Tom
10-23-2002, 02:55 PM
IPC Advanced Study Guide Page Reference: Pages 37 & 38, Section 1.5
Electro-magnetic compatibility (EMC) is concerned with electro-magnetic emission and susceptibility of electronic equipment to these conditions. Simply speaking, the subject of electro-magnetic compatibility is a concern for two issues: 1) the emissions, and 2) the susceptibility to be influenced by external emissions.
The basic rules for these should be:
1) Equipment design should be such that it does not emit electro-magnetic energy into the environment that causes interference with another assembly or equipment.
2) Equipment design should not be susceptible to electro-magnetic emission commonly found in the environment.
Susceptibility refers to a units ability to reject unwanted electro-magnetic interference and unwanted radio frequency (RF) signals prevalent in the environment. An automobile radio is a good example of a susceptibility problem. When the radio is tuned to a particular station and another station keeps “drifting in," the condition is normally referred to as "radio frequency interference." It shows the radio’s inability to reject the unwanted RF signal.
Emission refers to electro-magnetic energy that is radiated by a unit that might interfere with another piece of equipment. A laptop computer is a good example of the possibility of emissions occurring. The laptop has high frequency clock circuits that radiate electro-magnetic fields from the conductors, component leads, wires and cables. These emissions are strong enough to upset avionics equipment thus, the use of laptops or similar devices are prohibited from being used during aircraft takeoff and landings.
Radio frequency interference is defined as any undesired or annoying electro-magnetic signal in the RF frequency spectrum that competes with signals of interest. As a rule, the higher the frequency of operation, the more susceptible the circuit is to RF interference. Wavelength is the reciprocal of frequency; as frequency increases, wavelength decreases. When the length of the conductor exceeds 1/10 of the signal wavelength, the circuit is considered radio frequency. It should be noted that wireless communication such as radio, TV, and cellular phones, all fall within the radio spectrum.
Radio frequencies in the range of the electro-magnetic frequency suitable for wireless communication can be classified as follows:
- Very low frequency (VLF) that is 10-30 kHz
- Low frequency (LF) is 100 kHz
- Medium frequency (MF) 300 - 3000 kHz
- High frequency (HF) 3 - 30 MHz
- Very high frequency (VHF) 30 - 300 MHz
- Ultra high frequency (UHF) 300 - 3000 MHz
- Super high frequency (SHF) 3 - 30 GHz
All conductors and component lead wires act like antennas and can thus pick up qualified EMI interferences. The longer and narrower a conductor or lead wire is, the more likely it is to pick up these unwanted signals. The effort of design analysis, signal integrity, printed circuit board layout and electro-magnetic susceptibility analysis should be integrated into the design. The designer should use a much higher level of EMC compliance the first time a product is tested without design overkill.
Stray electro-magnetic fields can wreak havoc with the operation of printed circuits in electronic instruments by coupling with wires, conductors, and component leads, causing unwanted currents to flow. Shielding is one of the ways to protect printed circuits from the effects of stray electro-magnetic fields. The shielding could be as simple as adding a ground plane to a board, or as complicated as placing the entire assembly inside a solid metal enclosure with special gaskets that reflect or absorb the electro-magnetic energy before it reaches the sensitive circuit.
Many times shielding is accomplished through the use of multilayer board inner layer planes. The figure shows a ten layer configuration intended to help the performance of the printed board, electronic characteristics as well as providing EMI performance protection. See the "Inner Trace Layers" in the below pictures. That's where your "High Speed" conductors should be routed.
Electro-magnetic compatibility (EMC) is concerned with electro-magnetic emission and susceptibility of electronic equipment to these conditions. Simply speaking, the subject of electro-magnetic compatibility is a concern for two issues: 1) the emissions, and 2) the susceptibility to be influenced by external emissions.
The basic rules for these should be:
1) Equipment design should be such that it does not emit electro-magnetic energy into the environment that causes interference with another assembly or equipment.
2) Equipment design should not be susceptible to electro-magnetic emission commonly found in the environment.
Susceptibility refers to a units ability to reject unwanted electro-magnetic interference and unwanted radio frequency (RF) signals prevalent in the environment. An automobile radio is a good example of a susceptibility problem. When the radio is tuned to a particular station and another station keeps “drifting in," the condition is normally referred to as "radio frequency interference." It shows the radio’s inability to reject the unwanted RF signal.
Emission refers to electro-magnetic energy that is radiated by a unit that might interfere with another piece of equipment. A laptop computer is a good example of the possibility of emissions occurring. The laptop has high frequency clock circuits that radiate electro-magnetic fields from the conductors, component leads, wires and cables. These emissions are strong enough to upset avionics equipment thus, the use of laptops or similar devices are prohibited from being used during aircraft takeoff and landings.
Radio frequency interference is defined as any undesired or annoying electro-magnetic signal in the RF frequency spectrum that competes with signals of interest. As a rule, the higher the frequency of operation, the more susceptible the circuit is to RF interference. Wavelength is the reciprocal of frequency; as frequency increases, wavelength decreases. When the length of the conductor exceeds 1/10 of the signal wavelength, the circuit is considered radio frequency. It should be noted that wireless communication such as radio, TV, and cellular phones, all fall within the radio spectrum.
Radio frequencies in the range of the electro-magnetic frequency suitable for wireless communication can be classified as follows:
- Very low frequency (VLF) that is 10-30 kHz
- Low frequency (LF) is 100 kHz
- Medium frequency (MF) 300 - 3000 kHz
- High frequency (HF) 3 - 30 MHz
- Very high frequency (VHF) 30 - 300 MHz
- Ultra high frequency (UHF) 300 - 3000 MHz
- Super high frequency (SHF) 3 - 30 GHz
All conductors and component lead wires act like antennas and can thus pick up qualified EMI interferences. The longer and narrower a conductor or lead wire is, the more likely it is to pick up these unwanted signals. The effort of design analysis, signal integrity, printed circuit board layout and electro-magnetic susceptibility analysis should be integrated into the design. The designer should use a much higher level of EMC compliance the first time a product is tested without design overkill.
Stray electro-magnetic fields can wreak havoc with the operation of printed circuits in electronic instruments by coupling with wires, conductors, and component leads, causing unwanted currents to flow. Shielding is one of the ways to protect printed circuits from the effects of stray electro-magnetic fields. The shielding could be as simple as adding a ground plane to a board, or as complicated as placing the entire assembly inside a solid metal enclosure with special gaskets that reflect or absorb the electro-magnetic energy before it reaches the sensitive circuit.
Many times shielding is accomplished through the use of multilayer board inner layer planes. The figure shows a ten layer configuration intended to help the performance of the printed board, electronic characteristics as well as providing EMI performance protection. See the "Inner Trace Layers" in the below pictures. That's where your "High Speed" conductors should be routed.