|[AMRadio] Re: AMRadio Digest, Vol 49, Issue 31|
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
> 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
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
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.
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