[time-nuts] Another "atomic" clock question

Bob Stewart bob at evoria.net
Tue Mar 4 13:21:26 EST 2014

Hi Tom,

You may have missed TVB's post yesterday, quoted below.  A "hanging bridge" is an area on a timing receiver's plotted sawtooth correction value that stays on one side of phase zero for some period of time.  As a result of this "bias", a GPSDO that is not corrected for sawtooth will probably have at least a phase shift in its output frequency during a hanging bridge.  It's not so bad on the newer 10ns receivers as it was on the older +/-52ns Oncores.  If one could prevent these hanging bridges, an uncorrected GPSDO would likely track phase better.  Tom suggested to detect when the bridge forms and give it a shot of heat.  I suggested to vary the oscillator's frequency by periodically heating and cooling it.

Tom's post:

"Sure. In fact you can loosely phase lock it to GPS that way. Your xtal 
doesn't need to have an EFC pin. You are using external temperature as a
 replacement for EFC. Call it TFC (temperature frequency control) 
instead. You can't get much simpler than that. Make sure to use a plain 
XO (not a TCXO or OCXO).

I used a resistor heater to bust hanging-bridges: http://leapsecond.com/pages/vp/heater.htm


> From: Tom Holmes <tholmes at woh.rr.com>
>To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com> 
>Sent: Tuesday, March 4, 2014 12:07 PM
>Subject: Re: [time-nuts] Another "atomic" clock question
>Hanging bridge? What is it; where is it found; and how does it form?
>My guess is that a Google or Wikipedia search is going to come up with
>something named Golden Gate or maybe a musical term.
>Tom Holmes, N8ZM

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