you want to broadcast. In the AM (amplitude modulation) scheme, the carrier frequency stays fixed, and you vary the amplitude (intensity) of the sine wave to encode the amplitude (intensity) of the music. A high amplitude means the radio should produce a high sound pressure, and a low amplitude means the radio should produce a low sound pressure. Your speaker moves back and forth to reproduce those pressures, and you hear the music.

In the FM (frequency modulation) scheme, the carrier's amplitude stays fixed, and the frequency is varied by a few hundred kilohertz. A higher frequency means high pressure, a lower frequency means low pressure. Your speaker moves back and forth and reproduces those pressures and you hear the music.

Here are some pictures of the results of applying AM or FM modulation to a carrier: http://hyperphysics.phy-astr.gsu.edu/hbase/audio/bcast.html#c3

Let me know if you have any more questions.

- Warren

 

[Imparcticle]

How are sine waves superimposed?

 

[chroot]

In general, you use a device known as a "mixer" to add two signals. A mixer can be as simple as a plain ol' resistor.

- Warren

 

[Imparcticle]

Thank you so much chroot! I've been dying to know.

How does a mixer add two signals? What are the signals for?

 

[brewnog]

The "primary" signal is that which your radio tunes into, here, BBC Radio 1 is at around 99.5 MHz, (the primary signal kinda vibrates this many times each second). This is the frequency that you 'tune' your radio into. Think of this one as being like a big wave on the sea, just up and down, up and down (the sine wave Chroot mentioned).

The other signal (the superimposed one) is much smaller, and this one broadcasts the content of the actual radio station, so this is what you hear. Think of it as being a load of small ripples on the surface of the big wave you imagined earlier. The difference in FM and AM (again as Chroot explained) is the difference between the way that these small waves manifest themselves, - for FM the ripples get faster and slower to reproduce the speech and music, for AM they get bigger and smaller.

 

[Averagesupernova]

In general, you use a device known as a "mixer" to add two signals. A mixer can be as simple as a plain ol' resistor.

- Warren

Ummmm, not quite. A mixer or modulator requires a NON-LINEAR device. Using resistors will form an adder which is NOT a mixer.

 

[chroot]

Ummmm, not quite. A mixer or modulator requires a NON-LINEAR device. Using resistors will form an adder which is NOT a mixer.

I beg to differ. Many people use a simple resistor as a mixer in non-critical applications. Sure, it's a really horrible mixer, but it's doing the mixing.

- Warren

 

[Averagesupernova]

Nope, a linear circuit is incapable of true modulation. You will not get sum and difference frequencies out of a circuit that you describe. It may be capable of adding 2 signals together to send down the same wire or something but this is not a mixer. I've worked on a lot of RF equipment and I have never seen a mixer/modulator that didn't use some sort of non-linear device.

 

[rayjohn01]

mixer

The 'ideal' mixer has nothing to do with resistors it is a square law device
vo = ( a.sin(w1.t) + b.sin(w2.t) )^2
This produces harmonics and sum and difference terms
the required frequency can be selected by filtering or by using a more complex mixer in which some terms cancel , this is the preferred method in chip radios , and chip mixers.
If a 'mixer' is used on one signal it works as a rectifier which after filtering harmonics produces DC .
A mixer could be one diode , but is usually made of several bipolar transistors in a ' balanced' form look up any application notes on chip devices .
Ray.

 

[4newton]

Everyone here is right to some extent. No one it seems is old enough to remember vacuum tubes or old AM radio. This is before balanced mixers.

All AM modulation was done by adding voltage, or subtracting, from the average voltage of the final of the RF amp. You had to prevent over modulation by not allowing the audio voltage to go lower that zero.

100% modulation is when the audio voltage is just at zero and twice the average RF voltage. Any method that is able to add or subtract from the average RF voltage is able to modulate the RF.

FM is easy. All you need to do is shift the frequency of your oscillator. For the most part this has always been done at low level.

 

[Averagesupernova]

Everyone here is right to some extent. No one it seems is old enough to remember vacuum tubes or old AM radio. This is before balanced mixers.

Who's talking about balanced mixers? Incidentally, how is old AM radio different than new AM radio? Many AM transmitters on the air have been there for many years. I would be willing to bet even the new ones still run class C and modulate the power supply on the PA.

 

[4newton]

>how is old AM radio different than new AM radio? Many AM transmitters on the air have been there for many years. I would be willing to bet even the new ones still run class C and modulate the power supply on the PA.<

None of the posts said anything about high level modulation I thought it was worth while to mention it. Old AM radio is before TV and before DSB and carrier injection and low-level modulation. I agree there are still many old final modulators out there but very few on this site think about them or know they exist. It would be interesting to find out who is still using them. If you built a system today it would cost 1.5 to 2 times the cost of other methods. Also old radio receivers had tubes and the portables where 1 to 2 cu ft in size. Do you remember old radio?

 

[rayjohn01]

Well I am old enough , and it's still not addition but multiplication no matter how it's done ( for AM mod , or frequency changing ) , The point is that modulation is a NON linear process compared to linear mixing of signals as in a sound studio .
Ray

 

[Averagesupernova]

>how is old AM radio different than new AM radio? Many AM transmitters on the air have been there for many years. I would be willing to bet even the new ones still run class C and modulate the power supply on the PA.<

None of the posts said anything about high level modulation I thought it was worth while to mention it. Old AM radio is before TV and before DSB and carrier injection and low-level modulation. I agree there are still many old final modulators out there but very few on this site think about them or know they exist. It would be interesting to find out who is still using them. If you built a system today it would cost 1.5 to 2 times the cost of other methods. Also old radio receivers had tubes and the portables where 1 to 2 cu ft in size. Do you remember old radio?

Am I old enough? That's not really the point. I have restored 6 volt tube radios with vibrator contacts that run the high voltage power supply. Age means nothing. My grandparents were plenty old enough to remember it but couldn't tell you the first thing about modulation. I'll say it again, old AM radio is no different than new AM radio concerning the AM broadcast band unless you count AM stereo. You also mention double sideband (DSB). AM is inherently defined as double sideband. Amplitude modulate the carrier and you WILL end up with a double sideband signal with a carrier. When radio evolved is when they started to figure out how to remove parts of the signal such as the carrier and or one or a partial sideband before it went out on the air.

To the OP: Any of this making sense? You probably have new questions now? Like what is a sideband?

 

[chroot]

Averagesupernova,

"Sidebands" refer to the frequencies alongside the carrier that contain non-zero power. In AM, the frequency is not modulated, so there are no sidebands. The entire signal is only at one specific frequency, that of the carrier.

- Warren

 

[4newton]

To all:

I don’t think any of you are old enough to understand the term old AM radio. The term does not refer to the physics of AM it is in reference to a period of time, The early days of radio. If you were old enough you would not think of Old AM Radio in any other way.

>Modulation is a NON linear process<

I don’t know how you arrive at that conclusion. The intent of AM is to increase and decrease the peak voltage of the RF in a linear manner proportional to the applied audio. Many transmitters include feed back from the RF output to make the modulated output linear. All AM demodulators are intended to be peak detectors that follow the peak of the RF. A good demodulator will be linear.

>AM is inherently defined as double sideband. Amplitude modulate the carrier and you WILL end up with a double sideband signal with a carrier.<

It was found that if you eliminate the carrier and start just with the DSB you might then inject a carrier with less power and produce a more effective output. As you can see this method has many advantages and you are still able to receive this output with a standard AM radio.

>there are no sidebands<

I am sorry to disagree with you. The sidebands do contain energy and are located plus and minus the center frequency equal to the frequency of the modulation. This is why you are able to eliminate the carrier in DSB or SSB two sidebands are not necessary one sideband works fine for the transfer of information.
Look at a spectrum analyzer if you still doubt me.

 

[chroot]

4Newton,

You are correct -- I wasn't thinking about the fact that modulating the carrier's amplitude is the same as convoluting it with a lower-frequency signal. Sorry about that.

- Warren

 

[Averagesupernova]

To all:

I don’t think any of you are old enough to understand the term old AM radio. The term does not refer to the physics of AM it is in reference to a period of time, The early days of radio. If you were old enough you would not think of Old AM Radio in any other way.

Still not sure what that means. It has nothing to do with the original posters questions. If you want to start something with 'old AM' then do it someplace where it doesn't confuse people.

>Modulation is a NON linear process<

I don’t know how you arrive at that conclusion. The intent of AM is to increase and decrease the peak voltage of the RF in a linear manner proportional to the applied audio. Many transmitters include feed back from the RF output to make the modulated output linear. All AM demodulators are intended to be peak detectors that follow the peak of the RF. A good demodulator will be linear.

Yes, what you are saying is correct. Modulators and demodulators should be linear in the sense that the output should represent the input very closely. That is why there is feedback in the transmitter as you describe. But the output is then only as accurate as the demodulator that demodulates the output in order to compare the output to the input. As far as I knew feedback is mostly used in transmitters to keep the output clean of harmonics. For example in UHF transmitters they will do this but the feedback doesn't go all the way back to the audio. They are mainly concerned with avoiding harmonics of the main carrier. But the FUNCTION of modulation is not linear. A true linear amplifier will sum two signals and not create new frequencies. A non-linear amplifier will sum them AND create new frequencies. Creating new frequencies is what modulation is all about.

>AM is inherently defined as double sideband. Amplitude modulate the carrier and you WILL end up with a double sideband signal with a carrier.<

It was found that if you eliminate the carrier and start just with the DSB you might then inject a carrier with less power and produce a more effective output. As you can see this method has many advantages and you are still able to receive this output with a standard AM radio.


>there are no sidebands<

I am sorry to disagree with you. The sidebands do contain energy and are located plus and minus the center frequency equal to the frequency of the modulation. This is why you are able to eliminate the carrier in DSB or SSB two sidebands are not necessary one sideband works fine for the transfer of information.
Look at a spectrum analyzer if you still doubt me.

Yep. Note to chroot: A modulator and a mixer are the same thing. Not the type of mixer used in recording studios, the mixer used in radio work. They are optimized for the frequencies being processed though.

 

[Imparcticle]

What's an RF output?

 

[Averagesupernova]

What's an RF output?

Radio Frequency Output.

In the above posts it is the modulated signal that comes out the the transmitter and fed into the antenna.

 

[4newton]

I understand how you are just a little confused about mixing and modulation. From the way you talk about the two I don’t think you have had much exposure on the bench.


Using an oscilloscope:
Take two frequencies one that is higher in frequency by about 5 times. The difference is not important it only helps to see what is taking place. Mix the two frequencies in a summing circuit, two resistors. You will note that the higher frequency is adding and subtracting riding on the lower frequency.

If you look at the modulated output of an AM transmitter you will note that the modulated frequency is adding and subtracting from the total amplitude of the RF not riding on the carrier nor is the carrier riding on the modulation. Only the peak envelope of the RF is changing.

Both functions are linear. If you take the first condition and apply the signal to a non-linear transfer device the two frequencies modulate each other. The result is the same as the second condition.

You find this type of modulation when you tune a musical instrument. You are able to hear a beat frequency because the ear is acting as non-linear device.

I hope this clears things up for you.

BTW you do not need to be confused about anything. All you need to do is ask if you don’t know, as did Imparcticle.

 

[Averagesupernova]

Ummmm, 4Newton are you referring to me?

 

[4newton]

Averagesupernova, I was referring to your last post about this subject.

 

[Averagesupernova]

Ok 4Newton, it so happens I have a tremendous amount of experience on the bench. Concerning my comment on a mixer and modulator being the same, I am referring to AM. Sorry I didn't mention that. BOTH do the same thing. They create an upper and lower sideband through addition and subtraction of the 2 signals applied to the mixer/modulator.

I have about 12 years of experience as an electronic technician. Many of those years I spent aiding in the design and troubleshooting of field strength meters used in the cable TV industry. I did the same with oscilloscopes, video equipment which utilized modulators up through 800 MHz and stereo sound which I might add uses a slight variation of the standard multiplex encoding scheme which involves AM supressed carrier, lots of test equipment in general.

I OWN a fairly long line of test equipment as well. I have repaired a 1 MHz through 2 GHz signal generator that I purchased in a non-working state. I also own another generator which is a lesser quality rig but nonetheless is a synthesized generator. I own a Cushman CE15 spectrum analyzer, HP141 spectrum analyzer, several scopes, deviation meter, power meters, pads and directional couplers, tube testers (I don't remember 'old' radio but I still understand it). Do you really think I would invest the money in all this equipment without the knowledge of how to use it? The only problem is that I don't have the time I would like to spend with it and since I don't have the time cannot justify purchasing more or trading off what I have for better gear.

 

[Averagesupernova]

OK. I have taken the liberty of copying out of the 1996 ARRL Handbook. It is an amateur radio book put out by the American Radio Relay League. Chapter 15 covers: Mixers, Modulators and Demodulators.

Incidentally, I will keep the discussion leaning towards AM.

What is a Mixer?

Mixer is a traditional radio term for a circuit that shifts on signals frequency up or down by combing it with another signal. The word mixer is also used to refer to a device used to blend multiple audio inputs together for recording, broadcast or sound reinforcement. These two mixer types differ in one very important way: A radio mixer makes new frequencies out of the frequencies put into it and an audio mixer does not.

Mixing Versus Adding

Radio mixers might be more accurately called “frequency mixers” to distinguish them from devices such as “microphone mixers,” which are really just signal combiners, summers or adders. In their most basic ideal forms, both devices have two inputs and one output. The combiner simply adds the instantaneous voltages of the two signals together to produce the output at each point in time. The mixer, on the other hand, MULTIPLIES the instantaneous voltages of the two signals together to produce its output signal from instant to instant. Comparing the output spectra of the combiner’s output contains only the frequencies of the two inputs, and nothing else, while the mixer’s output contains new frequencies. Because it combines one energy with another, the process is sometimes called heterodyning, from the Greek words for OTHER and Power.

*****************************

I skipped the section about the multiplication. It contains formulas and also talks about how real world mixers are not perfect and are optimized to keep the distortion products down to a low level. Distortion products make for a ‘dirtier’ output as the book puts it.

There is a section called “Putting multiplication to work” that I will cover the last paragraph. The ‘multiplication’ refers to the multiplying that is discussed in one of the above paragraphs. The last paragraph and following text reads as follows:

*****************************
It turns out that the mechanism underlying multiplication, mixing, modulation, and demodulation is a pretty straightforward theing: Any circuit structure that NONLINEARLY DISTORTS ac waveforms acts as a multiplier to some degree.

Nonlinear Distortion?

The phrase nonlinear distortion sounds redundant but isn’t. Distortion, an externally imposed change in a waveform, can be linear; that is, it can occur independently of signal amplitude. Consider a radio receiver front end filter that passes only signals between 6 and 8 Mhz. It does this by LINEARLY DISTORTING the single complex waveform corresponding to the wide RF spectrum present at the radio’s antenna terminals, reducing the amplitudes of frequency components below 6 MHz and above 8 MHz relative to those between 6 and 8 MHz. (Consider multiple signals on a wire as one complex waveform is just as valid, and sometimes handier, than considering them as separate signlas. In this case, it’s a bit easier to think of distortion as something that happens to a vaveform rather than something that happens to separate signals relative to each other. It would be just as valid – and certainly more in keeping with the consensus view – to say merely that the filter attenuates signals at frequencies below 6 MHz and above 8 MHz.) The filter’s output waveform certainly differs from its input waveform; the waveform has been distorted. But because this distortion occurs independently of signal level or polarity, the distortion is linear. No new frequency components are created; only the amplitude relationships among the wave’s existing frequency components are altered. This is amplitude or frequency distortion, and all filters do it or they wouldn’t be filters.

PHASE or DELAY DISTORTION, also linear, causes a complex signal’s various component frequencies to be delayed by different amounts of time, depending on their frequency but independently of their amplitude. No new frequency components occur, and amplitude relationships among existing frequency components are not altered. Phase distortion occurs to some degree in all filters.

In a NONLINEARLY distorting circuit, however, the output’s root-mean-square (RMS) voltage or current values don’t track the input’s corresponding RMS voltage or current values in linear proportion. The distortion therefore varies with the signal’s amplitude (and in some nonlinear structures, polarity); the stronger the signal the greater the distortion.

Nonlinear distortion may take the form of HARMONIC DISTORTION , in which integer multiples of input frequencies occur, or intermodulation distortion (IMD), in which different components multiply to make new ones.

Any departure from absolute linearity results in some form of nonlinear distortion, and this distortion can work for us or against us. Any so-called linear amplifier distorts nonlinearly to some degree; any device or circuit that distorts non linearly can work as a mixer, modulator, demodulator or frequency multiplier. An amplifier optimized for linear operation will nonetheless mix, but inefficiently; an amplifier biased for nonlinear amplification may be practically linear over a given tiny portion of its input-signal range. The trick is to use careful design and component selection to maximize nonlinear distortion when we want it, and minimize it when we don’t. Once we’ve decided to maximize nonlinear distortion, th etrick is to minimize the distortion products we don’t want, and maximize the products we desire.

Why we call it amplitude modulation

We call the modulation process described in equation 8 amplitude modulation because the complex waveform consisting of the sum of the sidebands and carrier varies with the information signal’s magnitude. Concepts long used to illustrate AM’s mechanism may mislead us into thinking that the carrier varies in strength with modulation, but careful study of equation 9 shows that this doesn’t happen.

*****************************


Ok, I will quit copying here. There is a WHOLE LOT MORE to the chapter but I don’t feel like typing it all in. Just go buy the book. I also left the equations out but you can trust me that I typed it word for word. If you question the validity, once again, buy the book. You can contact the organization who puts out the book at:

American Radio Relay League
225 Main St
Newington, CT 06111-1494

Go ahead and tell them I plagiarized their book, they will love it since it is a possibility they will sell a book and recruit one more radio amateur.

4Newton: I recall your post saying: The intent of AM is to increase and decrease the peak voltage of the RF in a linear manner proportional to the applied audio. Hmmmm. As you can see this is not true. It is the apparent result of amplitude modulation, but the strength of the carrier does NOT vary.

 

[4newton]

Averagesupernova:

You are still confused. I have ARRL Handbooks. My oldest one, I think is 1952, but the last time I looked at that one it was falling apart.

You are confused about what you do when you modulate the final of the RF amp. When you modulate the carrier you are changing the voltage in a linear manner to change the amplitude of the RF wave. What you posted is the result of what happens when you modulate. What you do and what the result is are two different things. You are modulating the carrier in a linear manner the result is that the modulation energy is shifting to the side band. They are both correct but you are only doing the first.

 

[Averagesupernova]

Gawd. It's the chicken and the egg. Yes what you do and what the result is are not the same. But you make it sound like the actual amplitude of the carrier is changing at a rate represented by the audio. This is NOT the case. It looks like it on a scope because of the 'beat' between the 2 sidebands and the carrier. When you amplitude modulate a carrier you are technically NOT changing it's strength. You are distorting the shape of the main carrier sine wave which creates sidebands. Filter the sidebands out and you will find that the strength of the carrier is constant. I guess maybe the way to say it is that anytime you do ANYTHING AT ALL to the carrier you create new frequencies but you don't necessarily change the strength of the carrier itself.

Another thing I will point out. Take a balanced modulator such as the 1496 and give it a bias with no input modulation and you will have a carrier output. Then send in a modulating signal and you will have the standard AM signal. Now, reduce the offset bias and watch the signal on the scope. The valleys where the complex signal seems to almost pinch off to zero volts starts to form into crests again. It looks like 2 full wave rectified sine waves one on top of a second one which is inverted and the area in between shaded when the carrier is completely removed. The voltage looks like it goes to zero volts in the complex waveform, and technically it does. But filter either sideband out and you will find a pure clean waveform left which is the sideband that was not filtered out. There is nothing about any INDIVIDUAL signal whose strength is changing at the audio rate. The changing strength is an apparent effect.

Yes, to get an AM signal you do in fact do what you have described, otherwise you wouldn't call it AM. But many people get led in the wrong direction thinking that there are no sidebands and it is just a changing carrier just as chroot did. When in actuality you CANNOT DO ANYTHING TO THE CARRIER WITHOUT CREATING NEW FREQUENCIES. AM, FM, whatever it is, any time you add intelligence to a signal you create a bandwidth requirement. How efficiently you utilize the space is yet another discussion.

FM is similar. A lot of people think that a carrier that is frequency modulated at oh, say 1KHz with 25 KHz deviation will just sweep out 25 KHz either side from the carrier continuously. This is not the case either. There are sidebands every 1KHz and the spectrum in between these sidebands is empty. The name FM is also slightly misleading, although in my opinion not as misleading as AM.

Can we lay this to rest? Are we both saying the same thing in different ways? Do you just like to argue and try drive home the point that you have experience in remembering 'old AM' and about how you have a 1952 handbook? So what? I have old handbooks too.

EDIT:

Just thought of something after I posted. If you take 3 signals that are phase locked to each other, one at 900 Khz, one at 1 Mhz, and one at 1.1 MHz and sum them together you will have a complex signal which looks like a 1 MHz carrier AM modulated with a 1 KHz tone. So here we have arrived at a complex signal virtually identical to an AM signal formed in the conventional sense that has in no way varied the strength of the carrier in an amplifiers PA in order to arrive at it. When detected in an AM radio a 1 KHz tone will be produced. I tend to think in terms of what kind of signal(s) are actually there and not how it is arrived at because there are MANY ways to arrive at something.

 

[4newton]

Ummmm, not quite. A mixer or modulator requires a NON-LINEAR device. Using resistors will form an adder which is NOT a mixer.

Nope, a linear circuit is incapable of true modulation. You will not get sum and difference frequencies out of a circuit that you describe. It may be capable of adding 2 signals together to send down the same wire or something but this is not a mixer. I've worked on a lot of RF equipment and I have never seen a mixer/modulator that didn't use some sort of non-linear device.

But the FUNCTION of modulation is not linear. A true linear amplifier will sum two signals and not create new frequencies. A non-linear amplifier will sum them AND create new frequencies. Creating new frequencies is what modulation is all about.

The name FM is also slightly misleading, although in my opinion not as misleading as AM.

Can we lay this to rest? Are we both saying the same thing in different ways? Do you just like to argue and try drive home the point that you have experience in remembering 'old AM' and about how you have a 1952 handbook? So what? I have old handbooks too.

I only respond because you make statements that are not correct and leave the wrong idea with anyone reading the post. When I pointed out the linear function of a high level modulator you went into an analyses of waveforms that had nothing to do with the function of the modulator. You have a lot of information but you arrive at an incorrect understanding. With the knowledge you have I still think that time and experience will help you put things in the correct order. When you do you will know what it means to have a feel for the subject.

Yes I was trying to indicate that I have had some experience. I do so only with the hope that you take my comments with some weight. If you must know I have had over 55 years of experience. That does not make me the absolute authority. There are two types of people those with 10 years of experience and those with one year of experience 10 times. I may have some of the second.

 

[Averagesupernova]

I only respond because you make statements that are not correct and leave the wrong idea with anyone reading the post. When I pointed out the linear function of a high level modulator you went into an analyses of waveforms that had nothing to do with the function of the modulator. You have a lot of information but you arrive at an incorrect understanding. With the knowledge you have I still think that time and experience will help you put things in the correct order. When you do you will know what it means to have a feel for the subject.

Yes I was trying to indicate that I have had some experience. I do so only with the hope that you take my comments with some weight. If you must know I have had over 55 years of experience. That does not make me the absolute authority. There are two types of people those with 10 years of experience and those with one year of experience 10 times. I may have some of the second.

How can the analyses of waveforms you refer to have nothing to do with modulation? And how is a high level modulator any different than a low level one other than the obvious which is given away in the name?

The things you quoted me on you are implying are incorrect as I see it. It seems to me that they are pretty much in direct agreement with what is out of the ARRL handbook.

I don't know what else to say here. It seems you just want to get your 2 cents in and be able to say that you are more correct than everyone else, when in actuality you are not, just because you have 55 years of experience and are able to remember 'old AM'.

 

[4newton]

Averagesupernova:

How can the analyses of waveforms you refer to have nothing to do with modulation? And how is a high level modulator any different than a low level one other than the obvious which is given away in the name?
The things you quoted me on you are implying are incorrect as I see it. It seems to me that they are pretty much in direct agreement with what is out of the ARRL handbook.

I don't know what else to say here. It seems you just want to get your 2 cents in and be able to say that you are more correct than everyone else, when in actuality you are not, just because you have 55 years of experience and are able to remember 'old AM'

You are incorrect!. I used the circuit of the high level modulation in the hope that you would see how you are wrong. There is no non-linear device in high level modulation. There is no non-linear function in high level modulation. The amplitude of the RF changes with change of applied voltage in a linear manner. If you simply change the voltage on the output of the final stage of the RF amp you will change the RF voltage output in a linear manner. If you measure the RF voltage output of the RF amp and then increase the voltage applied to the RF amp by 25 percent and again measure the RF voltage you will see that the RF voltage has increased by 25 percent. This is by definition amplitude modulation. Modulation is the change of conditions not the rate of change of conditions. The simplest form of AM is CW.

It is interesting that I am able to see your point of view but you are unable to see mine, but then that is experience. Where is this everyone else you are taking about? I don’t think you will ever get the concept and if this were a private conversation I would have given up on you a long time ago. I am not able to let you confuse others in a public forum without trying to correct you. I owe that to anyone interested in science.

.

 

[Averagesupernova]

Where is the everyone else I am talking about?

Well I am old enough , and it's still not addition but multiplication no matter how it's done ( for AM mod , or frequency changing ) , The point is that modulation is a NON linear process compared to linear mixing of signals as in a sound studio .

Well there is one who seems to agree with me.

You are saying that by changing the voltage on the PA of an RF amp you will change the level of RF on the output. Hmmmm. Here is where things get interesting. On a high level modulated transmitter there is ONLY carrier driving a class C PA. The power supply is superimposed with audio. So according to your logic, then the carrier would be changing in strength. We know this is not the case. Yes, I agree with your method and such and I agree that the level of the carrier will change when the supply is changed and LEFT at the changed voltage. In other words, take a tube type transmitter with a 1000 volt plate voltage and then drop it to 500 volts. Yes, the carrier will then drop in level. I agree 100%. But modulation is a dynamic thing and when the plate voltage is in a changing state you are generating new frequencies. Once AGAIN, you imply that the strength of the carrier is changing. You don't specifically say so, but you say the amlitude of the RF changes with change of applied voltage in a linear manner. So can you please answer this one question: DO YOU BELIEVE THAT THE CARRIER CHANGES IN AMPLITUDE ON A MODULATED SIGNAL?

 

[4newton]

Ok maybe you are starting to get it. As you have stated the amplitude of the RF changes with applied voltage in a linear manner. Therefore changing the applied voltage to the final is a linear function. Modulation is just this change. The rate of change of the applied voltage is only a component of modulation not modulation in and of itself. Your view overlooks the basic function of modulation and only looks at the rate of change.

If you will take one of your instruments and look at the output of an audio modulated RF carrier you will notice that you have two sidebands, if you use a single frequency tone. Now take note of the amplitude change of the sidebands as you change the amplitude of you audio tone. You will note that the amplitude changes in a linear manner.

This is why I did not state that the carrier was changing and I did say that the RF output was changing in a linear manner with modulation.

You still have not explained where your nonlinear device is that makes modulation a nonlinear function. A nonlinear component is required to produce a nonlinear result.

Only one type of waveform addition is modulation. Modulation is the addition and subtraction of voltage to produce a change in the envelope of the carrier. The other is addition and subtraction on top of another wave. This is mixing. Again if you don’t understand the difference look at the waveforms on your scope.

..

 

[Averagesupernova]

Ok maybe you are starting to get it. As you have stated the amplitude of the RF changes with applied voltage in a linear manner. Therefore changing the applied voltage to the final is a linear function. Modulation is just this change. The rate of change of the applied voltage is only a component of modulation not modulation in and of itself. Your view overlooks the basic function of modulation and only looks at the rate of change.

If you will take one of your instruments and look at the output of an audio modulated RF carrier you will notice that you have two sidebands, if you use a single frequency tone. Now take note of the amplitude change of the sidebands as you change the amplitude of you audio tone. You will note that the amplitude changes in a linear manner.

This is why I did not state that the carrier was changing and I did say that the RF output was changing in a linear manner with modulation.

You still have not explained where your nonlinear device is that makes modulation a nonlinear function. A nonlinear component is required to produce a nonlinear result.

Only one type of waveform addition is modulation. Modulation is the addition and subtraction of voltage to produce a change in the envelope of the carrier. The other is addition and subtraction on top of another wave. This is mixing. Again if you don’t understand the difference look at the waveforms on your scope.

..

None of this is new to me. I have seen AM on a scope and spectrum analyzer hundreds of times.

You ask about the non-linear component.

Well it should be pretty obvious to you. In a class C high level modulated transmitter the non-linear device is the device used in the PA whether it be a tube or transistor. Class C is a NON-LINEAR configured amplifier. I'm quite surprised you missed that.

Ok, out of another book:

This book is a college textbook. Electronic principles 3rd edition. Author is Albert Paul Malvino Ph.D. It has an ISBN number of: 0-07-039912-3

Chapter 23 is entitled Frequency Mixing. The first part talks about non-linearity. I will skip it, but copy in the very first paragraph of the complete chapter. I assume we can agree that frequency mixing and amplitude modulation can be considered the same thing other than the fact that they are optimized for output in different parts of the spectrum relative to the input frequencies. They both generate new frequencies which is what mixing and modulation are all about.

It reads:



When a sine wave drives a non-linear circuit, harmonics of this sine wave appear in the ouput. If two sine waves drive a non-linear circuit, we get harmonics of each sine wave, plus new frequencies called the sum and difference frequencies. This chapter describes the theory and application of these new output frequencies.


That should be enough for you, but history throughout this thread tells me otherwise. Sooooooo...

Section 23-6 of the chapter entitled Frequency Mixers.

A frequency mixer is used in almost every radio and television receiver; it is also used in many other electronic systems.

THE BASIC IDEA

Figure 23-7 shows all the key ideas behind a frequency mixer. Two input sine waves drive a nonlinear circuit. As before, this results in all harmonics and intermodulation components. The bandpass filter then passes one of the intermodulation components, usually the difference frequency Fx - Fy. Therefor, the final output of a typical mixer is a sine wave with frequency Fx - Fy. In terms of spectra, the frequency mixer is a circuit that produces an output spectrum with a single line at Fx - Fy when the input spectrum is a pair of lines at Fx and Fy.



They call them 'lines' as the drawing shows a crude spectral display.

I will skip a small paragraph that talks more about Fx and Fy and get to the good stuff.



USUAL SIZE OF INPUT SIGNALS

In most applications, one of the input signals to the mixer will be large. This is necessary to ensure nonlinear operation; unless one of the signals is large, we cannot get intermodulation components. This large input signal is often supplied by an oscillator or signal generator.

The other input signal is usually small. By itself, this signal produces only small-signal operation of the mixer. Often one of the reasons this signal is small is because it is a weak signal coming from an antenna.

The normal inputs to a mixer, therefore, are:

1. A large signal adequate to produce medium- or large-signal operation of the mixer.

2. A small signal that by itself can produce only a small-signal operation.

TRANSISTOR MIXER

Figure 23-8 is an examle of a transistor mixer. One signal drives the base and the other signal drives the emitter. The resulting collector current contains harmonics and intermodulation components. With the LC tank tuned to the difference frequency, the output signal has a frequency of Fx - Fy.




Figure 23-8 shows a NPN transistor with voltage divider bias on the base and an emitter resistor. There is NO collector resistor, only the parallel resonant tank circuit. They show sine waves being fed onto the base and emitter through coupling caps. There are no component values given. I realize that this is not a high level modulator when the audio is superimposed on the plate/collector voltage, but it illustrates my point.

The chapter goes on to talk about diode mixers, conversion gain, mixers in AM receivers, spurious signals, and another section on PLL.

So are you happy yet? Are you going to continue to say that there is nothing non-linear about a frequency mixer/ amplitude modulator? Yes, you are correct that the 2 sidebands linearly follow the strength of the audio being used to modulate. That is not the point.

 

[4newton]

Averagesupernova:

Well it should be pretty obvious to you. In a class C high level modulated transmitter the non-linear device is the device used in the PA whether it be a tube or transistor. Class C is a NON-LINEAR configured amplifier. I'm quite surprised you missed that.

As always you seem to get things mixed up. The nature of the device that produces the RF is of no consequences. You only need to be concerned with the process of modulation, the transfer function. As you have pointed out the RF voltage varies in a linear manner with applied voltage. You also agree that the amplitude of the side band also varies in a linear manner with applied audio. If you really don’t understand modulation at this point you must be the only one on this forum that does not.

I assume we can agree that frequency mixing and amplitude modulation can be considered the same thing other than the fact that they are optimized for output in different parts of the spectrum relative to the input frequencies. They both generate new frequencies which is what mixing and modulation are all about.

Of course you know I don’t agree that frequency mixing and amplitude modulation are the same thing. You also know that it is not the same thing.

At this point I can see that you are starting to see this in a whole new light. I also know from what you have posted that you will never state that you are ever wrong.

I have no problem teaching electronics but I don’t care to do it in a confrontational manner. If you whish to know how mixers work or any thing else please change your approach. The approach I recommend is that you state you understanding listen to the other understanding, if different. then pose questions and give replies to the point. If you are confused on any point, resolve the confusion first without confrontation.

 

[enigma]

Can we please take the hostility down a notch, guys?

 

[Averagesupernova]

Averagesupernova:

As always you seem to get things mixed up. The nature of the device that produces the RF is of no consequences. You only need to be concerned with the process of modulation, the transfer function. As you have pointed out the RF voltage varies in a linear manner with applied voltage. You also agree that the amplitude of the side band also varies in a linear manner with applied audio. If you really don’t understand modulation at this point you must be the only one on this forum that does not.

Look at a 1 MHz signal modulated with a tone on a spectrum analyzer. The carrier does not change in amplitude and neither does either sideband as long as the strength of the tone is constant. What you see on the scope changing in amplitude is the complex waveform of the carrier in combination of the 2 sidebands. All of them combined produce the waveform. Remember when I mentioned taking 3 independently generated signals and summing them? The waveform you would see will look like the signal you are referring to.

As for the source producing the RF being irrelevant? Well, it IS relevant. If you run the PA transistor/tube linear such as class A/B and leave plenty of headroom then all that superimposing audio onto the power supply will do is SUM the RF and the audio. It will do this because you can lower the supply voltage and the sine wave won't change because it still has headroom left before it clips. (To a point of course) I will admit it is easy to forget that the PA in a class C output is running non-linear because the collector circuit has built in filtering and the output appears as a sine wave. It just seems like a good source for a sine wave but it is actually part of the modulator.

Of course you know I don’t agree that frequency mixing and amplitude modulation are the same thing. You also know that it is not the same thing.

Well, technically they are. I can recall seeing it in an ARRL publication. I'd have to dig it out but it was there. I can do it if you want, hopefully I have the book. I wouldn't have said it if I weren't sure.

At this point I can see that you are starting to see this in a whole new light. I also know from what you have posted that you will never state that you are ever wrong.

Why should I state I am wrong? You see I have this tendency to post about things I am certain of. I am not saying I have never been wrong but I don't open my mouth unless I am darn certain. You should try it. I will admit that there have been certain times in this thread that I have posted and assumed that the reader knows what I am talking about. The example I can think of is in a post when I said mixing and modulation are the same thing. What I should have said is that frequency mixing and AM modulation are the same thing. Technically you could read all modulation including FM was the same thing as frequency mixing. I am guilty of it in many threads. It is part of my personality.

I have no problem teaching electronics but I don’t care to do it in a confrontational manner. If you whish to know how mixers work or any thing else please change your approach. The approach I recommend is that you state you understanding listen to the other understanding, if different. then pose questions and give replies to the point. If you are confused on any point, resolve the confusion first without confrontation.

I don't need to know how mixers work. I already do. When did you assume you were the one doing the teaching? You seem to contradict things out of both books I have quoted now. How many books do you want to go for because as long as this keeps up I will quote out of books until I run out of them. And I will make a point of quoting things that prove your view wrong. Funny thing is though, the texts mention that in order to produce new frequencies the device has to be driven into it's non-linear region. You seem to ignore it. If new frequencies are generated with an AM modulator without non-linear operation then why can't the same be done with mixers? The book says it is not possible and from what I can recall working on video equipment modulators and mixers it is not.

 

[Averagesupernova]

Enigma I am very sorry if this is getting a little too hostile. I guess at this point I would like some more input from other users. I know that there are several other posters on here who should have some input. I know my tone can be a bit confrontational at times. But I don't see how quoting textbooks and having my opponent completely ignore facts from those books adds anything to this thread. So as an apology to everyone, I want to ask all the posters on this thread what they would like me to do? If posters have a specific question about something I have posted by all means ask.

 

[4newton]

Enigma you are right.
I am sorry that my frustration got out of hand.

having my opponent completely ignore facts”

I apologize Averagesupernova I should know better, but I did not realize, until now, that I was considered an opponent.

 

[enigma]

Thank you both.

 

[4newton]

Averagesupernova;

But I don't see how quoting textbooks and having my opponent completely ignore facts from those books adds anything to this thread.

I am sorry if you have taken offense at my not responding to your quotes but I did not feel that it was necessary. I do not disagree with your quotes. I thought I stated that once before. They are just not relevant to the disagreement about modulation.

I have tried to hold the discussion to the basics of modulation. I did not feel that it was of any use going in any other direction without that basic agreement between us.

I pointed out and you agreed that the voltage out of the RF amp was linear with applied voltage. I also pointing out to you and you agreed that the voltage in the sideband from the modulation was also linear with applied audio, I thought that should have resolved the issue. I still don’t know why it has not. It is axiomatic that a result that is linear is a linear function.

Maybe I should introduce you to the idea of a transfer function. The RF source maybe thought of as a black box. It has only three terminals. Common, supply voltage, and output. The only thing you can change is the supply voltage. The only output you have is the RF. I have shown. And you have agreed, that with any change of supply voltage or supply voltage change at an audio rate the output is always linear. If you disagree all you need to do is show that the output will change in a nonlinear manner with a change of supply voltage.

The black box could be a constant RF source that has a variable resistor the output of the box is linear with the position of the resistor. As you change the resistor, by definition, you modulate the output. This box will produce amplitude modulation with the same waveform of any other AM modulator. There is no nonlinear component. This is why the nature of the RF source is of no importance. Only the transfer function is relevant.

 

[Averagesupernova]

You didn’t realize you were an opponent? Well, I didn’t consider you so at first, but somewhere along the line….. And don’t we have opposing views? Anyway, on to the disscussion.

My goals have not changed. They are:

1) To convince you that mixing and amplitude modulation are considered the same thing.

2) To convince you that what you call the voltage out of the RF amp varying linearly with applied audio is nothing more than the carrier and 2 sidebands forming a complex waveform.

3) To convince you that the source of the RF is in fact relevant and that the output device is part of the modulator.

4) To convince you that a device running linear mode on the output would NOT be capable of modulation by superimposing audio onto the power supply.

I have once again turned to the good old ARRL to back my side of this whole deal. This time the book I am quoting is entitled:
THE RADIO AMATEUR’S LICENSE MANUAL. It is the 80th edition around 1984.

On page 2-19 there is a good columns worth of text that talks about amplitude modulation. Here we go.



Amplitude Modulation

The modulation system that gave rise to the carrier idea is called amplitude modulation (a-m). It’s a system for doing just what we have described above.



What they mean when they refer to what they described above is a description of sidebands and carrier. They descibe the mathematical relationship between the carrier, sidebands and frequency of the modulating audio.


But when we come to the actual mechanics by which the signal is modulated, it’s easy to thinik of it from a different viewpoint. That viewpoint is this: That the amplitude of the carrier (that is, the value of the carrier’s current or voltage) is made to follow faithfully the instantaneous changes in the audio-frequency voice voltage. The general idea is shown in Fig. 32. In the amateur bands, the carrier frequency is thousands of time greater than the audio frequency, so the carrier will go though a great many cycles during one cycle of even the highest audio frequency we want to transmit. This is shown by the shading in the figure; we couldn’t begin to draw the actual radio frequency (rf) cycles because there would be far too many to be printed.




Figure 32 is the typical AM 100% modulated signal as viewed on a scope.


You’re probably thinking that something is actually being done to the carrier. But this is only because it isn’t possible to draw a picture of more than one aspect of modulation at a time. The picture you see in Fig. 32 is really a composite one showing the result of the action of three separate frequencies in a circuit that will pass all of them. The three are the carrier frequency, the upper side frequency and the lower side frequency. They all add together in such a way as to give the appearance of a single frequency (the carrier) whose amplitude is changing just the same way that the signal doing the modulating is changing.


Isn't this what I've been saying?


But appearances can sometimes fool us. It would be more accurate to say that the actual modulation process is one of mixing, which we mentioned in our discussion of receiver circuits.



I do believe I have been saying this too.


Nevertheless, it’s easier to grasp some things about amplitude modulation with the help of a picture such as Fig. 32, and we’ll take advantage of it. Never forget, though, that the carrier does not actually vary in amplitude, and that the modulation is all in the two sidebands.

Averagesupernova;
I am sorry if you have taken offense at my not responding to your quotes but I did not feel that it was necessary. I do not disagree with your quotes. I thought I stated that once before. They are just not relevant to the disagreement about modulation.

The quote out of the above book should prove otherwise. The one about modulation actually being a process of mixing.

I have tried to hold the discussion to the basics of modulation. I did not feel that it was of any use going in any other direction without that basic agreement between us.

Once again, mixing, modulation, can't have one without the other.

I pointed out and you agreed that the voltage out of the RF amp was linear with applied voltage. I also pointing out to you and you agreed that the voltage in the sideband from the modulation was also linear with applied audio, I thought that should have resolved the issue. I still don’t know why it has not. It is axiomatic that a result that is linear is a linear function.

Ummm, not quite. I believe I said that the envelope follows the audio in a linear fashion. You talk about 'RF' coming out of the amp. You seem to refer to it as a quantity of 'stuff'. The 'envelope' is an illusion of sorts, a complex waveform made up of 3 signals as the text says.

I've done a little math. I will show you how modulation is not linear. Take a 20 volt peak to peak sine wave that is NOT modulated. It is driving a 50 ohm load. The power dissipated in the load is 1 watt. I think we can agree on that.

Now let's modulate it at a given frequency with a NEAR square wave. I hope you are ok with this method. We will modulate it 100%. So what we have is the 'RF output' going on and off. The voltage NOW swings up to twice the original peak to peak voltage but only half the time. The other half of the time the output is ZERO. Average power dissipated is 2 watts. Now let's modulate at 50%. The envelope peaks up to 15 volts now for a 30 volt peak to peak signal. Incidently, this condition is defined in the book I quoted as 50% modulation. So now we have power dissipated in the load at the peak of the envelope of 2.25 watts and the trough of the envelope of .25 watts for an average of 1.25 watts. Hmmmmmm. Is this linear? I don't think so.

Incidentally, notice that twice the power is dissipated at 100% modulation as with no modulation. Ever notice at 100% modulation using a tone the sidebands are each exactly 3 db down from the carrier when viewed on a spectrum analyzer? Each is half the power of the carrier which adds up to what we figured in the above math. I used square wave modulation for simplicity.

Maybe I should introduce you to the idea of a transfer function. The RF source maybe thought of as a black box. It has only three terminals. Common, supply voltage, and output. The only thing you can change is the supply voltage. The only output you have is the RF. I have shown. And you have agreed, that with any change of supply voltage or supply voltage change at an audio rate the output is always linear. If you disagree all you need to do is show that the output will change in a nonlinear manner with a change of supply voltage.

I just did that.

The black box could be a constant RF source that has a variable resistor the output of the box is linear with the position of the resistor. As you change the resistor, by definition, you modulate the output. This box will produce amplitude modulation with the same waveform of any other AM modulator. There is no nonlinear component. This is why the nature of the RF source is of no importance. Only the transfer function is relevant.

You can do that, but the math I did still proves it is not linear.


Now I want to ask you a question: I am going to take a class A or class A/B amplifier configured common emmiter and take the output off of the collector. I have set it up so that the voltage swing of + and - 5 volts and the average DC voltage on the collector at 10 VDC. I am using a 20 volt power supply. So I have some headroom left so to speak. Now I superimpose a tone on the powersupply but it never gets down to 15 volts. Just low 'modulation' so to speak. What would the output look like?

 

[4newton]

Averagesupernova

1) To convince you that mixing and amplitude modulation are considered the same thing.

Do you think summing two audio frequencies together with resistors is modulation or mixing?

2) To convince you that what you call the voltage out of the RF amp
varying linearly with applied audio is nothing more than the carrier and
2 sidebands forming a complex waveform.

Are you saying that the wave form you see on the scope is or is not the change of voltage with time and what you see is not taking place?

3) To convince you that the source of the RF is in fact relevant and
that the output device is part of the modulator.

4) To convince you that a device running linear mode on the output
would NOT be capable of modulation by superimposing audio onto the power
supply.

Did you follow the concept of transfer function? What is the transfer function of a class C amp with applied voltage? What is the transfer function of a class A amp with voltage?

That the amplitude of the carrier (that is, the value of the carrier’s current or voltage) is made to follow faithfully the instantaneous changes in the audio-frequency voice volttage.

What do you think that says? This is a description of a linear function.

They all add together in such a way as to give the
appearance of a single frequency (the carrier) whose amplitude is changing
just the same way that the signal doing the modulating is changing.

You note the word ADD.

Now let's modulate it at a given frequency with a NEAR square wave. I hope you are ok with this method. We will modulate it 100%. So what we have is the 'RF output' going on and off. The voltage NOW swings up to twice the original peak to peak voltage but only half the time. The other half of the time the output is ZERO. Average power dissipated is 2 watts. Now let's modulate at 50%. The envelope peaks up to 15 volts now for a 30 volt peak to peak signal. Incidently, this condition is
defined in the book I quoted as 50% modulation. So now we have power
dissipated in the load at the peak of the envelope of 2.25 watts and the
trough of the envelope of .25 watts for an average of 1.25 watts.
Hmmmmmm. Is this linear? I don't think so.

Are you aware that you went from voltage to power. How does power vary with a change of voltage in a resistor? Is changing voltage across a resistor a linear function? Look again at you example.

 

[Averagesupernova]

Averagesupernova
Do you think summing two audio frequencies together with resistors is modulation or mixing?

No. I have NEVER said that. In fact, I was the first one in this thread to dispute it. When I speak of mixing I mean frequency mixing. Maybe for someone who remembers ‘old AM’ I should call a mixer a converter. When I mean summing I will say it.

Are you saying that the wave form you see on the scope is or is not the change of voltage with time and what you see is not taking place?

Well the last time I looked up the definition of an oscilloscope I believe it was defined as a device which is primarily used for displaying a graph of voltage over time. Could the questions become any simpler?

YES, the waveform viewed is in fact a voltage changing over time and what is seen is the carrier and 2 sidebands summed together. Those 3 are in fact summed linearly. The non-linear part came before that and the 2 sidebands are the result of it. It is what I call a complex waveform.

Did you follow the concept of transfer function? What is the transfer function of a class C amp with applied voltage? What is the transfer function of a class A amp with voltage?

Yes I follow the concept of transfer function. However, we cannot agree on the behavior of a class C versus a class A amplifier when superimposing audio onto the power supply. At least I cannot assume we agree because you haven’t answered my question concerning the class A or class A/B amplifier from my last post. So what is the point of talking about transfer function? In order for us to agree on it we have to first agree on the behavior of the considered amplifiers.

What do you think that says? This is a description of a linear function.

How convenient for you to leave out the surrounding text. You KNOW that the paragraph is talking about how it is viewed on a scope and nothing else.

You note the word ADD.

Of course I do. What is your point? See my comment earlier in this post concerning the addition of 2 sidebands and the carrier.

Are you aware that you went from voltage to power. How does power vary with a change of voltage in a resistor? Is changing voltage across a resistor a linear function? Look again at you example.

Do you really think that I accidentally went from voltage to power?

I did some more math. The conditions are the same as my last post with the near square wave. Here is what I came up with:

Take a sine wave at 20 volts peak to peak driving a 50 ohm load. The power dissipated in it is 1 watt. Double the voltage and the power dissipated in the resistor is quadrupled. This is nothing new to me either, but I am glad you brought it up.

Excuse the crude chart, but it should get the point across.

% mod_____pk volts_____min volts_____pk pwr_____min pwr_____Avg pwr
0_________10__________10__________1__________1__________1
10________11__________9___________1.209______.81_________1.0095
20________12__________8___________1.44_______.64_________1.04
30________13__________7___________1.69_______.49_________1.09
40________14__________6___________1.959______.36_________1.1595
50________15__________5___________2.25_______.25_________1.25
60________16__________4___________2.559______.16_________1.3595
70________17__________3___________2.889______.09_________1.4895
80________18__________2___________3.239______.04_________1.6395
90________19__________1___________3.609______.01_________1.8095
100_______20__________0___________4__________0__________2

I think the chart is fairly self-explanatory. But I will go over it anyway. The first column should be obvious. The second column is the PEAK voltage on the envelope. The third column is the PEAK voltage in the ‘trough’ of the envelope. The fourth column is the AVERAGE power in watts during the crest of the envelope. The fifth column is the AVERAGE power in watts during the ‘trough’ of the envelope. The 6th column is the average total power of the whole envelope.

The load used is a 50 ohm load. Notice how NONE of the power columns wattage is linear with the % of modulation the same way it would be if you just were doubling the voltage on a resistor. Now, take and subtract one watt out on all entries in the average column (6th). This represents removing the carrier. You have 2 sidebands left. The power STILL does not increase the way a plain old sine wave would when doubling the voltage across a resistor. Now divide each entry in half in the 6th column. This represents removing on sideband. NOW the power is linear. The same way it would be by doubling the voltage of a plain old sine wave into a resistor. Which is exactly what it is. One single frequency. If you were to filter each sideband out you would find that each one increases in power linearly. We’ve agreed on that. In reality, there would be many sidebands because the modulating signal is not sinusoidal in my example. But the power in each group of sidebands would add up to the same power as if it were one single upper and lower sideband.

One last thing. I want an answer to my question about modulating the supply voltage on a class A or class A/B amp. What will the output look like?

 

[4newton]

Averagesupernova:

Yes I follow the concept of transfer function. However, we cannot agree on the behavior of a class C versus a class A amplifier when superimposing audio onto the power supply. At least I cannot assume we agree because you haven’t answered my question concerning the class A or class A/B amplifier from my last post. So what is the point of talking about transfer function? In order for us to agree on it we have to first agree on the behavior of the considered amplifiers.

The transfer function is the behavior of the considered amplifiers. A class A/B amp has a constant output and does not change with any change of voltage. It therefore is irrelevant to the question at hand. You can not modulate it you can not use it for summing. It is a constant output device. The transfer function of a class C amp changes linearly with applied voltage. Just as you show in you chart. The voltage changes linearly with the modulation. Your chart will show the same results if you have no RF at all and you put a step function that has the same differential voltage as you show in col. 2 and 3 into you resistor.

If we just stay with the oscilloscope I think we can make some progress. You keep drifting off to waveforms and devices that are irrelevant to the question. As you agree the oscilloscope is a true picture of what is taking place. Do you agree that the whole topic can be resolved in this manner.

 

[Averagesupernova]

So now you are saying that a class A amplifier has a constant output that doesn’t change with supply voltage. That is not what you said several posts ago. You said that the source of the RF is irrelevant. So you now see my point? Class A is in fact different than class C? If so, then the output transistor/tube has to be considered part of the modulator.

Yes, you are right. The instantaneous voltage on the envelope changes linearly with applied audio. But the AVERAGE voltage out does not. If it did, then the power would also, and it doesn’t. But that is sort of irrelevant because as I have said countless times now the envelope having a shape that follows audio is just the summing action of the 2 sidebands and the carrier. Take 3 signals. A 1 MHz carrier, and an upper and lower sideband each at half power of the carrier. Make the sidebands each oh, say 5 KHz either side from the carrier. Now combine these 3 signals with a summing circuit. What you will see is the same thing that is seen if you modulated a 1 MHz carrier with a 5 KHz audio tone at 100%. But, there is no 5 KHz tone. Just the 3 signals combined. So you cannot say that responsibility for generating the envelope in question is solely placed on the audio frequency. Because in this case there is no audio frequency.

Incidentally, why should we just stick to the oscilloscope? Right is right, and ALL instruments involved need to lead us to the same conclusion. You just can’t pick your test equipment based on whether or not you like the results that they give you. And discussing modulation while prohibiting discussing the sidebands and other involved frequencies is like discussing the methods for making chocolate chip cookies while prohibiting the mentioning of chocolate chips. I have never said that the scope is the complete picture of what is taking place. And I do not believe that the whole topic can be resolved just by looking at what is on the scope.

 

[4newton]

Averagesupernova:

Yes, you are right. The instantaneous voltage on the envelope changes linearly with applied audio.

Thank you. I don’t know what the other points are that you are talking about. This was the only difference I know of that we had

You said that the source of the RF is irrelevant.

That is right. It is only important that the transfer function is that same as it is for high level modulation. You can’t take a rock and say that modulation is not linear because the rock can not be modulated. You must have a device that can be amplitude modulated.

why should we just stick to the oscilloscope

The main reason is to decide on an instrument to prove the question. The oscilloscope will do the complete analysis, only of course if you know how to read an oscilloscope. But at this point the issue is settled. If you wish I will interpret the oscilloscope trace for you and show you how it includes the sidebands.

Have a Good day.

 

[Averagesupernova]

Nice cop out...

 

[4newton]

What is you problem? What point do we not agree on?

Ummmm, not quite. A mixer or modulator requires a NON-LINEAR device. Using resistors will form an adder which is NOT a mixer.

Nope, a linear circuit is incapable of true modulation. You will not get sum and difference frequencies out of a circuit that you describe. It may be capable of adding 2 signals together to send down the same wire or something but this is not a mixer. I've worked on a lot of RF equipment and I have never seen a mixer/modulator that didn't use some sort of non-linear device.

But the FUNCTION of modulation is not linear. A true linear amplifier will sum two signals and not create new frequencies. A non-linear amplifier will sum them AND create new frequencies. Creating new frequencies is what modulation is all about.

Your last statement.

Yes, you are right. The instantaneous voltage on the envelope changes linearly with applied audio.

I pointed out to you this function.

The black box could be a constant RF source that has a variable resistor the output of the box is linear with the position of the resistor. As you change the resistor, by definition, you modulate the output. This box will produce amplitude modulation with the same waveform of any other AM modulator. There is no nonlinear component. This is why the nature of the RF source is of no importance. Only the transfer function is relevant.

This shows that you do not need a nonlinear device and that a linear change produces AM modulation. You did not comment on this statement.

All your other posting were off topic to the disagreement. You can not resolve a disagreement by expansion to non related topics. All you need do is reduce to basics as I have done. Do you wish me to comment where you are wrong on other point that you brought up? If we start into that we will be here forever. Like I said before. You have a lot of information but you just don’t put it together right.

 

[Ouabache]

Edwin Armstrong

Very perceptive discussion! Fascinating to learn about AM & FM transmissions.
BTW, some light reading describing the subtleties of FM, may be found in an original paper written by Edwin Armstrong,
inventor of frequency modulation, published May 1936, Proceedings of the Institute of Radio Engineers