Our first recommendation is to try to reduce unwanted emissions from the oscillation circuit for the main clock that drives the microcontroller and other ICs in your system. Generally speaking, EMI levels grow as the clock speed rises. Noise from harmonic components can be inductively coupled within adjacent circuit patterns. Also, that noise is radiated through package leads and circuit board patters by antenna effects. To minimize these problems, review the oscillation constant and use as slow a clock frequency as you can. Also, examine the output of the oscillator at the normal supply voltage, as well as the extremes of the operating voltage range. If you find signal distortion, change the values of the components in the oscillation circuit to smooth out the output and eliminate overshoot and undershoot. For example, in the circuit shown here, adjust C1, C2, and R2.