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'.