[time-nuts] Frequency comparison
lists at cq.nu
Sun Feb 7 16:22:45 UTC 2010
The rubidium *should* be good at the 1x10^-11 level. The GPSDO can be better
than that. The issue is that neither one is very good in the short term.
Measurement periods of hours are not unusual to get things right. If GPS
with your antenna location is good at the 10 ns level, that's 1x10^-8 per
second. 10,000 seconds will get you to 1x10^-12. That's good enough to
calibrate the rubidium. The calibration standard should always be much
better than the thing you are calibrating.
No matter what you do, it's a "watching grass grow" sort of thing. Some sort
of automation is a good idea.
One approach (of *many*):
Your GPSDO probably has a 1 pps coming out of it.
Your Racal can measure time between two 1 pps pulses (if I remember that
Divide the rubidium down to 1 pps and look at it relative to the Racal.
Log data out of the Racal so you can go back and see what's happened.
That's not going to work very well if the Racal does not have some kind of
serial or GPIB output.
There are a lot of other approaches. Cost can become a factor. For some
the budget for this sort of thing is quite large. For others low cost /
build it yourself is what makes sense. Are you looking at setting up a lab,
or just want to get the Rubidium on frequency?
From: "Raj" <vu2zap at gmail.com>
Sent: Sunday, February 07, 2010 10:25 AM
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Subject: Re: [time-nuts] Frequency comparison
> 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!
> At 07-02-10, you wrote:
>>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
>>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.
>>> Raj, VU2ZAP
>>> Bangalore, India.
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>>> and follow the instructions there.
>>time-nuts mailing list -- time-nuts at febo.com
>>To unsubscribe, go to
>>and follow the instructions there.
> Raj, VU2ZAP
> Bangalore, India.
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> and follow the instructions there.
More information about the time-nuts