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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
