Digitally based coherent sweep generator

The author describes a digitally implemented coherent sweep generator, i.e. a sweep generator in which sweep rate, start frequency and start phase can be specified. By carrying out a digital integration twice, using a counter and an accumulator as the integrators, the quadratic phase function of a linear sweep is produced. The desired start frequency and start phase are introduced by presetting the counter and accumulator, respectively. The mod (2 pi ) of the quadratic phase function is extracted and used as address for a sine lookup table whose output is applied to a digital-to-analog converter. The system is capable of producing sweep signals from DC up to the lower MHz range, and the sweep rate can be varied over several orders of magnitude. By operating several digital sweep generators from the same clock, multiple coherent sweep signals can be produced. The digital sweep generator has been implemented with transistor-transistor logic, and generated waveforms are presented.<>