Analysis of the Wilkinson Coupler Under Different Input Conditions

In sub-terahertz receivers, it is indispensable to apply beamforming that combines multiple RF paths from the viewpoint of SNR (signal to noise ratio) improvement by increasing the directivity of the communication path. For beamforming in the case of mm-wave band application, the receiver is placed near the antenna. However, in the case of sub-terahertz band application, where the wavelength is short, the RF paths must be directly synthesized because space for a receiver is not available. It is desirable to apply a Wilkinson Coupler composed of simple passive elements to combine RF paths. When applied to beamforming, the amplitudes and phases of signals to be combined may differ greatly. It is also necessary to turn off some input signals for calibration.In this case, it is difficult to fix the impedance in the OFF state to a specific impedance. For this reason, it is desirable to have low sensitivity to the impedance in the OFF state. In this paper, we analytically confirm that the Wilkinson Coupler can operate normally even in such a special situation.

I. Introduction

When performing sub-terahertz communication, it is essential to apply beamforming using multiple RF paths [1]. When beamforming is applied, the signal amplitude and phase of each signal path are different (see Figure 1 (a)). Each path is equipped with a variable phase shifter that adjusts the phase of each path appropriately to maximize the level of the bundled signal to achieve beamforming. For this reason, it is necessary to correctly transmit combined information of the phase and amplitude of signals on each path. Furthermore, in order to carry out calibration, it is also necessary to operate each system independently and match the gain of each path to the same value (see Figure 1(b)). Wilkinson Coupler is widely applied to bundle multiple RF paths [2]. However, in many applications, the amplitude and phase of each input signal are almost the same, and the input impedance is also almost the same, such as in the power combining application for a power amplifier (see Figure 1 (c)) [3]. Although there is an example in which the Wilkinson Coupler is applied to the application of bundling signals for beam forming [4], the operation of the Wilkinson Coupler in that case has not been studied in detail.