| Low power and low cost are the primary requirements in WPAN (Wireless Personal Area Network) applications. These requirements lead to the pursuit of circuit simplicity in the system design stage, and the choices of a direct-conversion frequency plan, DSSS (Direct-Sequence Spread Spectrum) and non-coherent DBPSK (Differential Binary Phase Shift Keying) detection. Furthermore, a spectrum with little low frequency energy enables simple direct-conversion architectures. While some complementary spreading codes generate DC-free spectra when combined with DBPSK, they cannot satisfy the FCC spectral peak width requirement of 2.4GHz ISM-band DSSS systems. A new kind of spreading code, which we call an offset code, solves this problem and exhibits superior performance over PN (pseudorandom noise) spreading codes.; In a typical WPAN transceiver the frequency synthesizer consumes a large fraction of the total power. This work introduces an oscillator architecture that extracts the third harmonic of its oscillation frequency. This architecture allows us to lower the oscillation frequency and eliminate the prescaler (the first and most power-hungry stage in a frequency divider), to save power. A voltage-controlled coupled inverter-ring oscillator with 12 output phases and sharp current pulses generates both in-phase (I) and quadrature (Q) signals using this technique. A new analysis based on linear models, Barkhausen criteria and pole plots provides design insights and successfully predicts the stabilities of the various modes of the coupled-ring oscillator. An implementation in 0.18mum CMOS exhibits significantly reduced total power consumption for a 2.4GHz I/Q frequency synthesizer. |
