[time-nuts] What is the best counter for a Time Nuts?
Mike Monett
XDE-L2G3 at myamail.com
Mon Oct 13 02:59:02 UTC 2008
Hello Charlie,
I am very pleased to see your post. Measurements and good data are
very important, and much more valuable than handwaving.
"Myers, Charlie" <Charlie.Myers at ps.net> wrote:
> Hello to the Time Nuts,
> I have been reading the mail on this topic for the last week or so
> with great interest. Lots of interesting ideas have been put forth
> for measuring frequency to a high degree of precision and for
> comparing a 10 MHz clock's frequency to a highly accurate 10Mhz
> frequency "standard".
> The way I measure the frequency of a 10 MHz clock is to compare it
> to a second 10MHz clock of known accuracy and stability, not only
> with a frequency counter but also with a phase meter.
This is excellent procedure. Cross-checking is always a good idea. I
try to find as many ways to measure something as possible. If one
measurement doesn't agree with the others, it shows something is
wrong somewhere. So this is not a boring or stupid waste of time. It
is essential.
> I have several GPS disciplined OCXO's, one GPS disciplined
> Rubidium oscillator, and several free running rubidium
> oscillators. I measure the frequency of an unknown 10 MHz clock
> using a 2 step process.
What are the models? What GPS card are you using, and how are you
locking to it?
> First I measure the unknown 10 MHz clock using an HP 5384A
> reciprocal counter that employs my known 10 MHz clock as its
> external timebase. I set the gate time to 10 seconds and the
> counter delivers a frequency measurement with a resolution of less
> than 3 mhz (3 millihertz).
Do you happen to know any source for the manual? There is almost
nothing on the web.
> So, assuming my known timebase is "bang on", I know the frequency
> of the unknown 10 MHz source to an accuracy of roughly 3e-10 or 3
> parts in 10 billion.
> To get a more precise measurement of the frequency difference
> between the two 10 MHz clocks, I supply the known 10 MHz clock to
> the Channel A input of an HP 3575A Gain-Phase meter and the
> unknown 10 MHz clock to the channel B input of the Gain-Phase
> meter.
Another very difficult instrument to find any data on. The best I
could find shows it has an accuracy of +/- 0.5 degree and resolution
of 0.1 degree.
I think in this case the resolution parameter might be quite valid.
You are measuring a small change, not an absolute value. So if it
stable, I'd consider keeping it.
> I measure the change in the phase angle between the 2 input clocks
> over some convenient time interval (eg, 10, 100, or 1,000 seconds)
> and compute the frequency difference using the formula:
> Frequency Difference = [Change in Phase Angle (in degrees) /
> Measurement Duration (in seconds)] X [1 / 360]
> The frequency difference can then be converted to frequency
> accuracy using the formula:
> Accuracy = Frequency Difference / 1e7
> This seems like a pretty straight forward technique. Am I missing
> something?
> Charlie Myers
> WA3RAD
Charlie, this could be quite significant. A cross-check on your math
shows 1 degree at 10MHz is 1e-7 / 360 = 2.77e-10s, or 277
picoseconds. That's not bad at all.
In 1000 seconds, you could resolve 2.77e-10 / 1e3 = 2.77e-13. That's
pretty good. 1000 seconds is only 16.6 minutes, which is quite
reasonable. And that's truncating the resolution to 1 degree.
24 hrs would get you to 2.77e-10 / (24 * 3600) = 3.2e-15, which is
very acceptable. That puts you in the big leagues.
Now comes the big question - what kind of data do you get?
Can you give some examples of the measurements you have made?
Can you see any drift in the GPS time?
Thanks again very much for your post!
Best Regards,
Mike Monett
More information about the time-nuts
mailing list