A tail current-shaping technique to reduce phase noise in LC VCOs

This paper presents a tail current shaping technique to improve phase noise in differential LC VCOs. The tail current is made large when the oscillator output voltage reaches its maximum or minimum value and when the sensitivity of the output phase to injected noise is the smallest; the tail current is made small during the zero crossings of the output voltage when the phase noise sensitivity is large. The proposed technique does not dissipate any extra power, does not use extra noisy active devices and occupies a small area. Besides better noise performance, a VCO with tail current shaping has a better DC to RF conversion and thus larger oscillation amplitude compared to a standard VCO with equal power dissipation. Measurement results for a 2 GHz differential NMOS LC-VCO with tail current shaping fabricated on a 0.25 μm BiCMOS process are compared to measurements for a standard VCO in the same process. The phase noise improvement is more than 3 dB at the carrier frequency of 1.75 GHz. The measured phase noise at 3 MHz offset frequency is -134 dBc/Hz from a 1.75 GHz carrier and the VCO draws 1.5 mA from a 1.5 V supply.