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Passive EMI Filter Frequency-Response Calculator

Use complex ABCD matrices to calculate loaded-voltage insertion loss, transfer, and port impedance for C, L, LC, pi, T, and order-2–5 low-pass ladders.

FORMULAv1.2.0
INPUT PARAMETERS

Enter filter topology and termination conditions

Source/load resistance and all parasitic parameters must be entered by the user; no unsourced defaults are provided.

FILTER DESIGN ADesign A

L1 series inductor

C1 shunt capacitor

Curve sweep rangeDisplay setting; does not change the current-frequency calculation
CALCULATION RESULT

Calculation result

Calculated locally
∿

Enter the parameters and run the calculation to see results and model assumptions here.

FORMULA & TRACEABILITY

Formulas and applicability

Formulas, variables, and model boundaries are published. References last reviewed on 2026-08-30.

4 formulas · 4 references
CORE EQUATIONS

Core equations

These equations directly produce the primary results and define the tool’s core model.

2
F01

ABCD chain parameters for series and shunt elements

M_series=[1 Z; 0 1], M_shunt=[1 0; Y 1], M_total=ΠM_k

把元件按从源端到负载端的实际顺序相乘，以一个复数二端口描述完整滤波网络；通用低通梯形仅允许交替的串联L与并联C。

Z
Series-element complex impedanceΩ
Y
Shunt-element complex admittanceS
M_total
Total filter-network ABCD matrix—
Applicability
  • Linear time-invariant, small-signal sinusoidal steady state
  • A lumped model is used; element order must not be exchanged
  • 通用低通梯形限2–5个元件，且只支持交替的串联电感与并联电容
F02

Voltage transfer with source and load impedance

H=V_L/V_S=1/(A+B/Z_L+Z_S·C+Z_S·D/Z_L)

Calculate complex voltage transfer under the actual terminations directly from ABCD parameters and source/load resistance.

Z_S
Source resistanceΩ
Z_L
Load resistanceΩ
H
Ratio of load voltage to Thevenin source voltage1
Applicability
  • Source and load are purely resistive in this version
  • Voltage and phase use the same sinusoidal steady-state phasor convention
DERIVED & CONVERSION

Derived and conversion equations

Derive units, levels, and supporting engineering quantities from the core values.

1
F03

Loaded-voltage insertion loss relative to a no-filter reference

IL=20log₁₀(|H_0|/|H|), |H_0|=R_L/(R_S+R_L)

Compare load voltage before and after inserting the filter with identical source and load resistance. Positive means attenuation and negative means voltage peaking. This is neither termination-independent power insertion loss nor vendor S21.

IL
Insertion lossdB
H_0
No-filter voltage-transfer reference1
H
Voltage transfer after filter insertion1
Applicability
  • Source and load remain unchanged before and after insertion
  • This is a voltage-ratio definition and cannot be interpreted without its terminations
BOUNDARIES & RULES

Boundary and rule equations

Check model applicability, measurement conditions, and regulatory rules.

1
F04

First-order capacitor and inductor parasitic models

Z_C=ESR+j(ωESL−1/ωC), Z_L=(DCR+jωL) ∥ 1/(jωC_p)

Include capacitor ESR/ESL and inductor DCR/parallel parasitic capacitance to show the first self-resonance impact on filter response.

C_p
Inductor equivalent parallel parasitic capacitanceF
DCR
Inductor winding DC resistanceΩ
ESR / ESL
Capacitor equivalent series resistance/inductanceΩ / H
Applicability
  • Parameters are treated as constant across the sweep
  • First-order lumped approximation covering only the first self-resonance
REFERENCES

References

01Keysight Technologies · Application Note 5992-2693S-Parameters and Two-port MeasurementsOfficial application material · Describes matrix representations of two-port networks and the role of T/ABCD chain parameters in network analysis.
02Murata Manufacturing · EMC know-how, Chapter 6Guidelines for EMI Suppression: EMI suppression filtersOfficial technical material · States that actual noise suppression depends on surrounding circuit impedance and that 50 Ω insertion loss cannot be applied directly to arbitrary terminations.
03Murata Manufacturing · Noise Suppression Filter GuideBasics of Noise Countermeasures, Lesson 7: LC Compound-type EMI Filters2012 · Explains LC, pi, and T combinations and the principle of selecting filter structure according to input/output impedance.
04Coilcraft · Inductor application noteMeasuring Self Resonant FrequencyOfficial technical material · Explains parasitic resistance/capacitance in real inductors and modeling the first self-resonance using L in parallel with self-capacitance.
Engineering use notice

Results use the models and assumptions shown on this page for design estimates and pre-compliance risk review. Complex structures, dispersion, near-field coupling, and test setup can cause significant deviation.

FAQ

Frequently asked questions

How is EMI filter insertion loss estimated?

In an ideal 50 Ω system, a single-stage LC filter gives IL = 20·log₁₀|1 − ω²LC| (rising 40 dB/decade above cutoff). Real loss must account for source and load impedances.

Why does a filter measure worse than its datasheet?

Datasheets assume 50/50 Ω. When real source impedance (e.g., a low-impedance mains) and load impedance deviate, single-stage insertion loss can degrade by tens of dB.

Should common-mode and differential-mode filtering be designed separately?

Yes. Common-mode noise uses common-mode chokes with Y-capacitors; differential-mode noise uses X-capacitors with DM inductors. Separate CM/DM contributions before designing a fix.

PRE-COMPLIANCE SUPPORT

Need accredited testing or pre-compliance support?

These tools support design-stage estimates. For accredited EMC testing, compliance decisions, and troubleshooting, contact us.

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