RTL-SDR frequency step/accuracy

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edinsam
Posts: 1
Joined: Sat May 05, 2018 2:21 pm

RTL-SDR frequency step/accuracy

Post by edinsam » Sat May 05, 2018 3:38 pm

Is there somewhere on the net a technical description on how the R820T/RTL2832 sets its tuning frequency (i.e. from librtlsdr library calls)

I am using a NooElec dongle with 1ppm crystal accuracy, but need tuning accuracy to a few Hz.

At present I am achieving this by multiplying the IQ data from the dongle with a rotating complex sine pattern (in a feedback loop) which works OK and gives me the required precision, but it is rather time consuming in terms of CPU time on the RPI3 I am working with. I was wondering whether doing something with the RTL's own frequency correction calls would help somehow.

thanks

rrobinet
Posts: 2
Joined: Thu Nov 16, 2017 10:08 pm

Re: RTL-SDR frequency step/accuracy

Post by rrobinet » Wed Nov 07, 2018 6:58 am

I am using the RTL-SDR to receive WSPR signals on the 4342 MHz band. The internal XO is remarkably accurate and stable, the whole WSPR band is 200 Hz wide and the decoder tolerates no more than 2 Hz of drift during the 2 minute long packet reception.
To improve upon that I am feeding the RTL clock from a Leo Bodnar GPSDO which has better than 10e-9 accuracy (1 Hz at 1000 MHz). But configuring the receive frequency of rtl_fm to 432.3 Mhz, and a known accurate signal is output about 20 Hz too low. I have tried compensating for the 20 Hz in the tuning, but only changes of more than 100 herz result in a change in the received frequency.
So it appears that there is some truncation or rounding error in rtl_sdr or the librtlsdr libraries. Has anyone explored this problem?

alanzfq
Posts: 7
Joined: Thu Oct 04, 2018 11:18 am

Re: RTL-SDR frequency step/accuracy

Post by alanzfq » Thu Nov 08, 2018 8:05 am

100Hz steps for a digital television tuner seems overkill.
Is it is a limitation of the device itself?
Alan

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