S-band digital downconverter for radar applications based on a GaAs MMIC fast sample-and-hold

A digital coherent-downconverter configuration is described whose operation principle is based on the theory of subharmonic sampling for narrowband waveforms. This configuration uses a fast sampler, operating at microwave frequencies, for directly demodulating the in-phase and quadrature components of a complex input waveform, yielding a sampled sequence in a suitable format for subsequent analogue-to-digital conversion. Unlike the conventional double-conversion coherent superheterodine architecture, the proposed configuration offers attractive features in terms of cost and complexity and appears particularly suitable for the implementation of the front-end unit in a coherent receiver. An experimental prototype, based on a very fast GaAs MMIC sample-and-hold (SH) circuit, has been implemented for validating the operation principle. Complex demodulation of a 1 GHz input waveform is demonstrated, with 6-bit equivalent accuracy.