An oscillator includes oscillator circuitry (8) including a transconductance stage (2) and a resonator (3). A comparator (10) produces first (CLK) and second (/CLK) clock signals which indicate the timing of positive and negative phases of a differential output signal (VIN+-VIN−) produced by the transconductance circuit in response to the resonator. A synchronous rectifier (14) converts the differential output signal to a current (IRECT) in response to the first and second clock signals. A switched capacitor notch filter (15) filters the current in response to the first and second clock signals. A control current (ICONTROL) which controls the transconductance of the transconductance circuit is generated in response to the notch filter. The resonator may be a MEMS resonator.