Dynamic Duty Ratio Control of Oscillator for EMI Reduction
Original Publication Date: 1988-Jan-01
Included in the Prior Art Database: 2005-Feb-14
This article describes a technique for reducing Electromagnetic Interference (EMI) from logic circuits including crystal oscillator(s). The EMI reduction can be easily accomplished by means of changing a duty ratio of a pulse train output of a crystal oscillator with time. To change the duty ratio means to reduce the average EMI levels of harmonics of the output which can be detected with the temporal average or quasi-peak method. An example circuit for implementing this technique is shown in the figure. It comprises basic crystal oscillator OSC#1, a duty control oscillator OSC#2, a transistor TRQ, a Schmidt inverter IC1, a NAND gate IC2, a resistor R and an integration capacitor C. OSC#1 produces a rectangular pulse train having a repetition frequency fo and a duty ratio of 50%.