Introduction

In this chapter, we present on-chip filtering techniques based on active blocker cancellation that can potentially allow the removal of SAW filters in receivers. Active blocker cancellation can be in two major forms: feedforward blocker cancellation and feedback blocker cancellation. Both feedforward and feedback blocker cancellations can generate on-chip high-Q bandpass filters with the center frequency controlled precisely by the clock. Receivers in [15, 16] are two examples of feedforward blocker cancellation. The feedback blocker cancellation is used in [17, 18]. Although feedforward blocker cancellation is simpler architecturally than feedback blocker cancellation and has no stability issue, the gain and phase of the feedforward path must be well matched to those of the main receiver path. On the other hand, feedback blocker cancellation eliminates the tight gain and phase control requirements of feedforward blocker cancellation but introduces stability concerns. Both feedforward and feedback active blocker cancellations use the frequency translation technique to construct a high-Q bandpass filter using two identical low-Q baseband filters. To do so, a complex downconversion mixer clocked by the corresponding LO clocks of the zero-IF receiver, frequency shifts the incoming signal to the IF. The desired signal, which is centered around the LO, is downconverted to around DC, whereas the strong downconverted blocker sits at an IF that is equal to the separation between the blocker and the desired signal. The complex IF signal is passed through the two baseband bandpass filters, in which the strong blocker signal is located in the passband.