[Collins] AOR DDS-2A Revisited

[Sheldon Daitch]

I agree with Jerry on the proper tuning of SSB signals.

With skill, and careful listening, it should be very easy 
to tune an SSB signal to within a few Hertz.  When in the
days of working at an HF receiver site, I could 
routinely tune our receivers to well within a few hertz, 
simply by listening to the harmonics of the audio, as
detailed by Jerry below.  In most cases, the AFC circuitry
would flash only a few phase shifts away from the proper lock.
Granted, it was much easier to tune on music than voice, as
there were many more harmonics available.  (The carrier level
meter was also a giveaway.  These receivers had a sharp
crystal filter for the carrier frequency, which was
reprocessed for both the detectors and the AFC circuitry.)

One may claim that not knowing the voice characteristics of
the other operator makes it difficult to tune in properly,
but with very few exceptions, almost all voices have enough 
characteristics that is is not too difficult to "find" the 
right frequency.

About 30 years ago, AF MARS had used a frequency netting
device known as FATTO (fixed audio two tone oscillator,
IIRC).   It was a two tone audio oscillator, where the
two tones were harmonically related, say 1000 Hz and 2000 
Hz.  When the receiver was not on frequency, the
two tones had no harmonic relationship and a beat note
could be heard between the two tones.  As the receiver
was tuned closer and closer to the proper frequency,
the beat note was lowered in frequency until it nearly
disappeared.  

One could suggest that below, say 30 to 50 Hz, one could
not hear the beat note, and this is correct, except
that as got very close to the proper frequency the 
beat note became more of a phase difference, which, with a 
little training, was easily discernible.

I guess this could easily be described as similar to the
technique used to tune a guitar, by fretting up the
strings and tuning out the beat between harmonically 
related strings.

(Oh and this just reminded me of an old ERT feed we used to 
monitor, out of Greece.  The transmitter site had an audio
feed that apparently went through some FDM multiplex system
that was off by a few hertz, and it made the music sound
awful.  I could "tune out" this frequency shift by tuning
the receiver an equal number of hertz off frequency the
the other direction to put the audio back in the proper
frequency range.  The receiver was not stable enough to
maintain this frequency for long, and it required a fair 
amount of attention, and I also had to make sure the AFC
was off, otherwise, the AFC would pull the receiver back
to the "proper" frequency.

73
Sheldon
WA4MZZ/SV7

"Dr. Gerald N. Johnson, electrical engineer" wrote:
> 
> A huge fallacy of frequency setting of SSB is presuming one can set the
> receiver frequency to exactly the transmitter's suppressed carrier
> frequency.  Tuning halfway between the limits of understandability is
> only a first approximation. The proper scheme requires a high fidelity
> ear that can match received harmonic components to the true harmonics of
> the fundamental voice components. This gets the most natural sound (like
> AM or FM) but takes a lot of skill and stable equipment. MARS frequency
> tolerances tend to be unenforceable because of the uncertainty of tuning
> to an incoming signal.
> 
> I notice every year at FD that many new hams accustomed only to VHF FM
> activity rarely achieve tuning accuracy as close as 200 Hz and their
> contact rate suffers as their replies get lost in the adjacent QRM.
> 
> Another problem with 10 Hz digital readouts is that too many assume that
> if the signal is not copiable having tuned 340 (or 1000) Hz away that
> its a NEW frequency suitable for initiating communication on top of that
> already using that spectrum.
> 
> I did achieve a 1 ppm result in one ARRL FMT using nothing but my 75S3B
> and its built-in calibrator. I know from later tests that I won't hold
> frequency that close because 10 Hz is needed for successful unattended
> HF packet and it won't do that.
> 
> 73, Jerry, K0CQ, Technical Advisor to the CRA.
> --
> Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
> Reproduction by permission only.
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