By Michael Millhaem, Keithley Instruments, Inc.
Test equipment manufacturers are constantly challenged to develop new solutions for testing their customers' latest devices, but they've traditionally developed specialized hardware to meet this challenge. The communications market is still more challenging due to the rapid development of new standards, which often require new stimulus and measurement capabilities. To keep pace, test vendors must find new approaches that reduce instrument development times and allow instruments to adapt to new requirements. Software-defined radio is one technique that can help.
"Software-defined radio" (SDR) can be defined as a radio communication system that uses software to modulate and demodulate radio signals. Economics is the driving force behind the growing use of SDR. These systems can achieve high flexibility at a lower cost than traditional analog designs.
In the purest sense, digital-to-analog (D/A) and analog-to-digital (A/D) conversion would occur at the carrier frequency and no analog up- and down-conversion would be required. Today's SDR applications typically have at least one analog up- and down-conversion stage. Clearly, the A/D and D/A converters are key elements of an SDR system. The speed and resolution of the converters will determine how much analog frequency conversion is required. Converters need sufficient resolution (bits) to produce or capture the modulation data adequately, and more complex modulation formats will require converters with even greater resolution. The speed of the converters will limit the maximum signal frequency that can be created or sampled. Converter technology continues to advance, providing higher combinations of resolution and frequency.
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