[time-nuts] What is the best counter for a Time Nuts?
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Oct 8 21:25:14 UTC 2008
Tom Van Baak wrote:
> 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.
>
>
It is feasible to achieve a somewhat higher timestamp resolution (~20ps)
and continuous sampling rate (> 1.5MSPS) than those of the 53132A
relatively inexpensively provided that a DSP or similar processor is
used to process the time stamp data in real time.
It is also possible to use a SR620 or a 5370 to take 1000 timestamps/sec.
However may require using some external hardware (frequency divider).
For maximum accuracy and precision the resultant data has to be logged
and processed off line using a PC or other computer.
> (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
>
Some Pendulum (A Philips spinoff) and Fluke (rebadged Pendulum) counters
do use an improved (fit linear regression line to time sequence)
statistical technique.
Bruce
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