EMI contribution of the drive current and output phase lag in CMOS gate oscillator

Abstract

Any logic processor needs to run a clock frequency reference, and the oscillator which is most commonly implemented uses the linear characteristic of a CMOS gate. The behavior of such a gate in the linear region of the characteristic is like a quasi amplifier. This is the most simple way to build an oscillator with a digital process, but this type of oscillator is an important source of electromagnetic emission. It creates large loops of high frequency currents with external components and PCB power rails. The amplitude of the emitted electromagnetic field is a function of the current that flows in the power lines of the amplifier and also in the external components, crystal and load capacitors. At low frequency, the behavior of the gate is like a true inverter and the currents trough the two load capacitors are symmetrical. If the voltage gain value is low, the total ground current is close to zero. When operating to highest frequency value, over the cut frequency of the gate, the output signal lagging is more than 180 degrees. This spurious phase shift between the two currents that flow in he load capacitors can cause strong harmonics generation mainly if the gain of the oscillator is still important.