[time-nuts] Raspberry Pi TCXO Hat

[David J Taylor]

From: Richard Laager

On 11/2/19 1:15 AM, Hal Murray wrote:
>
> rlaager at wiktel.com said:
>> I'm curious if this would provide any meaningful improvement in system 
>> clock
>> accuracy, for NTP, if I'm already a GPS PPS hat. If there's a reasonable
>> chance this could be interesting, I'm thinking about ordering a couple 
>> and
>> "sacrificing" a Pi 3 and/or Pi 4.
>
> What are your goals?  NTP server?  Neat graphs?

Closer to the latter.

> The Pi 3 has Ethernet on USB.  That adds a layer of jitter to packet 
> timings
> so a Pi 3 even with GPS will never make a great NTP server.

Right, though a Pi 4 may be better.

> How stable is your temperature?  Self heating as the CPU load changes is
> important even if the room temperature is stable.  A TCXO won't help if 
> the
> temperature is already stable.

The graph in ntpviz shows the jitter and temp almost perfectly correlated.

> If you have 2 identical Pi-s, you could put the TCXO on one and run a
> side-by-side comparison and tell us what happens.

Right, that's the sort of test I had in mind.

Richard
=======================================

Folks,

As a matter of interest, I've just compared the reported jitter on a RPi 3B, 
RPi 3B+ and RPi-4B, all PPS synced with classic NTP, all in the same room, 
but with slightly different puck antenna locations.  The lowest 5-hour 
averaged jitter was:

RasPi-15  model 3B   0.98 us
RasPi-22  model 4B   1.08 us
RasPi-18  model 3B+  1.40 us

So either the antenna location (puck inside a room) is important, or these 
results are down in the noise anyway.  RasPi-18 has a puck antenna near a 
south-facing window so I would have hoped it was the best, but apparently 
not.  RasPi-15 and -22 have antennas in a similar location (pucks with a 
magnetic base on top of a Desktop PC).  These are all Wi-Fi connected RPi 
cards so I couldn't compare the Ethernet delay.

Maybe I'll try and setup a 3B/4B Ethernet comparison sometime - I'd like to 
know the result too!

I suspect that a good outside antenna might benefit at least one of these 
RPi cards.  Even the RPi model 3 would make an adequate server for most 
users considering that devices these days are mostly connected over Wi-Fi 
rather than Ethernet!

Where better timing is preferable, I've tended to add cheap PPS boards with 
built-in GPS antennas to assembled Raspberry Pi zero units (e.g. MMDVM 
hotspots), using only the PPS from the board (as other units want the serial 
port pins), and relying on Wi-Fi for the time-of-day information.

Cheers,
David
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