Wideband IQ modulator with RC/CR automatic quadrature network

摘要

An IQ modulator incorporates a quadrature network that is responsive to a frequency dependent control signal and that has in-phase and quadrature signals that are of equal amplitude and in exact quadrature over a wide range of applied frequencies. The quadrature network includes RC and CR phase shifters whose C's are fixed capacitors of equal value and whose R's are made equal to each other and to the capacitive reactance of the C's by the action of the frequency dependent control signal. The R's may be FET's. The frequency dependent control signal may be generated without express knowledge of the applied frequency by a servo loop that nulls out the amplitude difference between the in-phase and quadrature outputs from the quadrature network; it may also be generated from a look-up table as an express function of frequency. The frequency dependent control signal is split into separate instances that are applied to each R, and an offset may be introduced therebetween to provide extreme precision. The offset may be fixed or programmatically controlled as a function of frequency. Varactors may be used to provide variable C's in cooperation with fixed R's. The entire circuit may be fabricated as a GaAs IC for use in the frequency range of 200 MHz to 3GHz.