EMC Analysis: How to Calculate Filter Insertion Loss

Filter insertion loss is a measure of how well an EMI filter attenuates a signal as it passes through the filter. Normally expressed in decibels, filter insertion loss is the ratio of the input signal to the output signal. For additional background information, see our article, “EMI Filter Insertion Loss.”

When performing system EMC analysis, EMI Analyst readily accounts for the complex interaction of EMI filters, as well as cabling, parasitics effects, shields, resonances, and a bunch of other subtle effects.

However, what if you just want to calculate the attenuation of the EMI filter? EMI Analyst can do that too. Here’s how.

Filter Schematic

Let’s say we want to calculate insertion loss for a power line EMI filter that contains both differential mode and common mode components, for example, the filter shown schematically below. Its topology is typical of many off-the-shelf filter modules.

The filter contains a two-pole LC differential mode section and a two-pole LC common mode section. Short circuit corner frequencies for the filter are 50 kHz differential mode and 159 kHz common mode. However, when measured in a 50 ohm system, the corner frequencies shift, due to the loading effects of the source and load impedances.

Standard EMI Filter Measurements

EMI filters are measured by connecting a signal source across the filter input terminals and then measuring the signal amplitude across the output terminals. Differential mode insertion loss and common mode insertion loss are measured separately.

Because radio frequency test equipment normally has input and output impedances that are 50 ohm, the filter is said to be measured in a 50 ohm system. The 50 ohm source and load need to be simulated when calculating filter insertion loss with EMI Analyst so that simulation data is comparable to measured results.

– Differential Mode
Differential mode insertion loss is the attenuation of a signal applied across the filter input terminals, (I+) and (I-), as shown in the left-hand diagram above. The attenuated signal appears across the output terminals, (O+) and (O-).

– Common Mode
Common mode insertion loss is the attenuation of a signal applied between chassis ground and the shorted input terminals, (I+) and (I-), as shown in the right-hand image above. The attenuated signal appears between the shorted output terminals, (O+) and (O-), and chassis ground.

EMI Analyst Insertion Loss

Filter insertion loss can be calculated using two different methods in EMI Analyst. It can be done as a conducted emissions simulation using CE Analyst or as a conducted susceptibility simulation using CS Analyst.

We use CS Analyst here, but either method is equally effective. CS Analyst is a conducted susceptibility program within EMI Analyst. It simulates what happens to the circuitry when signals couple to cabling conductors.

– Analysis Method
The analysis approach is to inject a known voltage on the lines connected to the filter input and then compute the signal level on the filter output terminals. For differential mode (DM) we inject 1 volt in series with each input line, but with opposite polarity, as shown in the left-hand diagram above, applying net 2 volts total. For common mode (CM) we inject 2 volts on both input lines simultaneously, as shown in the right-hand diagram above.

At low frequencies, where the filter provides little filtering, the applied voltage divides equally across the 50 ohm source impedance and 50 ohm load impedance, 1 volt across each. As frequency increases, insertion loss increases and the voltage at the load decreases. The ratio of the output voltage to the input voltage gives us the filter insertion loss. In decibels (dB),

Setup Steps
The steps for calculating filter insertion loss using CS Analyst are quite simple.

– Differential Mode
1. Frequency Range: 1 kHz to 100 MHz
2. CS Limit: 2 volts
3. Threshold: No Threshold
4. Left-Hand Circuit: 50 ohm line-to-line, no chassis connection
5. Conductors: Wire pair over ground plane or wire pair, minimum length
6. Right-Hand Circuit: Contains filter schematic and 50 ohm line-to-line.

Calculated insertion loss shows the DM corner frequency shifted from 50 kHz to 11 kHz, as expected.

– Common Mode
1. Frequency Range: 1 kHz to 100 MHz
2. CS Limit: 2 volts
3. Threshold: No Threshold
4. Left-Hand Circuit: Short circuit line-to-line, 50 ohm line-to-chassis
5. Conductors: Wire pair over ground plane or wire pair, minimum length
6. Right-Hand Circuit: Contains filter schematic, Short circuit line-to-line, 50 ohm line-to-chassis

Calculated insertion loss shows the CM corner frequency shifted from 159 kHz to 200 kHz, as expected.

Wrap Up

The results above are for a filter having ideal components. A real filter has parasitic elements that are an unavoidable byproduct of physical inductors and capacitors and the packaging methods used to mount and enclose the filter. EMI Analyst has full capability to model these second order effects, so whether you are simply calculating filter insertion loss or modeling electromagnetic compatibility for a cable-connected circuit, simulation results reflect real world circuit behavior over the entire frequency range.

For more information, check out EMI Analyst at http://www.emisoftware.com.