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

[Bob Stewart]

Hi Mark,

I'm neither an engineer, nor an expert, but here are my comments.  

I think that the idea of 100ns/T is wrong.  There are several variables that control accuracy, but the time between pulses from your OCXO (assuming no phase or frequency drift) isn't one of them.  So, that gives 1/T.  Here the problem is that T must get large before your accuracy can be good.  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.  The theoretical maximum would be infinity, of course, but your oscillator won't be that stable.

Another big problem is the accuracy of the 1PPS pulse.  I'm using an Adafruit GPS receiver, and it's listed as accurate to within 10ns.  And it is, but you have to be wary of exactly what that means.  It doesn't mean +/- 5ns.  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.  And even then, you can't be 100% sure that it's not due to the quantization errors in your 1PPS
 signal.

The better GPS receivers will output a quantization error value every second.  But if you're using the 1/T method, there's nothing you can do with it, so you have to live with whatever quantization errors you get.

Anyway, those are my experiences.

Bob - AE6RV





>________________________________
> From: Mark Haun <haunma at keteu.org>
>To: time-nuts at febo.com 
>Sent: Wednesday, February 26, 2014 3:51 PM
>Subject: [time-nuts] GPSDO with all-digital phase/time measurement?
> 
>
>Hi everyone,
>
>I'm new to the list, and have been reading the recent threads on
>Arduino-based GPSDOs and the pros/cons of 10-kHz vs 1-Hz time pulses with
>interest.
>
>As I understand it, there are a couple of reasons why one needs a
>time-interval / phase measurement implemented outside the MCU:
>
>1) Time resolution inside the MCU is limited by its clock period, which is
>much too coarse.  The GPSDO would ping-pong within a huge dead zone.
>2) Software tends to inject non-determinism into the timing.
>
><snip>
>
>Thanks,
>
>Mark
>KJ6PC
>
>