[time-nuts] Programmable clock for BFO use....noise
kb8tq at n1k.org
Sun Sep 16 11:48:29 EDT 2018
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
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.
> On Sep 16, 2018, at 7:49 AM, Didier Juges <shalimr9 at gmail.com> wrote:
> Not the same part number but probably similar in terms of performance:
> 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 <lstoskopf at 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
>> 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?
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