As you drive down the road it is important that you do not over correct
errors and look ahead and start turning before the car reaches the turn,
your corrections have a delay in taking effect as well as a delay in knowing
where on the road you are. You know where the car was but more important is
where it will be when your correction takes effect.
Stanley
----- Original Message -----
From: "Chris Howard" <[email protected]>
To: <[email protected]>; "Discussion of precise time and frequency
measurement" <[email protected]>
Sent: Thursday, June 13, 2013 8:58 PM
Subject: Re: [time-nuts] Fw: New to list and GPSDO questions
I'm with Bob in that I don't really understand this description.
And what are we trying to be in phase with? the 1PPS?
Is it possible to reproduce the actual phase of the
clock in the GPS birds once it has been through my GPS
and come out as 1PPS?
I picture the EFC steering the car straight down the road.
And you want to adjust not just the drift to the right but
steer back to the center of the lane. I think I get
that part. I don't get who is setting the lane
and why I should care.
Do the statistics (ADEV, etc.) show that this is an improvement?
If I'm doing my slow zigzag down the center line or the
right shoulder, again, do I care?
Chris
w0ep
On 6/13/2013 5:51 PM, Richard H McCorkle wrote:
Hi Bob,
The VE2ZAZ controller is a frequency locked loop that keeps the source
on frequency but over time the phase drifts, as past corrections are
not compensated for. When the system is stable every 16 seconds the
16-bit counter rolls over 2441 times with an extra 26624 counts (6800
HEX). The previous count is subtracted from the current count, the
difference from 6800 HEX is used to update the EFC to correct the
frequency, and the current count is stored as the previous count for
the next sample. When the current difference is not exactly 6800 HEX
then a phase shift of 6.25ns has occurred over the sample period for
each count of the difference from 6800 HEX. The EFC corrects the
frequency to give the proper 6800 HEX over the following samples, but
the phase shift from the previous error during the correction period
remains.
If you add a little code to provide a correction history you can
add phase correction to the VE2ZAZ controller as well. Once lock is
established store the sample count as a phase reference count. Add
6800 HEX to the reference count every update and store the result as
the next reference value. Use the difference between the current and
previous samples from 6800 HEX to coarse correct the frequency as
currently done, but add the difference between the current count and
reference count to the EFC correction as a fine phase correction. If
hold or unlock occurs disable the phase correction routine until lock
is re-established. Then store a new reference and restart the phase
correction process. By updating the reference every update it provides
a phase history so the accumulated count error over time can be
removed and the phase of the source can be stabilized.
Richard
Atilla,
Isn't the VE2ZAZ circuit functionally equivalent to your example 3?
Granted, he's
not picking the 10 millionth transition and checking its phase
difference to the reference, but I've only got a 1PPS reference with a
1uS or so jitter from pulse to pulse. Bert is averaging over 16
seconds, and creating a PWM signal to drive an integrator (simulating a
DAC), which
will drive a Trimble 34310-T. And like I mentioned
earlier, I just like the way Bert did it. It has a kind of elegance
that appeals to my inner hacker. =)
Bob - AE6RV
----- Original Message -----
From: Attila Kinali <[email protected]>
To: Bob Stewart <[email protected]>; Discussion of precise time and
frequency
measurement <[email protected]>
Cc:
Sent: Thursday, June 13, 2013 3:39 PM
Subject: Re: [time-nuts] New to list and GPSDO questions
snip
[3] Describes how to use a clock
synchronizer to build a GPSDO. Probably
not the easiest and not the cheapest way, but definitly one with a very
low parts count.
snip
[3] The AD9548 as a GPS Disciplined Stratum 2 Clock, by Gentile, 2009
http://www.analog.com/static/imported-files/application_notes/AN-1002.pdf
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