Software Defined Radio, SDR

I simply had to try software defined radio based on a TV-USB-dongle! I am not a HAM-radio enthusiast, and have no license to transmit, but listening can also be interesting. Another aspect is the possibility to do spectrum analysis of various filters and oscillators. I found a lot of information on the site www.rtl-sdr.com where you can also buy various bits and parts. I wanted to keep the budget low, so I settled for the rather inexpensive approx. $25 package, including:

  • USB-stick: RTL-SDR R820T2 RTL2832U 1PPM TCXO SMA
  • 6cm to 20cm telescopic antenna
  • 20cm to 1.5m telescopic antenna
  • 4.5cm diameter magnetic mount base with 1 meter RG174 cable

This dongle is downwards limited to approx. 24 MHz (upwards to 1.7 GHz), and I am actually more interested in the lower long-wave bands. So I had to think of an “up-converter” pre-stage for the dongle, and found the Nooelec “Ham It Up v. 1.3” that will shift your antenna input up to 125 MHz. This one also comes with a noise source and a test oscillator. The noise source is feasible for spectrum analysis of filters, the feature I was also looking for.

Compared to the dongles, the up converters are generally more expensive, and the Nooelec was approx. $40 at www.nooelec.com/store/ham-it-up.html. I also bought a small SMA-male-to-male cable (pig-tail) to connect the converter to the SDR-dongle.

I know something about building radios from my work at the long gone “AP Radiotelefon” where I built a few mobile radio prototypes. I know how tricky HF can be, and when I got the parts for my radio I thought that some thorough shielding had to be done. This is how I solved it:

The STL-SDR is inside the little box inside a bigger box, and with the USB wired to an alternative USB-B going out of the side through both aluminium boxes. The SDR card is kept in place with a small copper bracket soldered to the USB-A connector and fastened with a screw to the box. 5V is taken from the USB and out to the upconverter (a small ferrite core on the way out).

LED’s and switches and jumpers are all brought out to the front of the outer box. From left you can see switches:

  • 125 MHz oscillator on/off
  • upconvert/passthrough
  • test-generator on/off
  • noise-generator on/off.

The LED’s are green for upconvert active, yellow for power on, red for +5V too low and blue for noise generator on (= +12V on). The SMA is the noise generator output, but the outputs for test generator and 125 MHz oscillator are missing on this photo.

As SDR-software I use an XP version of the HDSDR v2.70 www.hdsdr.de, run in a virtual environment isolated from the web (using Parallels Desktop). I found the HDSDR much easier to setup than a couple of other possibilities, including the SDR#. For spectrum analysis I have tried the “RTL dongle panorama” , but I don’t know yet if it is something I can use. Recently I found the app GQRX , and it works perfectly on my Mac directly from MacOS:

 

I have found it rather difficult to find hardware documentation. I don’t know if the schematics for the RTL_SDR are correct. but here they are:

 

rtlsdr1

rtlsdr2

Upconverter-125MHz-ADE_Schematic

Upconverter-125MHz-ADE_PCB_Preview