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[TenTec] Omni 6 receive audio

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: [TenTec] Omni 6 receive audio
From: Gary J FollettDukes HiFi <dukeshifi@comcast.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 10 Jan 2016 12:02:14 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
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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