Frequency-Offset Cartesian Feedback Based on Polyphase Difference Amplifiers

A modified Cartesian feedback method called "frequency-offset Cartesian feedback" and based on polyphase difference amplifiers is described that significantly reduces the problems associated with quadrature errors and dc offsets in classic Cartesian feedback power amplifier control systems. In this method, the reference input and feedback signals are down-converted and compared at a low IF instead of at dc. The polyphase difference amplifiers create a complex control bandwidth centered at this low IF, which is typically offset from dc by 200-1500 kHz. Consequently, the loop gain peak does not overlap dc where voltage offsets, drift, and local oscillator leakage create errors. Moreover, quadrature mismatch errors are significantly attenuated in the control bandwidth. Since the polyphase amplifiers selectively amplify the complex signals characterized by a + 90° phase relationship representing positive frequency signals, the control system operates somewhat like single sideband modulation. However, the approach still allows the same modulation bandwidth control as classic Cartesian feedback. In this paper, the behavior of the polyphase difference amplifier is described through both the results of simulations, based on a theoretical analysis of their architecture, and experiments. We then describe our first printed circuit board prototype of a frequency-offset Cartesian feedback transmitter and its performance in open and closed-loop configuration. This approach should be especially useful in magnetic-resonance-imaging transmit array systems.