Feedthrough capacitors are core EMI filtering components widely used at chassis boundaries, connector interfaces and shielded enclosure walls. For OEM engineers responsible for component evaluation and specification, insertion loss (IL) serves as the primary performance benchmark — yet it is also widely misinterpreted.
Feedthrough capacitor insertion loss represents the reduction in unwanted RF or EMI energy after the component is installed in a transmission path. It is normally expressed in decibels across frequency and is commonly measured under standardized source and load impedance conditions.
This article clarifies the definition of insertion loss, standard measurement procedures, the root causes of discrepancies between datasheet curves and real-circuit performance, and practical guidelines for selecting feedthrough capacitors with realistic performance expectations.
What Is Insertion Loss in EMI Filtering?
Insertion loss quantifies the degree to which a filter suppresses noise signals flowing through a circuit, measured against the signal amplitude when the filter is removed entirely. The unit is decibel (dB), and in RF and EMI testing, insertion loss is calculated from the two-port S21 scattering parameter.
A higher insertion loss at a specific frequency means better noise attenuation at that band. However, insertion loss varies drastically with frequency instead of being a fixed constant. A feedthrough capacitor offering excellent filtering at 100 MHz may deliver weak suppression at 1 GHz or 10 kHz. Engineers must always evaluate the full frequency curve rather than relying on isolated single-point data.
Critical preface: Insertion loss is not an intrinsic fixed index of the component itself. Identical feedthrough capacitors yield different IL results under varying source/load impedance, mounting methods and test configurations. This key factor will be expanded on in later sections.
Why Feedthrough Capacitors Behave Differently from Standard Capacitors
Standard two-terminal MLCCs and feedthrough capacitors adopt fundamentally distinct geometric structures, and this structural gap directly leads to divergent high-frequency filtering performance.
Three-terminal construction. In a feedthrough capacitor, the signal conductor passes through the capacitor body, with the capacitor itself providing a low-impedance bypass path to the chassis or reference ground. This is different from a standard two-terminal capacitor, where the signal path and the ground path share more of the same physical route.
Reduced equivalent series inductance (ESL). The feedthrough layout streamlines and shortens ground return paths, delivering much lower ESL than equivalent two-terminal MLCCs. ESL is the primary factor that weakens high-frequency noise attenuation. This explains why feedthrough capacitors sustain stable insertion loss at far higher frequencies than standard MLCCs with identical capacitance.
Reliant on high-quality grounding. The above performance edge is not inherent to the component alone; it relies heavily on solid, low-impedance mounting to chassis ground. A feedthrough capacitor fitted with long, resistive ground leads cannot achieve its theoretical maximum attenuation performance.
| Characteristic | Standard Two-Terminal Capacitor | Feedthrough Capacitor |
| Ground path | Share path with signal routing | Dedicated, typically shorter path to chassis |
| Parasitic inductance (ESL) | Generally higher | Generally lower |
| High-frequency attenuation | Degrades earlier with frequency | Tends to extend further into high frequency |
| Installation sensitivity | Lower | Higher — grounding quality matters more |
How Insertion Loss Is Measured
Insertion loss data listed on component datasheets are captured under standardized ideal test environments, typically 50 Ω source and 50 Ω load impedance. This test method follows MIL-STD-220C, the standard governing insertion loss testing for EMI filters.
This test configuration is useful for comparing components on a consistent, repeatable basis. However, it does not represent the impedance environment of most real circuits. Actual source and load impedances in power supplies, motor drives, or signal lines are frequently far from 50Ω and can vary with frequency, load condition, and circuit topology.
This mismatch creates the most widespread confusion among hardware engineers: a feedthrough capacitor with excellent attenuation in a 50 Ω test fixture often delivers inconsistent insertion loss inside end equipment. This does not mean the component is faulty or datasheet data is misleading. Insertion loss depends on the surrounding impedance network rather than being a fixed inherent characteristic of the capacitor alone.
Key Practical Measurement Notes:
- Failure to de-embed test fixtures and RF cables will skew insertion loss results, particularly at high frequencies.
- The mounting style used for testing (flange soldering, threaded installation, direct soldering) must match mass-production assembly. Inconsistent mounting is a frequent reason for gaps between bench data and real-device performance.
- The integrity of the ground reference in the test setup directly shapes the measured attenuation curve.
Key Factors That Affect Real-World Insertion Loss
| Factor | Effect on Insertion Loss |
| Source/load impedance mismatch (vs. 50Ω test condition) | Measured IL in actual circuit can differ substantially from datasheet curve |
| Mounting method (flange, threaded, solder) | Lower ground impedances generally improve high-frequency IL |
| Frequency range relative to self-resonant frequency (SRF) | Below SRF, capacitance dominates; above SRF, the device behaves inductively and IL performance changes |
| Temperature and DC bias (dielectric-dependent, e.g., X7R-type materials) | Can shift effective capacitance, which in turn shifts the IL curve, particularly at lower frequencies |
| Rated current / ripple current | Not a direct IL factor, but affects long-term stability and thermal margin, which can indirectly influence performance over time |
Two critical guidelines for design and component procurement:
- Larger capacitance does not guarantee better insertion loss.Capacitance dominates attenuation performance at low frequencies. At high frequencies, parasitic inductance and the component’s self-resonant frequency exert far greater influence than nominal capacitance.
- Impedance characteristics reverse once the operating frequency exceeds SRF.Above this threshold, component impedance no longer falls with rising frequency and instead climbs upward. This transition creates a clear inflection point and noticeable attenuation drop on datasheet insertion loss curves.
How to Read a Datasheet Insertion Loss Curve
When evaluating manufacturer-supplied insertion loss plots, analyzing the full curve trend across the entire frequency spectrum delivers far more useful insight than comparing isolated single-frequency data points between different components.
Common Pitfalls to Avoid When Evaluating Insertion Loss Curves:
- Judging components solely by a single frequency point.Two feedthrough capacitors may deliver matching insertion loss at 30 MHz, yet their performance diverges drastically at 300 MHz or 1 GHz due to structural differences.
- Taking for granted that datasheet test conditions match your circuit.Verify whether the attenuation curve was measured under standard 50 Ω / 50 Ω setup following MIL-STD-220C, or custom impedance conditions.
- Overlooking the mounting configuration used for the test.A curve generated with a flange-mounted, low-impedance ground connection will not necessarily represent performance with a different mounting method.
Selection Guide: Matching Insertion Loss Performance to Your Application
Since insertion loss varies heavily based on circuit operating conditions, component selection must prioritize real-world application demands instead of simple one-to-one spec comparison.
| Application Factor | Key Evaluation Items |
| Power line vs. signal line filtering | Rated current and voltage margins for power lines; signal integrity/bandwidth impact for signal lines |
| Target noise frequency range | Whether the relevant noise band falls below or near the part’s self-resonant frequency |
| Installation/mounting constraints | Whether the intended mounting method matches the method used in the manufacturer’s test data |
| Certification or standard requirements (e.g., MIL-PRF-28861 for RF/EMI filter devices) | Whether the specific part has documented qualification to the referenced specification — this should be confirmed directly with the supplier rather than assumed from general product category descriptions |
| Environmental conditions | Temperature range and, for ceramic dielectrics, DC bias effects on effective capacitance |
LCA supplies solder-in, threaded and miniature feedthrough capacitor configurations for different mounting, voltage, current and environmental requirements. Engineers should compare insertion-loss curves together with mounting structure, capacitance, rated voltage and mechanical dimensions rather than selecting by attenuation alone.
Conclusion & Engineering Checklist
Insertion loss is an essential benchmark for evaluating feedthrough capacitors, yet it cannot be treated as an absolute value independent of operating scenarios. All datasheet attenuation curves must be analyzed alongside their defined test conditions. Before locking in a feedthrough capacitor model, engineers should complete the following checks:
- Are the datasheet insertion loss plots measured under standard 50 Ω source / 50 Ω load? Can this impedance setup approximate your real circuit environment?
- Does the mounting method in the test data match your intended production mounting?
- Is your dominant noise frequency band below or close to the component’s self-resonant frequency (SRF)?
- Have temperature and DC bias effects on capacitance been considered for your operating conditions?
- If a military or industry specification is referenced, has qualification to that specification been confirmed directly with the supplier?
Frequently Asked Questions
Q1: What is a good insertion loss value for a feedthrough capacitor? There is no single universal target value. The appropriate insertion loss depends on the applicable EMI requirement, the specific noise frequency range of concern, and the actual impedance environment of the circuit.
Q2: Why does my measured insertion loss differ from the datasheet? This is most often explained by differences between the datasheet’s test conditions (commonly 50Ω/50Ω) and your circuit’s actual source/load impedance, as well as differences in mounting and grounding between the test setup and your installation.
Q3: Does higher capacitance always mean higher insertion loss? Not necessarily. At higher frequencies, parasitic inductance and the self-resonant frequency of the part tend to have more influence on insertion loss than the nominal capacitance value.
Q4: How does mounting method affect insertion loss? Mounting methods that provide a lower-impedance ground connection (such as flange or threaded mounting, depending on the part) generally support better high-frequency insertion loss than mounting approaches with a longer or higher-impedance ground path. The specific effect depends on the part and installation.
Q5: Can feedthrough capacitors be used for both power and signal lines? Yes, feedthrough capacitors are used in both contexts, but the selection criteria differ — power line applications require attention to rated current and voltage, while signal line applications require attention to bandwidth and signal integrity impact.
Next Steps
Selecting the right feedthrough capacitor depends on your specific frequency range, impedance environment, and mounting constraints — details that are difficult to generalize from a datasheet curve alone. If you’re evaluating options for a current design or comparing suppliers, our application engineering team can help review your specific requirements and test conditions.
