Langer EMV - 17 ESA1 set

17 ESA1 set

ESA1 set – Application example: Troubleshooting emission problems on electronics

Introduction

The PCB to be troubleshooted (DUT) is a microcontroller board with two cables connected. The first cable is used for the voltage supply, the second one is used for the connection to a communication partner via a serial interface.

DUT - Electronics with interference emission problems — **Figure 1:** DUT – Electronics with interference emission problems

Radiated Emission Spectrum

The radiated emission of this board and its cables was measured with an antenna in the far field and has the following spectrum, showing several limit violations.

Radiated interference spectrum - antenna measurement — **Figure 2:** Radiated interference in the far field – spectrum of the antenna measurement

Far-field measurement setup — **Figure 3:** Setup of the far-field measurement – antenna at a distance of 3 m from the circuit board

Step 1: Reproduction

The ESA1 set from Langer EMV-Technik GmbH is used for troubleshooting and can be set up directly at the hardware designer's desk. The circuit board is placed on the base plate of the shielding tent and the DUT is supplied with power via the HFW 21.

The HFW 21 is an RF current transformer. It measures the high-frequency excitation currents from the DUT that cause the antenna, consisting of cables and PCB, to radiate. The measured spectrum is recorded with the tent closed and differs slightly from the emission spectrum of the antenna measurement. The results are proportional to each other.

ESA1 measurement setup with HFW 21 — **Figure 4:** ESA1 measurement – setup with HFW 21

ESA1 spectrum recorded with HFW 21 — **Figure 5:** ESA1 measurement – spectrum recorded with the HFW 21

Step 2: Investigation and Modification

To locate the RF sources on the DUT, near-field probes are used in the open tent. Using an E-field probe, a strong electric RF field can be measured around the coil of the DC-DC converter. The communication cable is placed directly above the coil (Figure 1), coupling disturbances into the cable. The harmonics of the 500 kHz clocked DC/DC converter exceed the limit by 5 dB at around 100 MHz in the antenna measurement (Figure 2), so interference should be reduced by more than 5 dB (ideally 10 dB).

E-field probe locating RF sources — **Figure 6:** Use of an E-field probe to localise RF sources

To reduce electric field coupling, a shield is placed over the coil of the DC-DC converter and connected to the DUT ground.

**Figure 7:** Shielding the choke with copper tape

A magnetic field probe identifies the clock line of the serial interface in the ribbon cable as a source of interference (Figure 8). Spikes between 200 MHz and 400 MHz correspond to the antenna measurement frequencies (Figure 2).

Magnetic field probe locating RF sources — **Figure 8:** Use of a magnetic field probe to localise RF sources

To reduce these spikes, a 47 pF filter capacitor is added to the clock line.

Capacitor on clock line — **Figure 9:** Additional filter capacitor on the clock line of the ribbon cable

Step 3: Verifying the Effectiveness

Effectiveness is checked with an HFW 21 measurement in a closed tent. The high-frequency excitation currents in the supply cable were successfully reduced.

Shielding the coil reduces broadband interference at lower frequencies, while the capacitor on the clock line reduces spikes. With these modifications, another antenna measurement can confirm that the limit violations are eliminated.

HFW21 measurement comparison — **Figure 10:** Comparison of HFW 21 measurement spectra (red – unmodified, blue – with modifications)

Antenna measurement comparison — **Figure 11:** Comparison of antenna measurement spectra (red – unmodified, blue – with modifications)

The EMC problem is fully solved once a production-feasible implementation of these modifications is identified, for example by relocating the coil on the PCB assembly.