DPSK modulator with accurate phase transitions for mode-S radars

European air traffic control will be managed in future through the use of the Mode-S standard, as it is becoming to be nowadays in U.S. (Barker et al. (1)). These systems require a continuous phase varying DPSK modulator with very accurate characteristics of linear transitions in phase and constant amplitude, to provide selective interrogation to aircrafts. In this paper, a modulator is presented which uses a low-cost commercial monolithic QPSK modulator. In order to obtain the proper phase transitions according to the International Organization of Civil Aviation specifications, required I, I, Q and Q signals are derived. Specifications for above signals are obtained in time domain so, a method to synthesize networks under time domain constrains is used. This results in a high performance and low-cost modulator.