On 7/4/20 7:49 AM, David J Taylor via time-nuts wrote:
David,
I've seen your comparison in the list archives. However, none of the
approaches I have seen published so far (including yours) exploit all the
possibilities I mentioned.
The Raspi having better community support doesn't help if the platform
itself
is unfit for the purpose. It might be OK as a general purpose NTP server if
you don't have any special requirements to accuracy, but for a PTPv2
grandmaster, having the ethernet behind a USB interface is a no-go.
Regarding the RF interference - I'm running a Ublox M8T module directly
mounted on the BBB via my custom GPSDO cape. It is working just fine.
Disabling the onboard HDMI encoder might be the "secret sauce" for success.
BR,
Matthias
===================================
Matthias, my feeling is that if you want a precision source, neither BB
not the RPi is a good solution. Maybe with all the tweak you mentioned
the BB approaches precision (for some values of precision). I see the
RPi as something which can provide far better NTP than simply using an
Internet source, something which will be adequate for the majority of
users (for some types of user!).
The RPi 4 doesn't have E/net over USB hence my comment on seeing a
comparison. The RF interference is to other parts of the spectrum - not
specifically 1.5 GHz GPS - and has been documented. I noticed it on
either 145 or 435 MHz bands when testing. [Jim] Whether anyone has
bothered to do proper measurements I doubt.
I was wondering more about non-GPS band interference.. And whether
anything was hooked up to it.
I've got some RTL-SDRs here, and I've connected them both to an older
RPi and some BBs, and with a whip antenna about a meter away, I've not
noticed a huge difference between them, in a sort of casual way. They're
both noisy, but then, I don't know if it's the environment, or other
stuff hooked up to them (Ethernet or USB cables, or the USB power supply).
I wouldn't expect them to be hugely different from an RFI standpoint as
bare boards with nothing else connected. It's stuff that's connected is
more likely the source, by conduction from the board then radiation from
the wires. That's why I was asking about the details.
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