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