Feature: Interface design
To reduce the impact of intra- system and external EMI, placing common mode chokes on both, the transmit and receive paths of a diff erential interface, serve to reduce radiated EMI
a diff erential interface, serve to reduce radiated EMI; see Figure 2. An example of a suitable common mode choke is the Murata NFG0QHB242HS. This surface-mount device, available in a 0605 package, is rated at 5VDC operating voltage and a maximum withstand voltage of 12.5VDC. With a typical cut-off frequency of 8GHz, the common mode insertion loss characteristic is 33dB at 2.4GHz; see Figure 3. As shown, the common mode noise suppression
characteristics aim to improve the performance of 2.4GHz, 3.7GHz and 5.4GHz applications for differential signalling interfaces.
Improving Wi-Fi receiver sensitivity To illustrate the impact of using a common mode choke on a USB interface, Murata engineers investigated in a laboratory. The equipment under test (EUT) was an electronic device with a Wi-Fi antenna and a USB 3.2 interface connected via a plug, socket and USB cable to an external solid-state disk (SSD); see Figure 4.
The EUT was placed in an anechoic chamber, with the
device’s Wi-Fi antenna connected to a spectrum analyser via a pre-amplifier. By using the EUT’s own Wi-Fi antenna, the results would indicate the true extent of receiver desensitisation resulting from radiated noise from the USB differential signalling lines. Figure 5 shows the results of three different tests
conducted on the EUT. The first test, shown as a red trace, represents the noise spectrum recorded during EUT operation without any common mode choke on the signalling lines. The green trace indicates using a common mode choke coil (CMCC), Murata NFG0QHB372HL2, placed close to the USB external port, which is observed to be away from the noise source of the EUT’s CPU. The final test, shown as a blue trace, indicates the CMCC placed as close as possible to where the USB signalling lines leave the CPU. With the CMCC placed close to the CPU, the noise level improved by approximately 5dB. Similar tests to evaluate receiver sensitivity were
conducted using the EUT’s internal Wi-Fi module and antenna with an omni-directional antenna connected to a PC equipped with a Wi-Fi test-set. Figure 6 shows the results, with an improvement of 8.6dB by fitting a CMCC close to the noise source. As a final test, to ensure compliance against the USB-IF signal integrity standards, an eye-mask test was conducted with the Murata CMCC. The examination revealed that adding a CMCC in the signal path did not adversely impact the differential signal quality. As data rates continue to increase, the impact of EMI
from differential interfaces has the potential to be more severe. The investigation and evaluations performed in the above tests confirm that using common mode chokes reduces the impact of EMI with no signal integrity degradation.
Figure 6: Wi-Fi module reception sensitivity improvement with a common mode choke
www.electronicsworld.co.uk July/August 2025 27
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