Using built-in RTL-SDR Bandpass Filters

The R820T tuner used in the vast majority of RTL-SDR dongles (including Carl Laufer's RTL-SDR.com) features some High-Pass and Low-pass filters - by combining both, you can get a working band-pass filter.

librtlsdr team implemented serious and various enhancements that were implemented in other sparse initiatives in their librtlsdr fork.

Hayati Aygün extended the ExtIO (used by some software like HDSDR) library functionalities, implementing librtlsdr functionalities in a straightforward & easy-to-use, no-hassle user interface to command these powerful settings, directly, on-spot, without having to play with clumsy command lines.

The Bandpass filter

I use my panadapter to check for band activity. While the RTL-SDR total possible bandwidth does not span across the entire 2m or 70cm band, it is well beyond most HF bands. Take the 40m band for example:

The region of interest to me is the yellow square. However, the SDR also captures everything else and it is not without troubles: Strong signals create images, signal replicas, desensitize gain control - it is not desirable.

So what if I could wipe out everything else and narrow down my reception to the signal of interest?

That's what I tried to achieve by designing a hardware bandpass filter in my Full Release - with less than impressive results.

Hayati's ExtIO

In his ExtIO (TODO:LINK), Hayati provided all the functions to narrow down the receiving range from the full 2.4 MHz (automatic setting) to sharp 290 kHz.

And now the 40m band trimmed down to 290 kHz:

Installing Hayati's ExtIO

TODO

Configuring

TODO

Bonus points: GPIO handling

TODO