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A 3.1–4.2 GHz automatic amplitude control loop VCO with constant Kvco and <10mV amplitude variation

This paper presents an automatic amplitude control (AAC) loop voltage controlled oscillator (VCO) with a constant VCO gain (Kvco) for fractional-N frequency synthesizers in WLAN application in 65nm CMOS process. A novel AAC scheme based on a mixed-signal feedback loop has been proposed to minimize the amplitude variation. In order to achieve a low Kvco variation, an extra switched varactor array is also introduced to the LC tank together with the conventional switched capacitor array. The VCO achieves a tuning range of 23.7% from 3.1 to 4.2GHz, while consuming 4.6mA of quiescent current from a 1.2V power supply. The peak amplitude variation over the entire frequency range is less than 10mV, which can be otherwise as large as 100mV, or 22.2% without the AAC loop. The VCO shows a nearly constant Kvco of 60MHz/V, a low phase noise of −123.3dBc/Hz at 1 MHz offset, and a superior FoM of −187dBc/Hz from a 3.6 GHz carrier.