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

Achim Gratz Stromeko at nexgo.de
Sat Apr 14 06:28:36 EDT 2018


jimlux writes:
>> 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.

OK, some more details (I did say it was a bit non-obvious :-) on that
idea.  The R820T tuner in the RTL-SDR v3 only uses the I input of the
RTL2832U as it's a low-IF architecture, leaving the Q inputs free and
unused in the normal mode of operation, which the direct sampling
modification takes advantage of by populating the Q branch with a direct
input.  The other commonly used tuner, the Elonics E4000, uses both
inputs, because it's using a zero IF architecture.  Thus it should be
possible to switch the RTL dongle to zero IF mode, get the I/Q samples
as per normal and you get two perfectly correlated data streams from two
different inputs.  You can modify the hardware to also have a direct
input on the I branch normally used by the tuner.  Alternatively, don't
modify the hardware and use the tuner to downconvert an RF sync signal
to the I branch (you can test if that works by just using it in direct
sampling mode with the I branch selected, but I think the tuner might
get switched off by the rtlsdr library, so there'd need to be some
modifications to the software).

http://datasheetcafe.databank.netdna-cdn.com/wp-content/uploads/2015/09/RTL2832U.pdf
https://www.reddit.com/r/RTLSDR/comments/1uazsw/rtl2832_datasheet_deep_info/

Also, the RTL-SDR v3 does have a clock input, so you can modify the
hardware to use that and make its sampling coherent to a known clock
source.  That would allow you to align the sampling files only
sporadically and then rely on the known relation to the clock as long as
no USB frames are getting dropped.

https://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/

No, I haven't done anything of that yet, but what initially got me
interested is the idea of correlating DCF77 (through an upconverter)
with GPS.  Still working on the antenna setup, it might be possible to
also receive TDF, MSF and maybe even RBU.  I did get as far as buying a
GPS timing module with a frequency output that I can use for the
external clocking to eventually get coherent sampling.

>> 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.

My RTL-SDR v3 is still in transit, but I expect your result to indicate
that it was still warming up based on what my other RTL based dongles
do.


Regards,
Achim.
-- 
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