geojunkie ) writes:
I have noticed that some SSB tube transmitters only send a single side
band plus the carrier in AM mode. Does this provide a lower quality
signal for reception? What are the advantages and drawbacks versus DSB
plus carrier AM transmitters? It seems like DSB is the preferred mode
for the AM crowd.

Some receivers, such as my SX-101a, will demodulate only one side band
which must be selected. Does this provide lower fidelity than full
wave AM detectors? Does this compromise its performance as an AM
receiver?

Dan

One reason it's done this way in ham equipment is that it's cheap.
Just unbalance the balanced modulator, and you've got some sort of
AM for use with AM receivers. It takes a resistor off the mode switch,
which is really easy to implement. If you went and bypassed the sideband
filter, you'd get into more complicated switching (because you'd be dealing
with actual signals at the IF frequency).

In boatanchor era SSB equipment, AM is thrown in only for compatibility,
since it dates from a time when there was still significant AM activity.
Hence, that it may not be the best scheme (a low level signal going through
various stages of linear amplification) is irrelevant because anything more
complicated would be costly, for equipment that is really intended as
SSB equipment. A fallout from sending a single sideband, besides less
complicated circuitry, is that the signal chain doesn't have to amplify
the eliminated sideband, so more of the output signal is wasted; if both
sidebands were sent there would be a loss of actual output signal.

There was of course a period when AM pretty much disappeared from SSB
transmitters and transceivers, since there was little point in it since
AM was fading, and by that time, fewer with SSB wanted to talk to those
with AM. It's reappeared, to some extent, in recent years, likely more
fallout from manufacturers wanting to have as many bells and whistles.

In AM days, virtually all of it was generated at a high level (ie plate
or screen modulation of the final amplifier), and then there was no option
but to send out both sidebands. It's only when more complicated equipment
came along with SSB that many amateurs had equipment that could anything
but a signal with a carrier and two sidebands. But with an SSB transmitter,
given sufficient switching schemes, one could generate AM (a carrier with
two sidebands), DSBsc (Double SideBand with suppressed carrier), SSBsc (Single
SideBand with suppressed carrier) and SSBc (I don't think that's the proper
way to designate it, but Single SideBand with carrier). In other words,
there was never any real need to send the extra sideband except for
simplicity in the transmitter, and the carrier came along for the ride (and
of course it allowed for simpler reception). But since the two sidebands
are exact images of each other, one is redundant, yet the AM transmitter
has to devote some of its power to sending that redundant sideband.

And since the extra sideband is redundant, there is absolutely no loss
of "fidelity" if it's not sent, or the receiver ignores it. You can receive
an AM signal fine with an SSB filter, tuning issues aside. You will have
not lost one bit of actual content by stripping off the extra sideband
with an SSB filter (at the transmitter or receiver). Of course, one
traditionally used a wider passband in AM days, while with SSB once you had
the need for a filter to strip off the unwanted filter one might as
well use it to limit the bandwidth of the signal, so one might notice
some loss of "fidelity" when using an SSB filter. But this is due
to the bandwidth of the filter, not because one stripped off one of the
sidebands.

Michael VE2BVW