On 1/4/12 8:14 AM, Joe Gwinn wrote:
At 10:16 AM +0000 1/4/12, [email protected] wrote:
Date: Wed, 04 Jan 2012 00:58:24 -0800
From: Hal Murray <[email protected]>
To: Discussion of precise time and frequency measurement
<[email protected]>
Subject: Re: [time-nuts] crunching numbers from XOR phase detector
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<[email protected]>
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Neat. Thanks for sharing.
With a XOR, you can't tell which input is higher frequency. I think
you can
fix that with a second XOR and a delay line.
I think 90 degrees of delay will provide the most information. At 10 MHz,
that's 25 ns. I think that's about 15 feet of good coax.
Beware of teflon dielectric cable, as the teflon knee is centered around
room temperature, making delay cables of teflon quite sensitive to
slight changes in temperature.
<http://www.micro-coax.com/pages/technicalinfo/applications/27.asp>
See also
http://www.gore.com/en_xx/products/cables/microwave/phase-stability-whitepaper.html
http://www.gore.com/en_xx/products/cables/microwave/changes-insertion-loss-phase.html
Yeah, but the change is pretty small, (a few hundred ppm) and overall,
those cables have fairly low temperature coefficient. The latter Gore
writeup shows 0.2 deg/GHz/ft across the "knee"
For the 10 MHz scenario at 15ft, that's .03 degrees, 83 ppm
You sort of have a choice between a cable that has low overall
variation, but a step in the curve OR a cable that has a smooth
characteristic and no bumps.
Interestingly, other dielectrics don't show this effect. In particular,
the silica dielectric stuff is very stable.
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