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CMOS has made steady inroads into ICs for wireless applications. Building blocks implementing the RF and baseband circuits in a 9OOMHz wireless transceiver have been described [l]. New uses have been discovered in the RF context for some of the well-known properties of CMOS analog circuits, such as the cancellation of quadratic nonlinearity in balanced circuits, the capability of switched-capacitor circuits to handle large signals without distortion, the use of FET switches to commutate signals at RF, and the large signal swings possible in a transistor with an insulating gate [l, 21. l p m CMOS circuits have been shown to operate at SOOMHz, while 0.6pm channel lengths suffice at 2.4GHz. As feature size scale down to 0.35pm, 5GHz CMOS front-ends will be within reach.Building blocks alone do not account for the current interest in CMOS for RF applications. The more compelling reason is the opportunities CMOS affords for large-scale integration. Modern wireless transceivers will increasingly blend digital blocks into conventional analog front-ends for frequency synthesis, adaptivity, multi-mode operation, and sophisticated detection. This raises questions such as how well digital CMOS circuits can co-exist on the same substrate as the radio front-end, or whether there is sufficient on-chip isolation in a low-cost package to guarantee stable operation of a receiver with more than lOOdB of baseband gain, or how the power amplifier modulates the on-chip local oscillator. The future of CMOS transceivers may well depend on satisfactory answers to these questions. This paper presents design techniques to mitigate these problems in a single-chip 9OOMHz spread-spectrum transceiver implemented in l p CMOS, and measurements of the transceiver to validate their effectiveness ( Figure 1).The transceiver operates in time-division duplex, that is, at any instant of time it either transmits a data packet spread by frequency hopping across the ISM band, 902-928MHz, or it receives such a packet. The digital agile frequency synthesizer is active in both modes. The transmitter must deliver the modulated RF signal to the antenna relatively free of spurious tones arising from distortion, of the unwanted sideband, and of leakage from the local oscillator (LO). All these undesirable spectral components may fall within the ISM band, and therefore cannot be filtered. This requires linear upconversion mixers with high accuracy and matching, and very small parasitic on-chip coupling between the LO and the power amplifier (PA). The single-chip radio achieves excellent performance in all respects (Figure 2). A fixed-frequency 915MHz LO with inherently accurate quadrature outputs upconverts a digitally-synthesized baseband spreadspectrum, whose quadrature accuracy is susceptible only to the small gain mismatch in the two DACs. Four balanced passive FET switch mixers upconvert each baseband component by both quadrature LO phases, and an R-C polyphase filter suppresses the opposite sideband appearing at the 3rd LO harmonic. To maintain equal load...

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