Wideband Q-VCO using tail-current shaping based automatic amplitude control

A wideband Q-VCO circuit with two tail-current branches ranging from 1.54GHz to 2.8GHz is proposed by tail-current shaping based automatic amplitude control (AAC). In Q-VCO's outputs, the AC component combing with the negative peak are injected back to the tail-current transistors as biasing voltage so as to reduce the tail-current noise contribution and realize the AAC. Moreover, an extra current path is enabled at higher frequency in order to suppress the tail-current increment for further LC-tank amplitude reduction, which is connected between the two tail-current branches. The proposed circuit was analyzed by SpectreRF simulator using TSMC 0.18@mm CMOS technology with a power supply of 1.8V. The phase noise at 1MHz frequency offset is from -132.6dBc/Hz at 1.54GHz to -125.3dBc/Hz at 2.8GHz while the FOM is about -182dBc/Hz. Besides, within the tuning frequency, the maximum LC-tank amplitude variation is only 326mV, which is 2.25 times less than that of the Q-VCO without the AAC.

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