Direct-Conversion Receivers

Abstract

Old RF single-medium transceivers are mostly narrow-banded, and filtering at RF and IF helped to realize discrete RF systems. However, in modern integrated digital wireless systems, the lack of RF and IF filters heavily taxes on the performance of RF transceivers. In particular, channels with broadband spread spectrum and strong nearby blockers demand analog RF processing be extremely linear, and channel filtering highly selective. Direct frequency conversion to and from DC (Zero-IF) or low frequency (Low-IF) is the mathematical frequency translation using complex local (LO) carrier. On the transmitter (TX) side, key design factors are carrier leak, injection locking, out-of-band emission, spectral regrowth, and power amplifier (PA) efficiency. The carrier is suppressed using four-quadrant mixers, and the injection locking is avoided by dual conversion. The out-of-band emission is reduced below the spectral mask using RF filters or harmonic mixers, and the spectral regrowth issue doesn’t exist with low IM3. The PA efficiency is also traded for linearity. On the receiver (RX) side, there are self-mixing, blocker, offset, harmonic mixing, and image issues. The self-mixing with the local carrier is avoided using low-IF or dual-conversion architectures, and the blocker is rejected by digital channel filters with wide dynamic range front-ends. The offset in the zero-IF receiver is eliminated using the feedback that is active only during the data packet period. The low-IF receiver exhibits no offset problem. All issues related to the harmonic mixing and image problems in direct down-conversion receivers are discussed exhaustively using a generic complex frequency translation concept.