[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



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