**3.2 LED information board**

Measures to mitigate the galvanic coupling through AC mains wires and DC power supply of the LED modules to the LED information board are investigated. The LED information board enclosure is a galvanized steel with a protection rating of IP65 and anti-reflective protective glass. The LED information board supplies from a single-phase AC low-voltage network converted by DC-to-DC converters with a maximum current of 20 ampers. A total of three DC-to-DC converters are in the LED information board. For receiving and sending data in several ways, an optical converter, router, and ADSL modem are installed. A mini PC controls data processing and process control [14]. The results show that the EMI emissions in horizontal polarization are higher in comparison to vertical polarization. The following measures were sequentially used to reduce the EMI emissions:


**Figure 9** shows the interference level with three different EMI mitigation measures. Pre-compliance testing shows that the most critical frequency range is 30 to 130 MHz. From the initial measurements without EMI measures, the results

**Figure 9.** *Frequency characteristic of EMI level for different EMI mitigation measures.*

**Figure 10.** *Frequency characteristic of EMI level for combined EMI mitigation measures.*

show that the maximum EMI radiation is at 37.5 and 87.5 MHz. Other sources of EMI emissions are at 62.5, 100, 112.5, and 125 MHz. It is evident from the list that frequencies are multiples of 12.5 MHz.

The most optimal EMI reduction reach by the simultaneous use of several EMI reduction measures. An example of the measured EMI emissions levels for such a case shows **Figure 10**, where chokes on twisted pairs from AC-to-DC converters at both ends, impedance matching resistors, and data wires on PCB with SMD LC filters are used. Other variants used in testing had only a slight effect, a few tenths of dB. To compare the effects of EMI measures with each other, the vertical scales in **Figures 9** and **10** are the same.

#### **3.3 Audio equipment**

Sound equipment is still popular, and home theaters provide quality surround sound. Retro designs with small dimensions are popular. In the past, analog amplifiers have been used in high-performance audio devices to emphasize sound quality. In recent years, digital amplifiers have improved sound quality and become popular in home audio. Today, audio devices have implemented multiple communication and I/O technologies to provide comfort. This is associated with EMI problems in the development of new audio equipment.

The EMI sources are the display unit, switching power supply, Bluetooth, USB port, S/PDIF connection. Due to a large number of connections, the cabinet of the audio device contains many holes. Holes are places where the generated interfering electromagnetic fields radiate out into space. **Figure 11** shows a selection of pre-compliance measurements on audio devices in different operating conditions.

**Figure 11a** shows the EMI for an audio device (amplifier with compact disc (CD) drive) with a metal cover, Bluetooth active, CD playback on, the maximum power delivered to the rated load, and the connected S/PDIF cable. It is clear from the measured data that the device emits a discrete spectrum from 80 to 700 MHz, 930 to 1000 MHz, and a significant continuous spectrum between 220 and 300 MHz.

*EMI Pre-Compliance Measurements Reveal Sources of Interference DOI: http://dx.doi.org/10.5772/intechopen.99754*

**Figure 11.** *EMI pre-compliance measurements on audio devices in different operating conditions.*

**Figure 11b** shows the EMIs for the same audio device under the same conditions with a change in the use of the choke in the power conductors inside the cabinet of the audio device. Surprisingly, this measure has the opposite effect. Instead of the expected reduction of EMI field radiation, emissions are increased by 6 to 8 dB in the band 340 to 680 MHz. In addition, the use of a choke affected the width of the continuous spectrum. As a result, the continuous spectrum is narrower by shifting the upper limit from 300 to 270 MHz.

**Figure 11c** shows EMIs for an audio device (amplifier with CD drive) with a cabinet in which some parts are wood. In this case, the audio device is powered by a battery pack. The CD drive is empty without a CD inserted, and the audio cables are not connected. The aim is to determine the basic level of EMI radiation with the possibility of subsequently monitoring the effect of connecting cables to audio inputs and outputs and a CD drive with a CD inserted. The measured EMI in **Figure 11c** shows only the discrete spectrum of EMI field radiation. **Figure 11d** shows the EMIs for the same audio device with a CD inserted in the CD drive, and the device operates in "play" mode. S/PDIF cables are connected as well. The measured EMI emissions in **Figure 11d** show the continuous emission spectrum of EMI fields from 220 to 300 MHz. The same CD drive and CD drive control are used, as in the first measured audio device (see **Figure 11a** and **b**). Thus, a component generating continuous EMI emissions in an audio device is classified.

## **4. Summary and conclusions**

Electromagnetic smog is a problem for technical equipment and living organisms. The manufacturer of electrical equipment wants to gain a foothold in the market and sell the product. The successful sales of the product on the market require an EMC certificate of conformity, which increases the development and production costs of the product. To reduce equipment development costs, shorten time to market, and obtain a risk-free EMC certificate, pre-compliance measurements are needed. In addition, shortening the time to market accelerates the professional experience of engineers.

This chapter has provided three examples of EMI measurements using precompliance measurements. The first case concerns the generation of unwanted harmonic currents and the detection of EMI sources by conduction in a prototype of street LED lighting. The second case concerns EMI sources from the LED information board and how some measures reduce EMI interaction. Finally, the third case points to unexpected changes in EMI radiation during the development of audio equipment.
