gqrx, direct sampling configuration, shortwave/AM reception, etc

This is one of those articles that I write specifically to, you know, not forget it the next time.

I picked up a v3 RTL-SDR dongle from http://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/ a few weeks ago. It's a solidly looking aluminium can dongle with a much more useful RF connector. But, it still only tunes down by default to the same RTL-SDR limits as .. well, the rest.

So I go digging. I bought this thing because on Windows you're supposed to be able to tune down to a couple hundred KHz. I found out some useful stuff after a bit of google abuse:

• it's called "direct sampling mode";
• it's supported by the rtl_sdr driver/library that you normally use;
• you have to configure things up in a special way to get GQRX, etc to actually do the right thing;
• There's a specific thing called "Q-Branch" that you have to care about.

Ok, so the actual details.

Firstly, the NIC needs to go into direct sampling mode. So instead of being mixed with a VCO, you're bypassing all of that and just acting as a really fast ADC. Which, yes, will alias signals everywhere.

Secondly, you have to tell it to only give you "Q" samples, rather than both I and Q. If it's direct sampling, mixing doesn't come into it, so the hardware is just giving you ADC samples.

Then, you just tell GQRX, etc to ignore enforcing tuning limits (tick "No limits") and you're golden.

The specific string to add to the device string when you set up which rtl_tcp instance to talk to is:

direct_samp=2

(0 = no direct sampling; 1 = direct sampling on I, 2 = direct sampling on Q.)

Ok, so does it work? I'm still testing it. I will need to acquire some RF filters to try and filter out bands of HF frequencies to avoid aliasing as there is limited to on actual passband filtering going on here. But it did tune to AM radio and I picked up some data transmissions in 3MHz.

Baby steps!