[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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