Measurement/Searching techniques/Experiment The customer claimed that stripes appeared on the TV screen in one TV channel. The broadcasting company investigated the cause of this EMI problem and found that the disturbance was due to EM waves radiating from the telecommunication building. 1 Measurement of EMI source To find the source of this EMI problem, the following measurements were carried out: a) TV broadcast signal level of the TV receiver, and b) electric field distribution near the customer's house. 2 Results of the measurements 2.1 Received TV signal level at the customer's house The received level of the TV broadcasting waves was measured with a spectrum analyser connected to a TV antenna at the customer's house. Figure 1.12-2 shows the measured frequency spectrum at the customer's house. An interference signal existed between the video and audio signal bands, and its level was about 22 dBμV. One can see that the D/U (desired/undesired) signal ratio was about 30 dB. From the experience of similar EM problems, the required level for the D/U ratio should be greater than 40 dB. Therefore, it was considered that this interference signal influenced the TV receiver, resulting in the generation of stripes on the screen. 2.2 Measuring electric field distribution near customer premises Figure 1.12-3 shows the results of measuring the electric field distribution around this location. The results indicate that the highest electric field strength was observed near the building and that the measured level tended to decrease as the distance from the building increased. 2.3 Measuring emissions near the telecommunication equipment in the building As a result, it was confirmed that the disturbance came from the building in front of the customer's house. The source of this interference was investigated and it was found that the interference signal radiated from transmission equipment in the building. 60 Customer's house Audio signal Video signal Received level [dBμV] Interference signal 40 20 22.5 Electric field[dBμV/m] 47 Highest level 0 Miti(08)_F1.12-3 210 212 214 216 Frequency [MHz] Miti(08)_F1.12-2 Figure 1.12-3 – E-field distribution near the customer Figure 1.12-2 – Measured result of received signal level Case study 1.12 – 2 –