Low phase noise LC VCO with sinusoidal tail current shaping using cascode current source

摘要

AbstractTail current shaping is a known technique for phase noise reduction. According to this technique, close to outputs zero crossing points, where Impulse Sensitivity Function (ISF) is maximum, current supplying path to the transistor pair is disconnected. In this paper, tail current shaping is performed by two additional transistors that are placed in cascode configuration with the tail current source. Conventional tail current shaping is performed by two tail transistors with the gates connected to the oscillator outputs. The proposed circuit outperforms the conventional circuit in phase noise performance, due to four mechanisms: higher impedance seen from the common source of switching transistors due to cascode current source; better tail current shaping; higher oscillation amplitude with the same power consumption; and lower RMS value of the effective ISF. Also, the proposed circuit presents more stable bias point and output amplitude, compared to conventional circuit. In addition, higher ratio of start-up tail current to the minimum of shaped tail current for the proposed circuit, removes the need for additional start-up detection circuits. Post layout simulations predict a sample oscillator designed at 4GHz in 0.18μm RF CMOS technology presents 11.3 and 3.8dB less phase noise at 1MHz offset, compared to simple and conventional oscillators, respectively.]]>