SSB AM is not as good as full DSB AM but it is also not as poor as some
believe it is. A properly set up SSB AM rig can sound pretty good.
Modulation level does have to be kept down some as Don says or the
receiver
detector distortion goes up rapidly as you approach full modulation. As
you
near 100% modulation the receiver detector can't tell the difference
between
the carrier and the modulation signal. Second harmonic distortion results.
The only reason it even sounds fairly decent is that with the human voice,
the average amplitude level is equivalent to about 30% modulation. The
peaks that approach 100% are relatively infrequent, so the distortion is
tolerable.
When I say set up properly, there are few that know how to do that right.
Most that run SSB rigs on AM, in the DSB with carrier mode have a hard
time
making them sound good on AM and it is mostly the fault of not setting it
up
properly. Some sound very nice.
Most modern transceivers run true DSB in the AM mode. Theoretically, there
is no reason these wouldn't sound identical to a broadcast transmitter. The
carrier level setting is critical, and the audio level is critical. These
level settings must be so that the carrier is modulated 100% on voice peaks,
but enough headroom is left to allow the crest of the positive modulation
peak to reach 100% without flat-topping.
Want to see what good SSB AM sounds like? Just tune off to the side of a
good sounding AM signal with your receiver so that the filter cuts one
side
band off but not enough to reduce the carrier. The audio level will drop a
little and distortion may rise slightly but it will still sound pretty
good.
You are hearing the same thing as if it was transmitted with only one side
band. This same trick is often done when there is heavy qrm on one side.
You
listen to SSB AM and don't even realize it!
You still hear the distortion on heavy voice peaks. This would be better
described as equivalent to vestigial sideband, just like the video TV
signal. The lower frequencies, where the heaviest peaks usually occur are
not cut off with the filter, since the receiver is rarely tuned to one
sideband to the point that the carrier is reduced, so at the lower
frequencies with the most peak audio power, you are still demodulating DSB.
The part of the signal that is effectively SSB is in the higher frequency
range. With the human voice, these frequencies are naturally low amplitude,
so quadrature distortion from the SSB detection mode is usually not
apparent, unless the high frequency components are riding on top of heavy
peaks at lower frequencies. In the latter case, the distortion is apparent,
and comes through as intermodulation distortion.
As to Collins S line receivers sounding better than some of the rice box
receivers, I haven't heard one. I have several 75S-1, 75S-3, 75S3A etc.
receivers. All have more distortion than my Kenwood TS430. I measured each
with a distortion analyzer, and you can hear the difference, and the TS430
runs rings around any of the Collins receivers in audio quality. So just
because it's old doesn't readily mean that it is better.
From my experience, the audio section of all Collins amateur equipment from
the era of the 75A4 on, is an afterthought at best. I made my 75A4 sound
reasonably good on AM with several reversible modifications. This inclues
increasing coupling capacitance between low level stages, and using an
outboard audio power amplifier, with the AF signal picked off the AF gain
control. A plug-in module replaces the 1st audio stage (12AT7 IIRC), and
delivers the audio to the outboard amp via a cathode follower stage.
Mechanical filters are the limiting factor in the attainable audio quality
on any receiver that uses them. That is why the R-390 sounds so much better
on AM than does the R-390A.
The distortion that occurs with SSB + carrier can be easily and clearly
demonstrated using the standard rotating vector diagrams like the ones in
the 1960's era ARRL Handbooks and their series Single Sideband for the Radio
Amateur. I first became aware of it years ago when I built an experimental
SSB generator at 64 kHz using a telephone company multiplexing mechanical
filter, and looked at the envelope of the signal on a scope with zero,
reduced and full carrier. With full carrier, the familiar modulation
envelope of AM modulated with a sinewave became increasingly distorted as
the percentage approached 100%. At 100% negative modulation, the waveform
becomes a sharp point at the part of the envelope near the baseline.
Interestingly, quadrature distortion is almost a mirror image of the
distortion heard with a square-law detector. SSB + full carrier should come
thorugh distortion free if the signal level fed into the detector is such
that the true square-law curve extends from zero signal to the crest of the
100% modulated voice peak. Square law detectors were commonly used on late
1920's-early 1930's broadcast radios using triode grid-leak detectors.
That's what gives those radios that peculiar sound that is almost like
selective fading distortion. Later BC sets switched over to diode detectors
because the latter has a linear characteristic and not square-law.
It would be interesting to experiment with SSB-AM received with a grid leak
detector to see if it really does demodulate that mode without the usual
distortion.
Usually, if I have to tune to one side of the AM signal to dodge QRM, I turn
on the BFO and copy it as SSB. The inserted carrier into the product
detector produces a true product of the sideband and carrier, without
quadrature distortion.
When SSB is tuned in with a diode dectector, that's the reason the rf gain
has to be turned down to get a clear signal. Think of the SSB output from
the i.f. amplifier as the modulation, and the BFO as the carrier. If the
signal being fed into the diode decector is strong, the BFO is being
modulated at a high percentage, hence the distortion. When the rf gain is
reduced, the percentage of modulation is effectively reduced to a low value,
and the distortion is not apparent.
Don k4kyv
______________________________________________________________
AMRadio mailing list
List Rules (must read!): http://w5ami.net/amradiofaq.html
List Home: http://mailman.qth.net/mailman/listinfo/amradio
Partner Website: http://www.amfone.net
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:[email protected]
To unsubscribe, send an email to [EMAIL PROTECTED] with
the word unsubscribe in the message body.
