[time-nuts] crunching numbers from XOR phase detector

Jim Lux 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
>> Message-ID:
>> <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.
>
> <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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