[time-nuts] Frequency comparison

Joop lous at xs4all.nl
Mon Feb 8 21:26:44 UTC 2010

At 07-02-10, John Ackermann wrote:
>Hi Raj --
>You've already gotten some good answers.  If all you want to do is 
>measure the frequency offset rather than characterize the stability, a 
>simple approach is to first get as close as you can by adjust for 
>minimum march of the 10 MHz signals across the oscilloscope, then use 
>either the Racal counter or the digital o'scope to measure the delay 
>between the two signals and how it changes over time, preferably 
>measuring at 1 PPS rather than 10 MHz.
>In other words, measure the time difference between the leading edge of 
>the PPS signals, averaging for a while (depending on how close the two 
>already are) to improve resolution and reduce the noise.  Write down the 
>delay figure, note the wall clock time, wait a while, then come back and 
>measure the delay again.  The change in delay over the elapsed time will 
>tell you the frequency offset, e.g., if you see 1 microsecond per day, 
>that's 1.16x10e-11.
>Adjust and repeat.  As others have mentioned, being a time-nut requires 
>patience. :-)
>It's best to do this at a lower frequency than 10 MHz, and ideally at 1 
>PPS, as there's only 100 nanoseconds between cycle slips at 10 MHz, and 
>that limits how long you'll be able to measure before you've drifted a 
>complete cycle.
Ok, the past few days I have been working on exactly the same thing. That
is, adjust an LPRO to my homemade GPSDO.
Good to know I followed a proven procedure. Initially I wanted to build
two PPSDIV/TADD circuits but did not have the right PICs. Instead I used
two PIC12F devices and put them on the same veroboard. The software for
only a single 1Hz output was not too complex.

There was also a Racal 1992 I could use to measure and log the phase
shift. If that would not have been the case I probably would have tried a
XOR port to both (synched) outputs and use an R + C to measure the DC
voltage. That would make a nice little gadget for people not having a
high-res counter. Also the TADD 74AC04 drivers would not be needed this

Anyway, I finally dared to adjust the LPRO.
Can somebody tell if the following seems normal?
* The LPRO measured 1.77 E-10 high (before any adjustment)
* Lamp voltage is about 5.7V
* The GPSDO 1Hz pulse seems to move (noise like) + or - 15ns around its
linear regression line
* The LPRO takes more than 8 hours (perhaps even 24) to reach the 1E-11
The last point is guessing since my GPS signal is super stable. I thought
I managed to adjust it to that level after the LPRO had been powered on for
more than 24 hours. But powering it on two days later shows a higher drift
after 5 hours than where left before.

Also I would like to know if I have to repeat this procedure once it is
built into its final enclosure. Right now it is open on the bench and
clamped to a heatsink. Temperature might be different inside a box.


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