Analysis and characterization of PLL-based retrodirective array

In this paper, we present a new phase-locked loop (PLL) architecture that is capable of providing phase conjugation as required for retrodirective array action. An analysis is performed on the architecture, which shows that the presence of nonlinear mixing products gives rise to an additional and substantially larger RMS noise-induced phase-error component than is suggested by analysis based on ideal mixer properties. In addition, the analysis allows us to quantify the effect that a Doppler shifted signal has on phase-conjugation error. For the purpose of demonstration, we have constructed and characterized a six-element array operating using commercially available components at 1 GHz. Near-perfect retrodirective action is presented over a 360deg azimuth sweep, with 16-dB array gain. Due to the inherent isolation between input and output signals, and programmable retransmit frequency, the PLL approach introduced in this paper offers considerable practical advantage when used within retrodirective antenna arrays, as compared to the classical mixer-based approach where mixer losses and RF-to-IF frequency isolation are major issues