[time-nuts] How far can I push a crystal?
Ed Breya
eb at telight.com
Fri Jan 18 22:11:54 UTC 2013
Bob, please tell me more about cascading the DFFs. I was only using one
half of the '74, with the other inactive, so both are available for the
task. From your description it sounds like I just run the Q from the
first DFF to the D of the second, clock them together from the 1 or 10
MHz, and take the cleaned up difference signal from the Q of the second.
So, I think what it means is that the same information should pass
through, just delayed by one sampling clock cycle, and scrubbed of any
edge uncertainty of an analog nature, that would otherwise be passed to
the phase detector. Right? I would definitely do this if no additional
logic packages are required.
If a single-PLL type we discussed earlier is workable, I won't even need
to worry about the second PLL system. It all depends on whether the
phase detector frequency will be high enough. I'll be thinking through
that and trying a few experiments. It's simple enough that I could even
just build it and see what happens. If it's not right, then I'll just go
with the previous plan, with high confidence - and a two-stage sampler.
Regarding the oscillators - yes, having different signals present in
common packages is what got me into this trouble in the first place. As
I mentioned earlier, I had optimized the original design for compactness
and minimum package count, so I had every signal in the box going every
which way, all mixed up. I had used a different method for making the
10.7 MHz though - building it up by mixing various divided frequencies,
then filtering it with cascaded 10.7 MHz IF filters. Most of the stuff
went right around the filters anyway, since there was so much whizzing
around in there.
In case anyone is wondering why I'm so hung up on this 10.7 MHz thing:
For this particular tracking generator project, I just need to
synthesize one fixed, "correct" reference frequency with the simplest,
most compact scheme that performs well enough. The original design
evolved from using the 10.7 MHz base frequency, but it isn't actually
needed per se. If anyone comes up with sets of numbers that seem to work
in a single-PLL scheme, and fit the constraints evident in this
discussion, please let me know.
I have other tracking generator projects in the works though, that will
cover most or all of the 8566B span of ~0 to 24 GHz, and need to produce
various numeric and harmonic relationships for IFs and frequency control
- all of these can be readily integer-derived from the fundamental 10
and 10.7 MHz references. In all cases, the ultimate reference is the 10
MHz used or produced by the 8566B, so everything is phase locked.
Ed
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