[time-nuts] How does sawtooth compensation work?
Tom Van Baak
tvb at LeapSecond.com
Tue Jul 19 08:00:00 EDT 2016
> On the sub-set of receivers that send you the sawtooth correction *after* (as in 200 ms after) the PPS
Right. The 1PPS delay line method is not compatible with these chips.
> If the sawtooth says the edge was late, you can never get it back to correct (accurate).
The systems that use 1PPS delay chips solve your worry with an at-power-on adjustment to the receiver's antenna delay parameter.
So if you have a 256 ns 8-bit programmable silicon delay line, instead of telling your receiver that your antenna delay is X you tell it X - 128 ns. Then the delay chip acts like a -128 ns to +127 ns phase stepper instead of a 0 to 255 ns phase stepper.
----- Original Message -----
From: "Bob Camp" <kb8tq at n1k.org>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Tuesday, July 19, 2016 4:06 AM
Subject: Re: [time-nuts] How does sawtooth compensation work?
On the sub-set of receivers that send you the sawtooth correction *after* (as in 200 ms after) the PPS …. the delay
line correction thing does not work very well. Also in a “strict time nuts” sense, you can only delay the edge. If the
sawtooth says the edge was late, you can never get it back to correct (accurate). Not at all a big deal for a GPSDO.
It is a big deal if you are looking for a “perfect tick” time wise.
> On Jul 18, 2016, at 11:28 PM, Mark Sims <holrum at hotmail.com> wrote:
> 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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