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October 24th 03, 04:05 AM

On 24 Oct 2003 01:00:44 GMT, (Avery Fineman)
wrote:

In article , Gary Schafer
writes:

Along the same line consider that the envelope of an SSB signal has no
direct relationship to the original modulation the way that an AM
signal does.

This is why you can not use RF derived ALC to control the audio stage
of an SSB transmitter the way you can with an AM transmitter.

You can't use ENVELOPE detection on SSB the same way it is
done on conventional AM.

But, you CAN use RF-derived feedback - if mixed with a steady
carrier to recover the modulation content - to do that very well.

Agreed.

Or audio clipping that works on AM but does not work the same on SSB.

? Wrongly-done audio clipping on AM is just as bad as on SSB.

RF clipping circuits are quite another thing from audio.

In SSB RF clipping, the signal being peak limited is the same as that
being transmitted.
If you do audio clipping on an SSB transmitter you can not limit the
peak output as you can in AM or in RF clipping of SSB, because the
output of the SSB transmitter has no direct relationship to the audio
in.

Transmit a square wave on an AM transmitter and you see a square wave
in the AM envelope. Do the same with an SSB transmitter and you only
see sharp spikes in the envelope.

That depends on the frequency of this square wave. That also depends
on what is being used to view the RF envelope. A 50 MHz scope will
show the RF envelope of any HF rig.

With a 1000hz audio square wave into an SSB transmitter you will not
see a square wave envelope as you would from an AM transmitter. You
will see sharp spikes.
My point is that what you get out of an SSB transmitter is not
directly representative of what you put in, as it is in AM.
Even though the modulation process in the SSB transmitter starts out
the same as it does in the AM transmitter. Much phase modulation takes
place in the SSB process.

Put an electronic keyer on the SSB transmitter and transmit only dots
at a high speed setting. The SSB envelope will show the dots as
dots.

Same as a CW transmitter.

Conversely, if you put a high-purity sinewave audio into a SSB xmtr,
a spectrum analyzer display will show only a single frequency signal.

Put two pure equal amplitude audio tones into the SSB transmitter and
the envelope out at first glance looks like a 100% modulated AM
transmitter envelope that is modulated by a single tone. But a closer
look will reveal that the envelope is not a pure sine wave. The
envelope is folded over so as to produce a sharper crossover.

If this signal is looked at or listened to on an envelope detector the
detector will produce high 2nd harmonic distortion. (that's another
story)

Look at the signal on a spectrum analyzer and it will show 2 carriers
seperated by the seperation of the two tone frequencies.

No one can interchange frequency and time domains directly and
get an explanation. Envelope viewing is time domain. Spectral
analysis is frequency domain.

Agreed.

Len Anderson
retired (from regular hours) electronic engineer person

73
Gary K4FMX