A CMOS channel-select filter for a direct-conversion wireless receiver

This paper describes the design and performance of an SC channel select filter for a frequency-hopped spread-spectrum direct-conversion receiver operating in the 902-926 MHz ISM band. Data modulates a hopped carrier by one of two (or four) prescribed frequency offsets, such that the modulated spectrum is always contained within +200 kHz of the carrier. Following preselection of the 26 MHz-wide ISM band at RF, an agile local oscillator synchronized to the hopping pattern of the received signal downconverts the band to bring the sought channel to DC. The channel-select filter passband extends 220 kHz around DC, and the adjacent channel 320 kHz away from the carrier defines the filter's stopband edge. System simulations suggest that if all ISM-band users in a cell abide by power control, then to detect the desired channel with a sufficiently high signal-to-interference ratio, the filter must attenuate adjacent channels by at least 45 dB.