Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 11, NO. 6, JUNE 1999

This article describes the performance of a variable attenuator formed from a 3 dB quadrature coupler with tracking variable terminations on the coupled ports. To help understand its operation, we must recognize that the attenuator can be analyzed using simple two-port techniques. The resulting S-parameters are obtained for the general case where the terminating impedances are unequal and for the case where the terminations track each other per fectly. We then find that the effects of unequal terminations are departures from ideal non-reflective input/output port impedances and a variance in attenuation from the ideal.

The idea of creating a variable attenuator from a lossless 3 dB quadrature coupler is inspired by the fact that the isolated, or decoupled, port presents infinite attenuation to the input signal when the coupled ports are terminated with reflection coefficients equal to zero ( ? _c=0). It has zero attenuation of the input signal when the coupled ports are entirely reflective ( ? _c=1) and equally terminated. Between these two extremes, the input port to isolated port transmission is proportional to the magnitude of the reflection coefficient of the (tracking) coupled ports. Thus a variable attenuator is implemented by using a 3 dB quadrature coupler with the isolated port designated as the out put port while terminating the...