Two Brands, One Problem, One Booth
At the EMV exhibition in Cologne this year, one booth caught my attention for a different reason than most.
The banner above it read: POWER ELECTRONICS AND EMI DEBUGGING. To the right: ZES ZIMMER CONFORMITY TEST SYSTEMS.
Two brands. One booth. And on the oscilloscope screens in front of me, something I rarely see displayed this cleanly at an exhibition: a live power converter board with its EMI decomposed in real time into its two fundamental components — Differential Mode and Common Mode — simultaneously, on separate spectrum traces, while the converter was running.
This decomposition is not cosmetic. It is the technical prerequisite for knowing what to fix, where to fix it, and which filter topology will actually solve the problem.
Rohde Schwarz RTO G Series oscilloscope and ZES Zimmer LMG671 power analyzer joint booth showing differential mode common mode EMI debugging at EMC exhibition Cologne Germany 2026 Photo: Thomas · @SignalByThomas (X)
The Core Technical Concept: DM and CM Are Different Problems
Every conducted emission from a switching power supply can be decomposed into two orthogonal components. Understanding the difference is the foundation of effective EMI filter design.
Differential Mode (DM) noise flows in opposite directions on the two power lines — Line and Neutral. It is generated primarily by the switching current waveform of the converter: the rectangular current pulse drawn by the primary switch, with its fast edges and high harmonic content, creates a current that flows forward on one line and returns on the other. DM noise is attenuated by X-capacitors (line-to-line) and series inductors in differential configuration.
Common Mode (CM) noise flows in the same direction on both power lines simultaneously, returning through the safety earth. It is generated primarily by the parasitic capacitance between the switching node and the chassis or ground — as the switching node voltage swings rapidly, displacement current is injected into the ground plane, flowing back to the source via both Line and Neutral simultaneously. CM noise is attenuated by Y-capacitors (line-to-earth) and common mode chokes.
The critical engineering consequence: a differential mode filter does not attenuate common mode noise, and vice versa. An engineer who doesn't know the DM/CM split of their converter's conducted emissions cannot design an effective filter — they are guessing at which topology to deploy without knowing which problem they are solving.
Rohde Schwarz RTO G Series oscilloscope showing three simultaneous displays time domain waveform differential mode FFT and common mode FFT for power converter EMI debugging at EMC exhibition Cologne Germany 2026 Photo: Thomas · @SignalByThomas (X)
The R&S RTO G Series: Three Simultaneous Views
The R&S RTO G Series oscilloscope at this demo was running a three-layer simultaneous display that made the DM/CM decomposition immediately readable:
→ Top trace: time domain — the raw voltage waveform at the measurement point, showing the converter's switching waveform shape, duty cycle, and any visible transients
→ Middle trace: Differential Mode FFT — labeled explicitly "Differential Mode" — the spectral content of the DM component extracted from the measurement
→ Bottom trace: Common Mode FFT — labeled explicitly "Common Mode" — the spectral content of the CM component extracted simultaneously
The three traces are derived from a single synchronized acquisition. The DM and CM components are computed mathematically from the two-channel voltage measurements — by summing and differencing the two line voltages relative to ground, the oscilloscope software separates the signal components that flow in opposite directions (DM) from those that flow in the same direction (CM).
This is not a new mathematical technique. What the RTO G Series demo was showing is the execution quality: the decomposition happening in real time, live, with the converter running, fast enough to see the spectrum updating continuously. An engineer debugging a converter can change a component, observe the effect on both DM and CM spectra simultaneously, and immediately understand which component of the emission changed and by how much.
The probe set visible in the demo — the HZ-15 from ZES Zimmer — is the measurement accessory that enables this two-channel differential measurement at the power line input. High-voltage differential probes with matched characteristics on both channels are the prerequisite for a mathematically clean DM/CM separation.
ZES Zimmer LMG671 precision power analyzer and RI2415 reference impedance with LMG Test Suite compliance software for IEC EN 61000-3 harmonics flicker testing at EMC exhibition Cologne Germany 2026 Photo: Thomas · @SignalByThomas (X)
ZES Zimmer LMG671: The Power Measurement Side
On the right side of the joint booth, ZES Zimmer — A Rohde & Schwarz Company — was demonstrating the complementary measurement discipline: power quality analysis and harmonic compliance testing.
The LMG671 Precision Power Analyzer is ZES Zimmer's multi-channel precision instrument for power measurement. At the booth, it was paired with the Reference Impedance RI2415 — the standardized network impedance module used for IEC/EN 61000-3 harmonic and flicker testing. The RI2415 presents the reference impedance defined by the standard at the DUT's power input, ensuring that the harmonic current measurements reflect the behavior the DUT would exhibit when connected to the public low-voltage grid.
The large screen above the LMG671 was showing the LMG Test Suite software interface — ZES Zimmer's compliance automation platform. The tab structure visible — Scope 1, Scope 2, Transient, Plot 1, Plot 2, Harmonics 1, Harmonics 2, Vector — maps the complete measurement workflow for IEC/EN 61000-3-2 harmonic emissions testing: raw waveform capture, harmonic analysis, limit comparison, and report generation.
The standards covered by the LMG Test Suite, as detailed in the product literature at the booth, span the complete power quality compliance matrix:
→ IEC/EN 61000-3-2: Harmonic current emissions, equipment input ≤16 A per phase
→ IEC/EN 61000-3-12: Harmonic currents, equipment 16–75 A per phase
→ IEC/EN 61000-3-3 / -11: Voltage fluctuations and flicker
→ EN 50564 / IEC 62301: Standby power measurement
→ IEC/EN 61000-4-7: Harmonics and interharmonics measurement techniques
This is the regulatory framework that every product sold in the EU must demonstrate compliance with before CE marking. The LMG671 + RI2415 + LMG Test Suite combination is a complete hardware and software solution for that compliance workflow — from measurement through to the standardized test report format that regulators and notified bodies require.
Why These Two Systems Belong on the Same Booth
The pairing of the R&S RTO G Series (EMI debugging) with the ZES Zimmer LMG671 (power quality compliance) on a single booth under the "Power Electronics and EMI Debugging" banner is not accidental. It represents a coherent view of what power electronics development actually requires across the compliance lifecycle.
During development: The RTO G Series with DM/CM decomposition is the tool that identifies where the EMI is coming from and what type it is. The engineer uses it iteratively — change the layout, change a component value, change the gate drive timing — observing the effect on both DM and CM spectra in real time. This is the debugging phase, where speed and visibility matter more than standards compliance.
Before certification: The LMG671 with the LMG Test Suite is the tool that measures the final design against the exact parameters and limits the standard requires. Harmonic current amplitudes at each harmonic order. Flicker disturbance values. Standby power. These are not measurements an oscilloscope is optimized to perform — they require the precision, the standardized impedance reference, and the compliance-oriented software that the LMG671 system provides.
The EMI filter designer and the power quality compliance engineer are often different people. But the design decisions made during EMI debugging — which filter topology, which component values, how much DM vs CM attenuation — directly affect the harmonic current profile that determines IEC 61000-3-2 compliance. A converter optimized for radiated EMI with an aggressive CM filter may pass CISPR 32 and fail IEC 61000-3-2. Understanding both measurement domains simultaneously is the prerequisite for making informed design trade-offs.
The ESD Dimension: R&S RTO6 + K155
The application card visible on the demo bench — ESD Generator Pulse Verification Using High-Performance Oscilloscopes — introduces a third measurement domain present at this booth: ESD immunity testing per IEC 61000-4-2.
ESD pulse characterization requires very different instrument capabilities than EMI spectrum analysis or power quality measurement. The IEC 61000-4-2 standard specifies ESD pulse parameters — rise time (tr, 10–90%), first peak current (Ip1), second peak current (Ip2), current at 30 ns (I30), and current at 60 ns (I60) — with specific tolerance ranges. Verifying that an ESD generator produces compliant pulses requires an oscilloscope with