[time-nuts] Raspberry Pi TCXO Hat

[David J Taylor]

I stumbled across this product listing:
https://www.teradak.com/products/115.html

It's a Pi hat that contains two TCXOs, one at 19.2 MHz for the CPU and
one at 25 MHz for the Ethernet controller. It's a rather permanent
modification, as you have to desolder the stock crystals and replace
them with wires going to the hat. I'm not 100% sure how to order the hat
yet, but they gave a price of $46 (USD) via email.

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.

In terms of mounting both hats, I'm guessing that the TCXO is only using
power from that hat connector, so rather than use the hat connector, I
would just solder some wires to it and the Pi, such that I could stack
the TCXO on top of the GPS hat physically but not need to connect them
electrically.

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

Richard,

As a pure sacrificial experiment I would be interested to know your results.

In terms of NTP time keeping, I'm not sure you would gain a lot.  Best 
results here are obtained when the RPi is kept at a steady or very slowly 
changing temperature.  I have one which is in a normally closed cupboard 
butting onto a north-facing outside wall:

  https://www.satsignal.eu/mrtg/raspi1_ntp.html

and this is a Raspberry Pi 1 model B.  By contrast, here's one sitting near 
a central heating radiator:

  https://www.satsignal.eu/mrtg/raspi12_ntp.html

which is an RPi 3 model B.  Both have a constant CPU load (light on RasPi-1, 
14% average on RasPi-12.  Both have GPS/PPS sources attached.

Given that the path to the outside Ethernet world on both of those models is 
via a USB controller, I would expect to see very little improvement with a 
TCXO in a constant temperature environment.  Just perhaps with a RasPi-4 you 
/might/ see an improvement as the Ethernet has a more direct path, but 
that's a board where self-generated heat is greater, so the thermal issues 
might make things worse.

Try it and see!

Cheers,
David
-- 
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Email: david-taylor at blueyonder.co.uk
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