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

CISPR 25 Conducted Emissions - Current Method - MU Sources

CISPR 25 Conducted Emissions - Voltage - MU Sources 

CISPR 25 Radiated Emissions - ALSE Method - MU Sources

CISPR 25 RF EMISSIONS INPUT QUANTITIES - PROBABILITY DISTRIBUTION FUNCTION

CISPR 25:2021

CISPR-25 indicates that both CE-V and CE-I must be carried out to validate an automotive electronic device.

CE-V in dBuV is measured on B+ and GND lines using the LISN port.

CE-I in dBuA is measured using a “current probe” clamped at 5 cm, then at 75 cm from DUT’s connector. The probe is clamped on the whole harness, then on each connector separately. The RF noise measured may be coupled from DUT directly as well as from wire-to-wire along the 1.7 m test harness.

CISPR 25 is not very specific about supply lines CE “redundancy”, therefore we test everything for CE-I.

Chrysler is the only OEM that specifies in CS.00054 as exception from CISPR 25 to remove from “current probe” all Supply Lines (power and ground).

CS.00054 is asking to run CE-I on all wires not tested at CE-V, however measurements are aquired only at 5 cm from DUT's connector.

2022-06-29

Christian Rosu

 The EV does not mean the end of 12V automotive battery. For various safety reasons, complex modules are powered using two 13.5V / 200A batteries such that the Backup battery comes into play the moment the Main battery's voltage is outside operating voltage range.

The Ground Offset Test involves a voltage variation of +/- 1V on Supply Return line that may affect DUT circuitry referenced to an absolute 0V via remote ground. The diagram below shows how to use a combination of three power supplies to simulate the +/- 1V Ground Offset condition.

2022-06-29

Christian Rosu

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