[time-nuts] ADC sample voting algorithm?

[Nick Sayer]

> On Oct 5, 2016, at 4:52 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Nick,
> 
> Are you applying sawtooth correction to your phase measurement? 

Yes, these are post-correction observations. I have some confidence that my corrections are scaled appropriately for the ADC values because of their stability “most” of the time (meanwhile, the uncorrected values are bouncing around inside a 12 ns corridor).

> If not, are you merely seeing a hanging bridge that dissolves into at a normal sort of tick-tock movement?
> 
> Bob
> 
> From: Nick Sayer via time-nuts <time-nuts at febo.com>
> To: Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> Sent: Wednesday, October 5, 2016 6:45 PM
> Subject: [time-nuts] ADC sample voting algorithm?
> 
> This is tangentially on topic, I suppose. It’s for my GPSDO.
> 
> I notice periodically that the phase measurements seem “noisy.” You can see that over the course of several seconds the value doesn’t change, then it jumps a bunch and then comes right back.
> 
> My theory at the moment is that sampling the ADC multiple times in a row might help, but then what’s the best way to (quickly) pick which sample to use?
> 
> The mean would allow a bad sample undue influence.
> 
> At the moment, I’ve coded taking 3 samples, averaging them and picking the sample that is closest to the mean. If I’m right, and two of the samples happen to be very close to each other and a third is an outlier, then that seems like it would eliminate it.
> 
> I guess what I want is the mode, but with 3 samples, that’s going to be poorly defined (if at all).
> 
> Anyone have any suggestions (besides a larger sample size)?
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