[time-nuts] How does sawtooth compensation work?
Tom Van Baak
tvb at LeapSecond.com
Tue Jul 19 01:25:33 EDT 2016
I forgot to add -- see pages 12 to 19 of http://www.gpstime.com/files/PTTI/PTTI_2006.pdf where Rick uses a DS1020, especially page 16 where he compares both the traditional TIC-based sawtooth correction method with the [new] delay line method. The agreement is about 1 ns rms.
----- Original Message -----
From: "Tom Van Baak" <tvb at LeapSecond.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Monday, July 18, 2016 9:41 PM
Subject: Re: [time-nuts] How does sawtooth compensation work?
>> Or use the sawtooth compensation value to control an external variable delay line circuit
> Hi Mark,
> Right, one example is https://datasheets.maximintegrated.com/en/ds/DS1020.pdf or google for silicon delay line. Not sure they're in production still but you can find them at the reseller sites.
> This delay line idea came up in the early Oncore-VP era gps mailing list (pre time-nuts) by someone who first explored sawtooth correction and "hanging bridges"; and it's the method that Rick then chose for his CNS Clock product line. See: http://cnssys.com and http://gpstime.com for details.
> The advantage of the delay line method is that you don't need a nanosecond TIC in the box; you correct for the sawtooth error live on the 1PPS. Very simple and effective. The main GPS feed in my lab is a CNS Clock.
> This disadvantage is that if you already have a TIC connected to your GPS/1PPS, there's not much point in pre-sawtooth correcting with a delay line. The error is something that you're going to correct with arithmetic anyway so there's no need to correct it in pulse phase. Rick's TAC32 software (that many time nuts use) handles integration of serial TIC data (such as hp 53132) along with GPS binary data to provide sawtooth corrected measurements. Several of Rick's papers at the above sites explain this in fine detail.
> ----- Original Message -----
> From: "Mark Sims" <holrum at hotmail.com>
> To: <time-nuts at febo.com>
> Sent: Monday, July 18, 2016 8:28 PM
> Subject: [time-nuts] How does sawtooth compensation work?
>> Or use the sawtooth compensation value to control an external variable delay line circuit to move around the PPS signal from the receiver. This can get interesting to implement if the receiver can output negative values for the sawtooth compensation (hint: add a bias to the sawtooth value to make the compensation values always positive and adjust the antenna cable delay command to remove the bias value that you add. Oh, and for some receivers you have to reverse the meaning of positive and negative sawtooth corrections and/or cable delay values). It is even more interesting if the receiver outputs the sawtooth correction after the pulse it just generated... hint: get a different GPS receiver).
>>> A device that uses the sawtooth data shoves it into the control loop along with the measured early / late information on the PPS.
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