[time-nuts] Centroid pulse timing

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Sep 26 07:05:49 EDT 2006


Another way of determining the the arrival time of a pulse with high 
resolution is to use centroid timing techniques.
The input pulse is converted to a short pulse using a delay line timed 
monostable then the resultant pulse is low pass filtered by a discrete 
component RLC Gaussian low pass filter.
A sampling ADC continuously samples the low pass filter output at a 
fixed clock speed.
The centroid of the pulse can then be calculated from the resultant 
sequence of ADC samples.
Monostable output pulse width ~ 2x ADC sample clock period.
Low pass filter risetime ~ 2 ADC sample clock periods.
With a 10MHz sample clock a resolution of 100picosec or better can be 
achieved with a 12 bit ADC.
To avoid processing all the samples to find the pulse a synchroniser can 
be used to isolate 8 samples that straddle the pulse.
The synchroniser output also samples a continuously running counter to 
provide the coarse time stamp.
The processor combines the calculated pulse centroid position with the 
coarse time stamp

A delay line timed monostable is required for low output pulse jitter 
and good output pulse width temperature stabilty.
The output pulse centroid is delayed from the input pulse transition by 
monostable propagation delay plus 1/2 the monostable output pulse width 
plus the delay of the low pass filter.

If you have access to the GPS receiver PPS timing clock then the 
monostable can be replaced by a shift register and a couple of gates.

With a 10MHz sampling clock the monostable output pulse width should be 
about 200nsec and the (~1.75MHz) low pass filter has a risetime of about 
200ns.

With a 100MHz sampling clock and  matching ADC a timing resolution of 10 
picosec or better is possible.
A 1GHz sampling clock and matching ADC would allow subpicosecond timing 
resolution.

Bruce




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