An image-rejecting mixer and vector filter with 55-dB image rejection over process, temperature, and transistor mismatch

This paper describes an image-rejecting mixer and vector filter for use in radio systems with channel bandwidths in the range of 1 MHz. The circuit replaces the SAW filter and second downconverter commonly used in this style of radio. Because the output of the circuit is at an IF of 5 MHz, traditional demodulation methods including limiting and FM discrimination can still be used. The circuit is based on a quadrature mixer that guarantees good performance despite device mismatches and process variation. The circuit consumes 29 mA at 3.3 V,and achieves better than 55-dB image rejection despite device mismatches and process variation and is implemented in a single-poly triple metal 0.5 μm CMOS process with linear capacitor implants. The circuit is designed for input signals from 125 to 250 MHz. Input referred voltage noise is 900 μVrms. The in-band IP3 is 18 dBm. By changing an external reference frequency, the passband width of the filter can be varied from 3 to 0.5 MHz