The proposed circuit is a Highpass (3.5 kHz) with some Protection

When investigating in EMI Pollution on Powerlines, there are two possible approaches : A step-down transformer or a Highpass. As the step-down transformer attenuates the Noise as well, we decided to attenuate the 50 Hz (60 Hz) by using a Highpass. Assuming a Scope with an Input Impedance of 1 MΩ the Cutoff-Frequency is approx. 3.5 kHz.

Assuming a Powerline Impedance of 1 Ω the Attenuation at 50 Hz is 90 dB at the Scope.

The Circuit can be assembled to monitor the Phase as well as the Neutral Line. We investigate the Functionality when monitoring the Phase Line.

The Input coupling Capacitor is 2 x 1 nF, +/- 10% 3 kV (1808B102K302CT) in Series. This results in an effective Capacitance of 500 pF. The Protection of the Scope is done with two P6KE24A (TVS DIODE), in a back to back configuration. Parallel to that is an 4532-075-LF (Gas Discharge Tube, 75 V).

The Transient Voltage Suppressor Diodes (P6KE24A) limits the Voltage to approx. 25 V. This is lower than the maximal Input Voltage of the Scope. And this is higher than the attenuated Powerline.

Case is Crown Royal LMB CR #321.

The Scope says ... it's safe to connect the Spectrum Analyzer



The Spectrum Analyzer says ... some Noise here

Finding the evil device can simply be done by disconnecting suspect devices, until the Spectrum clears up.

Once, the EMI Polluter is identified, one can start in combatting the Noise generated by it. Very promising are EMI-Filters, like those from e.g. Schaffner. They promise high attenuation for common and differential mode between 10 kHz up to 300 MHz.



EMI Lowpass, a standard configuration

The EMI Lowpass filter functions like two low-pass filters. One is to attenuate common mode interference, and the other one is to attenuate differential mode interference.

The approach to succesfully silence your noisy load is empirically. You'll have to try several filters in order to find the best solution. In some serious conditions, the use of multiple filters deems recommended.

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