Whenever the topic of receiver sound quality comes up, it seems that the
discussion becomes contentious. I don’t understand why but that’s just the way
it appears to be. Maybe it’s just the suggestion that not everything about a
given Ten Tec radio is perfection personified that seems to rile some people.
My recent meandering thoughts about the Omni 6 receive audio is yet another
example of this tendency.
It might be helpful if I first express that everything I say about the issue is
purely my OPINION, not a statement of fact. I only make statements of FACT when
I can reference published, generally peer-reviewed literature to support my
position or when I have irrefutable experimental results to similarly support
any statements I might make.
That said, I want to clarify why I even care about the Omni 6 radio as opposed
to using something else, for my day-to-day listening.
It is my conviction that the overall design plan for the Omni 6 (actually
beginning with the Omni 5) was inspired. At that time, PLL synthesizers were
awful and there were no exceptions, at least not in the line of Ham gear. Even
the highly touted Drake TR7 synthesizer was pretty marginal in terms of phase
noise. The other kid on the block, the Collins KWM-380, was even worse. Thus
the choice to use crystal oscillators for the local oscillator made sense at
the time. The use of the VHF PLL, divided by 40, for the 5 MHz “VFO” was
equally inspired. This is the same method used by Hewlett Packard in the
famous, and quite excellent 8640B signal generator, which I use in my shop. The
division of the PLL frequency by 40 as implemented in the Omni 5 and 6 reduces
the PLL artifacts by 40, producing an overall premix local oscillator with
phase noise levels that are exceeded by only a handful of very recent radios.
The later addition of a temperature controlled PLL to the crystal oscillator
served to cure a minor problem of long term drift on the crystal oscillator
without introducing phase noise because this PLL did not need to be a fast
responding (high gain) loop to serve its purpose. Its only job was to correct
for very slow changes in crystal frequency and it did this very well.
To be continued...
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