A Reflection-Coefficient Sensor For 28ghz Beamforming Transmitters In 22nm Fd-Soi Cmos

Millimeter-wave beamforming transmitters use antenna arrays to increase the EIRP by focusing the radiation pattern into a direction chosen by programming the TX phase shifters. However, under sharp scanning angles, the antennas load each other (see Fig. 26.4.1) such that in each antenna path the load seen by an individual PA in the output matching network (OMN) deviates from the load corresponding to boresight transmission. This situation can be accompanied by other external factors, such as antenna blockage, that increase VSWR. To restore the PA performance under increasing VSWR conditions, one can use a tunable matching network (TMN) that is controlled by the observed VSWR. The latter is then measured by a VSWR sensor, which is presented here. The overhead of such a sensor in terms of form factor, power consumption, and TX performance degradation should be minimal. The sensor described here can measure reflection coefficients with a magnitude of up to 0.7 (corresponding to a VSWR of 5.7) at 28GHz. It detects the complex voltage difference over a 235fF sensing capacitor that is inserted in the signal path at the PA output. The magnitude and phase of this voltage vary with the complex reflection coefficient $\\Gamma$ and are detected separately. The loading effect of the rest of the sensor can be modelled as a 32fF shunt capacitor added to the matching network, equivalent to an extra insertion loss of less than 0.2dB.