[time-nuts] NEO-M8T TimePulse Phase Noise (was Re: Ublox M6T -M8T)

[John Ackermann N8UR]

I've been doing a lot of tests of the u-blox modules with their 
TIMEPULSE outputs set to RF-ish frequencies.  Attached are phase noise 
plots of the NEO-M8T in several groups of related frequencies.

The frequencies that are integer MHz values seem to be the worst.

4, 8 and 12 MHz have worst-case spurs in the -10 to -20 dBc range at 
about 30 Hz offset.  At 10 MHz the worst case spur is -34 dBc at 150 Hz 
offset; at 5 MHz it's about -42 dBc at about 850 Hz offset.  So based on 
biggest spur, the 5/10 MHz outputs are significantly better than the 
4/8/12 MHz.  Otherwise, the spectra of 4/8/12 MHz look quite *different* 
than 5/10 MHz, particularly beyond 1 kHz, but I don't know that they are 
better overall.

Some power-of-two frequencies seem to be quite a bit better.

It looks like 4.096 is the winner, with a maximum spur of -52 dBc at 1 
kHz (that value is from the spur table; on the plot it looks to be < -65 
dBc).  At 8.192 MHz, the peak spur is -35 dBc also right at 1 kHz.  By 
eyeball, the overall spectra of both look much better than those of the 
integer frequencies.

At 2.56 and 5.12 MHz the maximum spurs are also around -50 dBc, but 
there are more and bigger spurs in the range of 1 kHz to 10 kHz that 
make the overall spectra appear worse than at 4.096 MHz.

So contrary to the u-blox paper, it doesn't seem that sub-multiples of 
48 MHz are a "sweet spot", and in fact they generate the worst spurs. 
 From this set of an admittedly small number of test frequencies, 4.096 
MHz seems the best.  [ The u-blox paper tested the M6T while I am 
working with the M8T.  I don't know if there are changes between the two 
series that would change these results. ]

I'm scheduling similar tests of the ZED-F9T and NEO-M9N and should have 
that data in another week or two. Preliminary tests didn't show any 
surprises compared to the M8T results.

John
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