An X-Band RF Binary Phase Modulator Based on a Reflection-Type Phase Shifter

This paper presents a high-speed wideband RF binary phase modulator based on a reflection-type phase shifter for X-band applications. A PIN diode in the shunt arm, connected with a transmission line of characteristic impedance Z and electrical length $\theta^{\circ}$, is used as a load at the through and coupling ports of a conventional 3 dB branch line hybrid coupler (BLC) to minimize the parasitic effects of the diode at X -band frequencies. The load impedance can be switched between low and high by applying forward and reverse bias to the PIN diode, causing the phase shift $\left(S_{21}\right)$ to change by 180° for BPSK modulation for a continuous wave frequency of 10 GHz. The load network is further modified with an additional series transmission line to maximize the load impedance ratio in the ON and OFF states of the diode. The measured insertion losses are 2.17 dB and 1.18 dB in the ON and OFF states of the PIN diode, respectively. The measured $\left|S_{11}\right|$ is below $\mathbf{- 1 5} \pm 5 \text{dB}$ over the $\mathbf{8 - 1 2} \mathbf{~ G H z}$ range, and the measured phase difference between the ON and OFF states remains at $180^{\circ} \pm 10^{\circ}$ over the $\mathbf{9. 4 2 - 1 0. 2 3 ~ G H z}$ range.