[time-nuts] To Bert from Burt...

[Burt I. Weiner]

Bert,

I run my 3586B from an external GPS controlled reference.  There's no 
practical relationship for the product detector L.O. that would be 
easy or likely worth the effort.  Here's a link to my FMT Methodology 
that describes what I've done.  If you're interested in the I.F. 
pickoff point information let me know and I can e-mail to you 
information on "How I Did It".

See: http://www.k5cm.com/k6OQK%20FMT%20NEW.htm

Burt, K6OQK





>Hi Everyone,
>
>Some recent tests I have made on my HP 3586B Selective Level Meter have
>confirmed that the detected audio drifts a lot as a function of ambient
>temperature. A day/night change of 1.5 degree Celcius was clearly visible on
>Spectrum Lab samples. I could actually find out how many times and when the
>central home furnace had cycled in-out overnight. It gave a neat plot which I
>post here: https://www.onlinefilefolder.com/3sFcUl8Z8i8zy7
>
>So I have been looking at modifying my HP 3586B SLM so that it 
>becomes entirely
>synchronized off the external 10MHz reference. As you probably know, 
>all stages
>exept the final one, the SSB LO (BFO),  are derived from the 10MHz reference.
>The SSB LO chain is made of two free-running crystals used for LSB and USB
>detection. In my unit (3586B, option 003), the detection LO frequencies are
>13775 Hz and 17475 Hz. These two frequencies are not directly math-related to
>any other internal reference (at least I could not personally find any). BTW,
>the IF LO rate is 15625 Hz.
>
>I have substituted a function generator instead of the internal 
>oscillators just
>to see how much the off-centering would affect the audio quality, 
>and found out
>that this works well as long as it is not too far off the designed 
>LO rates. The
>closest 10MHz-derived integer rate I could find that will work for audio tone
>measurement is divide-by-726 (13774.104... Hz) in LSB. The closest USB rate
>would be divide-by-572 (17482.5... Hz), much farther than the LSB one. So the
>LSB rate difference of around 0.9 Hz can be neglected when keying in the
>frequency and listening to regular radio signals. When making absolute audio
>measurements, subtracting that delta is easy to achieve in post processing.
>Besides, the frequency and amplitude measurement capabilities of the 
>instrument
>are not altered by this mod. To me this would seem like an 
>acceptable compromise
>that is simple to implement and would add long-term stability. My 
>intent is to
>have a single chip (in this case, an 8-pin PIC divider) do the 
>trick, as we are
>dealing with tTL level signals here.
>
>But can I do better?
>How much more complex?
>Can I avoid PLLs?
>All mixer stages become synchronized to a single source. Is this an issue for
>reliable audio detection?
>Am I missing something here?
>
>As always, I truly enjoy your feedback and am quite convinced I will learn
>something new once more.
>
>Thanks,
>
>Bert, VE2ZAZ
>
>
>
>
>

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
K6OQK