[time-nuts] femtosecond jitter

Magnus Danielson magnus at rubidium.dyndns.org
Sun Apr 15 17:17:14 EDT 2018


On 04/14/2018 06:45 PM, John Larkin wrote:
> Hi, Magnus,
> We did a little PC board that has two Analog Devices CML comparators
> that feed the flop.
>   https://www.dropbox.com/s/05ti1c57eush0uq/99S394A.pdf?dl=0
> An external DAC tweaks the VBIAS voltage to slew the edge times across
> one another, and an external ADC looks at the averaged flop outputs. The
> jitter noise floor is probably dominated by the test signals, not the
> flop under test.

Ah, thanks. Much clearer now!

You more tweak the voltage than actual timing, it's the slope property
that does the timing, but interesting never the less.

> We considered something like a micrometer-driven
> differential trombone line... note that 1 fs is one PPM of a nanosecond,
> how far light travels in 12 micro-inches.

Yeah, that would be "interesting" and maybe more work than you wanted to
game for.

> The quantization is probably DAC resolution. The step function is just
> the integral of the histogram.


> This is going into a test set that needs maybe 1 ps RMS noise floor, so
> this flop is hugely better than what we need. It's a big deal to set
> this up, so I don't think we'll do any more measurements.

For that needed precision, this seems not to be your limiting factor,
for sure. Good work.

OK. it would have been interesting to characterize it further.

> As a bang-bang phase detector, with some lowpass filtering in the loop,
> this flop would have a noise floor in the attosecond range. You're
> right, temperature will dominate low-frequency noise, and not just in
> the flop.

Yes. With both sensor and steering you can temperature compensate it.
With modest thermal isolation you can keep changes slow enough for
compensation to have a chance.


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