Swap I & Q?

[n1gp]

I've got an RTL stick that seems to have the I & Q reversed.

Is there a setting in Gqrx for swapping I & Q or a way to pass that in
via the I/O config?

[Alexandru Csete]

Hi

There is no I/Q swapping function in gqrx because that setting is only
relevant for devices that work as generic soundcard input device and
these are not yet supported in gqrx. For other devices such as rtl-sdr
the order of I and Q is well defined between the input driver and the
rest of the application. Therefore, if there is a device that needs
I/Q swapping it should be taken care of in the driver. I'm not saying
that I/Q swapping wouldn't be handy in gqrx, but it would be the wrong
place to fix this for rtl-sdr devices.

What is the exact model of your device?

Before we start looking for bugs in the driver, can you please ensure
that it really needs swapping? Please make sure that you are not
looking at an image or something similar.

Alex

[n1gp]

Thanks for you reply.

I need to do some more investigating. Will post back later...

[n1gp]

I did some more investigation tonight on this. It appears that I only have the need to
swap I & Q if I take the 124.487Mhz 1st IF out of my Icom IC-7000. If I try using a
direct antenna to the RTP stick it doesn't need the swap.

I have tested receiving the WX broadcasts on the 162Mhz band both direct antenna
and through my IC-7000.

I don't understand why the need to swap when using the Icom?

[Robin Gape]

It's probable that the local oscillator is inverting the 1st IF of your receiver. So lower frequencies become higher & vice versa, as a result of subtractive mixing. Then swapping I & Q undoes the earlier swapping.

HTH,

73, Robin, G8DQX

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[Alexandru Csete]

Ok, I didn't think of that possibility. I will add an I/Q swap option
asap and will let you know when it's done.

Alex

[n1gp]

Thanks Robin, that's exactly what's happening!

[Alexandru Csete]

There is now an I/Q swapping option available under "Input controls"
If you can test it please let me know how it works for you.

Alex

[n1gp]

Yes Alex this works very nicely for me. When using my IC-7000's IF in to
the RTL I can swap the IQ and it works great. I go back to the antenna to
RTL and swap IQ back, works fine. Thank-you!

[Alexandru Csete]

Ok, thanks for the info Rick.
Just in case you missed that announcement, you can store the settings
in two different config files and choose one at startup using the -c
or --conf command line option. For example, if you save settings as
ic7000.conf you can start with it using:

$ gqrx -c ic7000.conf

This may become handy when you get an extra dongle.

Alex