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

Attila Kinali attila at kinali.ch
Wed Jun 17 03:08:24 EDT 2015

On Wed, 17 Jun 2015 09:32:23 +1200
Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> > 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.

Is there any fundamental difference there?

> > 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 
> well
> > 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.

Can you give a description what you would do differently?

And yes, the two authors look like fresh graduate students who were told by
their professor to see whether they can reproduce Paneks results without
using a SAW filter.

> 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.

Hmm.. but the diode switched current source would need a quite steep
input pulse, wouldn't it? So some kind of pulse shaping would be
needed for a general circuit.

I also played with the idea to use a overtone crystal oscillator instead
of an LC tank, as this would probably give a higher temperature stability.

				Attila Kinali

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Metastability is the little-death that brings total obliteration.
I will face my metastability. 
I will permit it to pass over me and through me. 
And when it has gone past I will turn the inner eye to see its path. 
Where the metastability has gone there will be nothing. Only I will remain.

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