[AMRadio] Physical Reality of Sidebands

Donald Chester k4kyv at hotmail.com
Sun Jan 16 20:00:33 EST 2005

>I don't believe that the carrier still exists because of tank circuit 
>ringing when 100% negative modulation is reached either. The Q of the tank 
>only allows for a couple of cycles at most when no carrier is present. It 
>decays very quickly. With modulation, even at voice frequencies, the 
>modulation frequencies are much lower and there are many more rf cycles for 
>each audio cycle. So one audio cycle would be equivalent to many rf.

It wouldn't be the ringing in the transmitter tank circuit that would fill 
in the "holes" in the carrier.  As you say, the rf tank would only smooth 
out the rf waveform of the carrier.  The bandwidth of the resonant tank 
circuit is tens of kHz.  It is ringing in the i.f. filter in the receiver 
that produces the effect.  The i.f. selectivity is ideally about the same as 
the bandwidth of the transmitted signal, and with today's receivers would 
rarely be more than twice the bandwidth of the signal.  The receiver 
bandwidth must be many times the signal bandwidthl for it to appear as an 
"amplitude modulated" carrier rather than steady carrier plus sidebands.

Neither approach (physical existence of sidebands or carrier of varying 
amplitude) is "wrong"; they are completely campatible with each other.  It 
is a matter of how you "look at" the signal.  With a wideband receiver such 
as an untuned crystal set or cathode ray tube deflection plate, the 
existence or  non-existence of sidebands is meaningless.  With a selective 
receiver, the carrier of varying amplitude becomes an impossibility as the 
frequency of variation approaches the receiver bandwidthr.

This is somewhat analogous to another concept of physics; the quantum 
theory.  Electromagnetic radiation such as rf and light have a dual nature 
in that they can be equally well thought of as particles (photons) or waves, 
but not simultaneously.


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