This is the first article of a three-article series devoted to the correlation between the insertion loss and input impedance of passive EMC filters. In this article, we focus on LC and CL filters. Based on the analysis, simulation, and measurement results, it has been observed that the frequencies where the insertion losses of the filters are equal are also the frequencies where the input impedances are equal. These frequencies define the regions where one filter configuration outperforms the other (with respect to the insertion loss). To determine these regions analytically, we compare the input impedances of the two filters. Subsequent articles will focus on π and T filters and cascaded LC and CL configurations.

**1. Insertion Loss Definition**

Consider the circuit shown in Figure 1.

Insertion loss of a filter can be defined as

(1a)

where *V _{L}* is the magnitude of the complex voltage

*. Since*

_{L}*V*>

_{L,without filter}*V*the insertion loss defined by Eq. (1a) is a positive number in dB. The insertion loss could alternatively be defined as [1],

_{L,with filter}(1b)

In this case, the insertion loss in dB is the negative of the loss defined in Eq. (1b). We will use this definition when plotting the simulation results and comparing the simulation results to the VNA measurements.

**2. Input Impedance to the LC Filter**

The input impedance, * _{IN}*, to the

*LC*filter is calculated from the circuit shown in Figure 2.

(2a)

or

(2b)

or, in terms of the frequency

(3a)

The magnitude of the input

impedance is

(3b)

**3. Input Impedance to the CL Filter**

The input impedance, * _{IN}*, to the

*CL*filter is calculated from the circuit shown in Figure 3.

(4a)

or

(4b)

or, in terms of the frequency

(5a)

The magnitude of the input impedance is

(5b)

**4. LC Filter vs. CL Filter – Input Impedance – Simulations and Calculations**

Figure 4 shows the simulation circuit used for the comparison of input impedances.

The simulation results are shown in Figure 5.

Note that the two input impedances are equal at the frequency of 1.0349 MHz. Figure 5 also shows the resonant frequency of the LC filter at 733.16 kHz. This result is confirmed by the calculations in Equation 6.

(6)

Next, let’s calculate the frequency at which the input impedances of the two filters are equal. Equating the expressions in equations (3b) and (5b) produces

(7)

This equation can be solved for ω, [2], resulting in

(8a)

or

(8b)

Which agrees with the simulation result shown in Figure 5.

**5. LC Filter vs. CL Filter – Insertion Loss – Simulations and Measurements**

Figure 6 shows the simulation circuit used for the comparison of insertion losses.

The simulation results are shown in Figure 7.

The insertion losses of the two filters are equal at the frequency of 1.04 MHz. This is the same frequency at which the input impedances of the two filters were equal! Note that up to the frequency of 1.04 MHz the insertion loss of an *CL* filter is larger (in the absolute sense) than that of the *LC* filter. Beyond that frequency the insertion loss of the *LC* filter is larger. This means that the *LC* filter is more effective than the *CL* filter beyond the frequency of 1.04 MHz.

We have arrived at a very important observation: once the filter components values *L* and *C* are chosen, we can determine the frequency at which the insertion losses of *LC* and *CL* filters are equal. This is the frequency at which the input impedances are equal and given by Eq. (8b).

To verify the simulations results of the insertion loss the measurement setup shown in Figure 8 was used.

The measurement results are shown in Figure 9.

Note that the measurement results agree with the calculated and simulated results.

**References**

- Bogdan Adamczyk and Dimitri Haring, “EMC Filters Comparison Part I: CL and LC Filters,”
*In Compliance Magazine*, December 2019. - Bogdan Adamczyk,
*Principles of Electromagnetic Compatibility: Laboratory Exercises with Lectures*, Wiley, 2024.