[TangerineSDR] [hamsci-grape] Re: 3-Channel VLF SDR Backend System

[John Ackermann N8UR]

On 9/8/23 10:56, John Gibbons via TangerineSDR wrote:

> The 1 PPS output has a variability of ~ +/- 50 nSec  - averaged over an 
> 8 Mhz GPSDO clock gives a timing error of +/- 6.25 femtoSec (6.25 x 
> 10^-15).  Crazy accuracy but "good enough"  🤔😁

Hi John --

I don't think that accuracy calculation is correct, for two reasons:

(a) The timepulse output is generated by the free running (TC)XO in the 
receiver, not directly by the navigation engine.

(b)  The navigation engine gates pulses to align the timepulse as 
closely as possible to the calculated top of the second (it doesn't 
steer the XO).  At PPS, the leading edge of the output is just the XO 
clock pulse occurring closest to the top of the second.  At higher 
frequencies, the CPU counts and lets through the right number of pulses 
per second needed to make the frequency.  In both cases the accuracy is 
limited by the clock granularity.

So you get an output that's on average correct, and it becomes more 
correct the longer you average.  But within each second, it's a free 
running crystal with pulses being dropped at various times.  The jitter 
at 10 MHz is about the same as at 1 PPS, just spread over more pulses.

(Can you speed up the navigation rate to get more frequent corrections? 
Tests show that 10 Hz navigation isn't any better, and u-blox recommends 
using 1 Hz navigation rate for timing applications.)

Attached is a zoomed in frequency plot showing a NEO-M8T with timepulse 
set to 10 MHz and navigation rate 1 Hz.  It shows about a half second of 
data at a 1000 sps rate.  You can see the jitter from millisecond to 
millisecond is on the order of +/- 0.5 Hz at 10 MHz.

I've also attached ADEV and phase noise plots of the same data.  From 
the PN you can see that you *do* *not* want to directly use the 
timepulse output as an RF source -- all those dropped pulses generate 
lots of spurs and you need a cleanup loop to make it safe for receive or 
transmit applications.

73,
John
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