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

Shielding Effectiveness

27. April 2016 01:39 by Christian in EMC/EMI
The generic shielding effectiveness requirement is 40 dB for magnetic field, electric field, and pla

The generic shielding effectiveness requirement is 40 dB for magnetic field, electric field, and plane waves. Depending on the application the frequency range can start from 10 Hz going up to GHz.

To predict shielding effectiveness (SE) of a metal sheet the following factors are summed:  Absorption Loss (A), Reflection Loss (R), re-Reflection Correction Factor (C).  SE = A + R – C (see MIL-HDBK-419A).

 

 

 

 

 

 

 

 

 

 

 

Absorption loss depends on material thickness, permeability, electrical conductivity, and the frequency of the incident wave.  It is the same for all electromagnetic waves.

Reflection loss depends on the distance of the EMI source to the material (different for electric, magnetic, and plane waves), material electrical conductivity, and the frequency of the incident wave.

Sources:
Christian Rosu

Automotive BCI Test Limits

3. April 2016 13:31 by Christian in
Calibration Levels per MIL STD 416F CS114:

The Bulk Current Injection (BCI) test method simulates a field-to-wire coupling from nearby low frequencies radiated fields induced onto a test harness small relative to wavelength.
The coupling from BCI probe will increase with test frequency when the cable is electrically short, and then flatten out when the cable approaches and exceeds a half-wavelength in length.

The transducers (RF current transformers) inject current into both sides of the test harness, therefore both DUT and Load Simulator are subject to test. RF radiation from load simulator cables is possible, but it can be reduced placing 20cm of clip-on split-ferrite RF suppressers close to the transducer.

The BCI common-mode current injected in the test harness simulates an illuminating RF field.
To simulate conducted differential-mode disturbances the BCI induced current is injected in individul conductors.

Using the Substitution Method the actual current injected in test cables can vary from what was initially  calibrated, being less likely to over-test but more real-life representative.

To ensure the repeatability of test results, the cable under test must be centered within BCI current probe, the test set-up must be consistent, especially cable routing, placement of the clamp, and proximity to metal structures.


BCI Calibration Levels per MIL STD 416F CS114:


BCI Probe Insertion Loss per MIL STD 416F CS114:


RF Immunity Ratio mA versus V/m per MIL STD 416F:


Ford RI 112 (BCI) Calibration Limits requirements per FMC1278:



CAN Bus Off Recovery

28. March 2016 01:35 by Christian in
CAN Bus Off is an error state of the CAN controller and it can be set only by the Transmitter Node w

CAN Bus Off is an error state of the CAN controller and it can be set only by the Transmitter Node when Transmit Error Counter is above 255. Such critical error is usually the result of a critical hardware issue (e.g. high level of electromagnetic field, bus wiring short-circuit, defective transceiver).


Methods to self-recover from a Node CAN Bus Off state:

1) Automatically after the CAN controller generates an interrupt.

2) Manually upon User request (ISO11898-1 §6.15).

In both the above  instances the bus turns back on after 128 occurrences of 11 consecutive Recessive Bits (BOSCH CAN 2.0B §8.12).

Auto-Bus-ON is not required by ISO 11989, therefore the CAN controller makers let the application to decide on its implementation. The automotive industry does not encourage the auto-bus-on feature.

If application's driver reports repeatedly the CAN Bus Off state the application should stop using the CAN.

Christian Rosu


CAN Bus Noise Tolerance

27. March 2016 09:49 by Christian in
The data is carried on the CAN bus as a voltage difference between the two signal lines. If both li

The data is carried on the CAN bus as a voltage difference between the two signal lines. If both lines are at the same voltage, the signal is a recessive bit. If the CAN_H line is higher than the CAN_L line by 0.9V, the signal line is a dominant bit.

Immunity to Ground Noise

The CAN bus does not use the ground as reference point for these two signal lines. Therefore the CAN bus transmissions lines are immune to any ground noise typically present in automotive applications.

Immunity to Electromagnetic Filed

The signals on the two CAN lines will both be subject to the same electromagnetic filed level. Therefore no differences in voltages between the two lines should become relevant under electromagnetic interference.

Using Twisted Pair Wires for Differential Signal Lines

Bad connectors are almost guaranteed to present an impedance discontinuity, and hence will cause reflections. Transmission line stubs of any length are also a source of reflections, longer the stub, the worse the impact of the reflections on lower data rate signals. Reflections are bad because they can cause destructive interference that can corrupt any transmitted data.