[time-nuts] HP 3586A/B/C entirely referenced to 10MHz: A solution

WB6BNQ wb6bnq at cox.net
Thu Feb 24 20:48:08 UTC 2011


Hi Bert,

I am wondering if you would not do better using a DDS.  Specifically, I am thinking
of the Analog Devices 9913 which has the ability to fractionally modify the
accumulator.  I hadn't really put much thought in it, perhaps a regular DDS would
work as well.

Bill....WB6BNQ


"Bert, VE2ZAZ" wrote:

> 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
>
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