After properly terminating your PCB trace, EMI can still be an issue. By altering the shape of the signal waveform, you can attenuate high frequency components of your signal by rounding the square wave. This can be achieved with an RC filter at the source-side of the PCB trace.

Figure 2-6 RC Filtering Scheme

First, determine what your output signal frequency is. The capacitor value is calculated based on the RC cutoff frequency:

This frequency represents the -3db cutoff point of the signal. A standard cutoff frequency to start with is twice your square wave frequency. If larger high-frequency attenuation is required, you can use a lower cutoff frequency. Figure 2-7 through Figure 2-9 show the effects of lower RC cutoff frequencies on a square wave.

Figure 2-7 Cutoff Frequency = 2 × Square Wave Frequency

Figure 2-8 Cutoff Frequency = Square Wave Frequency

Figure 2-9 Cutoff Frequency = ½ Square Wave Frequency