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