[time-nuts] Using CPLD/FPGA or similar for frequency

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jun 16 00:24:34 UTC 2015

Using an ADC to sample a triggered damped sinewave easily achieves 5ps 
resolution (eg Keysight Acquiris). With a better optimised waveform model 
and least squares fitting routine greater resolution is feasible.
The accuracy is dependent on the ADC sampling clock stability.
An optical frequency standard derived clock may be required to maintain 
ps accuracy for long time intervals.


On Tuesday, June 16, 2015 12:14:01 AM Attila Kinali wrote:
> On Thu, 11 Jun 2015 14:22:58 +0000
> Alan Ambrose <alan.ambrose at anagram.net> wrote:
> > A clever interpolator for frequency or TIC would kill it -
> > for TIC essentially a PICTIC on steroids. The PICTIC does 19pS with
> > a 10 bit ADC and a 66MHz clock, an SR620 does 4pS with a 12 bit ADC
> > and an 80 MHz clock - so ... cough ... Spartan 3E at 256MHz with 16
> > bit ADC - and 1pS should be easy
> Which architecture for the FPGA do you have in mind? The delay
> line method (which is the most common one for FPGAs) has an intrinsic
> limit around 10-20ps. But the SR620 and the PICTIC use both a time to
> amplitude conversion by charging a capacitor (both include a Nutt
> interpolator).
> Using this technique, it might be possible to get into the 1ps ballpark,
> if the design is done carefully (according to Richard McCorkle, the
> limiting factor for the PICTIC II was the ADC of the PIC, followed by
> the stability of the reference clock).
> 			Attila Kinali

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