Hi Coming back to the basics of the design:
If you are playing with a normal radio, a BFO that drifts under a few Hertz is going to be pretty much un-noticable. Drift is a bit of an elastic term in this case since it can cover a bunch of different parameters on an oscillator (temperature as things warm up / aging / retrace / voltage stability ….). For fun, lets say that 25.3 Hz over a day and 5C is “adequate” for the task. Oddly enough this makes the math easy. 25.3 Hz / 253 KHz = 1 / 10,000 = 100 ppm. A crystal that drifts 1 ppm / C is not a super duper part at room temperature. Over our 5C range, that’s only 5 ppm. It would have to be 10X worse to really eat into our budget. Aging / warmup / retrace wise, a crystal that moves a couple ppm in the first day is moving a lot. Again not a big hit to our budget. Voltage stability on a properly designed circuit with a normal voltage regulator should be very small compared to the budget. Maybe it’s a ppm, probably less. Bottom line - the crystal likely is doing >10X better than what our arbitrary spec would require. That’s why a lot of radios do just fine with an L/C based BFO. For even more fun, take a look at the likely drift of the mechanical filters involved. They are not going to be as stable as the crystal ….. Even a crystal filter at 250KHz isn’t going to be as stable as the AT cut based evaluation above. Bob > On Sep 16, 2018, at 7:49 AM, Didier Juges <shali...@gmail.com> wrote: > > Not the same part number but probably similar in terms of performance: > > http://www.ko4bb.com/getsimple/index.php?id=microprocessor-crystal-oscillator-comparison > > Bottom line: use a true crystal oscillator, or make your own PLL, not a > programmable "microprocessor crystal" > > On Fri, Sep 14, 2018, 11:15 AM <lstosk...@cox.net> wrote: > >> Off topic for this list, but you guys are experts in oscillator noise! >> >> Playing with some mechanical filters. Need USB and LSB crystals for the >> BFO. No one seems to make crystals anymore, especially in the 253 KHz >> range! >> >> Looking at the DigiKey Cardinal programmable oscillators. Cheap and >> available: CPPC1LZ A5B6 >> >> Anyone have an idea how noisy these would be after a division by 4 to get >> them in range? >> >> Thanks, >> >> N0UU >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.