Design and Analysis of Programmable Receiver Front-Ends Based on LPTV Circuits
Wide programmability is increasingly desirable in transceiver designs. Due to the proliferation of frequency bands that need to be supported in wireless standards, such as LTE, WiFi, etc., the continued reliance on bulky off-chip SAW/BAW filters and on-chip inductors in current receiver designs is increasingly untenable. Hence, more integrated approaches are necessary, especially for future radio concepts, such as cognitive radios (CRs), where the channel bandwidth and center frequency are dynamic and not known a-priori. Current state-of-the-art on-chip receiver designs rely on techniques such as discrete-time analog signal processing, N-path filtering, and the mixer-first receiver topology. Nevertheless, on-chip filtering still does not approach the selectivity or linearity of their off-chip counterparts. Hence, new approaches are necessary.
This work presents a programmable receiver front-end that uses the recently developed concept of Filtering-by-Aliasing (FA). By utilizing a linear, periodically time-varying (LPTV) circuit with a sampled output, sharp baseband filtering is achieved that can be upconverted to any desired LO frequency with an integrated passive mixer. Further, to achieve impedance matching with the antenna, an S11 constraint is added to the design of the FA filters, while time-interleaving is applied to further increase filter sharpness. Measurements on a fabricated prototype achieved a wide LO tuning-range of 0.1-1GHz, wide filter bandwidth (BW) tunability, a filter stop-band suppression of 70dB with a transition band only 4xBW, high close-in blocker tolerance with >21dBm IIP3 at only 1.2xBW offset frequency, and a wideband S11 better than -9dB throughout the LO range. Further, an analysis technique for general LPTV circuits is developed utilizing the concept of conversion matrices. By deriving conversion matrix-based frequency-domain equivalent circuits, LPTV circuits can be analyzed just like LTI circuits. Circuit laws, such as KVL and KCL can be easily applied to derive frequency responses. Example applications to mixer-first receivers, N-path filters, and the problem of impedance matching in LPTV circuits are highlighted to showcase the utility of the analysis technique.