[time-nuts] FMT on October 13

[Tom Van Baak]

> I guess it depends on signal to noise ratio. With reciprocal counters, you
> only need one period to measure as acurately as you need, but to have good
> acuracy, you need very good S/N, as there is no filtering possible. 
> 
> For example, the HP 5370 can measure a single period of a signal with a
> resolution of 20pS (excluding noise and trigger imperfections), so excluding
> these errors, the HP 5370 could measure a single period of a ~3.5 MHz signal
> with 7 x10-5 precision (if I have not goofed the calculations....) More
> periods improve the resolution proportionately to the quare root. Accuracy
> is another matter.
> 
> Didier KO4BB

The jitter on a single period is likely very, very high, especially
if it comes over the air. That's why one usually measures over
a duration of thousands or even millions of periods (effectively
called the gate time).

The HP 53132A makes something like 200,000 measurements
per second. As a result, for a certain range of frequencies, it
claims 12 digits/sec of resolution (vs. HP 5370 ~11 digits/sec).

/tvb