21.7 A Mixed-Signal Circuit Technique for Cancellation of Multiple Modulated Spurs in 4G/5G Carrier-Aggregation Transceivers

To enable high data-rates in mobile communications the LTE standard has implemented carrier aggregation (CA) to increase channel bandwidth. Several local oscillator (LO) signals are needed to drive a multitude of mixers to receive from a number of LTE bands in the different CA scenarios. Due to device nonlinearities in the LO signal distribution and cross-talk between LO paths spurs are generated inside the transceiver chip which may downconvert interfering signals located at the spurious frequencies into the receiver baseband (Fig. 21.7.1). These unwanted signals could either be the transceiver's own transmit signal when operating in frequency-division duplex (FDD), or external signals received by the antenna. An interferer downconverted by an RX LO-LO spur is referred to as a modulated spur and causes severe degradation of the signal-to-noise-and-distortion ratio (SNDR) of the desired signal.