Interpreting Results
CANBench TrueZ v1.1 — Fabricated prototype, sole built unit. V1.1 is electrically identical to the V1.2 schematic refresh but predates the InvenTree symbol-library migration; the schematic component metadata reflects legacy SCADYS naming. Testing and bench validation reference this V1.1 hardware.
Other versions: v1.2 — schematic refresh (next version)
The whole point of the TrueZ is to answer one question: is the DUT's conducted noise mainly common-mode or differential-mode? The answer tells you which mitigation to reach for, instead of guessing.
Reading the CM vs DM split
Overlay the CM-25Ω and DM-100Ω traces (both corrected for the low-end droop). For each problem peak, note which mode dominates:
| Dominant mode | What it means | Typical source in the DUT |
|---|---|---|
| Common-mode (CM trace higher) | Disturbance couples to both supply lines together, returning via chassis / earth | Parasitic capacitance from a switching node to chassis; cable-to-ground coupling; transceiver slew driving the bus against ground |
| Differential-mode (DM trace higher) | Disturbance flows line-to-line, returning through the supply pair | Switching-converter ripple on the supply rails; ground-bounce between the two rails |
| Both comparable | Mixed mechanism, or a mode-conversion path between them | Asymmetric layout / harness converting one mode to the other |
Choosing the mitigation
Match the fix to the dominant mode:
| If CM-dominant | If DM-dominant |
|---|---|
| Common-mode choke on the supply / bus pair | Differential (line-to-line) LC or π filter |
| Ferrite clamp / sleeve on the cable | X-capacitor across the rails near the source |
| Improve chassis bonding / reduce the parasitic-C path to ground | Reduce the switching-loop area at the converter |
| Shielding; reduce switch-node-to-chassis coupling | Reroute the harness to shorten the differential return |
After applying a candidate fix, re-sweep both modes and compare against the previous set — the value of the TrueZ is the relative before/after comparison, which directly shows whether the mitigation worked on the mode you targeted.
What "pre-compliance" means
The TrueZ + CANBench Duo chain is a pre-compliance / diagnostic setup. It will tell you which mode dominates, where in the band the offenders live, and whether a mitigation helped — in relative terms. It will not give a certified CISPR 25 pass/fail (that needs a calibrated chain, a Quasi-Peak detector at the prescribed RBWs, and an accredited lab). Treat the traces as engineering data, not a compliance verdict.
References
- J. Wang, F. C. Lee, W. Odendaal, Characterization, Evaluation, and Design of Noise Separator for Conducted EMI Noise Diagnosis, IEEE TPE 20(4), 2005 — the CM/DM separation method.
- IEC, CISPR 25 — the conducted-emissions measurement framework.
- For the topology that makes the split possible, see Circuit Design → CM & DM Separator.