Abstract

The synthesiser described is for use in a spread-spectrum communication system where a high degree of immunity to interference is required. A frequency resolution of 100 Hz across an RF bandwidth of 2 GHz is achieved by the combination of two phaselocked loops (Pili) with a relatively simple Direct Digital Synthesiser (DDS). The overall frequency resolution is determined by the frequency resolution of the DDS, which is potentially unlimited. Decimal, or base-10 synthesis is obtained by using a decimal DDS, the design of which is outlined. By linearly ramping the DDS output frequency under digital control, it is possible to get sub-10 microsecond switching speeds with no cycle-slipping, with manageable PIL loop bandwidths (fn of 700 kHz). The analysis is presented, together with detailed measurements of spurious levels from the DDS. Overall phase noise and spurious levels are presented.

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