[time-nuts] ADC sample voting algorithm?

Bob Camp kb8tq at n1k.org
Wed Oct 5 20:00:15 EDT 2016


What does the signal you are sampling look like? 

Does it (maybe) have a bit of noise on it? 

If it is the output of a “normal” TDC, then the answer is to sample once.


> On Oct 5, 2016, at 7:45 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
> 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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