[Collins] AOR DDS-2A Revisited

[Adam Farson]

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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