direct sampling with RTL2832U

friedtj at friedtj at
Mon Nov 14 21:51:22 UTC 2016

Following my recent inquiry about direct sampling using the newer
RTL2832U+R820T(2) DVB-T receivers:
1/ I know from past experience that the E4k based dongles work fine in
direct sampling. Since the E4k is zero-IF, I could just get rid of the 
I+/I- or Q+/Q- capacitors and feed the RTL2832U with the signal I want
to sample. This works fine.
2/ The proposed patch I got last week aimed at canceling the IF
frequency since R820T(2) receivers are not zero-IF. Here I was failing
to get any result.

Going back to basics, I realized that using my old E4k in direct
sampling mode (rtl=0,direct_samp=1 in Gnuradio-companion osmosdr
source) is also not working. This is an unexpected behavior since E4k
based dongle configuration should not change whether direct sampling is
used or not. Indeed using the same dongle *without* the direct_samp
option did generate the right results.

I tried yesterday hacking librtlsdr in an ugly way to get raw samples
from the 820T(2) based receiver (solving my short term concern of
getting data). I could indeed acquire relevant raw samples by getting
librtlsdr to believe I was using a E4k dongle by replacing line 
1580 of librtlsdr.c (right after the "found:" label) with
now this induces a change in all the init functions with the
function pointers just defined afterwards (&tuners[dev->tuner_type];).
I have not yet tracked the difference between the init
functions, but I think it would be more or less safe
to jump to the E4k code in case the direct_samp
option is given.


JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France

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