# [time-nuts] Frequency comparison

John Ackermann N8UR jra at febo.com
Sun Feb 7 16:39:19 UTC 2010

```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.

73,
John
----
Raj said the following on 02/07/2010 10:25 AM:
> Hi Bob,
>
>         I have a HP5350B with the oven (probably standard) continuously powered. The Racall-Dana is with a OCXO that I have added. The Racall has a phase comparison between A-B inputs.
>
>         I am just comparing 10 MHz frequencies. I see one cycle drift in greater than 100 seconds to 10 minutes, painful to watch and adjust!
>
> Raj
>
> At 07-02-10, you wrote:
>> Hi
>>
>> Which HP counter(s) do you have?
>>
>> The method you are using now with a scope can give you good accuracy if your clocks are close to each other and you watch things for a long enough time.
>>
>> Bob
>>
>>
>> On Feb 7, 2010, at 10:04 AM, Raj wrote:
>>
>>> Hi! all Time-Nuts,
>>>
>>>        For a newbie like myself at GPSDO & Rb oscillators etc. what method of frequency comparison would you gurus recommend ? Any articles on this subject ?
>>>
>>>        Available test instruments are: Scopes digital & Analog, counters with 0.01 Hz display at 10 Mhz (HP & Racall-Dana).
>>>
>>>        Currently I am comparing the phase between two 10 Mhz and adjusting for the lowest drift visually on the scope and phase drift on the Racall-Dana 1992.
>>>
>>>        Your suggestions I would like to use for calibrating the Rb oscillator with a GPSDO.
>>>
>>> Regards
>>>
>>> --
>>> Raj, VU2ZAP
>>> Bangalore, India.
>>>
>>>
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>>
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>

```