EMC Filters Comparison Part II: π and T Filters

In the previous article, the CL and LC filters were discussed. In this article, the performance of the π and T filters is evaluated and compared to that of the CL and LC filters.


π and T Filter Configurations

Low-pass π and T filters are shown in Figure 1.

Figure 1: π and T low-pass filters


The performance of these filters will be evaluated in the configurations where the source impedance is low (50 Ω) and the load impedance is high (1kΩ), as shown in Figures 2 and 3 (these are the same configurations that were evaluated in [1] for CL and LC filters).

 

Figure 2: π and T filters: source impedance low and load impedance high

 

Figure 3: T and π filter configurations – low impedance source, high impedance load


Verification via Simulations and Measurements

π and T Filter Comparison

Figure 3 shows the LT spice simulation schematic. The 50 Ohm source impedance is provided by the network analyzer at Port 1. The measurement made by the network analyzer at Port 2 is across its internal 50 Ohm impedance. The filter configurations are tested with 1 kΩ impedance on the load side.

Figure 4 shows the insertion loss of the two filter configurations.

Figure 4: Insertion loss of the two configurations shown in Figure 3


As can be seen from Figure 4, the π filter outperforms the T filter (except for a small range of frequency around the resonance point at 1 MHz). The insertion loss of the π filter at 10 MHz is about 29.5 dB higher than that of the T filter. This is consistent with the general rule that the inductor should be placed on the low-impedance side and the capacitor on the high-impedance side.

To verify the simulations results the measurement setup shown in Figure 5 was used.

Figure 5: EMC filter VNA measurement setup


Since a four-channel network analyzer was used, we could evaluate the two different filter configurations simultaneously. Figure 6 shows a close-up of the two PCB filter boards used in this measurement.

Figure 6: T and π filters used for measurements


Figure 7 shows the measurement results for the two configurations shown in Figure 3 and simulated in Figure 4.

Figure 7: Insertion loss (s21 and s34) measurements of the two configurations shown in Figure 3


The measurement results are consistent with the simulation results. In the frequency range 100 kHz – 10 MHz the simulated and measured results are remarkably close, as summarized in Tables 1 and 2.

Table 1: Simulated and measured insertion loss for π filter

 

Table 2: Simulated and measured insertion loss for T filter


π and LC Filter Comparison

Next, let’s compare the insertion loss of the π filter with the insertion loss of the LC filter discussed in Part I [1]. Figure 8 shows the LT spice simulation schematic.

Figure 8: LC and π filter configurations – low impedance source, high impedance load


Figure 9 shows the insertion loss of the two filter configurations.

Figure 9: Insertion loss of the two configurations shown in Figure 8


As can be seen from Figure 9, the π filter outperforms the LC filter (except for a small range of frequency around the resonance point at 1 MHz). The insertion loss of the π filter at 10 MHz is about 29.78 dB higher than that of the LC filter.

Figure 10 shows the measurement results for the two configurations shown in Figure 8 and simulated in Figure 9.

Figure 10: Insertion loss (s21 and s34) measurements of the two configurations shown in Figure 8

The measurement results are consistent with the simulation results. At 10 MHz the difference between the simulated insertion losses of the two filters is 29.78 dB which is close to the measured difference of 28.02 dB.

The insertion loss curve of the LC filter looks similar to the insertion loss curve of the T filter. Let’s compare these two filters.


T and LC Filter Comparison

Figure 11 shows the LT spice simulation schematic.

Figure 11: LC and T filter configurations


Figure 12 shows the insertion loss of the two filter configurations.

Figure 12: Insertion loss of the two configurations shown in Figure 11


Figure 13 shows the measurement results for the two configurations shown in Figure 11 and simulated in Figure 12.

Figure 13: Insertion loss (s21 and s34) measurements of the two configurations shown in Figure 11


It is apparent that the LC and T filter insertion losses are very similar. Since the LC filter contains one fewer inductor, it should be chosen over the T filter.


Conclusions of the filter studies in Part I [1] and Part II

Note: These conclusions are based on the filter study with the source impedance of 50 Ω, load impedance of 1 kΩ, and the component values L = 4.7 µH, C = 10 nF.

  1. LC filter outperforms the CL filter
  2. π filter outperforms T filter
  3. π filter outperforms LC filter
  4. Insertion losses of the LC filter and T filters are virtually identical


References

  1. Bogdan Adamczyk and Dimitri Haring, “EMC Filters Comparison Part I: CL and LC Filters,” In Compliance Magazine, January 2020.
  2. Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.
  3. https://link.springer.com/content/pdf/10.1007%2F978-3-642-27326-1_90.pdf


Dr. Bogdan Adamczyk
is professor and director of the EMC Center at Grand Valley State University (http://www.gvsu.edu/emccenter) where he develops EMC educational material and teaches EMC certificate courses for industry. He is an iNARTE certified EMC Master Design Engineer. Prof. Adamczyk is the author of the textbook “Foundations of Electromagnetic Compatibility with Practical Applications” (Wiley, 2017). He can be reached at adamczyb@gvsu.edu.

Brian Gilbert is a graduate assistant at Grand Valley State University. He works with Dr. Adamczyk and GVSU’s EMC Center to develop EMC educational content. He received his B.S.E. in Electrical Engineering from GVSU, and now pursues his M.S.E. in Electrical and Computer Engineering at GVSU. His interests include EMC and Signal Integrity in Embedded Systems, FPGAs and High-Speed PCB design.

About The Author

Bogdan Adamczyk

Dr. Bogdan Adamczyk is professor and director of the EMC Center at Grand Valley State University (http://www.gvsu.edu/emccenter/) where he regularly teaches EMC certificate courses for industry. He is an iNARTE certified EMC Master Design Engineer. Prof. Adamczyk is the author of the textbook “Foundations of Electromagnetic Compatibility with Practical Applications” (Wiley, 2017) and the upcoming textbook “Principles of Electromagnetic Compatibility with Laboratory Exercises” (Wiley 2022).

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