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Reduction in radio interference through pwm frequency dithering

Solutions to meet Electro-Magnetic Interference (EMI) limits (e.g., CISPR 25, GMW3097, etc.) can be quite expensive. In some systems 30% of the whole cost is related to EMI/EMC for large filters, shielding, etc. In current-controlled electric-motor driven systems the main contributor to RF noise is the Pulse Width Modulation operation. The PWM generates narrow-band noise in the form of a spike at every integer harmonic of the switching frequency. An automobile's radio is a serious victim to narrow-band noise, as the harmonics can appear as distinct tones at the output or a desired signal can be blocked. For this reason, the CISPR 25 noise amplitude limit is at least 19dB stricter for narrow-band noise than for broadband noise. The solution discussed here converts the narrow-band noise into broadband noise and reduces the interference in radios by varying the switching frequency within a small range.