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

[paul swed]

Bill you beat me to the punch.
I completely agree that a DDS would allow both frequencies with very very
small offsets.
Granted its more complicated and costly. But then it would be locked.
I have 5 3586s all running so am always interested in possible updates and
was aware of the bfo.
Regards
Paul.

On Thu, Feb 24, 2011 at 3:48 PM, WB6BNQ <wb6bnq at cox.net> wrote:

> 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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