EMC FLEX BLOG A site dedicated to Automotive EMC Testing for Electronic Modules

The goal is to EMC validate DUT's Hardware Design assuming that DUT's Software was developed to serve the DUT's Hardware. Following a successful EMC validation, the DUT software may be subject to multipe updates and upgrades to serve the original hardware design w/o altering the outcome of overall product EMC validation.

DUT's software is used to exercise and monitor I/O lines and functions. The use of DUT production software should not be mandatory since may not be finalized prior to validation. The use of specialized DUT software is recommended since the production software may not be efficient enough to otimize the EMC testing time. The use of specialized DUT software is allowed provided that:

1) SW diagnostic timers are set to minimum detection values such that during the maximum 2-second RF exposure time all DUT response error flags are being captured and reported. Using production intent software would extend the DUT activation dwell time beyond 10 seconds such that long duation functions and/or sequential activation of various functions becomes possible.

2) DUT's state and fault conditions are reported directly via communication bus or indirectly via cyclying the outputs (e.g. changes to PWM duty cycle, monitoring LED flash rate, inadvertent status change).

3) DUT monitored data, I/O status values, analog input voltages, operating state are queried via parameter requests to ensure bi-directional communication during RF Immunity. Monitoring functional status via DUT scheduled or periodic broadcast messges is not recommended.

Module to Vehicle Interface Connector and User Interface I/O

• directly via communication bus
• indirectly via cyclying the outputs (e.g. changes to PWM duty cycle, monitoring LED flash rate, inadvertent status change) 

• directly via Fiber Optic system
• indirectly via loop back method (e.g. monitoring the simulated load using a DUT input)

• directly via communication bus
• indirectly via cyclying the outputs (e.g. changes to PWM duty cycle, monitoring LED flash rate, inadvertent status change).

• directly via Fiber Optic system
• indirectly via loop back method (e.g. monitoring the simulated load using a DUT input)

• The analog properties of the bus electrical signal (e.g. V_dominant, V_recessive, etc. must be validated during RF immunity.
• This may require special software to decrease the data rate to be within the bandwidth limitations of analog fiber-optic transmitters.

• Received signals must be set to 3 dB above specified minimum sensitivity level.
• The RF level is to be established with DUT installed in test chamber.
• Bit Error Rate (BER) is the preferred metric with an acceptance threshold set by RF device specifications. 
• BER must be monitored directly through the communication bus via parameter requests (never via scheduled, or periodic, broadcast messages).
• Transmitted signals must be monitored by an appropriate RF receiver, again monitoring BER, acceptable threshold set by RF device specifications.

MCU Connector “Internal” I/O 

• MCU Analog Input is set to nominal operating value/condition for the specified test mode with the value reported either directly via the DUT communications bus or, indirectly through the DUT monitoring the input and changing the state of an output in a known, pre-determined manner. These functions are based on internal Printed Circuit Board (PCB) operating conditions and are not expected to
  be controlled or changed during testing; it is not necessary to force a mid-value for these inputs as with vehicle harness interface I/O.
• Digital Input - Non-dynamic (Steady State I/O). Examples include feedback fault indication, over/under current monitoring via discrete comparator circuit, etc. The input is set to the nominal operating value/condition for the specified test mode with the value reported either directly via the DUT communications bus or, indirectly through the DUT monitoring the input and establishing the state of an output in a known, pre-determined manner. Not to include reset, address/data lines and communication between the microprocessor and electronically erasable programmable read-only memory (EEPROM), etc.
• Digital State Input - Dynamic Cycling I/O. Requires state change between asserted to non-asserted back to asserted states during radiated immunity RF “on” exposure, reported directly by communication or indicate indirectly via output state change by detected input. Reset, address/data lines and communication between the micro and EEPROM are not included. The following MCU I/O types do not require direct monitoring since are indirectly monitored by the inherent operation of the device: Discrete outputs, Analog outputs, Internal communication bus.

Christian Rosu

Reference: Automotive OEM EMC specs

Differential-mode RF emissions in a CISPR 25 component level configuration occur due to the flow of current (IDM) via signal paths in which the forward and return conductors are not routed together, thereby forming a conductor loop. The resulting magnetic field from the conductor loop is proportional to the current IDM, the area of the loop and the square of the frequency of the RFI current.

Common-mode RF emissions occur due to undesired parasitic effects, e.g. due to inductances in the current return path or unsymmetries during signal transmission. If we connect a cable to a DUT of it may function like an antenna allowing a common-mode current ICM to flow. Both signal and power supply lines can function as efficient antennas. Here, our rule of thumb is that line lengths that do not exceed λ/10 are uncritical, whereas longer lines (e.g. λ/6) must be treated as potential sources of RF emissions.

The magnitude of the voltage drop on the ground plane and thus the magnitude of the common-mode current coupled into the connected line are determined by the parasitic inductance and the slope steepness of the signal.

We cannot assume that differential mode radiated emissions are not dominant nor an infinite ground plane. A ground plane with finite width has inductance.

Common-mode RF emissions can also occur due to differential mode signal transmission.
If the parasitic terminating impedances of a differential mode transmission path differ substantially, in addition to the desired differential-mode current IDM a common-mode current ICM will also flow via the ground plane that connects the transmitter and receiver modules. This unwanted ground current ICM can then also be coupled into lines connected to DUT and cause emissions in the far field.

The strength of the common mode current and the level of radiated emissions depend on the inductance of the ground plane. The value of this inductance depends on the structure of the transmission line.

The ground plane inductance in a symmetric structure is:
L21 = (µ0/2π) * ln((tπ/W)+1)
Where:
W is the width of the ground plane
t is the height of the harness

The ratio of the height of the harness and the width of the ground plane determines the GP inductance.

As the harness is closer to the edge of the ground plane, the measurement tolerances are higher since the ground plane inductance increases. The tolerances in RE measurments are acceptable when the distance of the harness to the ground plane edge is 10 cm.
Since common mode radiated emissions occur through the ground plane (or the whole setup), the length of the ground plane can impact the tolerances in RE measurments. Longer the ground plane, higher the radiated emissions level.

Christian Rosu, 2022-03-07

RF Boundary is the element of an EMC test setup that determines what part of the harness and/or peripherals is included in the RF environment and what is excluded. It may consist of, for example, ANs, BANs, filter feed-through pins, RF absorber coated wire and/or RF shielding.

RF Boundary is also an RF-test-system implementation within which circulating RF currents are confined

• to the intended path between the DUT port(s) under test and the RF-generator output port, in the case of immunity measurements (ISO 11452-2, ISO 11452-4, ISO 1145-9), and

• to the intended path between the DUT port(s) under test and the measuring apparatus input port, in the case of emissions measurement (CISPR 25),

and outside of which stray RF fields are minimized.

The boundary is maintained by insertion of BANs, shielded enclosures, and/or decoupling or filter circuits. The ideal RF boundary replicates the circuitry of the device connected to DUT in vehicle.

The standard test harness lenght for automotive EMC electronic components is (1700mm -0mm / +300mm). This 1.7m test harness runs between the DUT and the Load Simulator (Shielded Enclosure) that plays the role of RF Boundary.

If the Load Simulator enclosure does not include all DUT loads and activation/monitoring support equipment, additional support devices may be placed directly on the ground plane. The connection of additional devices to LS enclosure must be done via short wiring running on the ground plane.

Testing at subsystem level is preferable to any simulation. Whenever possible, use production intent representative loads.

Running long coax cables directly from DUT outside the chamber via SMA bulk filter panel would violate the 1.7m test harness length rule invalidating the test result. Ideally is to use Fiber Optic to exchange data with devices placed outside the test chamber.

Running long coax cables between Load Simulator and a support device placed outside the chamber is acceptable as long as the I/O line in question is not just an extension from DUT without proper RF boundary at the end of maximum 2-meter length of standard test harness.

It is critical to use the test harness length as defined by CISPR-25, ISO 11452-2, ISO 11452-4, and ISO 11452-9 to achieve valid compliance for your product. The length of the test harness as well as the grounding method (remote vs local) can result in different RF emissions level. Longer the test harness, higher RF emissions above 100 MHz due to its resonance pattern. The local grounding would show less magnitude variation across resonance peaks above 100MHz.

Christian Rosu

2022-02-20

An incorrect DUT grounding scheme can easily make the difference between compliance and non-compliance to CISPR 25 CEI limits. Sometimes we have to evaluate CEI from two modules, one used as DUT and the other one used as DUT's load (e.g. Module #1 is a PWM maker while Module #2 is an LEDs Lamp).

Christian Rosu, 2021-06-09

Based on FMC1278R3 and CS.00054:2018 Production Representative Hardware and Software should be used for all verification testing unless approved differently by OEM via EMC Test Plan. 

The Production Representative Test Sample is built using production representative hardware and software constructed using production representative processes, tooling, etc.

Following Software Changes in addition to PCB Changes re-validation for test methods like ESD, CISPR 25 RE, BCI, RI ALSE , Hand Portable Transmitters, Transients, Voltage Dips and Dropouts may be required. 

FMC require DV testing to be performed using production representative components but not necessarily components constructed from production tooling.

EMC DV1 Testing for PCM (Powertrain Control Module) is normally performed with test software mutually agreed to by FMC D&R, EMC and the supplier.
EMC DV2 Testing for PCM must be completed using Production Intent Hardware and the latest available Application Software.

FCA require DUT Validation Testing done on Production Intent Samples using Production Intent Hardware and Software.
Production Intent Components must be used for the inputs and loads including switches, sensors, pulse width modulated loads, solenoids and motors.