A calibrated phase and amplitude control system for phased-array transmitters

A system is proposed that allows the phase and amplitude of a signal to be accurately set and regulated over process and power supply variations. The intended application is in a phased array transmitter, where the phase and amplitudes of the array elements are electronically adjusted to shape and steer the beam of radiation in the chosen direction. Phase and amplitude errors introduced in the array elements lead to degradation of the array pattern through effects such as sidelobe growth and reductions in directivity.; The conventional means of setting the phase in a branch of a phased array is through look-up tables storing the control voltage - phase relationships for the phase shifter. This method does not compensate for changes in these relationships intro-duced by processing variations between fabricated devices or changes in operating conditions such as power supply voltage and temperature. Additionally, if the phase shifter has variable gain over the phase control range then the amplitude will vary depending on the phase setting. The proposed system uses a variable gain amplifier (VGA) in conjunction with the phase shifter to compensate for the variable losses of the phase shifter and simultaneously provide a means of adjusting the amplitude of the signal. Dual feedback loops are employed to set the control voltages for the phase shifter and VGA, allowing the phase and amplitude to be closely regulated across process variations and adjusted to compensate for changes in operating conditions.; The phase shifter is implemented as a reflective-type phase shifter which provides up to 310° of phase shift, and the VGA is a narrowband implementation with a resonant load that provides about 20 dB of gain and is intended to be used as a power amplifier driver. The phase feedback loop includes buffers to remove the amplitude dependency of the signal, high-speed dividers, a charge pump, and an active loop filter. The phase of the system is specified by digitally setting the charge pump currents. The amplitude loop consists of an RF peak detector used in conjunction with an active loop filter, and the amplitude is specified by setting an analog input voltage to the loop filter.; The complete system has been fabricated in a 0.18 mum CMOS process, and assem-bled on a custom printed circuit board using chip-on-board bonding to allow testing and characterization. The system operates at 1.9 GHz, and the phase can be set with 5 bits of control over a 240° range, and the amplitude can be varied over a 20 dB range. The amplitude feedback loop reduces the variation in |S21| across the phase control range from 12.1 dB for the open loop case to 0.4 dB. The phase feedback loop reduces the variation in ∠ S21 across the amplitude control range from 32.1° to 7.4°. The standard deviation of the phase across the phase control range is reduced from 9.4° to 1.8° for changing power supply voltages, and from 5.0° to 1.2° between different test chips.