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

For shielding unbalanced two-wire cables, it is critical to understand the impact of changes to ground connections to chasis vs signal return.

The signal ground for both the driver and receiver systems is defined as the internal "O V" reference for each system. 

SCENARIO #1: UNBALANCED DRIVER & BALANCED RECEIVER

If the output of one device contains a balanced driver and the input of the other device contains a balanced receiver, then the nearly ideal shielding scheme is the connection of the cable shield to both of the chassis at both ends. If there are no pigtails present, then this shielding configuration is nearly ideal since the entire system is enclosed by one continuous metal shell. The cable can be viewed then as an extension of the chassis. This scenario (#1) is not perfect since the two chassis might be at two different potentials. This system can still be corrupted and unbalanced by other sources. When the chassis for driver and receiver circuitry are at different potentials, ground current will pass between the chassis and through the cable shield, forming a ground loop. Depending on factors such as the frequency, this current can appear on the inside of the cable shield and chassis. This ground loop current and the field it generates can also interfere with other systems that are not completely shielded.

Although the driver and receiver are shown as balanced, they cannot be perfectly balanced, and some of this inner shield current can induce noise into the circuit. Because the length of the connecting cable is often large compared to the largest dimensions of the two chassis, the balancing of the wires inside the cable is probably most critical. However, any current that is present along the inner surface of the two chassis can also induce noise in other nonbalanced and other nonideal balanced circuits inside either chassis. 

SCENARIO #2: UNBALANCED DRIVER & BALANCED RECEIVER

In this configuration the cable shield is connected to only one of the chassis. If only one of the chassis are connected to their respective signal grounds, then the other signal ground should not be connected to the cable shield. Otherwise, noise currents induced on the chassis and cable shield would travel inside the device through this signal ground connection, partially defeating the purpose of the shielding. Although the two outputs and two inputs of the balanced driver and receiver are not connected to these signal grounds, the rest of the circuit will probably contain unbalanced circuitry that would use a signal return or ground. When both the driver and receiver chassis are connected to their respective signal grounds, then there is no clear best solution. Usually, the cable shield is not left floating. If there i no convenient way of connecting the cable shield to either chassis, the cable shield is connected to one of the signal grounds. Usually, to avoid excessive noise currents on the signal grounds, the cable shield is not connected to both of the signal grounds; one end of the cable shield is left disconnected or not tied to anything. Since the signal ground is likely connected to the chassis ground at a single point, the cable shield is at a low potential. There is a potential benefit of connecting the shield to the signal ground at both ends: the path of the noise current is well known and the crosstalk might be better controlled. 

When a balanced driver connects to an unbalanced receiver, the unbalanced receiver easily amplifies noise that is picked up by the system. With balanced receivers, the receiver rejects some of the commonmode noise. For this reason, a balanced driver and unbalanced receiver combination can be very troublesome. As with the previous configurations, the 1st ranked configuration shown in Table 18.4 involves connecting the cable shield at both ends to the chassis. Again, noise currents on the cable shield can induce noise on the two conductors inside the cable. To reduce this noise or crosstalk coupling to the cable's inner conductors, the cable length should be minimized. If this noise level is too great, it may be necessary to disconnect the cable shield from one of the chassis as shown in the ranked configurations. If connection of the cable shield to a chassis is not possible, then the cable shield can be connected to either signal ground (but not usually both) as shown in the 3rd ranked ronfigurations. Care should be taken when connecting either output of the balanced driver to the signal ground of the unbalanced receiver. Essentially, the corresponding output is short circuited by this connection, which can damage the output device and cause distortion. Even connecting the output of the balanced driver to the signal ground of the receiver can be troublesome. Balanced floating drivers are available to alleviate some of these "shorting" of the output problems. Although usinga balanced driver can help in reducing the emissions from the system, it does not help (much?) for decreasing the susceptibility of the system. 

SCENARIO #3: BALANCED DRIVER & UNBALANCED RECEIVER

The driver is unbalanced while the receiver is balanced. Connecting an unbalanced driver to a balanced receiver will decrease the common-mode rejection ratio (CMRR) of the entire system. As with completely balanced systems, potentially troublesome noise current can exist along the inner surface of the cable shield and chassis. Since the driver and receiver combination is not balanced in this case, the noise current can couple into the system more easily than in the case with the fully balanced system. For this reason, interrupting the conducting path for these noise currents can result in lower noise levels. Therefore, either of the 2nd ranked configurations may result in lower noise levels than the 1st ranked system. If the option of connecting the cable shield to one chassis is not available, then the cable shield should be connected to one of the signal grounds. Generally, the shield should not be left floating. It is probably a good idea not to connect the cable shield to the signal grounds at both the driver and receiver. Not connecting the shield to both signal grounds will tend to reduce the noise currents on the signal grounds by interrupting the conducting path. The 3'd ranked configuration is commonly used, especially when the actual grounding connections inside the driver and receiver are not known with certainty. 

SCENARIO #4: UNBALANCED DRIVER & UNBALANCED RECEIVER

 Typically, when an unbalanced driver is connected to an unbalanced receiver, coaxial cable or other two-conductor cable (no three-conductor cable is used). However, if shielded two-wire cable is used SCENARIO #4  can be referred to for grounding recommendations. The previous rationale also applies for these connections.
Generally, shielded two-wire cable when properly connected will be less susceptible to noise and have lower field emissions than unshielded two-conductor cable. The balance nature of the cable is also an important factor. Shielded two-wire cable, such as shielded twisted pair, is often balanced. To help in the balancing of the system, when connecting a balanced driver to a balanced receiver, it is highly recommended that the connecting cable also be balanced. Although shielded balanced cable is recommended, the following comments are provided in those cases where two-conductor cable is used.

Twisted pair that is not shielded is considered a two-conductor cable. Twisted pair is considered a balanced cable. When balanced twisted pair is used to connect a driver enclosed by a metal chassis to a receiver enclosed by a metal chassis, the cable shield conductor is not present to "continue" the metal enclosure of the chassis. Although not all possible combinations will be discussed, generally for twisted pairs:

•  When connecting a balanced driver to a balanced receiver, neither side of the twisted pair should be connected, if possible, to either the chassis or signal grounds; otherwise, the balance of the system will be affected.
• When connecting an unbalanced driver to a balanced receiver, one conductor of the twisted pair must be connected to the signal ground at the driver to provide a return path for the driver current. Since the signal ground is likely connected to the chassis at one point, this may imply that one conductor of the cable is connected to one chassis (but not both). Neither the signal nor the chassis ground at the balanced receiver should be connected to either of the two twisted-pair conductors. Otherwise, a conductive path is available for any noise currents on the ground system.
• When connecting a balanced driver to an unbalanced receiver, one conductor of the twisted pair must be connected to the signal ground at the receiver. Additional connections to ground are normally avoided.
• When connecting an unbalanced driver to an unbalanced receiver, one conductor of the twisted pair must be connected to the signal ground at both the driver and receiver. This implies that the signal (and noise currents will return on both the cable return conductor and signal ground path.

When systems are connected with coaxial cable, the outer conductor is acting like a shield. Unfortunately, this outer conductor is also the signal return conductor. Coaxial cable has the advantage of providing some shielding, but coaxial cable has the disadvantage of being an unbalanced cable. For this reason, when coax is used to interconnect a balanced driver and receiver, it will tend to decrease the balance of the system more than twisted pair. The outer conductor of coax should be used as the signal return conductor. For this reason, the outer conductor or shield of the coax should be connected to something on both ends and not left floating. The previous recommendations for unshielded twisted pair can be applied to coax. The two conductors for the coax are the inner conductor and outer conductor. The only major difference is that the outer conductor or shield is normally connected to the more negative side of the driver and receiver. 

 Christian Rosu, Nov 18, 2021

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.

Shielded enclosure example:

Shielded enclosure connector types:

See Troubleshooting RF Noise and Fixing Ground Loops

Shielded cable types:

• Foil provides high-frequency shielding.
• Drain wire carries most of the low- frequency current.

• Foil provides high-frequency shielding.
• Braid carries high-currents.

Why Cable Shielding?

• Serves different purposes in different applications.
• Sometimes carries intentional signal currents. This is an example of self-shielding.
• May prevent coupling of external electric or magnetic fields to signals carried by wires in the cable.
• Beware of transfer impedance data. It should only be used to compare similar cables for a similar application measured with the same test set-up.

Summary of Main Points:

• Electric field shielding, magnetic field shielding, cable shielding and shielded enclosures are very different concepts requiring different materials and
  approaches.
• It is important to be understand the coupling mechanism you are attempting to attenuated before coming up with a shielding strategy.
• Electric field shields terminate or redirect electric fields. Where they are “grounded” is often critical.
• Magnetic field shields redirect the magnetic field. Magnetic fields cannot be terminated. 
• Low frequency (<kHz) magnetic fields must be redirected with high permeability materials. 
• High frequency magnetic fields can be redirected with good conductors of sufficient thickness.
• Shields and imperfect shielded enclosures can significantly increase radiated emissions. 
• Shields reduce radiated emissions by disrupting the coupling from near-field sources and the “antennas” in a system. 
• In the near field, shields are either electric or magnetic field shields that redirect high-frequency current flow.

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.

Types of Electromagnetic Coupling: Conducted, Radiation, Magnetic Field, Electric Filed

Electric Field Coupling

The EF lines start on positive charge and end on negative charge from higher voltage conductors to lower voltage conductors. Any two conductors at different potentials (voltages) have electric field lines between them. EF shields are connected to “ground” to maximize their effectiveness.

     The electric field lines are passing through ungrounded metallic planes. 

Grounding a copper enclosure does not increase or decrease its shielding property but it reduces the crosstalk within the product itself. The ungrounded shield allows coupling signals from circuits within the shielded enclosure. If the device is connected via external cable to another module the ungrounded shield can serve to capacitively couple signals from outside the enclosure.