EMI Reduction using Capacitive Filter

EMI (Electromagnetic Interference)Issues

Standard stipulates that an electronic device functions satisfactorily without interfering with other equipment within its environment. High frequency clock signals are one of the major sources of EMI. One of the ways to ensure EMI compliance is by the addition of a simple RC low-pass filter to the output of high frequency clock signals. The filter slows down the rise and fall time of the signal, thereby attenuating the harmonic contents of the signal.

Analysis and discussions

The schematic of the system that was used to demonstrate EMI reduction is shown in figure 1. The U1 in the circuit of figure 1 generates 625Hz square wave clock signals shown in figure 2. The RC low-pass filter was designed with a cut-off frequency Fc = 1/(2*pi* RC) = 723Hz; where R = 2k2 and C = 100nF. The clock signal of the circuit of figure 1 was used for this demonstration! 

Fig 1: EMI reduction demonstration circuit

From figure 2, it can be observed that the waveform of the signal with RC filter has a slower transition than the signal without RC filter.  

Fig 2: Time-domain signals generated from circuit of figure 1 - with filter and without filter

The energy levels of the frequency components for the clock signal with faster transition and slower transition are analysed in figure 3 and 4 respectively. It can be observed that the power levels of the harmonic contents of the signals with RC filter is lower than that of the signals without RC filter. Also It can be seen from figure 4 that the higher harmonic components of the clock signal with RC filter is almost at the noise floor unlike clock signals without filter. Hence EMI is reduced with the use of RC filter. 

Fig 3: Frequency spectrum of clock signals (without filter)

Fig 4: Frequency spectrum of clock signals (with filter)