Large signal analysis of the Y-fed directional coupler

A mathematical model for the modulation (transfer) curve of the Y-fed directional coupler is presented. The model, basically a sine-series function, can easily yield closed-form expressions for the harmonic and intermodulation performance of the Y-fed directional coupler with large-amplitude multisinusoidal input voltage for different values of the parameter, s, that represents the ratio between the interaction length and the coupling length of the passive directional coupler. The special case of two-tone equal-amplitude input voltage is considered in detail. The results show that the third-order intermodulation is always dominant except for particular values of s that result in nearly suppressed third-order intermodulation. However, the results also show that the suppression of the third-order intermodulation is highly sensitive to the value the parameter s.