Measurement/Searching techniques/Experiment 1) Search technique (to find which equipment emitted the noise) The emission source was searched for using a hand-held amateur radio tuned to the affected frequency. The affected area could be searched roughly. The electrical equipment acting as the emission source was determined by turning off the power of the suspected equipment. It was found that the emission source was the core equipment of the telephone system. 2) Mitigation trial a) A ferrite core was installed in the power line and in the telephone line of the core equipment. b) The core equipment was wrapped in aluminium foil. c) An insulated conduction sheet was pasted onto the rear of the circuit board. As a result, the acoustic noise affecting the amateur radio decreased slightly, but still remained. 3) Search technique 2 (to find the noise source) When a noise is emitted, there must be both a noise source and a noise radiator. Both elements were searched for. a) The system was measured in an anechoic chamber and the noise level was checked. b) The noise source was searched for using a near-field magnetic probe and a spectrum analyser, as shown in Figure 1.4-2. In this case, a probe (Figure 1.4-3) was used. This probe had a coil consisting of several turns installed ahead of the coaxial cable. The coil was connected between the inner and outer conductors. Areas close to board components IC8 and Xtal4 had higher noise levels. c) The noise level was checked by trial and error in an anechoic chamber when a mitigation device such as a capacitor or ferrite core was installed. Spectrum analyser or receiver Coaxial cable Inner conductor Outer conductor Loop Miti(08)_F1.4-2 Circuit board Miti(08)_F1.4-3 Figure 1.4-2 – Searching for the noise source Figure 1.4-3 – Loop antenna Miti(08)_F1.4-4 Figure 1.4-4 – Result of noise search Case study 1.4 – 2 –