Switched-RC beamforming receivers in advanced CMOS : theory and design

In consumer electronic devices, analog radio transmitters and receivers provide the wireless links between devices. These radios are mostly omnidirectional, i.e. transmitters send their energy in all directions and receivers listen to signals from all directions. As a result, interference between devices and services is becoming an increasing problem as more wireless connectivity is added. In order to transmit and receive directional 'beams', it is necessary to add multiple antennas to the radios and to control their precise relative delays. For the implementation of such a system in consumer electronics, it is required to add additional time delay and/or phase shift circuits to the existing radio architectures. These circuits are challenging to fabricate in mainstream CMOS technology and must be very linear, as they are subject to the unfiltered interferers. A rigorous analysis reveals that a switched-resistor-capacitor current loop can be the building block for high linearity circuits. The transfer function and noise contribution of this loop depend on the ratio between the RC time constant and the switch-on time. Two distinctive operating regions are identified, one with low noise properties for mixer applications, and one with high bandwidth properties for sampling applications. Three designs are implemented during this research project. The first demonstrates a high linearity downconverter with a mixer-first topology, based on mixing region switched-RC loops. The second adds beamforming capabilities with a discrete-time vector-modulator phase shifter, based on sampling region switched-RC loops. The third design evolves the previous designs into an all-passive topology where the impedance matching for the mixer is achieved by the charge dissipation in the phase shifter. This design stands out in linearity performance and is easily portable to future CMOS technologies