Config gqrx for direct sample mode with RTL2832U

[Pedro Correia]

Hi all,

I've managed to get gqrx 2.7 to work with an rtl2832 in direct sample mode in ubuntu 16.04 LTS. I did it by configure the Device string field whit " rtl=0,direct_samp=3" for HF and "rtl=0,direct_samp=0" for >30MHz.
I am using an DXPatrol Ultra Wide-band Coverage SDR Receiver MK4 http://www.dxpatrol.pt/images/Docs/DXpatrol-SDR-Radio-MK4.pdf  and it only works in direct sample mode because it uses the I input for the VHF antenna and filters, and the Q input for HF, this way there is no need for an external switch...
It did work (apparently) but I don't really understand what is happening, this is not coherent whit the documentation for the GrOsmocom in http://osmocom.org/projects/sdr/wiki/GrOsmoSDR some enlightenment is welcome.
Is it possible to implement a feature in gqrx to provide this?

Thank you,

Pedro

[Robin Gape]

Pedro,

1) It's not very clearly explained in the DXPatrol Mk4 manual, the diagram on p.3 appears to indicate that VHF/UHF is via I only. That is clearly nonsense, since for VHF/UHF both I and Q inputs must be involved in the SDR processing chain. That is confirmed later on in the manual on p.10, where for direct sampling

• for HF the selection is  Q input (only)
• for VHF/UHF the selection is Disabled (i.e. the usual use of both I and Q inputs) [If that were not the case, then all VHF/UHF signals would appear twice, both above and below the local oscillator frequency.]
  

2) If you are looking for a simple way to switch modes between HF and VHF/UHF, then the simple way should be to set up 2 configuration files, one for HF and one for VHF/UHF. (Configure for a particular mode, then save the configuration and give it a suitable filename. The configuration may then be reloaded. Shortcut keys ^S for save, ^L for load.)

HTH,

Robin, G8DQX

[Pedro Correia]

Robin,

It certainly helped, thank you. I'll follow your suggestion on the mode switching.

The MK4 diagram on the manual is, at least, not very accurate.
I did some googling and it seams that this rtl chip has 2 I and 2 Q inputs, probably "direct_samp=3" means use input 3 that is a Q input not used on the VHF side. Sounds plausible because it's a good way not to mix the 2 antenna inputs.

Pedro
CT7AEZ