[time-nuts] Importance or otherwise of counter rms jitter (one shot resolution)

[jpbridge at aol.com]

Hi All,

At the risk of becoming a bore, while I'm searching out a good price on a high end counter I've been trying to determine how important various counter specs are to me for GPSDO testing.

The big headline one is the one shot resolution which for the three counters I'm considering are:

Keysight 53230A 20psec spec with 10-15 psecs measured
SR620 25 psec typical 50 psec max (according to spec sheet)
Tek fca 3100 (Pendulum CNT91) 50 psec max 30 psec measured.

It is obvious that the Keysight has the best spec but it also costs a lot more. The SR620 is the next best (but not by much on paper) and costs much less if bought second hand off ebay but quite a lot more if bought with warranty in the UK even second hand.

My question though, is that all these jitter values are probably too high for ADEV plots on a goodish oscillator so some sort of mixing arrangement will be needed.

Given that a mixer is to be used, isn't it the case that the one shot resolution becomes less important than the trigger jitter which depends on the counter noise (and signal noise) and the slew rate. On this measure the Tek fares much better:

Tek fca 3100 typical 200 uV max 500 uV
Keysight 53230A typical 350 uV max 500 uV
SR620 typical 350 uV

Is my thinking right - i.e. if I'm planning on using a mixer then the one shot resolution term is negligible compared to the dominant noise/slew rate term?

James