From: Richard Laager
On 11/2/19 1:15 AM, Hal Murray wrote:
rlaa...@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
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv
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