Low-pass filter for UWB system with the circuit for compensation of process induced on-chip capacitor variation

This paper describes the design and optimization of a Chebyshev 5th order low pass filter with included circuit for automatic process calibration and compensation. The filter is realized using lumped elements in 130 nm radio frequency (RF) CMOS process and is dedicated to cover lower sub-band (3.4 GHz - 4.8 GHz) of ultra-wideband (UWB) system. The proposed full on-chip calibration concept estimates MIM-Capacitor (Metal-Insulator-Metal) capacitance process-induced variation against more stable on-chip MOS capacitor reference. In order to estimate the capacitance value, a low frequency oscillator is designed, which uses both types of capacitors for generating the oscillations, one after another. The MIM capacitor value is determined in digital domain based on the ratio of two oscillation frequencies and its exact needed value is obtained using a compensation capacitor bank. Detailed mathematical optimization of the calibration method is presented. All RF, analog and digital circuits have been integrated on a test chip and fabricated in 130 nm RF CMOS process. The produced ICs have been on-wafer measured and compared to simulation results. According to obtained results, the proposed calibration concept lowers process-induced filter transfer characteristic variation from approximately 5 dB to 0.6 dB at the critical frequency. The calibration needs to be applied just once at the beginning of circuit operation. The total area of implemented calibration circuit is less than 0.1 mm(2). The same method and the compensation circuit can be employed for the calibration of all on-chip circuits whose performance is affected by MIM capacitance process variation.