This paper describes the issues and tradeoffs in the design and monolithic implementation of direct-conversion receivers and proposes circuit techniques that can alleviate the drawbacks of this architecture. Following a brief study of heterodyne and image-reject topologies, the direct-conversion architecture is introduced and effects such as dc offset, I/Q mismatch, even-order distortion, flicker noise, and oscillator leakage are analyzed. Related design techniques for amplification and mixing, quadrature phase calibration, and baseband processing are also described.

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Design considerations for direct-conversion receivers

Direct-conversion is an alternative wireless receiver architecture to the well-established superheterodyne, particularly for highly integrated, low-power terminals. Its fundamental advantage is that the received signal is amplified and filtered at baseband rather than at some high intermediate frequency. This means lower current drain in the amplifiers and active filters and a simpler task of image-rejection. There is considerable interest to use it in digital cellular telephones and miniature radio messaging systems. This paper briefly covers case studies in the use of direct-conversion receivers and transmitters and summarizes some of the key problems in their implementations. Solutions to these problems arise not only from more appropriate circuit design but also from exploiting system characteristics, such as the modulation format in the system. Baseband digital signal processing must be coupled to the analog front-end to make direct-conversion transceivers a practical reality.

Direct-conversion radio transceivers for digital communications

A monolithic 1.9-GHz, 198-mW, 0.6-/spl mu/m CMOS receiver which meets the specifications of the Digital Enhanced Cordless Telecommunications (DECT) standard is described. All of the RF, IF, and baseband receiver components, with the exception of the frequency synthesizers, have been integrated into a single chip solution. A description is given of a wide-band IF with double conversion architecture which eliminates the need for the discrete-component noise and IF filters in addition to facilitating the eventual integration of the frequency synthesizer blocks with on-chip VCO's. The prototype device utilizes a 3.3-V supply and includes a low noise amplifier, an image-rejection mixer, and two quadrature baseband signal paths each of which includes a second-order Sallen and Key anti-alias filter, an eighth-order switched-capacitor filter network followed by a 10-b pipelined analog-to-digital converter (ADC). The experimental device has a measured receiver reference sensitivity of -90 dBm, an input referred IP3 of -7 dBm, a P/sub -1 dB/ of -24 dBm, and an image-rejection ratio of -55 dBc across the DECT bands.

A 1.9-GHz wide-band IF double conversion CMOS receiver for cordless telephone applications

The concept of concurrent multiband low-noise-amplifiers (LNAs) is introduced. A systematic way to design concurrent multiband integrated LNAs in general is developed. Applications of concurrent multiband LNAs in concurrent multiband receivers together with receiver architecture are discussed. Experimental results of a dual-band LNA implemented in a 0.35-/spl mu/m CMOS technology as a demonstration of the concept and theory is presented.

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Concurrent multiband low-noise amplifiers-theory, design, and applications

In the type of circuits considered here, the input power is divided equally between two channels whose outputs are caused to have a very nearly 90° phase difference over a broad frequency range. Networks suitable for application at low frequencies which perform the above function have been widely investigated. This report describes a new type of 90° differential phase shifter which has a constant resistance input, and which is useful over bandwidths as large as 5:1 in the microwave region.

A New Class of Broad-Band Microwave 90-Degree Phase Shifters

This paper presents a third method of generation and detection of a single-sideband signal. The method is basically different from either the conventional filter or phasing method in that no sharp cutoff filters or wide-band 90° phase-difference networks are needed. This system is especially suited to keeping the signal energy confined to the desired bandwidth. Any unwanted sideband occupies the same band as the desired sideband, and the unwanted sideband in the usual sense is not present.

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A Third Method of Generation and Detection of Single-Sideband Signals

This paper presents a design procedure for the practical design of RC wide-band 90-degree phase-difference networks. The important part of the paper is the design procedure given in "cookbook" fashion with a numerical example to clarify it. This procedure consists of many steps, each step being a relatively simple one. This should make it a useful design tool for people without much background in network theory. A brief theoretical introduction is presented but this is far too sketchy to be of much use in understanding just what is behind the design procedure. Some practical hints on construction and alignment are given.

Donald K. Weaver

Papers

A Third Method of Generation and Detection of Single-Sideband Signals

Design of RC Wide-Band 90-Degree Phase-Difference Network