Ham radio in the 21st century
The technology used in ham equipment has evolved significantly. Most high-performance HF/VHF transceivers now use digital-signal-processing technology for at least some of the modulation, demodulation, and filtering functions. A careful partitioning of both analog- and digital-signal processing achieves the best performance, and today’s radios offer excellent sensitivity and 100-dB dynamic range, with digital-signal-processing-enabled selectivity. Although most radios still maintain the traditional format of a front panel with a large knob to control the frequency and lots of other buttons and knobs, some newer SDRs (software-defined radios), such as those from FlexRadio, abandon this tradition in favor of keyboard and mouse operation; they have no front-panel controls (Figure 4).
However, not all hams buy their equipment off the shelf. Some prefer to build their own equipment. Ham operators have always been enthusiastic tinkerers, often building their equipment from discarded pieces of consumer electronics they find in their neighborhoods. Many hams understand concepts such as intermodulation distortion and phase noise, for example, because they have heard the effects of these signal imperfections, and they understand what happens when a nominally linear power amplifier enters hard compression.
Home-brewed radios can range from extremely simple transmitters and receivers to true state-of-the-art SDR systems. At the low end, one creative ham disassembled a compact fluorescent light bulb and discovered a high-speed, high-voltage switching transistor and assorted capacitors and inductors. By adding a 3.579-MHz TV colorburst quartz crystal, which sits conveniently in the middle of the 80m amateur band, he was able to construct a 1.5W CW transmitter from the parts (Reference 2).
Simple receivers are also easy to construct. Ham operator Charles Kitchin has developed a series of super-regenerative receivers that are easy to build and that work surprisingly well (Reference 3).
An engineer interested in developing his own SDR radio can build or buy an RF front-end/quadrature downconverter and connect it to the audio input of a PC and buy or write appropriate software for the demodulation and detection functions. Connecting the baseband in-phase and quadrature outputs of the radio to the left and the right inputs of the PC completes all of the hardware work. Some hams have constructed SDR front ends in the form factors of USB memory sticks and draw their power from those sockets.