Gqrx Single Hz Tuning / Sub Hz PPM Correction

[Kev Walton]

Hi

I am new to Gqrx but have had a search of the issues log and the website and can't find an answer to my issue.

I have a RTL-SDR dongle with a 120Mhz (actually about -119.999350) upconverter and am using it to decode WSPR - which has a band of 200Hz - so you see that accurate tuning is pretty important to get the band in the right place to decode.

The issue I have is that LNB LO setting appears to only operate in 100Hz increments (ie -119.999300 MHz or 119.999400 MHz), and the alternative, the PPM setting, is only settable in 1 PPM increments, which with the upconverter is also a 100Hz or so change.

How can I get acurate tuning that isn't 50Hz out please?

Currently running 2.14.4 built locally.

Thanks very much

Kevin

[Jeff Long]

The rtl-sdr library takes an int value for ppm correction. GQRX allows a more precise value, but the rtl can not accept it.

You can tune to 1Hz precision using the frequency at the top of the screen. Any tuning that can not be done (or you do not want to be done) by the hardware is done by a software frequency converter.

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[Kev Walton]

Thanks very much Jeff for the reply, unfortunately even the tuning at the top of the page is only working in 100hz or so jumps for me.

I can step up 10hz (for example) at a time, but not till i get from 600hz to 740 hz does the output tone change.  This is repeatable and the same 140hz change required each time.

Thanks very much

Kevin

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[Jeff Long]

Are you looking at the frequency centered above the frequency plot? Not the one in the control panel. It's definitely able to tune to 1Hz. The hardware tuning does have larger steps depending on the sdr, but the overall tuning does not.

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[Kev Walton]

Hi All

Please see the attached screenshots.

1) 14.095.600, wsjt-x waterfall showing 1425Hz

> Tune up in 10Hz steps by clicking on ten's number in frequency in the main window just below the toolbar

2) 14.095.720, wsjt-x waterfall showing 1425Hz

> Tune up in 10Hz steps by clicking on ten's number in frequency in the main window just below the toolbar

3) 14.095.740, wsjt-x waterfall showing 1320Hz

> Tune down in 10Hz steps by clicking on ten's number in frequency in the main window just below the toolbar

4) 14.095.650, wsjt-x waterfall showing 1425Hz

What am I missing / doing wrong?

Thanks very much

Kevin

[Jeff Long]

Gqrx and wsjtx can interact. Measure the frequency on gqrx without wsjtx running. Out of ideas after that.

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[Kev Walton]

Thanks Will, same issue with no wsjt-x running, you can just see it at max zoom...

[Jeff Long]

I was able to replicate this stepping problem using a RTL. An AirspyHF+ does not have this problem. Now I admit I have no clue why this is happening. This is with the most recent GQRX and GNU Radio libraries. I'll let you know if I figure anything out. Maybe someone else has an idea.

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[Kev Walton]

Someone else reproducing it is a great start, means I am not going crazy (yet).😁

Anything you need me to test just let me know.

Thanks very much 

Kev

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[Jeff Long]

Solution, though not intuitive. Tune to the approx frequency with the main control, then fine tune with the input under receiver options. It is still likely that there will be some offset because gqrx doesn't know (or isn't checking?) that the rtl isn't tuning to exactly the frequency it asked it to. Not sure what layer (gqrx, gr-osmosdr, rtl-sdr) has the disconnect, but this should get you going. Unfortunately, you will have to come up with your own calibration. 

[Jaap Viergever]

Op do 25 mrt. 2021 20:04 schreef Jeff Long <will...@gmail.com>:

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[Kev Walton]

Thankyou, I can confirm that whilst awkward, this does work, to single hz resolution.

Do you want me to open an issue for it on github?

Thanks very much

Kevin

[Jeff Long]

I opened https://github.com/csete/gqrx/issues/951.

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[Robin Gape]

I've also seen a stepping issue on an RTL-SDR and, if memory serves,
with a Funcube Dongle Pro Plus (FCDPP). Checking with the local 23cm
(1296MHz) beacon, the smallest tuning step is several hundred Hz, IIRC.
So this is a genuine problem.

One suspects that there are issues with support libraries. (I haven't
investigated further.)

Unfortunately current circumstances get in the way of much further
investigation. However, the repository at https://github.com/csete/gqrx
has an issues list. It's also possible that these issues have been
raised on this list previously, you might find it useful to check.
[RTL-SDR support comes, AFAIK, from the Soapy project.]

HTH, 73, Stay Safe,

Robin, G8DQX