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EMC problem on next generation network (NGN) trial

Disturbance.

Conducted emission.

TV receiver.

Customer premises.

Radiated emission, TV receiver, IP set-top box (STB).

2008-02-29

Figure 1.13-1 shows a system configuration in a next generation network (NGN) field trial. In this trial service, the home networks of trial users are connected to the NGN by an optical network unit (ONU) and a home gateway (HGW). High-quality video delivery services can be provided when both an IP-STB (IP set-top box) and a TV receiver are connected to the HGW. Analogue TV broadcasting is also provided by the traditional broadcasting equipment.

Figure 1.13-1 – A system configuration in NGN field trial

As shown in Figure 1.13-2, video noise occurred in a channel (91.25 MHz) of analogue TV broadcasting when the IP-STB was turned on. This problem was solved when the IP-STB was turned off or when the component video cable was disconnected. The trouble source was considered to be the conducted disturbance wave from the IP-STB.

Figure 1.13-2 – Examples of TV screen: Video noise occurred
when the IP-STB was turned on or off

Measured result of the conducted disturbance wave

The TV broadcasting signal and conducted disturbance wave were measured at the measurement point shown in Figure 1.13-1. Figure 1.13-3 shows the measured spectrum of the received signals. The desired signal to undesired signal (D/U) ratio was 25 dB when the IP-STB was turned on, and 45 dB when the IP-STB was turned off. As the D/U ratio of 40 dB is required for analogue TV broadcasting, this video noise on the TV picture was caused because the required D/U ratio was not being achieved.

Figure 1.13-3 – Measured results of received signal spectrum

Mitigation method

As this problem was caused by the conducted disturbance wave from the IP-STB, four ferrite cores (Attenuation > 20 dB @ 10-100 MHz) were installed on the component video cable as shown in Figure 1.13-4. Figure 1.13-5 shows the variation of the D/U ratio after the installation of those ferrite cores. As the figure shows, the D/U ratio rose to about 45 dB, and this problem was solved.

Figure 1.13-4 – Mitigation by ferrite cores

Figure 1.13-5 – Variation of D/U ratio after mitigation

Conclusion

In this NGN trial, communication and broadcasting equipments are connected by cables such as a video cable; therefore, both equipments can become disturbance sources. By checking the spectrum of measured signals, in this case, it was found that the IP-STB was a disturbance source. The requirements of video ports should be defined to prevent EMC problems on NGN services.