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

paul swed paulswedb at gmail.com
Fri Feb 25 15:55:13 UTC 2011


OK here is the place I get myself in trouble. Because I have really not gone
through the math though I certainly did play with the total LSB to USB diff
of 3700 Hz etc.
Simple thought.
Why not preset counters down that loop and reset at zero as an example.
Preload them with the usb/lsb number granted these could be 10 or 100 x
higher then needed and then divided to have a symmetrical output.
The clock would be either 1 or 10 mc from the ref.
Granted I still like the DDS option actually but this approach might be
easier for those of us that like 14 and 16 leg dead bugs. That can be seen
and soldered to. ;-)
Regards
Paul

On Thu, Feb 24, 2011 at 4:09 PM, paul swed <paulswedb at gmail.com> wrote:

> 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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