[time-nuts] A new take on the all-hardware GPSDO concept

Nick Sayer nsayer at kfu.com
Mon Sep 12 14:41:59 EDT 2016

I was talking with someone at AD about a question I had about one of their TinyDACs when they mentioned their HMC1031 chip. It looks like the ideal building block for a clean-up oscillator.

It struck me just a touch later that the Venus838LPx-T has by default a 10 MHz output that’s phase locked to GPS time. It’s not good quality, but I wonder if it’s good enough to be the reference for this particular chip. They do talk about the ability to be driven by a “noisy” or "jittery” reference.

I think I’m going to take a crack at an OH300 GPSDO based on this design concept. Actually, first I’m going to actually try to quantify the jitter on the 10 MHz output from the Venus. From the HMC1031 datasheet it appears that if it’s not confined to a ±3 ns corridor that the lock indicator may not work (or work well). That would be a bummer.

I can foresee a GPSDO with the miniDIN 4 jack presenting the PPS and serial I/O from the GPS module and two LEDs on the front - the “FIX” LED from the GPS module and the lock LED from the PLL along with two BNC jacks. It would have some downsides. For one, I believe in the absence of GPS reception, it wouldn’t be able to properly hold-over at all. But it’ll be interesting to see if it can work as well as the micro-controller driven variant does.

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