A Delay Spread Cancelling Waveform Characterizer for RF Power Amplifiers

A two channel 65-nm CMOS RF-waveform characterizer is presented that enables multi-harmonic adaptive matching networks or adaptive digital pre-distortion in RF-power amplifiers. The characterizer measures the dc component and the first 3 harmonics of RF signals by applying a DFT to 8 (ideally) equally spaced quasi-dc output voltages. Conventionally in these types of systems accuracy is limited by sample timing accuracies, which in our case are mainly due to delay cell mismatch. We introduce a novel way to cancel delay cell mismatch, that significantly increases measurement accuracy at the cost of only a small power and area increase. The RF-waveform characterizer achieves 6.8-bit measurement linearity together with a (clock feedthrough limited) 24 dB SFDR. The measured power consumption for our proof-of-principle demonstrator is 18.6 mW at a maximum input signal frequency of 1.1 GHz under continuous operation.