Which GPSDO did you settle on Jim? I have a BG9 clone and a Bodnar.
The BG9 osc has drifted
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beyond where it can be corrected but I'm still using the NMEA for
Bob - wd6dod
On 7/19/2021 8:55 AM, Jim Pennino via
In my testing I have tried various combinations of multiple
receivers and I do NOT recommend attaching more than one reference
clock, i.e. a GPS, to a machine.
NTP will work with more than one, but I have seen some strange
things happen when both devices are of equal quality.
For devices of equal quality, it appears to take ntp longer to
decide which one to select and occasionally marks both as false
tickers. I am guessing that is because the developers of ntp never
envisioned someone could actually afford having more than one high
quality source and the selection algorithms are having difficulty
deciding between them.
For devices of unequal quality, ntp will quickly pick the better
device and the second device from then on serves no purpose, e.g.
with a USB GPS and a PPS serial GPS, ntp quickly selects the PPS
I suggest moving one GPS to another machine or just keep it as
back up in the remote case of failure.
As for delay times, the actual delay will depend on ALL the
hardware involved. My value of 30 milliseconds is a rough average
for all the systems I've tested and is just a starting point. Of
all the devices I've tested, the VK-162 showed the least delay and
the device with the largest delay was about 750 milliseconds, so
yes, the delay can be quite large. If the delay pushes 1,000
milliseconds or goes over that (I have seen one that did which got
binned), I would not trust the device for time keeping and use
something else. Your numbers look reasonable to me.
A couple of other miscellaneous notes while I'm at it:
If one assumes the velocity factor of a USB cable is about 80%,
that means about 250 microseconds of propagation delay per meter.
As delay times are in the milliseconds, then a 4 meter USB
extension cable should add about 1 millisecond of delay, which is
down in the noise level of the jitter. Some quick testing of
extension cables shows this to be true. I would, however, only use
shielded cables in a RF environment least the cables become
During my career I often worked with clients that had a real need
for accurate time and it was not unusual for a client to spend
tens of thousands of dollars in today's money on hardware plus my
fees to achieve this. In my retirement I became interested in how
far the state of the art has progressed and wondered about the
quality of time keeping hardware available for the arbitrary limit
of $50 as well as what useful things can be done with a Raspberry
Pi, thus all this testing.
I decided to up the game and have ordered a GNSS disciplined OCXO
oscillator. While the oven controlled oscillator just guarantees
an accuracy of a few nanoseconds, at $180 it is about $600 cheaper
than a rubidium oscillator and there are limits to retirement
hobby spending. As a bonus this will provide a 10 MHz reference
that is +/- 0.0002 Hz for my frequency counter, or anything else
with an external reference input.
If anyone is interested in either/or high accuracy time keeping or
frequency measurement, I can write up the results after I have
done testing of this device.