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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jun 16 17:32:23 EDT 2015

On Tuesday, June 16, 2015 10:01:09 AM Attila Kinali wrote:
> Hoi Bruce,
> On Tue, 16 Jun 2015 12:24:34 +1200
> Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> > Using an ADC to sample a triggered damped sinewave easily achieves 
> > resolution (eg Keysight Acquiris). With a better optimised waveform 
> > 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 
> > ps accuracy for long time intervals.
> Do you mean the technique that Panek et al. [1]  are using?
Not quite he used an impulse to excite a saw filter rather than switching 
off the dc current feed to an inductor or the equivalent.
> IIRC he got that down to 0.5ps RMS now. And yes, the major
> source of error is the oscillator, according to [2].
> Ripamonti et al. showed in [3] that using an LC tank instead of an SAW
> filter will result in something in the order of 2-10ps RMS (after
> temperature compensation). So this system is in the same region as an 
> designed time-to-amplitude converter based system.
The curve fitting algorithm they used is somewhat deficient as is the 
switching method employed one can do much better  provided one has 
sufficient time or computing resources available. My crude testing using a 
somewhat simplified diode switched current source powered by the signal 
itself achieved a fitting noise of around 5ps with a 14 bit ADC. A better 
driver and higher resolution ADC with a lower noise input amplifier than the 
input amplifier of the oscilloscope I used should improve the results 
somewhat as would a better model for the damped sine signal.
> I really wonder which one would be easier to build.
Keysight as far as I can tell used a discrete LC circuit to produce a damped 
sine wave rather than the conventional TAC approach used in the lower 
resolution Acquiris models.
> 			Attila Kinali
> [1] "Time interval measurement device based on surface acoustic wave 
> excitation, providing 1ps precision and stability", by Panek 
> 2007 http://dx.doi.org/10.1063/1.2779217
> [2] "Random Errors in Time Interval Measurement Based on SAW Filter
> Excitation", by Panek, 2008
> http://dx.doi.org/10.1109/TIM.2007.915465
> [3] "High frequency, high time resolution time-to-digital converter
> employing passive resonating circuits", by Ripamonti, Abba, Geraci, 2010
> http://dx.doi.org/10.1063/1.3432002

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