[Collins] AOR DDS-2A Revisited

[Sheldon Daitch]

Adam,

In terms of the intent of the discussion, correctly tuning in 
SSB signals, no, I don't think you've missed anything, but 
you have brought up some points that bring our discussion of SSB 
comms into the modern day of fixed channel radios generally 
associated with the non-amateur world. 

My discussion on tuning SSB was related more in using receivers that
are not set up for channeled operations.  Actually, tho, it also 
applies to synthesized frequency control receivers that have enough 
discrete steps to allow "fine" tuning to a signal.

You are correct, in that RIT or clarifier really helps, in transceiver
operation, and I think one of the first applications for RIT in the ham 
bands was to keep two stations from "walking" across the band as one 
station followed the other in frequency.  Part of that situation may 
have also been a function of not correctly tuning in the other 
station to that they were essentially on the same frequency.  With
separate transmitters and receivers, using independent frequency
control, of course, no RIT is necessary, it is simply a function
of the separate receiver.

How close does one have to be in frequency?  Well, one list 
responder said if he could understand the other guy, that was close
enough.  And perhaps if they are happy, well, who am I to complain.

You mention applications where a reduced carrier is being transmitted,
and as I noted in a previous post about the AFC circuitry in some ISB
receivers that I used (note, these were 1960s vintage units), and
stable synthesizers were not really available on the market for
receivers.  Point is, we used a carrier to keep the receivers on 
frequency, even in our ISB operations.  The AFC system in the RCA SSB-R3
not only kept the receiver on frequency, but the regenerated carrier
inside the receiver system was in phase with the transmitted carrier.

The use of the equipment really drives the need for accuracy in the
system.  Obviously, if only understanding the person at the other 
end is the criterion, then the standards might not be as strict.  If,
as you point out, communications modes that require phase accuracy,
then the stability standards are tightened up.

I took a look at the Harris RF-3200 here and it does have a clarifier
but I realized the clarifier steps are no better than the steps of the
main frequency control, 10 Hz steps.  The advantage, as discussed, with
the clarifier is that it tunes only the receiver, and with 10 Hz 
steps, you would not be more than 5 Hz off from another station,
anyway.  

This unit will also transmit in R3E, it doesn't specify the carrier 
reduction, I'll have to explore the maintenance manual to see what
that it, but I think it applies only to the ALE, automatic link
establishment option.  Also, another use for R3E is found on the
HF air traffic control links, during the SELCAL tone transmit
periods.  No carrier is present on voice, but when the SELCAL 
tones are transmitted, there is a reference carrier also transmitted
for proper tone detection at the receiving end.

Sheldon







Adam Farson wrote:
> 
> Hi Sheldon,
> 
> In terms of the evolution of HF-SSB radio communications equipment, is
> something not being missed here?
> 
> The requirement for frequency stability, accuracy and repeatability as a
> prerequisite for ease of copying SSB radiotelephone signals drove the shift
> from free-running LC oscillators to synthesisers controlled by stable master
> oscillators. A "clarifier" or "RIT" control allows the operator to
> compensate for small differences. Ever-tightening national and ITU-R
> standards were another factor in this transition.
> 
> The ITU recognises two SSB emission types; J3E (SSB suppressed-carrier), and
> R3E (SSB reduced carrier, with the carrier at -6, -12 or -20 dBr). As you
> mentioned, R3E was widely used in single-channel applications in the early
> days, with AFC on the receiver LO. As master-oscillator stability improved,
> J3E largely supplanted R3E. Today, R3E is showing up again in the maritime
> radio services, to support modes such as HF e-mail which require phase
> coherency between transmitter and receiver. J3E is still acceptable for
> single-channel voice radio-telephony in all radio services; the "clarifier"
> or "RIT" covers a multitude of sins!
> 
> Cheers for now, 73,
> Adam VA7OJ/AB4OJ
> 
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