Tuesday, January 20, 2015

Prototype Radio

I have been wanting to build up some general purpose modules for radio home brewing and began this weekend with a LM386 module. This modular radio construction method is inspired by M0XPD's Breadboard Bitx project,

I am using the Busboard.com boards. This is sort of  a "Manhattan style" construction. These boards have a ground plane which should make them excellent for radio work. Because of the pad dimensions, these boards can use DIPs as well as surface mount devices.

I had obtained several plug boards at the TRW swap meet and used the double stick backing to attach them to a piece metal flashing from Home Depot ($0.86) with some rubber feet on the bottom. This made a nice large working area and I intend to make some magnetic brackets that will attach to the metal side areas for control knobs, displays, etc.

On the protoboard in the picture is a Teensy 2.0, an Adafruit Si5351 Clock Generator Breakout, and my LM386 module. The plan is to use the Si5351 with the Teensy to form a VFO for a simple Direct Conversion receiver.

Tuesday, January 13, 2015

Spectrum Surveys using rtl_power

I have been doing various experiments with my RTL SDR dongle lately and wanted to see if there where any new software packages for these devices. I was primarily interest in scanners or methods to look at large areas of the spectrum, even if it took time to do the capture. There are some great capabilities within SDRSharp for scanning and capturing usage of those frequencies in the database, but I was interested in more of a continuous sweep.

It turns out that within the the osmocom drivers that most all RTL SDR dongle use there is a tool called rtl_power.exe. More details on rtl_power can be found HERE.

There are some GUI front-ends for rtl_power.exe that make it easier to use but running it at the command line has the most power. Use the GUI front-end to figure out what you want to do and then refine your capture at the command line was my approach.

The best way to get started using rtl_power with your RTL SDR dongle is to have a running installation of SDRSharp on you system. This is a quick way to know your dongle is working and to get all the drivers in place. Next, setup RTLPan per the instructions. This will get rtl_power on your system. Then using some of the examples on the rtl_power scripting web site you can begin your exploration. This was all done on a Windows 7 system but is available on Linux as well.

Here is a capture I did of the whole 2 meter band. Using the following command line:

rtl_power -f 144M:148M:5k -g 50 -i 10 -e 8h 2Mband.csv

The graphic below was processed using a Python script available HERE. The python script lets me view sections of the data collected above. Since I collected 8 hours of spectrum in the above capture, I used the following options in the Python script to produce this one hour segment:

python heatmap.py --begin "2015-01-11 05:00:00" --end "2015-01-11 06:00:00" 2Mband.csv 2Mband2.jpg

What you are looking for in the heat map plot is broken streaks that show usage of repeaters, etc. Since I was just using the tiny whip that came with the dongle it only hears strong signals. I did capture the usage of the 147.21 Mhz Sunset Repeater that is part of the WINS here in Southern California as well as another system streak that is the Keller Peak repeater on 146.385 Mhz towards the bottom center.

I think this is amazing capability for these inexpensive dongles and with the RTLPan GUI, a spectrum display is available as well. This could be useful for use with an HF up-converter on the dongle for checking the spectral response of HF transmitters and filters (poor mans spectrum analyser!).

Monday, January 5, 2015

Heathkit HR-10B as a SDR front-end?

A fellow ham has an old Heathkit HR-10B receiver and based on some recent experiments I thought it would be interesting to see if using SDR software with an old radio like this, it would make it more useful.

Since the radio had sat in a garage for many years the first step was to clean it up a bit. The chassis was covered with about an inch of dust and dirt. I removed all the tubes and inspected and cleaned the sockets and I was surprised to find the radio was now working! There is a bit of hum in the audio so the electrolytic capacitors are in need of replacement.

Since I wanted to connect this to a computer I will need an audio interface to be safe since there are high voltages in the radio up to about 300 volts. This gave me a reason to finish my audio interface that I started back in October HERE.

This is the completed audio interface cabled to the radio. I just used the RX AF to TO PC side of the interface to connect it to SDR-RADIO.COM v1.5 software.

I was surprised with this setup and with a lot of fiddling I was able to view about 6 khz of spectrum and tune CW and SSB signals just fine. The radio does drift but after an hour or so it is not too bad and you can listen to a SSB QSOs, CW, etc. and only need to adjust it every 5-10 minutes. I even decoded a PSK-31 signal. It will only hold on frequency for 10-30 seconds with PSK-31 but it can be done.