[AMRadio] Physical Reality of Sidebands


Bob Bruhns bbruhns at erols.com
Mon Jan 17 08:36:01 EST 2005


I am convinced that sidebands exist.  But as for whether the carrier goes
away at 100% negative modulation...

How's this: a carrier should be considered to exist during the period when
it has recently been detected, and we are actually receiving modulation from
it.  It should not be considered to exist when it is clearly not being
generated.

So in the example of an AM signal at the instant of 100% negative
modulation, we should consider the zero to represent zero percent of the
carrier, averaged over time during recent history.  We can see that this
will result in appropriate demodulation of the actual signal.

But when the AM transmission is over, either after the end of a
transmission, or after the end of a broadcast day, or in most cases if the
signal fades out due to propagation, etc, we should consider the carrier not
to be present.  This gets a little tricky, because we can not really be
certain (Don's Heisenberg Uncertainty Principle thought here) that the
transmitter is not sending us a very, very long zero.  But we can be pretty
sure, pretty quick.

A synchronous detector is a good flywheel that tracks the known carrier
frequency and holds the reference for us during zeros and noise bursts.  We
set up a synchronous detector to detect the carrier and hold the reference
for a short period of time.  This works pretty well for the signals we
actually transmit.

Now take the example of a 1 KHz sinewave transmitted as double sideband
suppressed carrier.  You get pulses of alternating carrier polarity.  During
any given pulse, for about 500 microseconds at a time, you get an AM signal
with a carrier, transmitting one half of a sine wave.  When the pulse is
over and the signal passes through zero, it goes negative and the next pulse
appears - pretty much identical to the first pulse, but with reverse carrier
polarity.  You would not know the carrier polarity reversed except for your
flywheel reference.  But demodulating with the flywheel reference gives you
the 1 KHz sinewave.  It is evident to the observer that the flywheel
reference was correct.

With a very low modulating frequency, a DSBSC signal would just look like a
fading carrier.  The transmit frequency stability and the reference flywheel
precision would have to be very high to determione that the carrier polarity
had reversed.  At some point this becomes irrelevant, because the
transmission path varies in length, there is drift and phase noise in TX and
RX, and the signal is not received well enough to know for sure that the
carrier polarity flipped.  (Uncertainty again.)

And although with DSBSC we keep getting pulses of carrier, they keep
reversing, and on average they balance out to zero.  We accept that the
carrier is suppressed, we don't hear a heterodyne where we would usually
hear one, etc, yet we see the carrier dancing on the oscilloscope.  But over
the appropriate integration time, its frequent polarity reversals cause it
to balance out to zero.

For most real signals, a very long time base is inappropriate.  In some
cases though, such as slow synchronous CW, a long time base is appropriate.
So the receiver time scale should be appropriate to the signal being sought.

So it's a reality check issue.  Surely a carrier was not transmitted for all
time, just because it existed for less than a second at some point.  But
just as surely, the instant of 100% negative modulation should not be
reproduced as a glitch.  The application of the appropriate time scale is
the key, and it is up to the listener to determine what happened.

  Bacon, WA3WDR





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