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

[Tim Shoppa]

You know Nick, the loop time constant typically used with the HMC1031 loop
filter is typically 5 milliseconds. I'm sure some bigger R's and C's can
used for a longer time constant, and I'm sure that'll help clean up the
awful 10MHz output of the Venus838LPx-T. But it is hardly what I'd call a
"GPSDO".

Tim N3QE

On Mon, Sep 12, 2016 at 2:41 PM, Nick Sayer via time-nuts <
time-nuts at febo.com> wrote:

> 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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