What is a transient limiter?

A transient limiter is a protective device used during EMC-conducted emission testing to protect the sensitive RF input circuitry of a spectrum analyzer or EMI measurement receiver from high-level transients/transient spikes coming from the Line Impedance Stabilization Network (LISN). LISN is a device used in EMC-conducted emission testing (figure). If the transient limiter is not used, the transients that come from the LISN will damage the input circuitry of the RF input circuitry of the spectrum analyzer or EMI measurement receiver. The transient limiters can also be used in other measurements where input signal levels are unpredictable or accidental overloads may occur.

Basic EMC Conducted Emission test setup

Functionally, a transient limiter product typically incorporates HPF (high pass filter) followed by an attenuator and peak level limiter. When connected in series with the coaxial line connecting the LISN to the Spectrum Analyzer or EMI receiver, the transient limiter blocks/attenuates potentially harmful, short-duration, high-level transients coming from the RF port of LISN, thereby protecting the input of spectrum analyzer or EMI receiver. The high pass filter helps to suppress the harmonics of the main supply voltage or attenuates out-of-band signals. 

Key specification details of transient limiter:

Frequency: Represents the operating frequency range of (KHz to MHz) the transient limiter.

Attenuation: It represents the attenuation level (in dB) provided by the transient limiter.  

Input power: It represents the input power level (in W) that can be handled by the device.

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