A 4.2-5.8GHz Noise Circulating LC Oscillator with Implicit Common-Mode Resonance

An LC oscillator, operating at 4.2-5.8GHz is presented in this paper. Capacitive feedback, instead of inductive coupling, is adopted for the noise circulating structure to reduce the noise current injecting into the LC tank. The phase noise (PN) is further suppressed by using the implicit common-mode (CM) structure of the LC tank to manipulate the output waveform less sensitive to noise current. The proposed oscillator achieves excellent performances through the whole operating band benefiting from the capacitive coupling, which introduces less parasitic capacitance compared to transformer coupling. The PN is below −124.5dBc/Hz while figure of merit (FoM) is above 189.5dBc/Hz at 1MHz frequency offset. The designed oscillator implemented in 40nm CMOS occupies 0.16mm ² chip area and consumes 8.4mW at 1.1V supply.