[TenTec] Omni 6 receive audio

[Gary J FollettDukes HiFi]

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...