[time-nuts] Frequency comparison

Raj vu2zap at gmail.com
Mon Feb 8 04:15:36 UTC 2010

Thanks John. Let me see how I can do this.

Our VU time standard at Delhi (National physical labs) has vanished, nearest is China, occasionally I hear KH6 signals so I needed something better and reliable. That led me to FE5680A and your web pages. As I mentioned in another email, you left a lurking infection in my shack !! I am enjoying and thanks for the help.

I was using the Rb oscillator to correct the OCXOs in my counters. Now the bug bit to check the Rbs properly. Will let you know off group!


At 07-02-10, you wrote:
>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.
>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!
>>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 time.
>>>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.

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