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.
The LEA-6T is a good receiver, but it does have a sawtooth bug. From time to time, when the sawtooth is approximately 10,299 (or is it 10,399?) ps, the sign of the sawtooth is wrong. It's an easy fix: Just check whether the sawtooth makes "this" corrected measurement worse than the previous corrected measurement. I haven't checked the LEA-M8T for the bug, as I turned off the notice message when I corrected it.
Bob -----------------------------------------------------------------
AE6RV.com
GFS GPSDO list:
groups.yahoo.com/neo/groups/GFS-GPSDOs/info
From: Mark Sims <holrum@hotmail.com>
To: "time-nuts@febo.com" time-nuts@febo.com
Sent: Monday, July 18, 2016 10:28 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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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.
/tvb
----- Original Message -----
From: "Mark Sims" holrum@hotmail.com
To: time-nuts@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.
On Mon, 18 Jul 2016 21:41:51 -0700, you wrote:
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.
/tvb
Was their a specific reason to use an integrated variable delay line
for this versus a DAC, ramp generator, and comparator?
I ask because I have never seen a schematic for PPS jitter correction
which used the later.
Mark,
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.
/tvb
----- Original Message -----
From: "Tom Van Baak" tvb@LeapSecond.com
To: "Discussion of precise time and frequency measurement" time-nuts@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.
/tvb
----- Original Message -----
From: "Mark Sims" holrum@hotmail.com
To: time-nuts@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.
Hi
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.
Bob
On Jul 18, 2016, at 11:28 PM, Mark Sims holrum@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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Bob,
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.
/tvb
----- Original Message -----
From: "Bob Camp" kb8tq@n1k.org
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Tuesday, July 19, 2016 4:06 AM
Subject: Re: [time-nuts] How does sawtooth compensation work?
Hi
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.
Bob
On Jul 18, 2016, at 11:28 PM, Mark Sims holrum@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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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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