LOW-COST WIDE-BAND I-Q VECTOR MODULATOR

One of the applications of QPSK modulators is to control th amplitude and phase of an RF signal, working as a vector modulator. Recently the re are several monolithic commercial circuits that can be easily used to frequencies of > 4 GHz. First, the input signal must be decomposed into two quadrature components, which is usually done in a 3 dB 90 deg rees hybrid coupler. Using microstrip techniques, this I-Q vector modu lator exhibits narrowband characteristics, due to the degradation of t he hybrid performances with frequency. The authors show how the workin g bandwidth can be extended up to an octave by means of a versatile co ntrol system as described here, as an alternative to a new design of t he RT stage.