Fundamental Principles of Conducted Emissions Measurement

The proliferation of electronic and electrical equipment across all industrial and consumer sectors has necessitated stringent electromagnetic compatibility (EMC) regulations. A primary EMC concern is conducted emissions—unintentional radio-frequency (RF) energy coupled back onto mains power lines. This energy can disrupt the operation of other devices connected to the same network and violate regulatory limits. The Line Impedance Stabilization Network (LISN) is the fundamental transducer enabling accurate, standardized, and repeatable measurement of these emissions. This article details the operational principles of the Teseq LISN, its critical role in the compliance testing ecosystem, and its integration with advanced measurement instrumentation such as the LISUN EMI-9KC EMI Receiver.

The Role of the LISN in Stabilizing Measurement Impedance

A core challenge in conducted emissions testing is the variable and unknown impedance presented by the AC mains network at RF frequencies. This impedance can fluctuate with location, time, and the aggregate load on the grid, rendering direct measurements non-repeatable and non-comparable. The LISN solves this by inserting a known, stable, and standardized 50 Ω impedance between the Equipment Under Test (EUT) and the mains supply at the frequencies of interest. It performs three simultaneous functions: it provides a stable RF impedance for measurement; it supplies clean, DC or AC power to the EUT; and it isolates the EUT from ambient RF noise present on the mains, ensuring the measured signals originate solely from the EUT.

Analyzing the Teseq LISN Circuit Topology and Frequency Response

A typical Teseq LISN for single-phase applications, compliant with standards such as CISPR 16-1-2, ANSI C63.4, and MIL-STD-461, employs a specific passive network topology. The line (L) and neutral (N) conductors each contain a series inductance (typically 50 µH) that presents a high impedance to RF signals from the EUT, preventing them from propagating onto the mains. A coupling capacitor (e.g., 0.1 µF) on each line shunts the RF energy to the measurement port via a 50 Ω resistor, which establishes the standardized impedance. A second, larger inductor and capacitor form a low-pass filter to decouple the mains frequency from the sensitive measurement receiver. The network’s frequency response is carefully designed to provide the specified 50 Ω impedance across the standard measurement range (e.g., 9 kHz to 30 MHz for CISPR A band, 150 kHz to 30 MHz for CISPR B band).

Table 1: Representative Teseq LISN Specifications for CISPR 16-1-2 Compliance
| Parameter | Specification |
| :— | :— |
| Frequency Range | 9 kHz – 30 MHz |
| Impedance | 50 Ω ±20% (9 kHz – 30 MHz) |
| Rated Current | 16 A, 25 A, or 100 A (model dependent) |
| Voltage Rating | 250 VAC, 50/60 Hz |
| Insertion Loss | < 0.5 dB (150 kHz – 30 MHz) |
| Compliance | CISPR 16-1-2, ANSI C63.4, MIL-STD-461 |

Integration with the LISUN EMI-9KC EMI Receiver for Precision Measurement

The LISN’s measurement port outputs the RF disturbance voltage present on the power lines. This signal is fed directly into a precision measurement instrument, such as the LISUN EMI-9KC EMI Receiver. The EMI-9KC is not a simple spectrum analyzer; it is a specialized receiver designed explicitly for EMC compliance testing per CISPR 16-1-1. It incorporates standardized detector functions (Peak, Quasi-Peak, Average, and RMS-Average) and utilizes precisely defined intermediate frequency (IF) bandwidths (e.g., 200 Hz, 9 kHz, 120 kHz). The synergy between the LISN and the EMI-9KC is critical: the LISN provides a stable, repeatable signal source, while the receiver performs the weighted, bandwidth-limited measurement that correlates with the interference potential perceived by other equipment.

The EMI-9KC’s specifications are tailored for this application. Its high sensitivity (typically better than -150 dBm) ensures detection of low-level emissions. Its wide dynamic range and pre-amplifier options allow it to handle signals from sensitive medical devices to high-power industrial drives. The receiver’s scanning speed, coupled with its fully compliant detector algorithms, enables efficient pre-compliance and full-compliance testing, significantly reducing validation time for products in the Automobile Industry or for Power Equipment manufacturers.

Application in Diverse Industrial Compliance Testing Scenarios

The operational methodology of the Teseq LISN and EMI-9KC system is universally applied, though test setups and limits vary by product standard.

• Lighting Fixtures & Household Appliances: Modern LED drivers and switch-mode power supplies in these products are prolific sources of conducted noise. Testing involves measuring disturbances on both line and neutral conductors, often requiring an artificial hand network for handheld appliances. The system verifies compliance with CISPR 15 (lighting) or CISPR 14-1 (appliances).
• Industrial Equipment & Power Tools: These devices often contain variable-speed motor drives and high-power switching circuits, generating significant broadband and narrowband noise. High-current LISNs (e.g., 100A) are employed to handle the load, and measurements focus on ensuring heavy machinery does not pollute the factory power network, per CISPR 11.
• Medical Devices & Intelligent Equipment: For patient-connected medical equipment and sensitive IoT devices (Intelligent Equipment), emissions must be extremely low to prevent mutual interference. The measurement system’s sensitivity and ability to discriminate noise from signal is paramount, guided by standards like IEC 60601-1-2.
• Automotive & Rail Transit: Component-level testing for the Automobile Industry and Rail Transit often uses a DC LISN (or AN) to evaluate disturbances on 12V/24V/48V DC supply lines, as specified in CISPR 25. The LISN/EMI-9KC system assesses components like ECUs, infotainment systems, and charging modules.
• Information Technology & Communication Equipment: Testing per CISPR 32 involves measurements on telecommunication ports in addition to mains ports. While the LISN is used for AC mains, the EMI-9KC also measures disturbances on data lines using current probes and impedance stabilization networks for asymmetric and symmetric voltages.

Advanced Measurement Considerations and System Calibration

Proper operation requires attention to several advanced factors. The grounding of the LISN chassis to the reference ground plane is critical to establish a consistent RF return path; poor grounding leads to measurement errors. The physical placement of the EUT, cabling, and the use of ferrite clamps on non-under-test cables are part of the standardized test setup defined in relevant standards to ensure repeatability.

System verification is performed using a calibrated signal source and a known 50 Ω load to validate the LISN’s insertion loss and the EMI-9KC receiver’s amplitude accuracy across its frequency range. This end-to-end calibration traceability to national standards is non-negotiable for accredited laboratory testing in fields like Aerospace and Medical Devices.

Comparative Analysis with Alternative Measurement Methodologies

While voltage probes can offer a non-intrusive measurement, they do not provide the impedance stabilization of a LISN and are thus unsuitable for standardized compliance testing. Current probes are used for measuring disturbance currents on cables but target a different coupling mechanism. The LISN remains the mandated instrument for direct measurement of conducted disturbance voltage on power lines for the majority of commercial and industrial EMC standards. The integration of a LISN with a fully compliant receiver like the EMI-9KC represents the benchmark methodology, offering unparalleled accuracy and regulatory acceptance compared to screening methods using general-purpose spectrum analyzers.

FAQ: LISN Operation and EMI-9KC Integration

Q1: Why is a 50 Ω impedance standard used in LISN design?
The 50 Ω impedance represents a compromise between power handling and signal loss for coaxial RF systems and has been adopted as the reference impedance for most RF measurement equipment. Standardizing on 50 Ω ensures that measurements taken with a LISN in one laboratory are directly comparable to those taken with a different LISN in another laboratory, as both present the same load to the EUT’s noise sources.

Q2: When testing three-phase industrial equipment, how is the LISN configuration modified?
For three-phase EUTs, multiple single-phase LISN units are deployed—one for each phase conductor and often one for the neutral. Each LISN’s measurement port is sequentially connected to the EMI receiver (e.g., the EMI-9KC) via a switch matrix, or multiple receivers are used, to characterize emissions on all relevant conductors as required by standards like CISPR 11.

Q3: Can the EMI-9KC receiver be used for pre-compliance testing without a fully compliant shielded room?
Yes, the EMI-9KC is highly effective for pre-compliance diagnostics. Its real-time spectrum analyzer functionality, fast scanning speeds, and compliant detector emulation allow engineers to identify emission sources and evaluate design fixes on the bench. However, for final, legally binding certification, testing must be performed in a controlled environment (e.g., a shielded room with a ground plane) using the fully calibrated LISN/Receiver system as per the mandated standard.

Q4: How does the EMI-9KC handle the different detector functions required by various standards?
The EMI-9KC implements true hardware-based detectors for Peak, Quasi-Peak (QP), Average, and RMS-Average measurements. The QP detector, with its specific charge and discharge time constants, is particularly important as it weights emissions based on their repetition rate, modeling the human ear’s response to interference. The receiver automates the selection of appropriate detector and bandwidth based on the selected frequency range and standard, simplifying the testing process.