[time-nuts] Of rubidium life and piggy-bank anemia....

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Dec 1 05:00:14 EST 2007


If you need higher resolution than achievable with a gate array but
lower than that achievable with a dual mixer system then, a pipeline ADC
(per channel) and a low pass filter can be used to achieve a resolution
of 100ps or better.

Just low pass filter the input transitions so that the transition times
(rise and fall) are about 3 or 4 ADC sampling clock periods.
Then by processing the ADC samples the transition can be timestamped
with a resolution of better than 1/100 of the ADC clock period.

Just count the ADC samples that aren't close to the transition and
process a small number around the transition to perform the interpolation.
One method is to subtract the ADC sample taken say 4 clocks ago from the
current one transforming the transition to a pulse and calculate the
location of the pulse centroid.
Do this for all ADC samples (in hardware using a gate array or a
suitable DSP) if the difference is too small just count the samples.  If
the ~ 8 sample differences of interest are logged along with the sample
count corresponding to the first sample difference then the PC can be
used to calculate the centroid position.

With a 100MHz ADC sampling clock a resolution of a few tens (20-30) psec
should be achievable.

The low pass filter need not be more elaborate than either an RC filter,
or an LC filter, using low tempco components.

Bruce



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