[time-nuts] GPSDO with all-digital phase/time measurement?

[Bob Stewart]

Hi Charles,

The problem is that the only information available is the fact that a phase crossing occurred and whether there were more than 10M counts (or less) since the last PPS.  The phase error value is not available to me, nor is the sawtooth value; which would of course be of no value.  So, if I have a + phase crossing and then a - phase crossing, what do I know?  If they are closely spaced, I can guess that the reason for the bouncing is the jitter on the PPS.  If they are not closely spaced, then I can't really conclude anything other than that there is a phase offset in one direction or the other..  I could count the number of crossings over time and estimate the angle of the phase crossing, but I can't really be sure of the direction.  Also, since this is not a timing receiver, it tends to wander around about 10-20 ns.  So, that wander might be the only reason for a +/- count.  In the case where there are two + crossings, or two - crossings in a
 row, it is a 1/T question.  And with 1/T, it may be a long time until the next crossing, depending on how close you are in frequency, and how much the receiver wanders around.

I'm aware of the limitations of the hardware.  That's why I'm working on a TIC daughterboard.  I could have used someone else's board, and a different GPS receiver, and on and on, but what fun would that have been?  My goal is to do as much as possible with as little extra as possible using this particular board, learn as much as possible, and enjoy myself.

I hope that helps.  It's entirely possible that I've made some newbie mistake and that there's a good answer available.  But, in that case, I would think that someone else would have already applied it to this board.

Oh, and my granddaughter has been pestering me the whole time I've been writing this, so I hope I haven't been a bit short.  =)

Bob




>________________________________
> From: Charles Steinmetz <csteinmetz at yandex.com>
>To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>Sent: Friday, February 28, 2014 12:29 PM
>Subject: Re: [time-nuts] GPSDO with all-digital phase/time measurement?
> 
>
>Bob wrote:
>
>> You can achieve very good accuracy, but at the cost of waiting thousands of seconds between "phase points"; i.e. where your 1PPS coincides with the 10 millionth OCXO pulse.
>> 
>> So, as your 1PPS pulse bobs back and forth, you will often encounter an OCXO pulse up to 10ns early, or up to 10ns late.  So, might you count 9,999,999 pulses from the OCXO immediately followed by 10,000,001 pulses.  Neither of those, by itself is a signal to change the EFC voltage to your OCXO.  In fact, it is normal for your count to alternate between the two for long periods, if you are very very close to exactly 10MHz, just from the quantization error on the 1PPS.  It is also normal for 1/T to control the time between phase crossings.  So you have to wait for two miscounts in a row in the same direction to make a change.
>
>I have been puzzled more than once by your comments about only changing the DAC count every several minutes or more.  I am not familiar with the circuit you are using, but in a digital PLL the errors (assessed every second) typically feed a digital filter that drives the DAC.  So, there is generally a very small correction every second according to the long running average of the individual errors, rather than a large correction after hundreds or thousands of seconds.  If you only adjust the DAC every two miscounts in one direction, you are guaranteed to get slipped cycles (which appeared to be one of the problems you were having when comparing oscillators).  This is a reasonable way to get an oscillator roughly on frequency if it is substantially off to start with, but it is not a good way to hold an oscillator within ppb of the desired frequency, and no way at all to hold it in phase lock with the reference.
>
>If that is really the way the circuit you are using works, perhaps it would be better to implement a proper all-digital PLL with digital filter than trying to get better results out of the circuit you are using than it is capable of delivering.
>
>Or, perhaps I'm not understanding what you are doing?
>
>Best regards,
>
>Charles
>
>
>
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