5. Direct analog synthesis

Direct analog synthesis consists in generating a series of multiple or sub-multiple frequencies from a single stable source by multiplication and division, then combining them by subtraction and addition. The main advantage of this type of synthesis is its low acquisition time, which can be less than 10 µs. This depends essentially on the speed of frequency switching and the bandwidth of the filters used. However, to produce an instrument covering a wide range with a small frequency step, direct synthesis requires a large number of mixers and filters. Today, techniques using phase loops and digital circuits are preferred. What's more, without the filtering inherent in the phase-locked loop, the spectrum obtained by direct synthesis may contain numerous spurious lines at the intermodulation frequencies produced by successive mixes.