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What exactly is Amplitude Modulation?

Discussion in 'General Electronics Discussion' started by scroth1994, Jul 23, 2012.

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  1. scroth1994


    Jul 23, 2012
    First off, I wish to say that I am not exactly an expert in electronics. Though I do consider myself to have a basic knowledge.

    I'm finding that I can't seem to understand what AM truly is.

    I was thinking about how I would go about possibly making a very basic AM transmitter at home (I'm talking very short range as I would hate to do anything unlawfully).

    My basic (and probably stupid/impossible?) design was using an alternator to produce a high frequency carrier wave. I would then use a microphone which varied resistance, like a carbon microphone, with sound. I would pass the carrier wave directly through the microphone and the varied resistance would, so to speak, alter the amplitude of the carrier wave produced by the alternator.

    Now, if I sent this wave out a few metres and used a receiver which rectified the carrier wave and smoothed the result with a capacitor, would I gain any intelligible data from it?

    I think I understand that what I just described is not what people would consider AM. But then what is AM?

    Thanks in advance!
  2. duke37


    Jan 9, 2011
    AM is amplitude modulation i.e. the strength of the signal goes up and down.
    Your method of producing it was used in early transmitters. The main problem is that the alternator will have far too low a frequency for radio transmission. There is a very old, very high powered transmitter still in existance which is fired up once a year. This has many poles and runs very fast to get the frequency. The modulation is generated by altering the alternator excitation.

    There are designs to make a very low powered transmitter so that old medium wave radios can still be used. These are made with either valves or transistors. They have been given the name of pantry transmitters.
  3. GreenGiant


    Feb 9, 2012
    I agree that using an alternator would not achieve appropriate frequencies to make an AM wave, you would need a crystal oscillator ideally.

    My other suggestion would be to purchase one of the microphone kits that has the circuit already designed for you, all you have to do is put it together, usually they work on FM frequencies/properties though but I would say that is a valid option.
  4. duke37


    Jan 9, 2011
    Have a look at Wikipedia, Alexanderson alternator. The Swedish one is still operational once each year. I believe they had problems this year and the transmission ws delayed
  5. scroth1994


    Jul 23, 2012
    Thank you for your replies!

    I did see the Alexanderson Alternator before, that's what got me thinking about a possible miniature version.

    But if it is not possible, it isn't possible.

    You could probably make small alternators that could produce waves fast enough for transmitting small distances?

    It's interesting never the less!
  6. rogerk8


    Jul 28, 2011
    I know nothing concrete about AM-modulation (or radio) but here is my attempt. You will need a carrier wave i.e the high frequency (c). Before digital TV-broadcasts the picture (here in Sweden anyway) was amplitude modulated (AM). This meant that the picture could be somewhat hard to see if the signal wasn't strong enough. Nowadays the picture just disappears. Progress? Well I don't think so. Anyway, when you have a carrier, you can modulate it like an envelope and make use of the carrier to make the message propagate thru the air. Here is the academic formula: e(am)=(ec+m(t))cos(wct) where m(t) stands for the message (I had to look this one up in one of my books :) I do not recon this will be of any help but I hope so!
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