PCB design for Real-world EMI control

Primary resonance can be moved to a higher frequency(where it is not going affect amissions in the frequency range of concern). It is clear from these simulations that one or two contact points should be avoided, since the resonances are likely to be in the range of the first or second harmornic of the processor clock frequency(where the most energy exists). Eight contact points provided the most significant improvement of all the configurations tested.

PCB reference connection to Chassis reference

If we have a system with a completely enclosed chassis with no I/O cables leaving the enclosure, it makes no difference how, or if, the internal circuits are connected to the

enclosure chassis inside the enclosure. Any internal fields from the PC board are contained within the enclosure. This is seldom the case, however, since most real world products include openings and I/O cables.The most common cause of external emissions in typical

products is unwanted common-mode voltages from unintentional signals on the I/O cables and wires relative to the chassis. From an external point of view, we can consider the enclosure chassis to be reference, and the voltage on the I/O cables to be the feed for an antenna, we might even consider the enclosure and wire(with the source between them) to be a kind of lumpy, irregularly-shaped dipole antenna. PCB Assembly Regardless, the combination of I/O wire and enclosure creates an antenna that will radiate quite effectively at some frequencies.

A low inpedance connection between the PC board and the chassis, when done correctly, can reduce the emissions from these unintentional signals.

I/O Area connection

Consider the case with two I/O wires, such as a signal line and its return(ground),(for example, an audio speaker connection to a persional computer). The return wire is usually connected to the ground-reference on the PC board. Any filtering on the audio signal line is referenced to the ground-reference on the PC board. The reference for the external rediation is the chassis and not the ground-reference on the PC board. Any impedance between the chassis and the PC board gound-reference will allow a voltage to be created between them. This voltage will appear to be source on the I/O reference wire and the I/O signal wire. Figure 4-19 illustrates this problem.

The connection between the PC board reference and the chassis is often accomplished with stand offs and machine screws. The placement of these stand offs is mostly arbitrary and seldom selected for best EMI emissions control. Figure 4-19 shows a connection impedance between the PC board reference and the chassis. This impedance consists of the resistance of the connection and the inductance of the connection. If we consider the loop inductance of the connection between the chassis and the PC board, we will want to keep the loop area as small as possible. Figure 4-20 shows the loop area for the side view and a top view. The total three dimersional loop area is important.

Many I/O connectors have shields that serve two purposes. First, they shield the connector pins from unwanted signals coupling directly onto them from fields inside the enclosure. Secondly, and more importantly for this discussion, they provide a low impedance and low loop area path from the chassis to the PC board. This connector becomes the main connection between the ground-reference plane on the board and the chassis.

PCB Summary

Unfortunately, it is unlikely that the word "ground" is removable from designer's vocabulary. The concept of "ground" is easy to understand and emotionally comforting. Once the frequency range of the signals is above about 10KHZ, however, the "ground" concept is not a good way to think about the physics of the current flow. Consideration to the return current flow path is vital to controlling "ground" currents.

The term "ground" is often misused to mean a number of different reference. It is much better to consider the power-reference, the signal-reference, the chassis-reference. Cable shield-reference,etc. Once these various electronic assembly references are clearly described, then the proper connections can be established, and the return currents controlled.

PCB design for Real-world EMI control

Primary resonance can be moved to a higher frequency(where it is not going affect amissions in the frequency range of concern). It is clear from these simulations that one or two contact points should be avoided, since the resonances are likely to be in the range of the first or second harmornic of the processor clock frequency(where the most energy exists). Eight contact points provided the most significant improvement of all the configurations tested.

PCB reference connection to Chassis reference

If we have a system with a completely enclosed chassis with no I/O cables leaving the enclosure, it makes no difference how, or if, the internal circuits are connected to the

enclosure chassis inside the enclosure. Any internal fields from the PC board are contained within the enclosure. This is seldom the case, however, since most real world products include openings and I/O cables.The most common cause of external emissions in typical

products is unwanted common-mode voltages from unintentional signals on the I/O cables and wires relative to the chassis. From an external point of view, we can consider the enclosure chassis to be reference, and the voltage on the I/O cables to be the feed for an antenna, we might even consider the enclosure and wire(with the source between them) to be a kind of lumpy, irregularly-shaped dipole antenna. PCB Assembly Regardless, the combination of I/O wire and enclosure creates an antenna that will radiate quite effectively at some frequencies.

A low inpedance connection between the PC board and the chassis, when done correctly, can reduce the emissions from these unintentional signals.

I/O Area connection

Consider the case with two I/O wires, such as a signal line and its return(ground),(for example, an audio speaker connection to a persional computer). The return wire is usually connected to the ground-reference on the PC board. Any filtering on the audio signal line is referenced to the ground-reference on the PC board. The reference for the external rediation is the chassis and not the ground-reference on the PC board. Any impedance between the chassis and the PC board gound-reference will allow a voltage to be created between them. This voltage will appear to be source on the I/O reference wire and the I/O signal wire. Figure 4-19 illustrates this problem.

The connection between the PC board reference and the chassis is often accomplished with stand offs and machine screws. The placement of these stand offs is mostly arbitrary and seldom selected for best EMI emissions control. Figure 4-19 shows a connection impedance between the PC board reference and the chassis. This impedance consists of the resistance of the connection and the inductance of the connection. If we consider the loop inductance of the connection between the chassis and the PC board, we will want to keep the loop area as small as possible. Figure 4-20 shows the loop area for the side view and a top view. The total three dimersional loop area is important.

Many I/O connectors have shields that serve two purposes. First, they shield the connector pins from unwanted signals coupling directly onto them from fields inside the enclosure. Secondly, and more importantly for this discussion, they provide a low impedance and low loop area path from the chassis to the PC board. This connector becomes the main connection between the ground-reference plane on the board and the chassis.

PCB Summary

Unfortunately, it is unlikely that the word "ground" is removable from designer's vocabulary. The concept of "ground" is easy to understand and emotionally comforting. Once the frequency range of the signals is above about 10KHZ, however, the "ground" concept is not a good way to think about the physics of the current flow. Consideration to the return current flow path is vital to controlling "ground" currents.

The term "ground" is often misused to mean a number of different reference. It is much better to consider the power-reference, the signal-reference, the chassis-reference. Cable shield-reference,etc. Once these various electronic assembly references are clearly described, then the proper connections can be established, and the return currents controlled.