Understanding the Basics of an AM Transmitter

[alanveron]

can anyone tell me how AM transmitter works?thanks

 

[NewScientist]

I believe they work by creating a rapidly changing electric current, for example, attaching a deattaching a battery from a circuit will produce radio waves.

-NewScientist

 

[dlgoff]

can anyone tell me how AM transmitter works?thanks

Here's a simple transmitter. http://www.scitoys.com/scitoys/scitoys/radio/am_transmitter.html

 

[alanveron]

thks New Scientist & dlgoff!any idea how to make a powerful AM transmitter??is it depends on the inductor and transistor?

 

[NewScientist]

Well just a note of warning - are you actually going to make one?

If you are, I cannot legally tell you how to do so, as broadcasting on AM wavelengths in most countries is illegal

-NewScientist

 

[Averagesupernova]

AM has nothing to do with wavelengths. AM is a type of modulation. Although there are countless places it is not legal to transmit.

 

[NewScientist]

Average Supernova- where is it legal?! And the legality of transmitting has a lot to do with waelengths.

Part 15 of Title 47 of the Federal Code of Regulations addresses the construction of homemade AM band transmitters. The three most germane paragraphs follow:

§ 15.5 (General conditions of operation)

(a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or for power line carrier systems, on the basis of prior notification of use pursuant to § 90- 63(g) of this chapter.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by an other intentional or unintentional radiators by industrial, scientific and medical

(ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

(d) Intentional radiators that produce Class B emissions (damped wave) are prohibited.

§ 15.23 Home-built devices.
(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.
(b) It is recognized that the individual builder of home-built equipment may not possesses the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equipment.

§ 15.219 Operation in the band 510-1705 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100\milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

-newScientist

 

[faust9]

Average Supernova- where is it legal?! And the legality of transmitting has a lot to do with waelengths.

Part 15 of Title 47 of the Federal Code of Regulations addresses the construction of homemade AM band transmitters. The three most germane paragraphs follow:

§ 15.5 (General conditions of operation)

(a) Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or for power line carrier systems, on the basis of prior notification of use pursuant to § 90- 63(g) of this chapter.

(b) Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the operation of an authorized radio station, by an other intentional or unintentional radiators by industrial, scientific and medical

(ISM) equipment, or by an incidental radiator.

(c) The operator of a radio frequency device shall be required to cease operating the device upon notification by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.

(d) Intentional radiators that produce Class B emissions (damped wave) are prohibited.

§ 15.23 Home-built devices.
(a) Equipment authorization is not required for devices that are not marketed, are not constructed from a kit, and are built in quantities of five or less for personal use.
(b) It is recognized that the individual builder of home-built equipment may not possesses the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable. The provisions of § 15.5 apply to this equipment.

§ 15.219 Operation in the band 510-1705 kHz.
(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100\milliwatts.
(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

-newScientist

The point ASN was making was that AM is a type of broadcast and as such is not tied to the band one broadcasts within. You can legally use AM transmitters in the ISM bands if you choose to. The legal code you state above says as much BTW. The only time one cannot use an ISM band is when use of said band endangers a life or interferes with pre-existing licensed device operating in the same band.

In fact, a lot of low powered low BOM transmitters(Nordic, Atmel and Microchip make these as do other manufacturers) give a designer the option of using ASK(AM) or FSK(FM) transmitting modes.

There are legal barriers to the power level and the frequencies at which one may broadcast without a license, but there are no legal walls preventing someone from using AM at 915Mhz or 2.45GHz.

That was the point ASN was trying to make. AM is not a wavelength. AM is a way of transmitting data. FM is not a wavelength. FM is a way of transmitting data. The same goes for PM, ASK, FSK, BPSK, etc.

 

[Averagesupernova]

Thanks faust. I also wonder where in my post I said it was legal to transmit AM? Reread my post newscientist. I think you missed a word or 2.

 

[NewScientist]

Ye - speed reading doesn't work well does it! :P sorry for any problems!

-NewScientist

 

[Rogue Physicist]

Amplitude Modulation works like this:

First you have a radio frequency, and you run it through a circuit called a modulator, which is really just an electronic volume control. This electronic volume control, (which obviously has a variable amplitude amplifier as part of its circuit) is then controlled directly by the audio signal (in a typical broadcast).

http://lentils.imagineis.com/rouge/Amplitude-modulation.jpg (click on this link for diagram)

The changes in the (much lower frequency) audio signal turn the volume up and down on the radio (carrier) frequency. There is no *real* audio component in the final signal, just a radio signal that is varying (relatively) slowly in volume as it copies the audio signal. Later, another radio (receiver circuit) takes the radio signal and pumps it through a capacitor, which "shorts out" all the radio frequency harmonics, and re-connects the 'dots' to reconstruct the audio signal on the fly. This is then amplified by an audio circuit and sent to a speaker.

http://lentils.imagineis.com/rouge/Amplitude-modulation2.jpg (click on this link for diagram)

 

[Averagesupernova]

You're a bit off rogue. You make no mention of sidebands, the fact that the carrier does NOT change in amplitude when modulation is applied, various other tidbits...

 

[Rogue Physicist]

You're a bit off rogue. You make no mention of sidebands, the fact that the carrier does NOT change in amplitude when modulation is applied, various other tidbits...

aren't you thinking of FREQUENCY modulation?

can anyone tell me how AM transmitter works?thanks

 

[Averagesupernova]

aren't you thinking of FREQUENCY modulation?

No I am not.

 

[Rogue Physicist]

You make no mention of ...the fact that the carrier does NOT change in amplitude when modulation is applied...

Okay you've really lost me:
Amplitude Modulation is EXACTLY about having the carrier change in amplitude by applying modulation. Whatever can you mean, if you are not referring to some other method of transmission, like 'single-sideband', or 'Frequency Modulation', or 'Pulse Modulation'?

Please explain? You're starting to remind me of the song, "I remember it well" from the movie "Gigi".

 

[Averagesupernova]

Okay you've really lost me:
Amplitude Modulation is EXACTLY about having the carrier change in amplitude by applying modulation.

No it isn't. That is the perceived result as well as the perceived method of obtaining a result.

 

[dlgoff]

No it isn't. That is the perceived result as well as the perceived method of obtaining a result.

Say what? Here's a good definition of Amplitude Modulation.
Regards

 

[Integral]

AM can be produced in severl different ways the far and away most common in modren radios is a Superheterodyne circiut.

Here is what wikipedia has to say about Super Superheterodyne

As you can see from the block diagram, the key work is done with mixers.

 

[Averagesupernova]

There was a discussion about modulation several months back in this forum which I took part in. I would suggest reading it. I finally lost interest.

 

[Rogue Physicist]

What's all the snobbery about? Amplitude Modulation is about modulating amplitude. It's not just 'perceived' action, its the principle upon which the audio signal is encoded onto the radio carrier.

I still don't see what was wrong with my simple easy to understand explanation, and the accompanying improved diagrams made from the Wikipedia ones.
I added simple passive component circuits which are actually used in building a crystal radio...

So what is wrong with being clear and concise in a form that anyone with an education extending into high-school can understand? Isn't that what helping people is all about, as opposed to showing off your 'knowledge' about dipole expansions and advanced radio transmission techniques?

 

[Averagesupernova]

Snobbery? Puuuuuleeeeez...

In amplitude modulation the amplitude of the COMPOSITE signal varies with the modulating signal. Suppose we feed an antenna with an unmodulated carrier that has a signal strength of 100 watts. Then we add modulation. The strength of the carrier signal itself does NOT vary in amplitude after the modulation is added. It is still 100 watts. New frequencies are created. These frequencies are the upper and lower sidebands. The sidebands themselves are what change in amplitude with the modulating signal. At 100% modulation the average signal strength of our theroretical carrier in combination with the 2 sidebands will be 200 watts.

I'm sorry if the way things really are causes you to go into a internet forum fit. It sure appears that is what has happened.

Edit: The strength of the sidebands don't actually change either when feeding a transmitter with a constant tone. If you modulate a carrier at 100% with a 1000 hertz tone you will form sidebands 1000 hertz above and below the carrier. When you turn the modulation down to say 50%, the strength of those sidebands will be reduced.

The reason I said to find the other thread is because it seems I am repeating all of it here.

 

[Rogue Physicist]

Of course any varying sine-wave can be broken down into it's lowest harmonic (the fundamental) and all the others above it. Since in reality, the harmonics don't continue infinitely, (and that includes the carrier) The Fourier Analysis is an abstraction too, in which we conceive of the single varying amplitude of the complex wave as a series of harmonics or sidebands to the carrier. Either representation is as real as the other, and the case for the complex shaped single waveform (a carrier varying in amplitude) it has that added preference that it is the simplest representation (Occam's razor) and it is 'realistic' in that a single complex wave reflects how for instance a driven speaker will move physically, not knowing or caring about 'harmonics' but rather instantaneous force.

The strength of the carrier signal itself does NOT vary in amplitude after the modulation is added. It is still 100 watts.

You have also spoken as though amplitude and power were identical, or have confused them. Of course the power, or rather average power of the signal is constant. The amplitude (i.e. the instantaneous voltage traced by an oscilliscope as the signal varies via modulation) is something else again, and varies (alot) according to the content of the modulating signal and the percentage of modulation.

You've talked about 'another thread' but posted no link either. How helpful is that?

Your description of simply applying a '100 watt' carrier signal directly to an antenna seems novel. I don't doubt you could 'amplitude modulate' two signals by applying a pure carrier to an antenna, along with a complex, pre-modulated and processed signal created from the audio signal and other components at a strong power level to the same antenna, but this seems disengeniuous.

In most radios, there are several (pre-)amplifier stages, and also frequency generators, which are used very early in the signal path to blend the audio and radio signals together into a composite AM signal long before it is applied to an antenna. Usually, you only get to talk about 'watts' in the final power stage of a transmitter or receiver. In the transmitter, the signal has already been pre-modulated (sometimes more than once to create Intermediate and final Radio Frequency signals), and in the reciever, the audio has already been extracted before being amplified to significant wattage.

The dozen or so AM transmitters I have built using tubes and transformers work as I have described, not according to the method you suggest. This would include the hundreds of AM radios I have repaired over the years too. I have radio books and magazines going back to 1920, and as far as I know, AM modulation is as straightforward as I have described it, at least up until the invention of transistors and digital circuits. Perhaps you are thinking of RADAR.

 

[dlgoff]

This may be the thread he's thinking about: https://www.physicsforums.com/showthread.php?t=59170&highlight=modulation
Regards

Don

 

[Averagesupernova]

Actually, that's not the link. When I have more time I will try to dig it up unless someone else does first.

 

[Averagesupernova]

Ok, here's the link: https://www.physicsforums.com/showthread.php?t=38928

Rogue, you are making things more complicated than necessary. I think you misunderstand what I am talking about when I say apply a carrier to an antenna with 100 watts and then add modulation and out comes a total of 200 watts. The modulation I am talking about adding is not from some other source that we hook to the antenna. It is audio fed into the transmitter just how the normal AM transmitter is designed to be used. Look at the signal on a spectrum analyzer, you will find what I am saying is true. I'm sorry I've been a bit brief, I didn't think it would be that difficult to find the other thread. Anyway, give it a read.

 

[Rogue Physicist]

Thank you for your civil response: I will take a careful look at your thread.

I wonder if the original poster of the question got anything out of the posts people have contributed. It would be most helpful if he could tell us what if anything finally answered his question, and what (if anything) simply left him confused.

 

[Pumblechook]

One form of AM TX uses high level modulation. The final class C stqage (valve or transistor conducts for less than one half cycle) has audio superimposed on its DC supply. A 100 Watt (carrier) TX would require a 50+ Watt audio amlplifier to do this.



Another form essentially does this at a lower power and then linear amplifiers(s) run in class AB are required. Class AB amps conduct for a little more thn half a cycle and with produce an amplified replicar of the input. Class C amps can't do that but are more efficient.

Doherty modulation (carrier valve and peaking valve) is often used these days which is more efficient than both of the above.