# [time-nuts] What is the best counter for a Time Nuts?

Myers, Charlie Charlie.Myers at ps.net
Sun Oct 12 04:09:23 UTC 2008

```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.

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.  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).  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.  I measure the change in
the phase angle between the 2 input clocks over some convenient time
interval (e.g., 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