Reducing the noise of a power supply was the task the student wanted to achieve. On the other hand, some CRT 1 GHz Spectrum Analysers were looking for a new job. This inspired me to go for a EMC-Precompliance Test-Setup. As commercial solutions have astronomical price-tags, we went the diy way. And yes, approx. 20 dB cheaper :-)

The design is similiar to others out there. The MIL-STD-461F calls for 50 µH. The main coworkers are the IH15EB500K (50 µH, Vishay Dale 20 mΩ 15A) and the R53BW48205000K (8.2 µF, 10% 800 VDC from Kemet). The circuit looks like this :

COMPONENT	VALUE	SPECS
C1	8.2 µF	R53BW48205000K from Kemet
R1	5 Ω	2 x 10 Ω e.g. CFR200J10R
L1	50 µH	IH15EB500K from Vishay Dale
C2	220 nF	PPE0,22ST RU FRA, axial 220nF 630VDC/400
R2	1 kΩ	≥ 0.6 W

BOM - Bill Of Materials


The circuit is used twice, once for the phase (Phase) and once for the return path (Nulleiter). The GND is also called protective earth (Schutzerde). The Device has no fuse. This must be provided by the isolation transformer.

The Noise coming from the Powerline ist shorted by R1,C1. The Noise coming from the DUT is ac-coupled to the BNC connectors to be measured by the Analyser. And the inductor L1 avoids shorting the DUT Noise by R1,C1.



A look inside ... not much there

The isolation from the powerline works as expected below approx. 5 MHz. Then the situation worsens, as stray coupling of L1 causes higher frequencies to pass. But overall an isolation better than 20 dB up to 30 MHz is achieveable.

The output coupling works nice over the entire frequency range from 100 kHz to 50 MHz.
Loss is below 1 dB.

An isolation transformer is used during EMC measurements to block external power-line noise, prevent ground loops, and safely decouple the test setup from the local building earth. This ensures you only measure the true emissions or immunity of your device without interference from the broader electrical grid.

We designed an airplane - friendly device which you can inspect here. (SOON)

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