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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
