Millimeter-wave MEMS-loaded transmission-line phase shifters

Abstract

Beam forming and scanning at microwave and millimetre-wave frequencies (1 GHz – 100 GHz) using electronic steerable array (ESA) antennas, requires phase shift and time-delay units. RF path phase shifters provide an attractive solution, which enable RF transceivers of reduced complexity compared to phase shifting in the LO path. RF phase shifters based on ferrimagnetic, ferroelectric or semiconductor materials usually exhibit a high insertion loss, small bandwidth, poor linearity, or high power consumption. RF MEMS phase shifters using dielectric substrates can overcome these drawbacks and compete with state-of-the-art phase shifters fabricated in other technologies. This dissertation addresses the concept of distributed MEMS-loaded transmission-line phase shifters and uses dielectric substrates. A rigorous analysis of the electro-mechanical switching behaviour gives insight into the mechanics in static and dynamic cases. The results allow estimation of the power handling limitations due to self-actuation, switching time and bouncing versus actuation voltage, and the influences of process tolerances or changing operating conditions. Given an arbitrary actuation waveform, the mechanical and electrical switch responses can be studied. The energy balance shows the model accuracy, and relates the stored and dissipated energy to the total power consumption. The described fabrication process provides good chemical and thermal compatibility throughout all process steps, reduces process tolerances, prevents contact sticking, and enables realization of uniform and stress-free electroplated films. Proof of process is demonstrated by various fabrication runs carried out over a period of three years. Electrical measurements have been carried out to characterize the MEMS switches as a basic building block of the phase shifter. These comprise: switching time, switching and contact hysteresis, insertion loss, return loss, forward loss and isolation, lifetime predictions as well as the power handling capabilities of the switch. The measurement results are compared to the simulations. Following a new design procedure, RF MEMS-loaded transmission-line phase shifters have been realized. The common lumped element design equations using element values of prototype filters have been adapted for the synthesis of distributed element networks. Low-pass filters with programmable cutoff and insertion phase have been designed and fabricated using the Chebyshev coefficients for weighting of the loaded-line sections. The sections can either exhibit predominate reactance or susceptance loading, which results in a relative change of the insertion phase and characteristic impedance. Analytical descriptions are derived for S-parameters and T-parameters, which consider the length of the loaded-line sections and Bragg reflections. The limitations of the analytic approach are illustrated by comparison with Momentum simulation and discussed. The developed phase shifters are characterized by means of RF measurements. The phase shifter bits 1 to 5 designed for the insertion phase differences of 11.25°, 22.5°, 45.0°, 90.0°, and 180.0° showed a maximum phase error of 3° between 60 and 61.5 GHz. In the on state, the corresponding insertion losses of the phase shifter bits were measured to 0.0 dB, 0.1 dB, 0.3 dB, 1.3 dB, and 2.0 dB. The return losses between 30 GHz and 62 GHz of the phase shifter bits were better than 26 dB, 24 dB, 20 dB, 22 dB, 20 dB. The estimated insertion loss of a 5-bit phase shifter consisting of the phase shifter units 1 to 5 is below 3.7 dB. The average phase error over all 32 states within the ISM band is approximately 1°