After working on the FE-5680A discipline board, I came away with the presumption that longer time constants were universally better. After all, the tighter reign you keep on phase, the more you're going to have to horse around the frequency to do it.

To that end, the OH300 firmware of late has had three time constants - 100, 400 and 800 seconds.

I gathered some stats on the most recent unit I've built, and (alas I didn't save a screenshot), what I found was that its ADEV at low tau was better, but to make up for that, there was a giant "lump of suck" at around tau 2000s or so - up to 3E-11. Formerly, the results were more like a very rounded peak at 2E-11 from around 80s to 800s.

As an experiment, I changed the time constants to 100, 200 and 400 and ran it overnight again, and now it seems to have the best of both worlds - the low tau ADEV has a negative peak of around 6E-12 at tau 3s and rises up to that plateau of 2E-11 from roughly tau 100-1000, then starts the GPS dominated downward angle towards 3E-12 at tau 10,000s and onwards from there.

In comparing it to previous results taken some time ago, the low tau ADEV negative peak is lower now, but the medium tau hump is a touch higher and longer than before. It appears that that's the tradeoff - do you want lower ADEV at around 3s or lower ADEV and quicker dovetailing with GPS? It's a judgement call, but I think I'll go with this tuning. Perhaps in the future there might be a way to allow this tuning to be adjusted without uploading new firmware. I'll have to think about that some more.

I'm going to check that change in, as well as check in the new 'v4' firmware for the new design with the SkyTraq modules with sawtooth compensation message parsing.