[time-nuts] Interval Timer Recommendation
Magnus Danielson
magnus at rubidium.dyndns.org
Sat Jan 12 13:40:53 UTC 2013
On 01/12/2013 01:37 PM, Azelio Boriani wrote:
> OK, but I wasn't able to find a single shot resolution specification in its
> datasheet. Looking at the formulas to determine the accuracy, it looks like
> the minimum resolution is 250pS... anyway, by reading the user and service
> manual I found out that the PM6685 has an EEPROM memory and no battery is
> needed.
At 250 ps you use interpolators. The typical coarse counter would be 100
MHz and that means 10 ns in raw resolution, but you would then use an
interpolator to get higher resolution. By getting a start and stop
measure, roughly tau time inbetween each other you get
f = (t_stop - t_start)/tau
assuming t_start and t_stop trigger from the same source.
For frequency measures you can overcome the single-shot resolution by
using multiple start and stop locations and process the estimates.
BTW, some counters use quite different approaches for triggering and
routing their TI measures and their frequency measures, to such degree
that frequency trimming requires separate calibration. SR620 is one such
example. Typically would the calibration error cause a static time error
between start and stop, and scaling with tau is a good way to track it.
Measuring it's own time-base is a good way to measure this calibration
error and trim it out.
Cheers,
Magnus
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