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

The DUT Performance Functional Verification is based on a bench test software that does not account for EMC specific considerations and is normally performed prior and following each EMC test method.

Using the same Activation & Monitoring Method and Pass/Fail Criteria for ENV and EMC is not practical. DUT’s functions must be grouped in “operating modes” that are in line with the scope of EMC Test Method. When assessing the level of RF emissions we want the DUT to exhibit the highest level of noise possible as in vehicle. During RF Immunity evaluation we expect the following from a good activation/monitoring software:

• Capability to activate and have realistic data traffic on all I/O lines as well as individual I/O lines.
• Electrical Transients or RF coupled in supply voltage and I/O lines may not always trigger repeatable anomalies. Therefore we need a visible flag/indicator to immediately stop the actual EMC test method process for anomaly thresholding (e.g. level vs frequency). As we reduce applied stress level the DUT’s behavior may change, then at some point the anomaly should disappear.
• The same DUT operating mode may be feasible for one or more EMC test methods but definitely not for the entire test list. We need capability to configure what functions belong to each operating mode including live monitoring method. Log files are not useful during test, we still need them following the test for troublesooting. 
• All functions must resemble vehicle intent usage. Not all I/O lines will be active simultaneously in vehicle. Therefore do not use unrealistic I/O cables scenarios to facilitate testing since this can generate false current loops and other issues.
• The functions used by DUT activation & Monitoring Software are not meant to assess complaince to USB, E-Net, LVDS standards.

 Keep in your mind that:

1. Electrical Transients on supply lines can hard reset the MCU (e.g. dips/dropouts). Is there a test in your monitoring software that captures such condition?  Any other anomaly is not relevant once a hard reset occurs.
2. Do you have a function to verify that there is no memory loss following inadvertent hard/soft reset?
3. The RF can be coupled in both supply lines and I/O lines resulting in data traffic interruptions leading to a soft reset. Is there a monitoring function to capture such event?
4. If CAN bus is used in a design I would consider at minimum two critical errors: CAN BUSOFF & DTC SET. What we normally use is a pass fail criteria that can be adjusted such that is possible to determine if the anomaly occurs with every data transmission attempt or it happens only each 100/1000 attempts.

DUT support software

Do not waste time/money to tweak operating modes during EMC validations. In many EMC labs the cost for one hour of ALSE chamber is $500 regardless to how you spend this time. The operating mode must be pre-selected, yet adjustable if needed during troubleshooting. The "anomaly found" visual indicator is used by EMC test operator to stop the actual EMC test software. If we have a time stamp in log files, there is no need to stop the activation/monitoring support software.

DUT Activation Dwell Time

All functions within the same operating mode must be completed and repeated every 2 seconds. We call these 2 seconds Dwell Time, and it can make a huge difference in test duration and cost. For a 2-second dwell time and only one Operating Mode you can expect RF Immunity in ALSE chamber to last 4-5 days (one shift). For a 4-second dwell time it may last practically 8-9 days and so on. Bottom line, if the initialization of Load Simulator or DUT is time consuming it will cost a fortune to re-initialize following each incident/anomaly. Hard/soft reset must always be the last resort to resume operation. The goal is to minimize the number of operating modes and DUT orientations.

Types of DUT support software 

• Load Simulator EMC support testing software with specific sections for each Operating Mode.
• Load Simulator Functional/Parametric verification testing done before and after each Test Method.
• Full DUT Functional/Parametric testing before and after full EMC validation that is not typically done using the EMC Load Simulator but rather EOL like testers.

 DUT Operating Mode

Ideally is to include in Operating Modes only those DUT functions that are active while driving the vehicle. Functions used for diagnostics at the car dealer shop are not relevant. The worst case scenario is when vehicle is in Run Mode (speed >0) but we also have to simulate the Standby Mode (speed = 0) and Sleep Mode (current consumption < 1 mA).

Christian Rosu, Jan 12, 2021

Antenna Polarization (Vertical & Horizontal)

A requirement for CISPR 25 Radiated Emissions and ISO 11452-2 ALSE RF Immunity.

• The 1.7m test harness runing parallel with the edge ground plane will generate horizontal polarized emissions.
• Portions of 1.7m test harness reaching connectors positioned above the 5cm thick Styrofoam on DUT and Load Simulator would generate vertical polarized emissions requiring vertical antenna polarization to be captured.
• LS support equipment cables running over the edge of the metalic table may generate a combination of horizontal and vertical emissions.
• Folded LS support cables tend to cancel the field generating very low vertical emissions if the folding is very tight.

It is critical to eliminate the common mode currents on both 1.7m test harness and LS support cables for lowering the noise floor to minimum 6 dBuV/m under CISPR 25 limits.

In automotive EMC the DUT is normally remote grounded in one point via supply return line to the negtive pole of the 12V battery. Local grounding for DUT with metallic housing is not practical given the risk of grounding loops and rusty connections as the car is aging. Unwanted common mode currents may run along the outside of the cabe's shild: 

• The cable's shield should be connected to non-current carrying parts of DUT. If the emissions noise is actually on the shield of the cable, ideally is to use connectors that have provisions for connecting or clamping the cable shield in a 360-degree bond. Using pigtail connections is a less efficient way to connect cable shields to their connector shield terminations. The longer the pigtail used, higher the expect emissions, thereore it’s recommended to use multiple short pigtails to the connector shield surrounding the internal wires. This will tend to cancel the resulting fields.
• Bonding the cable's shield to DUT's shielded enclosure may work if local grounding is acceptable for that design. Most of the time the shielded enclosure or the heatsink is capacitively decoupled from supply return.
• adding common mode chokes to DUT PCB design to minimize common mode noise sources.
• istalling an external common mode choke around DUT's end of the I/O cable.
• Expensive connectors have provisions for connecting or clamping the cable shield in a 360-degree bond, which is ideal. 

Ground Loop

A noise current sharing a common return impedance with a signal current.

Confined System

When connecting signal line cables within a confined system, the shield is connected at both ends in order to provide a signal return current path. 

1. For high frequency digital signals above (10 to 100 kHz), proper magnetic field shielding requires a connection at both ends of the cable shield. This provides a return path for the high-frequency currents to flow back along the signal path.
2. For frequencies greater than 10 to 100 kHz, the return current wants to travel the path of least impedance – that is back through the cable shield – due to mutual impedance coupling.
3. For electric fields, connect only one side of the shield at the noise source (or sensitive analog) end.

Distributed System

For a system distributed across a larger area, with potential differences in the reference returns between one end of the cable and the other, the shield is connected only at the signal source end. The potential difference between the main controller digital return and and various sensor returns can be quite different. The result would be noise currents flowing in the shield. Such type of hybrid ground is used where a series capacitor is used to connect the non-source end of the shield to signal return (e.g. 300 feet long cables in aerospace industry). 

Opto-isolators, differential pairs, common-mode chokes are useful to “break” any noise currents in the shielded twisted pair of sensor cables.

Audio or power line frequencies

1. For fixing a ground loop issue, grounding one end of the shield or blocking the low-frequency (or DC) component with a capacitor might work best. Isolation transformers may be used for both line and audio applications.
2. For signal currents greater than 10 to 100 kHz, use a solid ground bond at each end of the cable shield. Ground loops just don't tend to occur above 10 to 100 kHz. 

NASA spec mention to:

1. Ground one end (or use some form of isolation to break the loop) for low frequency ground loop fields.
2. Ground both ends for shielding against external high frequency fields.

DUT with shielded enclosure using unshielded cable

• Minimize the common mode (noise) current loop through either diversion (back to the noise source) d or blocking with some impedance. Break (or block) the loop with common-mode chokes at the I/O connector signal lines. Add transient protection devices to guard I/O connections against ESD and other pulse-type signals.
• Insert a common-mode ferrite choke in the power and it's return lines. It's always good EMC practice to design in common-mode chokes in both the signal and power lines. 
• Ensure each signal and signal return wire pair within the cable is twisted. This will achieve several dB of shielding effectiveness by itself.
• If using a ribbon cable, make sure there are adjacent signal (and power) return wires for each corresponding signal (or power) wire.
• If running a clock signal, make sure there are clock return wires on each side of the clock wire.
• If all else fails, use a clamp-on ferrite choke around the cable, positioned right at the I/O connector.

DUT with plastic (unshielded) enclosure

There will inevitably be common-mode noise sources on the PC board. To keep these noise currents off our I/O and power cables:

1. block the currents from getting to the cables with a ferrite choke or
2. divert the noise currents back to their source.
3. A combination of blocking and diversion is the best method. Higher-end handheld consumer products use a diversion plate under the PC board. It is a thin meallic plate or metalized film with one end bonded or clamped well to the I/O and power connector ground shells. This offers a low impedance path for the common-mode currents to flow back to the source through distributed capacitance. It also protects sensitive circuitry from external ESD currents injected at the I/O connectors. In addition, it serves as an image plane which helps reduce radiated emissions. The cable shield must be bonded in some way to the digital ground (if a signal or I/O cable) and power ground (if a power cable). Ideally, all I/O connectors and power connectors should be grouped together on one side of the board. If they are spread all around the perimeter, then any noise sources on the PC board are potentially driving the midpoint of a dipole antenna.

Low Voltage Differential Signaling (LVDS)

Switches about 1.2V at very fast edge speeds. Theoretically differential signals should never radiate, but ANY unbalances in line length or routing can cause common-mode currents to form.

Solutions:

• use flat ferrite chokes
• shielding the cable and connecting the shield back to digital return in several places at each end of the shield.

Troubleshooting:

• use ferrite
• install copper tape to one side of the cable to provide a path for any unbalanced common-mode currents to return to their source.

Shielded Enclosures and Gaskets

Both the compression of the shields and gaps/cracks in the gasket may may affect slot emissions. It’s really a factor of both the manufacturer’s recommended compression, plus how well the gasket installation is designed. Minimize the length of any gaps between any two pieces of metal enclosure. The leakage can be measured using a near field probe and sliding it along all the enclosure seams. Preferable to be done in ALSE chamber.

Earth Grounding Rod

In EMC testing is needed for establishing a voltage reference, discharge high transient voltages, static discharge, personnel safety.

Pigtail connectors 

Are an insulation displacement connector that are filled with a di-electric grease to prevent moisture from getting inside the connector. No need to strip the ends of  the wires, just insert them into the connector, then squeeze the blue cap down with a pair of pliers.

Connectors

When the source of the radiation is from common currents on external cables such as those that connect to peripherals, using a “better” cable often has no impact at all on the radiated emissions. That’s because the common currents are flowing on the shield of the cable.

It only takes 3 μA of common current flowing on the shield of a cable, 1 m long, to cause an FCC class B failure. 

The most important driving voltage for these common currents that causes EMC failures is ground bounce in the connector attaching the cable to the chassis.

Ground bounce is the voltage generated between two regions of the return path due to a changing current flowing through the total inductance of the return path.

The total inductance of the return path is related to the total number of field lines around the conductor per amp of current flowing through it. When the dI/dt of the return current flows through the total inductance of the connector, it generates a voltage, and this voltage between the chassis and the cable’s shield is what drives the common currents on the cable, which results in an EMC failure.

A coax cable will have no ground bounce because there no external magnetic field around it.

The signal current generates an external magnetic field composed of circular rings of field lines circulating in one is direction.

The return current, if symmetrical about the signal path, generates the identical rings of magnetic field around the cable, but circulating in the opposite direction. These two sets of magnetic field lines exactly cancel out and there is no external magnetic field.

But suppose at the connector, the return current is not perfectly symmetrical about the signal current. Maybe there is a pigtail, maybe the clam shell is not well metalized, or maybe the connector only makes contact at one or two points to the chassis.

Any asymmetry will mean the magnetic field lines from the signal current and return current will not perfectly cancel out. There will be some net magnetic field lines and this will result in some total inductance of the return path. 

In a 50 Ω coax cable, with a 1V signal, having a 1ns rise time, the signal and return current is about 1 V/50 Ω = 20 mA.

Even if the asymmetry is so light as to generate only 0.1 nH of total inductance around the return path of the connector, the ground bounce voltage generated would be 2 mV. 

If the impedance the common current sees returning through all those fringe field lines is about 200 Ω, this 2 mV of ground bounce voltage will drive I = 2 mV/200 Ω = 10 μA.

It only takes 3 μA of common current to fail an EMC certification test.

This ground bounce driven current in the cable shield will cause an EMC failure.

Test Procedure:

• what the activity is (DUT type)
• who is to perform the activity (EMC Testing Laboratory)
• when the activity is to take place

This is more of an EMC Test Plan Template document that:

• defines the DUT classification and category
• lists required EMC Test Methods defined by the automotive OEM specs or International Standards (ISO, CISPR, SAE, etc.).

Test Method:

• how  the actual EMC testing is to be carried out (test equipment configuration)
• defines measurable data format for reporting and acceptable stress level limits 

This more of a Work Instruction outlined by automotive OEM EMC specs and international standards.

Christian Rosu, Dec 11, 2020.

The automotive OEM specs do not specify how to configure the DUT during CISPR 25 chamber ambient measurements. DUT must be unpowered, all other DUT support equipment must be powered and as much as possible functional to correctly evaluate RF emissions noise floor before start testing. This leaves at least three scenarios for how to configure the DUT.

1.  Test harness connectors are removed from
2.  DUT is unpowered.
3.  The 1.7 m test harness is unterminated on DUT side, no potential ground loops with Load Simulator.
4.  The 5uH LISN remains present.
5.  The Load Simulator and all support equipment remains powered.

The accuracy of RF Field Level during ALSE RF Immunity per ISO 11452-2:2019 Substitution Method relies on the Field Probe calibration factors. An incorrect Field Probe Calibration may result in significant deviations from the field levels called by automotive OEM specs. The Field Probe Calibration Report provides correction factors that are introduced into RF Immunity Test Software (e.g. TILE, NEXIO). Using calibration factors acquired at 15 V/m instead of 300 V/m can force the RF Amplifier output to maximum w/o the Field Probe to report expected Field Level. Moving transmitting antenna 10 inches closer to the Field Probe would allow the probe to report the expected field level, however this level is in fact higher as consequence of using bad correction factors.

RF Field Probe Selection for EMC Testing

Calibration Factors: corrections are provided as dB adjustments & multiplication factors. Maximum field measurement accuracy is achieved when the detailed 3-axis calibration is applied.

     Probe Calibration Certificate

     A) filed level applied via calibration antenna (V/m)

     B) filed level reported by probe (V/m)

     C) calculated multiplier factor

          A = B * C (e.g. 100 V/m = 120 V/m x 0.8333 where 0.8333 is the correction factor)

Sensitivity/Dynamic Range: e.g. (0.5 – 800V/m for 0.5 MHz – 6 GHz)

Linearity: the measure of deviation from an ideal response over the dynamic range of the probe that may vary as a function of the applied field level. (e.g. ±0.5dB 0.5 – 800 V/m).

Overload: the field level where damage can occur to the probe (e.g. 1000 V/m CW).

Isotropic Deviation: the variation of the probe’s response from ideal as it is rotated in the field. The minimal isotropic deviation of spherical probes (±0.5dB 0.5 MHz – 2 GHz).

Response time: the time a probe takes to respond to an applied RF field (e.g. 20 ms).

Sample rate: the rate at which information can be retrieved from the probe (e.g. 50 samples/second). 

Probe Type: refers to the configuration of the probe sensors. 

• An isotropic RF filed measures the total value of the field level and is unaffected by field polarity. This is accomplished by summing measurements from three different sensors placed orthogonal to each other. 
• Non-isotropic probes measure fields in one polarity at a time for electric field or magnetic field. 

IEEE 1309:2013 is the Standard for Calibration of Electromagnetic Field Sensors and Probes (Excluding Antennas) from 9 kHz to 40 GHz. 

The EMC lab must inform the calibrator about critical requirements imposed by automotive specs/standards for proper field calibration factors:

1. The frequency range or center frequencies as delineated by automotive OEM EMC specs (e.g CS.00054, GMW3097, FMC1278).
2. The filed level for each frequency band (e.g. 80V/m, 100V/m, 200V/m, 300V/m)
3. Field Probe orientation (all three axes X, Y, Z facing antenna).
4. Use 1 meter antenna distance to Field Probe. This is not always possible, therefore using a lower distance in far field  (e.g. 30 cm) should be acceptable.
5. Calibrate the probe using CW with transmitting antenna in both horizontal/vertical polarization.

IEEE 1309:2013 A.2.4.3 Field strength: if the probe or sensor linearity is better than ± 0.5 dB, the frequency response calibration of the probe can be performed at any field strength level, but preferably close to the field levels used in the EUT tests. It is also required that the same probe range and/or gain settings as used in the EUT tests are used in the probe calibrations.

IEEE 1309:2013 A.2.4.4 Linearity check for probe or sensor:

For applications needing multiple field strength calibrations, e.g., 3 V/m, 10 V/m, and 18 V/m, the linearity tests shall be performed for each level. Note that for automotive EMC testing the above e.g. translates to levels like 100V/m, 200V/m, 300V/m.

IEEE 1309:2013 A.2.4.5 Probe isotropic response

For isotropic probes using three orthogonal elements, it is recommended that the frequency response and linearity response measurements be performed for each axis individually. Each axis should be aligned with the incident field successively to provide a maximum response. Probe calibration in a single orientation, such as only the orientation used in a UFA calibration, is not recommended, because the transmitting antennas, separation distances, and the end-use environment are typically not the same between the two setups.

AR_App_Note_44_RF_Field_Probe_Selection.pdf (352.8KB)

Rhode & Schwarz Equipment Calibration Interval:

https://gloris.rohde-schwarz.com/anonymous/en/pages/toplevel/calibration-process.html