| Summary: | 碩士 === 元智大學 === 電機工程學系甲組 === 107 === For the application of 5G and the next generation wide-band communication system, this thesis proposes a programmable wide-bandwidth small-area low-pass-filter (LPF) and its corresponding passband auto-calibration circuit. By applying both analog bias and digital control code, the passband of the proposed LPF can achieve continuous tuning and with a wider tuning range.
Instead of the conventional trans-conductance pair, the passive resistors and the grounded passive capacitors, the proposed LPF uses the inverters biased at their threshold levels, the active inductors and the active Miller capacitor. The parallel connection of active inductors and capacitors create an under-damping second-order impedance response, thereby reducing the transition band. The parasitic inductance of the active inductors could be adjusted via external digital controls. The miller capacitors are made of tri-stage inverters, the metal-insulator-metal (MIM) capacitors, the voltage-controlled MOS resistors. By analog control voltage, the effective capacitance of the miller capacitor can be adjusted continuously.
In order to let the proposed LPF meet the specified passband frequency among different process corners and different operation conditions, we added an auto calibration circuit. The parasitic inductance of the active inductor and the parasitic capacitance of the active miller capacitor were overly adjusted to cause serious underdamp behavior, and the proposed LPF becomes a resonant oscillator. According to simulation and chip measured results, the passband frequency of the LPF during normal filtering operation and the oscillation frequency caused by overly tuning have a fixed ratio. As a result, once we use a feedback loop to control the oscillation frequency, the passband frequency would be proportional shifted to another specific frequency. The pass-band calibration circuit includes a replica LPF cell, a digital comparator, a counter and a pass-band control finite-state-machine (PC FSM), analog-to-digital converter.
To verify the proposed LPF and the auto-calibration circuit, a 4-stage LPF template was realized in 0.18um CMOS technology, occupies an active area of 0.048mm2 and consumes a power of 3.45mW under 1.8V of supplies. The test chip measurement results show that the -3dB frequency can be continuously varied form 0.6MHz to 53.7MHz with a 3-bits of digital controls and a 0.7V-to-1.1V of analog controls. In addition, the simulation results show that under a 1MHz of reference clock, the auto calibration system takes 16 clock cycles to finish the passband calibration.The calibrated passband frequency has an error about 6% to the target frequency. The power consumption of the calibration system is about 3.7% to the LPF.
Keywords: Low-Pass-Filter, LPF, Active Inductor, Miller Capacitor, Resonation, Oscillator, Auto-Calibration
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