[time-nuts] Fw: New to list and GPSDO questions

Chris Howard chris at elfpen.com
Fri Jun 14 01:58:10 UTC 2013


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 <attila at kinali.ch>
>>> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency
>>> measurement <time-nuts at febo.com>
>>> 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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> 
> 
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