A software-defined radio is one in which functions such as tuning, filtering and modulating/demodulating are performed by software rather than hardware. The ease with which this is now possible is due largely to advances in digital electronics.

   ==> The need for special purpose hardware is greatly reduced
   ==> New functions can be added to the radio by changing or adding to the software

The cost of SDR devices is continually falling, and it has recently reached the point where price is no longer an obstacle to the beginner. While high quality SDR transceivers still cost over US$1000, a fair quality (but very usable) plug-and-play receive-only software radio can be had for less then $70. For example, I purchased the Softrock Ensemble II SDR from Tony Parks, KB9YIG. The kit is bascially a simple downconverter, but combined with the right software, can become a very powerful radio receiver with a good antenna. Various SDR boards and complete solutions are shown below. Links are included to go directly to their respective websites.

Softrock Ensemble II	NetSDR	AIRSPY
RTL-SDR	Hermes HPSDR	SDR Cube
Funcube	Ham It Up	RFSpace SDR-IQ
SDRplay	HackRF-One	Kiwi SDR with Beaglebone

One of my colleagues at Tektronix, Alan Wolke W2AEW, posted an absolutely fablulous video about the Softrock Ensemble II that he built. Both videos are highly instructional and I encourage you to view them if you are building the Softrock and really want to learn how the product works. Thanks Alan for these great videos. The videos can be viewed below.

Other than a computer, a typical software radio requires these things:

   ==> An SDR device to convert analogue RF signals into digital data. Wikipedia has a good list of SDR devices

   ==> A computer program to demodulate the digital data. See the List of SDR programs

   ==> An antenna suitable for receiving RF bands of interest.

   ==> An optional transverter, downconverter, or upconverter to "move" a section of the RF spectrum into a range the SDR device is able to tune to, if you wish to receive transmissions outside the SDR's native tuning range

Block Diagram

This receiver implements a quadrature sampling detector to produce low frequency I and Q signals for input to the stereo line in inputs of a PC sound card. The I and Q signals are the product of the quadrature sampling detector (QSD) stage, in which bandpass filtered "chunks" of RF are mixed with quadrature clock signals to produce the down-converted I and Q signals.

These products (the I and Q pairs) are identical to each other in all respects except phase, where they are 90 degrees apart.

The I/Q products of the QSD ("mixer"), when input to the appropriate SDR program through the PC's STEREO line-in soundcard input, result in a spectrum display on the PC which will show signals arrayed around a "center frequency". This "center frequency" is the frequency of the I/Q outputs from the Quadrature Clock Generators. The bandwidth of the signals either side of the center frequency will be approximately equal to the sampling rate of the PC's sound card. Thus, if the local oscillator is tuned to produce 28.4 MHz to the Quadrature Clock Generators, they will output two signals (I and Q clocks) at 7.1MHz (the "center frequency"). If the PC's sound card has a 48 kHz sampling rate, then the SDR program can translate the QSD's I/Q outputs into a chunk of spectrum that is 24 kHz either side of the center frequency of 7.1 MHz: i.e, 7.076 - 7.124 MHz. If the LF Option is built (by eliminating the HF Jumper and installing the second 74AC74 IC to allow for a divide-by-16), the center frequency resulting from a local oscillator frequency of 3.5 MHz (the lowest for the Si570) will be approximately 218 kHz.

As the user tunes the receiver, varying the frequency of the local oscillator, the micro-controller tracks the frequency and switches the appropriate bandpass filter into the RF chain. The SDR program's display will update to show the new center frequency and adjust the scale to reflect the current +/- bandwidth around that center frequency. At all times, the operator can see all signals that are within this movable "window" (whose total width is 48, 96, or 192 kHz, depending upon the sampling rate of the PC's soundcard).

The receiver is controlled via a USB connection from the PC. This USB connection provides a "USB 5V" bus for the local oscillator and micro-controller. A separate 3.3 V voltage regulator on the 5 USB 5 volt bus provides power to the programmable oscillator, the Si570.

The RX has an Atmel ATTiny85 micro-controller unit which, acting as a USB device, and on the "USB 5V" rail, controls the frequency output of the programmable local oscillator (Si570) and provides two switching signals which can be used to select one of four filter banks in the band pass filter.

The output of the local oscillator is at a frequency which is 4 times the desired center frequency of the receiver and is consumed in the Quadrature Clock Generators.

The Quadrature Clock Generators divides the local oscillator frequency by 4 (or, for the LF option, by 16) to produce two clock signals - QSE Clk 0 and QSE Clk 1 - which will be used to clock the QSD stage. These I and Q clock signals are identical in all respects but phase (they are in quadrature - 90 degrees phase separation).

Rf at the antenna jack is filtered through the Bandpass Filter Stage, where one of four "chunks" of the HF (or LF) band is selected by the micro-controller, based upon the tuning of the programmable Local Oscillator. The filtered RF is passed as input to the QSD Stage.

The Quadrature Sampling Detector (QSD) Stage acts very similar to a mixer. It incorporates a high-speed switch that is clocked by the two QSD clock signals from the Quadrature Clock Generators and switches the incoming RF into a RC sampling network. The result is two outputs at low frequency and also in quadrature, which are the down-converted, baseband analogs of the incoming RF signals.

The outputs of the QSD stage are then fed into a pair of high gain Operational Amplifiers to produce the I and Q baseband signals which will be input to the PC soundcard's stereo Line In.