Introduction
In our previous article, How to Choose the Right Feedthrough Capacitor for EMI Suppression, we introduced the fundamentals of feedthrough capacitors and their role in reducing electromagnetic interference (EMI).
However, in real EMC projects, selecting a feedthrough capacitor is rarely as simple as choosing a capacitance value from a catalog.
Many EMC issues are aggravated when feedthrough capacitors are selected based only on capacitance or voltage rating, while factors such as noise frequency, insertion loss characteristics, grounding impedance, installation location, and overall filter topology are not fully considered.
This guide focuses on how EMC engineers evaluate and select feedthrough capacitors for industrial, aerospace, medical, RF, and high-frequency applications.
Why Feedthrough Capacitor Selection Often Fails
According to Henry W. Ott’s Electromagnetic Compatibility Engineering, filter performance is heavily influenced by installation practices and overall system design, not just component specifications.
In many failed EMC tests, engineers discover that:
- The capacitance value is not appropriate for the actual noise spectrum.
- The insertion loss performance does not align with the dominant interference frequency.
- The feedthrough capacitor is installed too far from the enclosure entry point.
- Ground impedance is higher than expected.
- The selected filter topology does not adequately address the noise source.
Successful EMI suppression requires understanding both the characteristics of the interference and the behavior of the filtering solution.
Step 1: Identify the Noise Frequency Before Selecting a Capacitor
The first question should never be:
“What capacitance value should I use?”
Instead, ask:
“What frequency range am I trying to suppress?”
Different electronic systems often generate EMI in different frequency regions, but the actual interference spectrum can vary significantly depending on switching topology, PCB layout, cable routing, grounding structure, operating conditions, and system architecture.
The ranges below should therefore be considered general engineering references rather than fixed design limits.
| EMI Source | Typical Frequency Region |
| Switching Power Supplies | Often concentrated in the low MHz range |
| Motor Drives & Inverters | Commonly span from hundreds of kHz into tens of MHz |
| Industrial PLC Systems | Frequently extend into higher MHz regions |
| RF Communication Equipment | Typically involve frequencies above 100 MHz |
| Radar & Aerospace Electronics | Often operate into the GHz range |
EMC engineering studies and practical industry experience consistently show that filter effectiveness depends on matching attenuation performance to the actual interference frequency.
Without understanding the noise spectrum, feedthrough capacitor selection becomes largely guesswork.
Step 2: Read the Insertion Loss Curve, Not Just the Datasheet
One of the most common selection mistakes is comparing capacitance values while overlooking insertion loss performance.
Insertion loss is one of the key indicators used to evaluate EMI attenuation performance.
Two feedthrough capacitors with similar capacitance values may exhibit different insertion loss characteristics depending on:
- Internal construction
- Electrode geometry
- Mounting configuration
- Grounding effectiveness
- Test conditions
Therefore, engineers should evaluate insertion loss curves rather than relying solely on nominal capacitance values.
When reviewing a feedthrough capacitor, attention should typically be given to:
- Insertion loss at 1 MHz
- Insertion loss at 10 MHz
- Insertion loss at 100 MHz
- High-frequency attenuation performance for RF applications
Insertion loss data provides a much clearer indication of real EMI suppression capability than capacitance alone.
Step 3: Match the Capacitor to the Application
Industrial Automation
Industrial control cabinets, PLC systems, servo drives, and VFD equipment often generate broadband conducted EMI.
Recommended priorities include:
- Effective attenuation across the relevant interference spectrum
- Reliable grounding
- Robust mounting structure
- Long-term durability in industrial environments
Medical Equipment
Medical devices must comply with IEC 60601-1-2 EMC requirements.
Selection priorities typically include:
- Low leakage current
- Stable electrical performance
- Long-term reliability
- Regulatory compliance
Aerospace and Defense Systems
Aerospace electronics commonly require compliance with MIL-STD-461.
Engineers often prioritize:
- High-frequency attenuation capability
- Vibration resistance
- Extended temperature operation
- High reliability under harsh environmental conditions
RF and Communication Equipment
RF systems are particularly sensitive to parasitic inductance and unwanted signal coupling.
Discoidal feedthrough capacitors are often preferred in RF and high-frequency applications because their geometry can help reduce parasitic inductance and improve attenuation performance at higher frequencies.
In addition to the application scenarios discussed above, feedthrough capacitors and feedthrough filters are also widely used in a variety of industrial, RF, aerospace, and harsh-environment applications. The table below provides a general reference for matching typical applications with commonly used product types:
| Typical Application | Product Type |
| PLC, VFD, Industrial Cabinets | Thread Mount Feedthrough Filters |
| PCB Electronics | Solder-In & Press-In Feedthrough Filters |
| General EMI Filtering | Tubular Capacitors |
| RF and Radar Systems | Discoidal Capacitors |
| Aerospace and Harsh Environments | Hermetically Sealed Filters |
| Microwave and RF Applications | Miniature RF Filters |
Step 4: Evaluate Installation Conditions
Even a high-performance feedthrough capacitor may not deliver the expected results if installed improperly.
Recommended installation practices include:
- Install the filter directly at the enclosure penetration point
- Minimize conductor length
- Maintain a low-impedance grounding path
- Ensure continuous bonding between panels and shields
Poor grounding, excessive conductor length, or improper mounting can significantly reduce real-world EMI attenuation performance.
According to established EMC design practices, installation quality is often just as important as the electrical specifications of the filter itself.
When Is a Feedthrough Capacitor Enough, and When Is Additional Filtering Needed?
A feedthrough capacitor is a mechanical and electrical configuration rather than a specific filter topology.
Depending on the design, feedthrough filters may incorporate:
- C filtering
- LC filtering
- Pi filtering
- T filtering
In many applications, a feedthrough filter itself already functions as a complete EMI filter assembly.
However, systems with exceptionally high conducted emissions, multiple noise sources, or demanding compliance requirements may require additional filtering elements, such as:
- Common-mode chokes
- Ferrite components
- Multi-stage LC filter networks
- Customized EMI filter assemblies
The appropriate solution depends on the interference characteristics and system-level EMC objectives.
LCA Engineer’s Feedthrough Capacitor Selection Checklist
Before selecting a feedthrough capacitor, verify:
✓ Dominant noise frequency identified
✓ Required insertion loss determined
✓ Voltage rating verified
✓ Current rating verified
✓ EMC standard identified
✓ Grounding structure reviewed
✓ Installation location confirmed
✓ Environmental requirements evaluated
Following this process can significantly improve first-pass EMC compliance success rates and reduce costly redesign cycles.
Conclusion
Successful feedthrough capacitor selection is not about choosing the largest capacitance value available.
It is about matching filter performance to the actual EMI challenge.
By evaluating noise frequency, insertion loss characteristics, installation conditions, and application-specific EMC requirements, engineers can improve conducted EMI suppression and enhance overall system reliability.
Need Help Selecting the Right Feedthrough Capacitor?
Every EMI challenge is different.
LCA’s engineering team can help evaluate your noise spectrum, EMC requirements, and installation environment to recommend the most suitable feedthrough capacitor, feedthrough filter, or customized EMI filtering solution.
Contact LCA today for one-to-one technical support and professional EMI suppression recommendations.
References
[1] Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.
[2] Clayton R. Paul, Introduction to Electromagnetic Compatibility, Wiley, 2006.
[3] IEEE Transactions on Electromagnetic Compatibility, research on conducted EMI suppression and insertion loss measurement.
[4] IEC 60601-1-2 Medical Electrical Equipment – Electromagnetic Compatibility Requirements.
[5] MIL-STD-461G Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment.
