[time-nuts] getting accurate timing on RTL-SDR output

Achim Gratz Stromeko at nexgo.de
Fri Apr 13 16:39:50 EDT 2018


Jim Lux writes:
> So now the challenge is to "line em up".  An obvious approach is to
> transmit an inband pilot tone with some sync pattern, received by all,
> and I'm working on that too.

A maybe not-so obvious approach would be to use RTL-SDR that have been
modified for direct sampling (usually via the Q branch) and inject your
timing pulse there.  That would limit the disturbance of the actual
signal while still relatively easy to extract from the data stream.

> But right now, I have the idea of capacitively coupling the 1pps pulse
> from the GPS to the antenna input - the fast rising and falling edge
> are broad band and show up in the sampled data.

The trouble is that you are going to impair the already low dynamic
range.  The ENOB on the I/Q ADC is around 7bit only.

> And you can see, no surprise, that the sample clock in the RTL isn't
> dead on - over the 10 seconds, it looks like it drifts about 30- 50
> microseconds - that is, the RTL clock is slow by 3-5 ppm.

Not all of these are created equal.  Several manufacturers claim to
factory calibrate their TCXO to better than 0.5ppm.  I have currently
two RTL-SDR that certainly are within 1ppm.  These things get quite hot,
so it definitely takes some time before they stabilize even if they do
have a TCXO in them.

> SO here's the question for the time-nuts hive-mind...
> What's a good (or not so good) way to develop an estimator of the
> timing/frequency error. Post processing minutes of data is just fine..

There is a program called rtl_test that just checks how many samples it
gets in a certain amount of time.  Let it run for a few hours on a PC
with a GPS-disciplined PC clock and it'll give you a pretty accurate
estimate of the mean sampling clock deviation.

The other method is to tune to a signal of known frequency and check
what it reads as.  There is a program floating around that uses a GSM
station for that purpose.


Regards,
Achim.
-- 
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