[time-nuts] What is the best counter for a Time Nuts?

[Tom Van Baak]

> Big question: If the single shot resolution is only 3-4 times better
> that that of the 53131 and much inferior to the 20 ps of my SR620, how
> does the 53132 manage to be THAT GOOD A PERFORMER? Magic? Black art? 

Hi Ulrich,

My understanding is that 1) the 53132A has a single-shot
resolution of about 150 ps, but also 2) it can do something like
200,000 phase measurement per second for CW RF inputs.

In frequency mode, the frequency computed and displayed
each second is the result of internal statistics on those
200,000 samples, which in general is much better than two
single TI measurements made 1 second apart.

In the best case of no correlation between input and timebase
the frequency resolution would be 150 ps / sqrt(200 000), which
is sub-ps.

In reality, depending on the numerical character of the input RF
frequency, you don't get the full sqrt(200000) factor. Page 3-13
of the 53132A user manual tries to explain this. But for most
input frequencies it is in the low picoseconds, which is why it
can be called a 12-digit per second counter. Google for
53132a user manual for a copy of the manual.

(As an aside, if you use a 53132A on a slowly warming OCXO
you can see the display resolution change when the magic n/m
threshold, mentioned in the back of the manual, is crossed).

The bottom line is that the SR 620 or hp 5370 is a better counter
for TI (single-shot) measurements (e.g., comparing 1 PPS ticks).
But for CW frequency measurements, the 53132A is much quicker
due to this extreme internal oversampling. To be fair, I'm pretty
sure that other modern counters (e.g., Philips, Pendulum) also
use this frequency resolution trick.

/tvb