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 <[email protected]> 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 > > > > _______________________________________________ > > 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. > _______________________________________________ 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.
