Amplitude Modulation is the method of superimposing intelligence on a carrier wave by means of varying the amplitude with the wave shape of the intelligence. When one looks at an AM signal on a scope with a linear horizontal sweep and a wide band pass it appears as one signal varying in amplitude. This is known as viewing the envelope. But when the band pass is narrowed keeping the carrier frequency as the center, we begin to see that the higher frequency modulation disappears, and as we narrow the band pass further, more and more of the modulation disappears, until all that is left is a carrier of unvarying amplitude.
This is because the products of modulation are actually upper and lower sidebands. The only thing that they have in common with the carrier is it's exact center reference point in the band spectrum and with the carrier as reference the two sidebands are mirror images of each other in both frequency and phase. This is a very important characteristic as we will explain later.
The reason the signal on the scope appears to vary in amplitude is because the scope shows the sums of all the signals it receives. At the peaks on the scope, the phase of all of the signals are the same so they add together to make one large pattern. At the trough of the scope's picture, the sideband's phases are such as to subtract from the carrier resulting in a small amplitude at that point of the pattern.
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It is important that the sidebands each act on the carrier in the same way at the same time. That is to say that if the upper sideband is headed towards being in phase with the carrier then the lowerside band must also be acting in the same manor or a true reproduction will not be possible. This is not to say that it will not be intelligible. It will just not be a true reproduction of the modulating intelligence. This is why the carrier must be in the exact center of the two side band's spectrum with respect to both frequency and phase.
As a matter of fact, if the carrier is shifted 90 degrees with out moving the side bands then envelope detection would yield virtually no intelligence at all. The intelligibility would increase however if the carriers phase was to continue to change passing the 90 degree point and would be best again at 180 degrees. This occurs because the side bands work best and together against the carrier if it is dead centered between them with respect to frequency and phase. At the 90 degree point the side bands will not work together against the carrier at all. At the 180 degree point, the intelligence would be good but the detected intelligence would also be 180 degrees from where it would otherwise be.
This is all true as long as both side bands are present. If one side band is not transmitted then the phase of the carrier no longer matters but the frequency of course and amplitude do. See SSB for more details.
Generally speaking, if the carrier that you used to create the modulation
is transmitted with the side bands and then used as the reference carrier
for the purpose of detection all the phases will be good and the intelligence
will be best. There are however a few things that
can cause distortion of the transmitted signal both in the equipment and
in the propagation path that need to be considered.
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