April 3, 2012 3 Comments
Balint Seeber developed a software library that links HDSDR, a software-radio receiver program (and a few other related programs) with USRPs, a family of FPGA-based radios. I have a USRP1, the original USB-based motherboard, a BasicTX and a BasicRX daughter boards (which are basically just transformers linking the input coax connectors to the amplifiers that feed the ADCs), and an RFX400 transceiver (up/down converter to 400-500Mhz).
The USRP radios are best supported by GNU Radio, an open-source signal-processing toolkit. I have already used GNU Radio. The software is very capable, but it is somewhat difficult to install and it does not come with complete sophisticated radios. There are also drivers that interface USRPs to Matlab/Simulink; I was unable to get them to work. Therefore, I was delighted to learn that Seeber wrote the library that allows USRPs to be used with HDSDR and decided to give it a try.
HDSDR is a radio program that was derived from earlier programs (Winrad and WinradHD). It supports a wide range of RF frontends through the use of software libraries called ExtIOs. Seeber wrote an ExtIO library that can control the USRP and can transfer RF samples from it.
Downloading and installing Seeber’s ExtIO library and HDSDR was easy, thanks to an installer that Seeber wrote. Seeber’s software can also switch the USB-based USRP between two different drivers, which is a very useful (my experience in trying to switch these drivers manually has not been good).
After installation, I gave the software a quick try by tuning to a UHF and checking that the RFX400 can receive a signal from a handheld transceiver; it did.
I decided to try to USRP on HF next. I connected one of my amplified receiving loop antennas to a bias tee, then from there to a low-pass filter, and from it to the input of the BasicRX board. The low-pass filter is a high-quality 39 MHz surplus unit. It’s role is to eliminate aliasing of VHF signals into the HF spectrum. Ideally, the stopband should have started at 32 MHz (since the USRP samples at 64M samples/second), but a 39 MHz unit was all I had. I was not sure whether I would receive many signals, since between the BasicRX input and the input of the ADC there are only 20dB of amplification. In the picture you can see the USRP (in the background, in a cardboard box), the filter, the bias tee, and the 12V supply that feeds the bias tee. The USRP and the antenna are shown at the bottom of the post.
It turned out that this is sufficient to receive a great number of interesting signals all over MF and HF. Here are some screenshots. In the first screenshot I was listening to BBC World at 1323 kHz, an AM medium-frequency station. The USRP digitally down-converts the high-rate RF samples and decimates the RF samples, in this screenshot to 250k complex samples per second, so we see 250kHz of RF spectrum.
In the next screenshot, the radio is tuned to a station at the low end of the AM range, but the decimation is set to a lower value. We now see a full MHz of spectrum, all of the AM range plus a bit to spare. Very neat; all the AM stations on one waterfall display.
Next, we see the entire 14MHz amateur band. The band is densely populated at this early-evening hour. On the left we see a bunch of CW stations, PSK stations near 14070 kHz, and a large number of SSB stations starting at 14100 kHz.
A few days later I tried UHF reception with the RFX400 daughter board. Local FM stations were received well. Satellites proved to be more of a challenge. I was not initially able to receive satellites with fairly weak signals like FO-29 and CO-58 (which I can hear reasonably well with the FT-857D), but I was able to receive the CW beacon of RS-22, as you can see in the following screenshot. The Doppler shift is clearly visible both in the RF waterfall display and in the audio waterfall. You can also see the very narrow signal in the audio spectrum display. The signal was received with my small VHF/UHF Yagi.
A second attempt to receive FO-29 was more successful. I was able to hear both SSB and CW signals clearly enough to understand them. Linking HDSDR to the satellite tracking program of Ham Radio Deluxe via DDE provided automatic Doppler correction, which worked out very nicely. Still, the RFX400 is probably not ideal for receiving UHF satellites. Its noise figure is not documented but I don’t think it’s particularly good, and its response rolls off below 440 MHz and above 460 MHz (as reported by Patrik Eliardsson and others). An external preamplifier would probably help.
Seeber’s library also supports a low-cost USB dongle that can sample RF signals between 64 and 1700 MHz. The dongle is sold as a digital-TV and radio receiver, but somebody discovered a way to cause it to send to the PC unprocessed RF samples, which can then be processed in programs like HDSDR and GNU Radio. I ordered a such dongle for about $20 from eBay and I’m looking forward to trying it out. Its analog-to-digital converter provides only 8 bits per sample, a lot less resolution than the much more expensive FUNcube dongle, but it supports much higher sampling rates than the FUNcube dongle.