[time-nuts] Programmable clock for BFO use....noise
Richard (Rick) Karlquist
richard at karlquist.com
Fri Sep 14 16:24:11 EDT 2018
I encountered these oscillators on a circuit I inherited
from another engineer. The spectrum of these is quite
dirty and they should only be considered as digital clock
oscillators. An additional annoyance is that they are
not marked with the frequency they are programmed to,
so if you have USB and LSB you'll have to put a dot
of paint on them or something to tell them apart.
Dividing by 4 or N will reduce spurs by 20 LOG N as
any time nut knows. If you get a programmable oscillator
at a frequency around 32 MHz and divide it down by 128
to ~253 kHz, you might get enough clean up for your
purposes. 20 LOG 128 = 42 dB.
Alternately, find a conventional clock
oscillator that can be divided by an even integer to hit
your BFO frequency. For example, 20 MHz divided by
78 = 256.4 kHz. 20 MHz divided by 80 = 250 kHz.
Divide by 39 followed by divide by 2 or divide by 40
followed by divide by 2, in order to get a square wave
at the output.
On 9/14/2018 9:14 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 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?
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