Why is HP 5370/5345 Time Interval Measuring less accurate trhan Period Measuring

I use a HP 5345B and a HP 5370A for time interval measuring ( separate as well as common mode ). The 5345A has a jitter stability ( useable resolution ) of 2 nS and the 5370A a jitter stability of 20 pS. Both the instruments behave OK. However, I see that the stability of the period is far better than the accuracy of the time interval.

Whatever tweaking I have done and whatever signal source I use ( e.g. a e-11 frequency reference )and at whatever frequency I perform the measurements, any time interval will be no more stable than about 5e5. Thus, measuring ( 5370A )( common TI mode ) a 10 Mhz frequency reference signal the stability shows about +/- 40 pS jitter which is very good and far within the specs.

Measuring at 1 Mhz ( reference locked ) however, the stability is +/- some hundreds of pS. And measuring at 10 Khz , the stability is +/- 50 nS

I get about the same ( relative ) results when using the 5345B instead of the 5370A ( be it that the jitter stability and resolution of the

5345B is 100 x less ).

However, measuring the period ( instead of the TI mode ) I get the expected results of about e-11( which is cf the specs if locked to a better than e-11 freq.ref ).

In the frequency modes both the instruments perfom far within specs ( stability resolution better than e-11 ).

What is it that TI measurement ( in these particular HP gear )is so much less stable than period measurent ?

Since both the instruments show this very phenomenum and for both even in the same relationship at varying frequencies, I guess this must be system feature and no flaw.

Any HP guru in this group ?

I bent my mind for days but I cannot resolve this one. Thanks for help.

Peter, PE1E ex PE1ECM

There is a built-in time uncertainty of one internal clock oscillator period in any counter. This is the resolution of your counter.

For an TI measurement you are measuring a single event, so your resolution on that measurement is your uncertainty.

For a period measurement, many instruments will measure over N periods and then divide by N to get the period. The same resolution above applies to the total time measured, but this makes the uncertainly for a single period 1/N times the measured uncertainty.

Often you can select N when you set up the instrument; it will be related to the gate time that you use when you measure frequency. Just as your frequency measurement gets better if you average over a longer gate period, so will your period measurement.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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