Brainerd Area Amateur Radio Club  

Software-Defined Radio    Thursday, March 29, 2007

Software-Defined Radio – A beginner’s guide

There has been a lot of talk about software-defined radio (SDR) over the past few years, but you don’t hear too many of these radios on the air. A full-function ham SDR transceiver like the FlexRadio SDR-1000 can cost upwards of $1700 when you include the cost of a high-quality sound card. That assumes that you already have a suitable computer that can be dedicated to the radio interface. Although the performance figures for the SDR-1000 are truly outstanding, many hams (like me) are reluctant to spend that much just to see what the technology has to offer. This article gives a brief description of what software- defined radio is and is not, what it can do, and how you can get a taste of this new technology without breaking the bank.

First of all, what is the difference between a computer-controlled radio, a radio that relies heavily on internal software (or “firmware”), and a “true” software-defined radio? The distinctions can be a little blurry and may become even less clear in the future. A number of computer-controlled receivers and transceivers are currently available.  Also most recent ham HF transceivers have CAT (computer-automation-technology) capability, allowing some or all of their functions to be controlled remotely by a computer. An example of an excellent freeware radio-control program is Ham Radio Deluxe at http://hrd.ham-radio.ch/, which will interface to a number of radios and has many other bells and whistles in addition to the control function. The fact that you can tune the radio, tweak most of the knobs, select filters, etc. by remote control indicates that the hardware and software are closely tied. However, with computer control the radio’s functionality is still determined primarily by the radio hardware.

Ham transceivers are relying increasingly on internal “software” to define their capabilities. For example, the Icom 756 PRO series allows the user to define IF filter characteristics and to customize the appearance of the multi-function graphic display. But I wouldn’t call that truly “software defined” because you can’t obtain upgrades to the firmware from Icom or install a different “operating system” from a third party.

If you look at the physical appearance of a radio like the SDR-1000 at http://www.flex-radio.com/ and study the block diagram, a number of traditional components are missing--like knobs, a display, IF filters… So now we are definitely in software-defined territory. The actual definition of a software-defined radio varies, depending upon whether we are talking about radio systems in general or about ham radio transceivers. Wikipedia has a good general definition and description of software defined radio at http://en.wikipedia.org/wiki/ Software-defined radio" or http://en.wikipedia.org/wiki/Software-defined_radio. When narrowed to ham transceivers, my own definition is: “A significant portion of the traditional radio hardware functions (such as frequency generation, mixing, filtering, modulation and demodulation) are performed in re-configurable software.”

As pointed out in the Wikipedia article, an ideal SDR system would have an analog to digital converter (A/D) that takes signals directly from the antenna, digitizes them, and feeds them to a computer for receiver processing. In transmit mode, processed data from the computer would go into a digital to analog (D/A) converter and then into the antenna. That kind of implementation is not practical at ham frequencies (not yet, anyway), so we still require some hardware to convert the signals into a limited frequency range that the A/D and D/A converters can handle.

System requirements for an SDR radio include the A/D and D/A hardware, digital-signal- processing (DSP) circuitry, lots of computing power, and, of course, some way for the user to see, hear and control the radio functions. Fortunately, a PC with a sound card takes care of most of those tasks. The PC has to be a fairly recent one with at least a 1 GHz Pentium processor or its equivalent. Additionally--although a sound-card upgrade will be necessary to get maximum SDR performance, the card that comes with a typical PC is good enough to get started, at least for receiving. (That is, if it has a stereo-line input. My recently acquired notebook computer, like many other notebooks, does not!) So if you already have the PC, all you need is the frequency-conversion hardware, or “front end,” which contains bandpass filters and special I/Q (in-phase and quadrature) mixers.

In order to have an all-band transceiver that makes the top of the charts in performance, you need really good RF front-end hardware, and that can be costly. But if you want to experiment on a couple of bands to get a feel for what software defined radio is like, there are some very inexpensive options. I haven’t played with any hardware other than the “SoftRock” series of kits, so I will limit my discussion to them. However, there may be other good kits and assembled units out there, and more are probably on the way.

Full details on the SoftRock kits can be found on the Yahoo SoftRock40 user’s group at http://groups.yahoo.com/group/softrock40/. It may be necessary to join the group to have access to all the files, but the joining process is free and painless. Starting at the bottom, SoftRock Lite single-band receiver kits are available for 160 through 20 meters at a price of $10 postpaid. That price includes the PC board and all parts, but not the case and connectors. Be advised that there are some surface mount capacitors and integrated circuits in the kits. If you haven’t worked with surface-mount devices before, this is an opportunity to gain some experience! Next up the ladder in the SoftRock series is the v6 receiver kit, available in dual-band versions for 40/30 meters and 80/40 meters. There is also a single-band version for 160 meters. The price for these kits is $14 postpaid. By the way, if you want to use a SoftRock receiver and SDR software as an external IF processor for your receiver, v6 and Lite kits are available for some of the common IF frequencies at an additional cost of $2. Moving up to transceive capability will cost you some bucks--$32 to be exact. The RXTXv6.1 kit is available for 40/30, 80/40, or 160 meters. Kits can be ordered using PayPal to Tony Parks, KB9YIG. Tony’s  e-mail address is kept secret in order to protect him from Spam, but his user name is raparks and the remainder of the address is ctcisp.com.

How well will these kits perform? If you’re an old geezer like me who remembers broad-as-a-barn receivers with a tuning dial that barely told you what band you were on, the SoftRocks will knock your socks off. Furthermore, even if you are already used to modern receivers with digital frequency readout and a good selection of IF filters, you are still likely to be impressed. The hardware does an outstanding job, but the real secret is in the software--more on that later. Each kit comes with a crystal (the transceiver comes with two) that provides a local oscillator in the CW portion of the band(s) that the receiver is designed for. For example, the 40/80 v6 receiver comes with a crystal that gives center frequencies of  7.056 and 3.528 MHz. If your computer has a sound card with a maximum sampling rate of 48 kHz, you will be able to use the SDR software to tune plus and minus 24 kHz from those frequencies. With an upgrade card that supports 96 kHz sampling, you can tune +/- 48 kHz from center, and cover almost the whole CW portion of the band. If your interest is in SSB operation, you will need to find your own crystals. The v6 receiver and RXTXv6.1 transceiver kits for 80/40 meters use overtone crystals in the 28 to 32 MHz range, which are divided by 4 or 8 (jumper selectable) to get them into the 40 or 80 meter bands. In order for the quadrature sampling detector (the type of mixer used in these kits) to function properly, the crystal frequency must be divided by at least a factor of 4. Crystals in the 14-16 MHz range can be used for 80 and 75 meters, but they will require a slight modification to the oscillator circuit. Fortunately, I had some crystals in my junkbox that provide a reasonable selection of frequencies for 40 and 80 meters. Surplus Sales of Nebraska and Ken’s Electronics have a variety of crystals that should work, although I can’t guarantee that!

Now to the software. There are several excellent programs (all freeware) that will work with the SoftRock kits and other hardware "front ends.” These are the ones I have tried:

http://www.m0kgk.co.uk/sdr/index.php  KGKSDR software -- This is the software I have been using for CW and SSB transceive operation.

http://www.weaksignals.com/ Winrad and SDRadio are excellent receive-only programs.

http://www.dxatlas.com/Rocky/ A very compact but fully functional program. CW transmit capability is available, and SSB transmit capability may be added soon.

http://www.flex-radio.com/ The PowerSDR software developed for the SDR-1000 transceiver will also work with the SoftRock receivers, limited, of course, to the frequency range that they cover. Versions of this software that support transmit/receive capability with the RXTXv6.1 kit are available if you search the SoftRock 40 user’s group files and messages. Since the available versions (and their links) change rapidly, I won’t attempt to include a link here.

In addition to being free, the various SDR programs have several other things in common. All of them allow you to enter the local oscillator frequency of your receiver, so that the band display and digital readout show the actual operating frequency. The programs all offer at least USB, LSB and CW modes, with multiple-filter bandwidths and/or user-definable filters. Another common SDR software feature is a real-time band scope that shows a panoramic display of all the activity within the tuning range. You can select a spectrum analyzer or waterfall type of display. Tuning is accomplished in various ways such as dragging the mouse in the display, using the arrow keys and page up/down, the mouse scroll wheel, or by pointing and clicking the mouse on a signal in the display. During a CW contest, it’s fun to wait for a signal to pop up, and try to catch it in time to copy the call. Well, some of us are easily entertained. Rocky has an automatic search feature that will jump instantly to the next signal up or down the band.

Since the signals coming in from the hardware “front end” are within the passband of the sound card, it is possible to record them like any other audio file. All of the SDR programs I have mentioned have the capability to play back and process the recording as if it were coming in from the antenna. Not just the audio that was coming out of the demodulator, but all of the signals that were in the display. So you can tune around the band after the fact, and find out what rare DX stations you missed. Recording can eat up quite a bit of disk space, though. With a 96 kHz sound card and 24-bit sampling, the recording requires about 20 megabytes per minute.

73, Lyle KØLR

Jim Talbott
WØYA VP
& Program
Chairman

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