Digital generation of coherent sweep signals

The authors describe 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 integrators, a quadratic phase function of a linear sweep is produced. The desired start frequency and start phase are introduced by presetting the counter and accumulator. The mod (2π) of the quadratic phase function is extracted and used as an address for a sine look-up 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 megahertz range, and the sweep rate can be verified over several orders of magnitude. By operating several digital sweep generators from the same clock, multiple coherent sweep signals may be produced, or quadrature signals can be produced with two modules. The digital sweep generator has been implemented with transistor-transistor logic (TTL). Generated waveforms are presented