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

jimlux jimlux at earthlink.net
Fri Apr 13 18:05:52 EDT 2018


On 4/13/18 1:39 PM, Achim Gratz wrote:
> 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.

That's where it's being injected.. I'm using the RTL-SDR V.3, which has 
the RF input fed right to the Q input.


> 
>> 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.

Well, so far, after DDC, it's coming out about 1/5th of the dynamic 
range, and I can always adjust the size of the capacitor.



> 
>> 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.

Could well be.. I just turned it on, waited for the beagle to boot, 
captured the data, and moved on.



> 
>> 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.

I'm not so concerned about the frequency measurement - that's "easy".. 
What I'm interested is figuring out the precise timing (in absolute 
terms) of the samples.





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