[time-nuts] crunching numbers from XOR phase detector
jimlux at earthlink.net
Wed Jan 4 17:22:21 EST 2012
On 1/4/12 8:14 AM, Joe Gwinn wrote:
> At 10:16 AM +0000 1/4/12, time-nuts-request at febo.com wrote:
>> Date: Wed, 04 Jan 2012 00:58:24 -0800
>> From: Hal Murray <hmurray at megapathdsl.net>
>> To: Discussion of precise time and frequency measurement
>> <time-nuts at febo.com>
>> Subject: Re: [time-nuts] crunching numbers from XOR phase detector
>> <20120104085824.8426E800037 at ip-64-139-1-69.sjc.megapath.net>
>> Content-Type: text/plain; charset=us-ascii
>> 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.
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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