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

When comes to achieving EMC compliance for automotive specs a 2-layer PCB works only for very simple designs. Modules having CAN/LIN, USB, E-Net, LVDS are normally using 4-layer PCBs. If adding CPU and memory a 4-layer PCB may not be good enough.

Ford (FMC) mentioned 20 years ago in their EMC design guide for 2-layer PCB to use the “Ground Grid Technique” for top and bottom side of PCB. They also recommended the use of "Faraday Cage" by installing ground vias around the perimeter of the PCB every say 15mm that are connected together on both PCB sides by 0.4 mm thick traces.

The problem with CE-V is that even using a “ground grid” or “faraday cage” won’t prevent DUT’s noise to be coupled into Supply Lines. Assuming that in your case CE-V are higher on GND line I would:

1. determine potential sources of noise with harmonics in FM band
2. determine the noise coupling method (e.g. common mode current, common return path, ground loops)
3. Clamp ferrite corres on the entire test harness to lower CE-V noise below limit. Clamp the same ferrite separately on VBATT or GND line to determine which one is more affected. 
   

RF filters are effective for emissions exceeding the limit with 2-3 dB. For conducted emissions exceeding the limit with 10 dB you better try to fix the layout first.

Christian Rosu Feb 8, 2020.

CAN Bus Termination

The termination is used to match impedance of a node to the impedance of the transmission line being used. When impedance is mismatched, the transmitted signal is not completely absorbed by the load and a portion is reflected back into the  transmission line. If the source, transmission line and load impedance are equal these reflections are eliminated. This test measures the series resistance of the CAN data pair conductors and the attached terminating resistors.

1. Turn off all power supplies of the attached CAN nodes.

2. Measure the DC resistance between CAN_H and CAN_L at the middle and ends of the network.

The measured value should be between 50 and 70 Ω. The measured value should be nearly the same at each point of the network. If the value is below 50 Ω, please make sure that:

- there is no short circuit between CAN_H and CAN_L wiring

- there are not more than two terminating resistors

- the nodes do not have faulty transceivers.

If the value is higher than 70 Ω, please make sure that:

- there are no open circuits in CAN_H or CAN_L wiring

- your bus system has two terminating resistors (one at each end) and that they are 120 Ω each.

CAN_H/CAN_L Voltage Verification

Each node contains a CAN transceiver that outputs differential signals. When the network communication is idle the CAN_H and CAN_L voltages are approximately 2.5 volts. Faulty transceivers can cause the idle voltages to vary and disrupt network communication. To test for faulty transceivers, please

1. Turn on all supplies.

2. Stop all network communication.

3. Measure the DC voltage between CAN_H and GND

4. Measure the DC voltage between CAN_L and GND

Normally the voltage should be between 2.0 V and 4.0 V. If it is lower than 2.0 V or higher than 4.0 V, it is possible that one or more nodes have faulty transceivers. For a voltage lower than 2.0 V please check CAN_H and CAN_L conductors for continuity. For a voltage higher than 4.0 V, please check for excessive voltage.

CAN Bus Ground Verification

The shield of the CAN network has to be grounded at only one location. This test will indicate if the shielding is grounded in several places: 

1. Disconnect the shield wire (Shield) from the ground.

2. Measure the DC resistance between Shield and ground.

3. Connect Shield wire to ground.

 The resistance should be higher than 1 M Ω. If it is lower, please search for additional grounding of the shield wires.

CAN Transceiver Resistance Test

CAN transceivers have one circuit that controls CAN_H and another circuit that controls CAN_L. Experience has shown that electrical damage to one or both of the circuits may increase the leakage current in these circuits. To measure the current leakage through the CAN circuits, please use an resistance measuring device and:

1. Disconnect the node from the network. Leave the node unpowered.

2. Measure the DC resistance between CAN_H and CAN_GND.

3. Measure the DC resistance between CAN_L and CAN_GND.

Normally the resistance should be between 1 M Ω and 4 M Ω or higher. If it is lower than this range, the CAN transceiver is probably faulty.

Understanding the role of return ground is essential in EMC testing.

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See Differential Mode vs Common Mode Current

 

 

CISPR-25 Generic Test Setup for compliance to CS.00054:2018

A few months ago I was surprised that UL lab (Novi, MI) runs RI 115 (Immunity to Hand Portable Transmitters) leaving the ALSE chamber door fully open. I have requested the test engineer to confirm that he follows correctly UL lab internal test procedure. The response was that this is practically near field RF immunity, therefore there is no concern to interfere with other lab test equipment.