3.6 A noise-cancelling receiver with enhanced resilience to harmonic blockers

By employing two passive-mixer-based downconversion paths, the frequency-translational noise-cancelling receiver (FTNC-RX) achieves a low noise figure and can tolerate most out-of-band blockers up to 0dBm with little performance degradation [1]. However, like most wideband passive-mixer-based designs, the architecture is far less tolerant of harmonic blockers, that is blockers located at or around precise integer multiples of the LO frequency. In a typical M-phase passive mixer, shown in Fig. 3.6.1a, most out-of-band blockers are heavily attenuated by large shunt capacitors at the inputs of the baseband TIAs. Harmonic blockers are an exception and do not experience this attenuation since they are downconverted inside the TIA bandwidth, are amplified along with the wanted signal, and are only rejected by the subsequent harmonic-rejection circuitry. Since TIA gain is generally large in order to maintain a low noise figure, moderate harmonic blockers will saturate the TIAs and consequently the receiver.