Digital Residual Alias Cancellation for Filtering-by-Aliasing Receivers

The filtering-by-aliasing (FA) receivers have demonstrated sharp analog finite impulse response (FIR) filtering by combining periodically time-varying (PTV) circuit elements with uniform sampling. Seen at the sampled output, the impulse response of an FA receiver can be controlled by the PTV resistor’s resistance that varies over time, which realizes very sharp filtering with 50–70-dB stopband rejection. However, the finite stopband rejection and the inherent sampling in FA result in unwanted in-band residual blocker aliases, which cannot be suppressed further by downstream linear time-invariant (LTI) stages, unlike in a conventional receiver, and the overall blocker rejection may be insufficient in some applications. This paper describes a digital residual alias cancellation technique tailored for the FA receivers. By using a second channel to capture the blocker and equalizing the blocker in the FA and the second channels with digital baseband filters, measurement results, with the cancellation algorithm implemented in MATLAB, show that the residual blocker aliases can be suppressed by another ~15 dB, in addition to the FA analog filtering.

I. Introduction

Wideband programmable receivers are a key enabling component for emerging software-defined and cognitive radio applications [6]. Conventional linear time-invariant (LTI) circuits have been facing numerous obstacles on the path to achieving high-performance programmable receivers. In particular, their reliance on off-chip SAW/BAW filters and on-chip LC filters, which are generally not very programmable, has greatly limited their tunability. In contrast, in recent years, periodically time-varying (PTV) circuits, such as the -path filters (NPFs) [7] and mixer-first receivers [8], have drawn a lot of attention. Toward building versatile programmable receivers, several works on improving multiple aspects of receivers have been published: [9] demonstrated a frequency-translational noise cancellation technique to efficiently lower the noise figure (NF) to 1.9 dB; [10] and [11] reported very sharp analog filtering; [12] and [13] focused on improving the linearity of the receiver and reported +44-dBm out-of-band (OOB) IIP₃ and over +15-dBm in-band (IB) IIP₃, respectively; and [13] and [14] realized an RF bandwidth (BW) of over 100 MHz. Additionally, PTV circuits have also been studied for other emerging applications, such as full duplex [15], [16], [17]; while other variants of PTV circuits using off-chip high-performance filters in conjunction with on-chip translational circuits have also been implemented [18]. Receivers based on discrete-time charge-domain IIR filters using switched capacitors have also been demonstrated with high-order filtering [19], [20]. The prior art primarily employs PTV circuits for frequency-translation purposes and has shown promising results.