[time-nuts] Measuring 10 MHz accurately.
brooke at pacific.net
Wed May 7 17:28:34 EDT 2008
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.
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Ulrich Bangert wrote:
> 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
> Best regards
> Ulrich Bangert
>>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.
>>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
>>I'm trying to explain it to a friend, and can't find a nice
>>with some diagrams, formulas etc. I know HP did one and
>>Standard did as
>>well, but can't find any quickly.
>>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
>>10 MHz to the GHz frequencies, but never manage to improve on
>>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
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