Radio and the antenna tuner
The selected 190-kHz bandwidth
Just recently I was shopping for a new communications receiver for ham radio and general shortwave listening. Lots of transceivers are available, but I only needed a receiver since I use a home-built QRP transmitter. While there are fewer separate receivers out there, the selection is interesting. I was on the verge of buying the popular Icom IC-R75-12. This general-purpose receiver covers frequencies to 60 MHz, which includes the 6-m ham band. It is a triple-conversion superhet with DSP filtering.
After some investigating, though, I could not ignore the increasing number of available software-defined radio (SDR) models. After a good look, I bit the bullet and bought one of the latest models, the RFSpace SDR-IQ. Here is a look at my experience with this SDR receiver.
TESTING THE SDR-IQ
Figure 1 shows my new radio and the antenna tuner I used. The SDR-IQ has a standard 50-Ω BNC input for the antenna. I didn’t have a 50-Ω antenna available so I just used a 30-foot piece of hookup wire with this MFJ L-network antenna tuner to handle impedance matching. It was hard for me to believe that this full-coverage receiver is so small. But because it uses a PC as the DSP and the display, it only requires the analog-to-digital converter (ADC) and filtering to get the complete receiver functionality. Even the power is supplied through the USB port of the PC.
Inside the box, a set of passive filters narrows the input range from 0 to 5 MHz, 5 to 15 MHz, or 15 to 30 MHz. The filters feed the low-noise amplifier (LNA), and then the ADC. No, this is not a superhet. No mixing or down-conversion is used. Instead, an Analog Devices 14-bit ADC sampling at 66.66 Msamples/s digitizes the entire bandwidth. Converter output is sent to a digital down-converter that isolates a 190-kHz window around the selected center frequency and generates the I and Q signals for the DSP. The digital data stream is sent through the USB port to the PC, where the Pentium or Athelon processor quickly performs a fast Fourier transform (FFT) spectrum analysis, does more filtering, and implements the display.
The SpectraVue software that is used is available with other SDR receivers. What you get is a full display of the selected 190-kHz bandwidth (Fig. 2) around the frequency you tune to. Below the spectrum presentation is the waterfall display, which represents frequency on the horizontal scale and time on the vertical scale. The display constantly “falls” over time and the signals are shown in colors representing different signal levels. I haven’t really learned the true purpose of this functionality, so I cannot appreciate its usefulness at this point.
The radio is tuned with the PC mouse. Users can also key in a specific frequency. A signal can be selected on the display by pointing and clicking. The receiver accommodates all the normal modulation methods, such as AM, SSB, CW, and FM. Of course, audio output is sent through the PC’s speakers or headphones if you want to use them.
The receiver works just as well as any other general-purpose radio, but you really do need a good antenna. It has taken me some time to get used to tuning with the mouse and keyboard. After decades of using knobs and switches on Hallicrafters, Hammarlund, Heathkit, and Yaesu radios, I am still uncomfortable with the SDR tuning, but I am getting there. I love the spectrum display because it really shows you what is going on around you, which is pretty cool!
Of course, anyone who has used a cell phone has experienced SDR. Most use a direct-conversion radio with a software baseband for the modulation, demodulation, and other functions. It is truly invisible.
SOME OTHER SDRs
There plenty of other SDR radios out there today. RFSpace also has a more expensive SDR-14 model. U.S. manufacturer Ten-Tec offers its RX-320D SDR radio, which is strictly a black box that has a USB port connected to a PC that handles the DSP and the display.
You can also get a software demodulator for Digital Radio Mondiale (DRM), the digital shortwave technology used mostly in Europe. Another interesting SDR radio is Micortelecom’s Perseus. It too covers 0 to 30 MHz and uses a 14-bit, 80-Msample/s ADC with an FPGA digital downconverter.
The Winradio WR303i downconverts the input to an IF from 1 to 15 kHz and then performs the DSP demodulation and filtering in the PC. The radio also offers a DRM demodulator. Winradio offers models that actually plug into the PCI bus of the PC as well.
As for transceivers, the big gun is still FlexRadio, with its stalwart 5000 model, and a newer 3000 model.
If you like to experiment, you may want to check out the kits from HPSDR. This is an open hardware/software standard created by hams for hams or SW listeners who want to build and experiment. The company offers a series of modular kits and boards you can assemble into a receiver and/or transmitter. Check out its products at www.openhpsdr.org.
A great deal has been written about SDR, and much of it infers that it is still to come. The reality is that it’s here right now—big time—and in many forms. You may want to experiment yourself.