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本文提出了一种用于滤波器截止频率调谐的方案，采用1位开关电阻结构替代多位电阻阵列，通过200MHz调谐脉冲的占空比控制有效RC时间常数，并利用ΔΣ调制和抖动抑制杂散效应。该方案应用于蓝牙收发器的信道选择滤波器，实现了6.3kHz步进的截止频率控制和小于1.8kHz的精度。
In this paper, a scheme for tuning cutoff-frequency of filters is presented. Instead of using a multibit resistor bank, a 1b switched-resistor structure operating at 200MHz is utilized. The effective RC time constant is controlled by the duty ratio of the 1b tuning pulse. ∆Σ modulation with dithering mitigates the spurious effects. Compared to the conventional tuning scheme, finer cutoff frequency tuning and more linear control are achieved. The tuning scheme is applied to a channel-select filter in a Bluetooth transceiver resulting in 6.3kHz-step cutoff -frequency control and less than 1.8kHz of differential linearity. The transceiver is implemented in a 1.2V 90nm CMOS process. Since CMOS process suffers from large variations of device characteristics, wide-range tuning of RC time constant is essential in high-order analog filters. A typical channel-select filter requires more than 4 bits in a multibit
Hideaki Majima, Mototsugu Hamada
Toshiba, Kawasaki, Japan