[AMRadio] Re: AMRadio Digest, Vol 49, Issue 31


D. Chester k4kyv at charter.net
Fri Feb 8 11:04:12 EST 2008


> With only one side band and carrier the detector can't make up its mind
> which signal is supposed to be the carrier and which is the modulation 
> when
> you have higher levels of modulation. The result is high levels of second
> harmonic distortion generated in the detector. As long as the modulation
> percentage is kept to a lower level the distortion is minimal and it 
> sounds
> fine.

>Gary K4FMX

Those waveforms can be demonstrated using rotating vectors.  Each sideband + 
carrier generates variations in phase as well as variations in amplitude. 
Any SSB  signal with or without carrier can be resolved into fundamental AM 
and PM componennts.  With DSB, it can be shown with the rotating vectors 
that the phase variations in the resultant are zero because the PM of the 
upper sideband exactly cancels the PM of the lower sideband.

At lower percentages of modulation the distortion of the resultant due to 
the phase variations in SSB with carrier is low, because those phase 
variations are swamped out by the high amplitude carrier.  But at  higher 
percentages, the phase variations have a greater effect on the resultant, 
thus the signal is distorted.  This distortion of the resultant is called 
"quadrature distortion".

We have exactly the same phenomenon when copying SSB on a receiver using the 
diode type AM detector.  Remember, you have to reduce the RF gain and turn 
up the AF gain to get the greatest clarity in the audio signal.  Think of 
the BFO in  the receiver as the carrier (it is, in fact, the reinserted 
carrier).  With the rf gain  high, the SSB signal "modulates" the carrier at 
a relatively high percentage, and distortion is produced.  Turn down the RF 
gain control and the percentage of modulation of the carrier (=BFO output) 
becomes lower, and the distortion is less noticeable.  Since the percentage 
of  modulation is less, you have to turn up the audio gain to recover the 
lost audio signal amplitude.

The only reason SSB with carrier can be somewhat tolerable to listen to on a 
diode detector is the nature of  human voice.  Voice peaks are high, while 
the average percentage of modulation remains relatively low.  (That is also 
the reason why p.e.p. is not an appropriate method for determining 
transmitting power - it is the AVERAGE power, not occasional voice peaks, 
that determines the loudness and interference-causing capability of a 
signal.)  So, the human ear tends to tune out the highly distorted voice 
peaks @ near 100% modulation, while the average voice level, at somewhere 
between 20 and 30 percent, is readable at the diode detector, with tolerable 
distortion.  Try listening to the voice announcements at CHU an 7335 kHz 
sometime.  They announce in SSB + carrier.  Listen to the voice quality with 
the receiver in normal AM mode.  Now switch to SSB mode and carefully 
zero-beat the carrier.  Notice how  much better the voice sounds.

As Gary said, with SSB + carrier, the detector can make no distinction 
between carrier and  sideband.  It is simply two signals mixing together at 
the diode detector, with whatever resultant the vectors produce.  Copying 
with a product detector theoretically gives zero distortion because the 
effective "percentage of modulation" of the BFO approaches zero, while the 
audio output voltage is still equal to the signal input voltage.  A two-tone 
audio test  signal applied to a SSB transmitter produces the equivalent of a 
SSB-with-carrier signal with 100% tone modulation.  Either tone can be 
thought of as the carrier while the other is thought of as the sideband.

Again, the only practical use for SSB + carrier is to give a PLL-controlled 
BFO in the  receiver something to lock onto to eliminate the usual SSB 
frequency error when tuning be ear.  This "pilot" carrier need be no greater 
amplitude than 20 dB below p.e.p. of  the sideband.

Don k4kyv 



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