[time-nuts] Measuring 10 MHz accurately.

Brooke Clarke brooke at pacific.net
Wed May 7 17:28:34 EDT 2008

Hi Ulrich:

The SR620 has a 1 kHz Reference output that's derived from the 10 MHz external 
clock input.  It also has a Gate/Arm function.

Appendix B in the PRS10 Manual explains how to use these two features to make 
1,000 time interval measurements per second thus averaging the 20 ps one shot 
way down.  http://www.prc68.com/I/PRS10.shtml

The time interval armed 1,000 times per second and the difference between zero 
crossings of a pair of 10 MHz signals are measured.  So the range of 
measurement is 100 ns before rollover.  I think at least one of the test 
signals needs to be 10 MHz to drive the external clock input to get the 1 kHz 
reference.  But you could use Toms PIC divider to generate a 1 kHz signal from 
about any test signal and use the same method.

Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam

Ulrich Bangert wrote:
> Martyn,
> 2 parts in 10E-12 in a second's gate time would require to measure the
> time interval with an 2 ps resolution. Since the SR620 (at least mine)
> features a 20 ps single shot resolution, i fear your number is more
> likely "20 parts in 10E-12". Or what you are doing is to make >= 10
> measurements / s of phase and computing the arithmetic mean of them.  
> There are not really many companies to offer you direct sub-picosecond
> measurement resolution and if at all the devices may in the same price
> reigion as your company's boxes. There ARE some measurement schemes that
> enable you kind of "artificial magnification" of the time differences to
> be measured, for example the Dual Mixer Time Difference Method. In
> principle they all look VERY EASY and may give you the impression that
> you can make measurements with that on the kitchen table. I have tried
> out a few of them just to find out that they do indeed work (in
> principle) but that their technical realisation has some strange
> requirements that are not too easily given on standard workbench.
> Nevertheless I am still working on the problem using a not so well known
> proposal.   
> Best regards
> Ulrich Bangert
>>-----Ursprungliche Nachricht-----
>>Von: time-nuts-bounces at febo.com 
>>[mailto:time-nuts-bounces at febo.com] Im Auftrag von Martyn Smith
>>Gesendet: Mittwoch, 7. Mai 2008 18:52
>>An: time-nuts at febo.com
>>Betreff: [time-nuts] Measuring 10 MHz accurately.
>>Two questions for all of you today.
>>1st Question
>>I can measure 10 MHz frequency to an accuracy of about 2 
>>parts in 10E-12 in 
>>a one second gate time.
>>I use a SR620 time interval counter and make a timeAB measurement.
>>I make two measurements of phase (between my UUT and my 
>>reference)  and then 
>>calculate the frequency.
>>I have some nice software that controls the SR620 and does 
>>all the math.
>>I'm sure most of you understand my measurement technique, 
>>since it's as old 
>>as the hills.
>>Does anyone have a good application note explain this 
>>standard procedure? 
>>I'm trying to explain it to a friend, and can't find a nice 
>>application note 
>>with some diagrams, formulas etc.  I know HP did one and 
>>Standard did as 
>>well, but can't find any quickly.
>>2nd Question.
>>I really need to measure 10 MHz to an accuracy of 1 part in 
>>10E-13 / second 
>>(ignoring the accuracy of my workshop standard).  I've tried 
>>multiplying the 
>>10 MHz to the GHz frequencies, but never manage to improve on 
>>the technique 
>>I mention in my first question.
>>Any ideas for a relatively cheap way of doing this.  I 
>>actually represent a 
>>company that can measure to parts in 10E-15 in one second, 
>>but their boxes 
>>costs $50k.
>>Best Regards
>>time-nuts mailing list -- time-nuts at febo.com
>>To unsubscribe, go to 
>>https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts
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