[time-nuts] Man with too many clocks.

Tom Van Baak tvb at LeapSecond.com
Fri Nov 4 20:09:05 EDT 2016


Hi David,

Your solution is fine. Most time interval counters can only make 10 or 100 or at most 1000 measurements per second, so what you did is exactly the right thing. When using a divider + TIC nothing is lost and everything is gained. Even 'scopes cannot retrace 1000's of times a second. So using a digital divider is what almost all of us do.

What you don't want to get in the habit of doing is comparing 10 MHz (100 ns cycles) against 1 PPS. Why? Because it works most of the time, except when it doesn't. Which is to say a good GPS/1PPS is within +/-50 ns almost all the time, except when it isn't. So it's more robust to phase compare a GPS/1PPS against another 1PPS, or even 1 kHz or 10 kHz, but not 10 MHz. This drastically reduces to practically eliminates chances of cycle slip. It's one reason why the PIC divider chips (or equivalent) are so useful (www.leapsecond.com/pic).

/tvb

----- Original Message ----- 
From: "David" <davidwhess at gmail.com>
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Friday, November 04, 2016 4:30 PM
Subject: Re: [time-nuts] Man with too many clocks.


> My simple solution to this was to divide the 1 PPS signal down so the
> jitter from the uncorrected GPS was a smaller part.  Of course then
> each measurement takes proportionally longer.
> 
> On Fri, 4 Nov 2016 11:35:59 -0400, you wrote:
> 
>>I gave up on trying to use the GPS 1 PPS signal to calibrate the 10 MHz 
>>OCXO's that
>>I have.   The reason that others have pointed out is that the 
>>uncorrected 1 PPS
>>signal from the GPS is has just a little too much a jitter to use it for 
>>calibration
>>with your eye using a scope.   If it were sawtooth corrected then it 
>>would be better
>>but you really need a GPS disciplined oscillator.
>>
>>...
>>
>>Pete.
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