Sunday, April 23, 2017

rtl-sdr direct sampling on linux

There's a nicely put together rtf-sdr receiver box available on Ebay from China for about AU$45 with the direct sampling modification for HF already done for you and separate inputs for HF and VHF+.

There are several sellers so shop around.

You can see it on the right here and the box above it. (Click to enlarge)

Internally, it's an RTL2832U.

On my recent weekend visit to Kevin, VK2KB, I helped to get it going under Windows using SDR# software and this prompted me to get mine going at home under Fedora linux.

One mistake I've made before is that some USB cables seem to come wired only for charging and don't wire up the other USB connections. I wasn't seeing the device in lsusb and when I tailed the log (which, incidentally, has changed under Fedora since I last looked, to be sudo journalctl -f), there was no chatter as the device was plugged in. Trying alternate cables fixed this.

It turns out that GQRX finds the device just fine. To switch on direct sampling from the Q input, you set it like this:


CubicSDR saw the device but couldn't receive until I blacklisted the built-in TV reception driver by creating a file /etc/modprobe.d/blacklist.conf and entering blacklist dab_usb_rtl28xxu in it. Then I rebooted.

After that, the CubicSDR device config looks like this:


And it receives 40m SSB like this:


(There's not much on at this time of day).

5 comments:

Girardi said...

Dear Mr. Marks,

I have bought the same SDR but the results for
HF are very poor.

Not even the strongest medium wave stations
can be listened.

The antenna is just a random wire (2 meters).

Could you comment the details of the complete set (computer, antenna, cables) ?

Maybe my SDR has some broken part...

Yours.

Peter Marks said...

Dear Girardi,

2 meters of wire is not really enough for HF reception. It will probably be fine for FM (100Mhz) but not for 1MHz etc.

First check that you can hear local FM stations so make sure your SDR is working, then try a long wire, perhaps 10m running out the window to a near by tree or structure.

Yours.

Girardi said...

Dear Mr. Marks,

Ok, thank you!
I can listen the local FM station, so part of the circuit is fine.
I will try a longer wire.

The antenna you tested with was also a long wire or a professional
one ?

Yours.

Peter Marks said...

Dear Girardi,

The antenna I use is a long co-ax run to a dipole strung up between a tree and the house.

Yours.

Girardi said...

Dear Mr. Marks,

I made another test with a longer wire (10 meters) and now I can
receive some ham transmissions and few broadcast stations.
I am really disappointed with this sdr performance since
my very cheap shortwave radio with 30 cm telescopic antenna
is more sensitive :)

Thank you a lot for you help,

Mauricio Girardi.