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

CABLE SHIELDING

18. November 2021 18:28 by Christian in EMC/EMI, EMC TEST PLAN, Grounding, PCB, Shielding
Near-field interference (crosstalk) is a major issue in electronic devices and systems when comes to EMC compliance.


Near-field interference (crosstalk) is a major issue in electronic devices and systems when comes to EMC compliance. To reduce crosstalk, as well as far-field interference, the transmission lines can be shielded. The length of transmission lines and spacing between the conductors should be as small as possible. The steel tube around the untwisted pair is superior to both the aluminum and copper tube due to its magnetic properties.

 

 Untwisted Pair

The length of the conductors and the spacing between them must be short.

Untwisted Pair Inside Copper Tube

Less susceptible to near filed magnetic noise. Copper is non-magnetic with μr (relative permeability) = 1. A very small induced bucking current in the Cu tube will generate a small  counter magnetic field.

Untwisted Pair Inside Grounded Aluminum Tube

Less susceptible to near filed magnetic noise. Aluminum is non-magnetic, 0.61 of copper conductivity. Therefore, the magnitude of the counter magnetic field is less. The grounded Al tube reduces near-field electric emissions susceptibility and static charge buildup on the shield being less expensive.

Untwisted Pair Inside Steel Tube

The steel tube & untwisted pair is better than Al or Cu tube due to its magnetic properties (μr = 1000 @ low frequencies):
(i) it increases the absorption of the magnetic fields
(ii) redirects the magnetic fields away from the tube's interior

 Twisted Pair

Twisted pair without any shielding is ranked higher than the untwisted pair in a steel tube. Per Lenz's law, the magnetic field will induce a voltage in a loop of wire. The orientation of the loop affects the sign of this voltage. Twisting the two wires forces the induced voltage in neighboring loops to be of opposite polarity. By summing all of the induced voltages from each of the loops generated by the twisting, the net induced noise voltage is significantly less than without the twisting. The sum is theoretically zero for an even number of loops. Twisting the wire is probably one of the least expensive methods to decrease the susceptibility of a cable to magnetic fields.

Twisted Pair Inside Steel Tube

Twisted pair inside a steel tube is the least susceptible to magnetic fields. The steel tube absorbs and redirects the magnetic fields. Steel is relatively inexpensive, but it is heavy. To avoid rusting it should be galvanized.

 

 

 

  •  The shielded twisted pair with both the source and load grounded offers only 2 dB less susceptibility to low-frequency noise. Although the grounding of the shield will reduce electric field emissions, many of the advantages of using twisted pair are lost since the load and source are not balanced (load & source are both single-ended grounded).
    The return current path is divided between the return conductor of the twisted pair and the ground plane.
    At low frequencies, a majority of the current tends to return via the low-impedance ground plane. At higher frequencies, the current tends to return along a path nearest to the forward signal current - the twisted pair conductor.

 

  •  The 'least immune, or most susceptible, cable of those listed to low-frequency noise (both electric & magnetic) is the coaxial structure where both the source and load are grounded and the shield is only connected to the source ground. Although the grounding of the shield will reduce electric field emissions from the center conductor, the outer shield has virtually no influence on the magnetic field susceptibility.

 

  •  The return current for both the signal and noise must be via the ground path between the load and source. This current-path loop can be large. The effective area for either magnetic emissions or magnetic pickup can therefore also be large. This dosed current path through the ground is referred to as a ground loop.

 

  •  A significant improvement in performance is obtained when a twisted pair is used and the load is balanced.
    The load is floating with neither end of the load connected to ground.
    At low frequencies, the parasitic coupling between the load and nearby grounds is small, and the signal current should mostly return via the return conductor of the twisted pair. The effective pickup area of the complete current path is small, and the twisting produces alternating polarity induced voltages in each of the loops. Since there is no surrounding shield, there is no electric field shielding. However, if the line is balanced, the capacitive coupling (i.e., near-field electric coupling) to each line should be about the same, and the net electric-field induced noise across the load should be negligible.

 

  •  When the shield or outer conductor of the coaxial cable is grounded at both ends (source & load), the return current will divide between the shield and ground plane. If the frequency of the signal is much greater than the cutoff frequency of the shield, then most of the current will return via the shield. If the frequency of the signal is much less than the cutoff frequency of the shield, then most of the current will return via the ground plane. This scenario is slightly better than the shielded twisted pair arrangement shown above dur to lower resistance of the shield relative to the return conductor of the twisted pair. Noise current passes through the shield and returns through the ground path. Most of the signal current, not shown here, returns via the return of the twisted pair conductor. A potential disadvantage of multiple ground points is that noise currents can exist along the shield. In addition, since the shield has a nonzero impedance, the noise voltage can also vary along the shield. Since capacitive coupling exists between the shield and each of the twisted pair conductors, noise will be induced across the load unless the line is perfectly balanced. The capacitive coupling from each conductor to the shield should be nearly the same. This noise will be a function of the distance along the line.

 

  •  The symmetry of the concentric shield is utilized when the load is floating and the shield is connected to the load. Furthermore, the voltage at the load end of the shield is closer to the voltages along the twisted pair conductors at the load. The currents from the shield to the twisted pair conductors via the parasitic capacitance are less since the voltage across the parasitic capacitance is smaller. DISAVANTAGE: any noise current that couples into the system can now exist on the shield and signal return. The conservative approach is to avoid noise currents on the signal and return conductors even when the system is (partially) balanced.

 

Christian Rosu, Nov 18, 2021.

Automotive LIN bus

10. November 2021 17:32 by Christian in EMC/EMI, EMC TEST PLAN, Test Equipment
In automotive EMC testing the LIN bus master/slave nodes must be terminated to replicate vehicle loading.

In automotive applications the LIN bus is used as communication bus for various functions:

  • Steering wheel: Cruise control, wiper, climate control, radio
  • Comfort: Sensors for temperature, sun roof, light, humidity
  • Powertrain: Sensors for position, speed, pressure
  • Engine: Small motors, cooling fan motors
  • Air condition: Motors, control panel (AC is often complex)
  • Door: Side mirrors, windows, seat control, locks
  • Seats: Position motors, pressure sensors
  • Other: Window wipers, rain sensors, headlights, airflow

A master node loops through each of the slave nodes, sending a request for information - and each slave responds with data when polled. The data bytes contain LIN bus signals (in raw form).

For EMC component level testing purpose, the LIN bus termination must be properly terminated to simulate vehicle loading scenario. 

 

 

Christian Rosu, Nov 10, 2021

EMC Test Plans compliance tricks

8. November 2021 10:49 by Christian in EMC/EMI, EMC TEST PLAN, OEM Specs, Test Methods
How to cheat EMC requirements

FMC1278R3 specification mention that CI 280 (ESD Test Methods) must be carried out prior to any other test methods. If CI 280 fails, continuing the rest of EMC validation must be decided by FORD. The same 2 samples must theoretically withstand all FMC1278R3 test methods selected by EMC Test Plan.

The order of the test methods is critical, and the test results listed by laboratory report applies only to the two samples provided (Part Number, Serial Number, HW/SW revision). Some Europeans & Japanes automotive makers allow the use of multiple groups of samples to be used for simultaneously running test methods, probably to to speed up the completion of validation. This means that no sample is exposed to the full validation test list leaving room for insufficient EMC compliance evaluations.

Example of potentially destructive test methods:

  1. ESD on the first group of samples.
  2. Transients on supply lines on a second group of samples.
  3. Reverse Polarity on a third grtoup of samples

To compensate somehow such selective test methods allocation, the EMC Test Plan authors would require 3 samples per group instead of 2 samples per full validation. 

A parametric test is required following each immunity test method, and this may reveal some tolerances being pushed to one extreme if not outside the acceptable range. In a real scenario, following ESD powered one unit out of three was measured with 12 KΩ impedance on B+ line versus 16 KΩ prior to test. Other than that everything was functional, the DUT current consumption was the same before and after ESD. Theoretically this unit survived ESD and based on EMC Test Plan was not supposed to be tested for Transients on Supply Lines. By mistake this unit was tested for JASO Pulse B-2 (-260V) and the outcome was "sample damged on the second pulse". This EMC test plan trick was used to hide a poor DUT design performance for Honda that otherwise would have never pass FMC1278R3 spec.

  

 

Christian Rosu, Nov 8, 2021.

ISO 7637-2 Pulse #1 & Droputs monitoring tricks

5. November 2021 20:04 by Christian in EMC/EMI, EMC TEST PLAN, OEM Specs, Test Methods
EMC Test Plan tricks

ISO 7637-2 Pulse 1

Conducted Immunity to Transients on battery lines.

Pulse 1 (Us = -150V, Ri = 10Ω, td = 2 ms, tr = 1µs, t1 = ≥ 0.5s (repetition rate), t2 = 200 ms, t3 < 100µs) can upset functionality of electronic modules. Most automotive OEM specs are accepting Class B response (DUT self-recoverable deviations), others are asking Class A response (no deviations) during Pulse 1.

In this particular case the pass/fail criteria was Charging Voltage remains 5V ±0.5V for 12V Battery dropouts ≤ 500µs. The EMC test plan asked the use of DMM to monitor the USB charging function for a Class A expected response:

  • This was a simulation of a mobile phone charging event.
  • DMM can only detect 5V Charging Voltage dips/drops ≥ 250 µs. A FLUKE can be set to count MAX and MIN voltage peaks, otherwise to monitor 5V fast voltage fluctuations is not practically possible.
  • The EMC test plan allowed the use of oscilloscope only for information.

Download this movie to see how the charging function was monitored simultaneously on both oscilloscope and DMM:

 

5V_Charging_during_P1.mp4 (127.57 mb)  

 

A similar monitoring equipment limitation was imposed the EMC Test Plan for dropouts test. Download this movie to see how the charging function was monitored simultaneously on both oscilloscope and DMM:

 

5V_Charging_during_500_microSec_dropout.mp4 (30.91 mb) 

 

 Christian Rosu, Nov 8, 2021

CISPR 25 LISN Types

The latest revision of CISPR 25 is looking into various types of Artificial Networks used in today&#39;s

The latest revision of CISPR 25 is looking into various types of Artificial Networks used in today's automotive EMC.

1. Artificial Network (AN): used for LV power supplies;
2. High Voltage Artificial Network (HV-AN): used for high voltage d.c. power supplies;
3. Direct Current charging Artificial Network (DC-charging-AN): used for d.c. power supplies;
4. Artificial Mains Network (AMN): used for a.c. power mains;
5. Asymmetric Artificial Network (AAN): used for signal/control port lines and/or wired network port lines.

1. Artificial Network (AN)

  

 

 

Measurement ports of HV-AN(s) must be terminated with a 50 Ω load. The HV-AN impedance ZPB (tolerance ± 20 %) in the measurement frequency range of 0.1 MHz to 100 MHz. This table above shows the nominal impedance and upper/lower tolerances in tabular form. It is measured between the EUT HV and ground terminals with a 50 Ω load on the measurement port and with the supply line HV and ground terminals short circuited.

2. High Voltage Artificial Network (HV-AN)

 

 

3. Direct Current charging Artificial Network (DC-charging-AN)

 

4. Artificial Mains Network (AMN)

Power mains must be applied to the vehicle through 50 μH/50 Ω AMN(s). The DC resistance between the ground of the AMN measurement port and the ground plane must not exceed 2,5 mΩ.

 

5. Asymmetric Artificial Network (AAN)

 

 

 

 

Christian Rosu

2021-06-22