RadioBanter - View Single Post

[email protected] · January 14th 07, 09:10 PM posted to rec.radio.amateur.policy

From: (Michael Black) on Sun, Jan 14 2007
12:24 am

"Dee Flint" ) writes:

Most CW computer programs are set up so that for transmission you set the
radio to CW mode and then run a line from a computer serial port to the
straight key jack on the radio. Therefore you are using an actual A1A
transmission. Right off hand, I don't know any CW programs that feed a tone
into the mic jack although I suppose there could be some out there.

On the other hand, there was a time when some commercial SSB rigs
did use an injected audio tone to send CW. Whether or not they
actually sent A1 would have been determined by the purity of the tone
oscillator, and the carrier suppression and unwanted sideband suppression
of the sideband rig.

Most of the ready-built "CW" or SSB HF transceivers in use
today do that sort of keying. Major reason is keeping the
PA at the same bias for all modes selected; makes for a
simpler mode selection control.

A more common occurance was RTTY, when AFSK was often used to send
FSK on an SSB rig. (I suppose it was more common since it was
easy to unbalance a balanced modulator and just key a stage for
an SSB rig, especially when it came from the factory that way, while
commercial rigs did not tend to have built in FSK ability and of course
frequency shifting often resulted in slight variation of how much shift
occurred depending what you modified and what you shifted.

The first RTTY radio circuits, circa 1930-1960, used
separate exciters to feed Class C biased transmitters.
The exciters (not an exciting name for a separate box)
literally shifted their carrier frequency from Mark to
Space. Those were, generally, crystal controlled but
with an adjustment for the "shift" (of Mark to Space).
Around 1950 the first "VFO" style of FSK exciters
appeared on the radio market.

Doing RTTY via audio frequency initial Mark-Space
shifting is simpler, more stable, but requires a SSB
transmitter system to translate the audio spectrum
into the HF spectrum...which is exactly what a SSB
voice modulator does. Once spectra are translated
there isn't any real difference in frequencies at RF.
Once again, there need be no change in PA biasing
between voice, data, or "CW."

The internal microcontroller of practically every
ready-built SSB transceiver takes care of the shifting
used during a translate of spectra. That's a
relatively simple programming task and, essentially,
invisible to the operator. It should be noted by
operators (but seldom understood) since the adjustment
of "carrier frequency" for operator display versus
mode varies between manufacturers.

This is precisely why two-tone oscillators are needed for testing
SSB transmitters. Because only then are you actually modulating
the output. Otherwise, it's just a carrier.

Quite true and succinctly put, Michael. A single
frequency from any source, translated to HF, will
still be a single frequency. Two frequencies close
together (a "two-tone" source) will still translate
to two RF frequencies close together...those can
simulate a carrier and its single steady-amplitude
AM tone content. [relative amplitudes of the pair
will simulate anything from percentage modulation
(as with AM) or the carrier suppression (of SSB).

What happens at AF to RF translation in THIS group
is the emotional-baggage tie-in to the mythos of
morse such that direct RF on-off keying is somehow
a "pure way" to send "CW." Those lost in the mythos
will contentiously state that audio tone generation
(with on-off keying of the audio) translated to RF
is "false" or "artificial." Those folks just
haven't made the connection to spectral content of
ANY modulated signal...a few even contend that "CW"
(on-off keying) "has no sidebands" because "it is
just turned on or off!" :-)

Perhaps worse is the group that believes
all-Class-C transmitters are "pure" in their
spectral content (as if those had no harmonics)!
Sigh... :-(