[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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