Do AM waves have symmetrical amplitude?

  1. I've been finding that many videos online depict AM waves (the modulated wave that is transmitted from a circuit/received by your aerial before demodulation) differently. Some show the waves as having symmetric amplitude so at a point (p) on the wave, the voltage is equal to +a and -a. Overall the wave has a voltage of zero (I'm not sure if I've used the correct term here can someone advise me?); I've heard this is why crystal radios have a diode - because one half of the wave must be removed to avoid the wave cancelling out. However, I've seen other sources which don't show an identical wave. Can anyone help me?
     
  2. jcsd
  3. marcusl

    marcusl 2,114
    Science Advisor
    Gold Member

    A wave cannot simultaneously have amplitudes +a and -a. The voltage oscillates rapidly at the carrier frequency f0 (which is 1000 kHz for "AM station 100"), while the audio appears as a slow modulation of the carrier amplitude. Thus, when the modulation has the value a, the actual signal oscillates between +a and -a every 1/f0 seconds.
     
  4. Drakkith

    Staff: Mentor

    In addition to what marcusl said, remember that the AM transmission also has sidebands which will vary in frequency and amplitude as well.
     
  5. Yes, that is one way to think of it. Once the + or - side is removed you can then filter the rf out and recover the original audio.

    [​IMG]

    This image shows an rf signal modulated by a single audio tone. You cannot see the individual rf waves in this image because the sweep frequency of the oscilloscope is too slow to resolve it. But you can clearly see the audio signal increasing and decreasing it's amplitude. Thus the term "amplitude modulation".

    [​IMG]

    Is This the other way you have seen it? This image shows the frequency spectrum of the same am signal. It's just another way of viewing the same thing. The horizontal axis is the frequency and the vertical axis is the amplitude. The peak in the center is the carrier and the two peaks on the left and right of it are the sidebands produced by the audio modulation. The difference in frequency between the carrier and the two sidebands are the same as the single tone audio modulation. The crystal diode in the receiver mixes the carrier with the sidebands to produce a difference and a sum frequency, of which the difference frequency is the original single tone audio.
     
  6. Wes Tausend

    Wes Tausend 130
    Gold Member

    ...

    When you see equal opposing amplitudes, you are actually looking at an "envelope" AM (amplitude modulated) waveform such as TurtleMeister has shown in his top pic above. As he mentions, the actual RF fine-lined high frequency dosn't show up here, inside the modulation envelope, because the CRT scope "drawing" resolution is too poor.

    In AM, the broadcast transmitter takes the coarse low-frequency audio envelope wave you see directly from a microphone (or such) and increases, or decreases, (modulates) the envelope at this corresponding audio frequency. The antenna "pulses" it into the air. Since the top and bottom seem to cancel out (average out) at audio frequencies, they are cut in half (decoded) by a diode and the high frequency half "RF filler" discarded. The remaining envelope-half rises and falls from zero, drives a speaker, and you hear the sound wave only. Since the modulation process is a pulsing voltage, pulses from lightening often interfere.

    Some crude depictions might show the envelope already cut in half, or even just the audio, and claim it is the AM signal, so keep this in mind.

    FM (frequency modulation) works similar except the envelope is a set of continuously straight (instead of wavy) lines above and below the contained high frequency RF. In this case, one would need to see the fine RF lines in between the outer envelope limits to notice that the inner high speed waveform spreads (audio freq lowers), or contracts tighter (audio freq goes up) to recognize from the picture that a lower modulated frequency has been imposed upon the RF as an audio signal. Since the decoding receiver does not respond to pulses or voltage spikes (just frequency variations), lightening, and other sparks, have very little effect making a nice quiet background for the signal.

    EDIT:
    [​IMG]
    From wikipedia http://en.wikipedia.org/wiki/Amplitude_modulation

    You can see that "AM waves do not have symmetrical amplitude" in that each RF wave alternates up, then down.

    Wes
    ...
     
    Last edited: Aug 3, 2014
  7. Thanks everyone, this cleared nearly everything up!
     
  8. sophiecentaur

    sophiecentaur 13,695
    Science Advisor
    Gold Member

    Read as much as you can about this. There are many really half baked descriptions and 'explanations' to be found on the Web. There is a mix of theoretical (ideal) explanations and practical examples where things are far from ideal. One thing you should bear in mind is that it is very rate, in practice, for an AM signal to be carrying just a simple tone so the envelope may look any shape.
     
  9. Perhaps you are seeing something being confused with SSB (single side-band) signalling, though that is a kind of FM rather than AM. No, the signals you get from the air must have a zero DC component, which means they are symmetrical, and rectification (what used to be called "detection") is required in order to receive them. DC cannot be transformed, nor can it be transmitted.
     
  10. sophiecentaur

    sophiecentaur 13,695
    Science Advisor
    Gold Member

    SSB can be regarded (and, indeed generated) as a combination of AM and Phase Mod. (Not FM)), both mods are required.
    "Symmetry" is not an essential consequence for a transmitted wave. Odd order distortion will produce asymmetry. This does, of course, require a very wide RF bandwidth in order to transmit it but it's easy to produce and display on a scope.

    DC information is actually possible to send in the form of the mean carrier amplitude.
     
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