EMC-Electromagnetic Compatible System Requirement
We have seen in our home that when DC motor or mixer is connected with AC mains and powered ON while watching TV, it creates noise on the TV screen. We have also noticed that lightening creates noise in the TV receiver. The DC motor, lightning, fluorescent lights are sources of electromagnetic interference. There are two types of electromagetic emissions viz. radiated and conducted. The radiated emission refers to undesired EM wave radiations resulting into malfunctioning of the nearby system. The conducted emission refers to undesired radiations through electric power lines resulting into disturbance into the other system connected with the same power line.
Electromagnetic Interference (EMI) is the term refers to interference caused to one system resulting from undesired EM radiations by another system. Electromagnetic compatible (EMC) refers to ability of one system not to radiate undesired EM waves so that another systems will function as desired without any problem.
In other words, an electromagnetic system is said to be electromagnetically compatible with its environment, if it is able to function compatibly with other electronic systems and does not produce or receive unintentional radiation.
The essential requirements to make a system electromagnetic compatible are as follows:
• The system itself will not radiate any unintentional radiation i.e. it will not cause interference to other systems.
• The system will not receive any unintentional radiation i.e. it should be susceptible to the emissions of other systems.
• The coupling paths between the systems will be as inefficient as possible.
• Any component or module of the system will not produce interference in the same system itself. The same effect have been observed when oscillators are not properly shielded or DC supply lines to RF circuits are not properly designed.
Due to increase in interference in the digital systems, FCC in the US have specified limits for electromagnetic emissions. If the system fulfills these limits then it is said to be EMC compliant system. There are two types of devices based on their applications viz. class A (for business use) and class B (for residential use). FCC have defined conducted emission and radiated emission limits at different range of frequencies for these class A and class B device. Refer conducted and radiated limits for more.
If these undesired radiation is not minimized it will couple with other nearby circuits and will create different types of problems such as cross talk, propagation delay, parasitic effects, false triggering etc. Hence while designing PCB based on microstrip as well as other RF structures following need to be considered.
Electromagnetic Compatible System Design
Following are important aspects need to be taken care while designing electromagnetic compatible system:
• Grounding problem: As we know conductors used for grounding will have certain amount of impedance, Hence any current passes through imperfect ground conductor will have different potentials at different points of the ground conductor. A ground grid structure is used to minimize this problem. The grid must be connected at all the points using via holes in order to make it more effective.
• Emission Models: The unintentional antennas in a system are the wires, PCB lands and other metallic structures. Depending on current flow through two close wires or lands there are two modes viz. common mode and differential mode. Differential mode will have less radiated field compare to common mode of emission considering the same frequency and same line length. But differential mode emission field depends on frequency and hence it increases with the increase in the frequency. The differential mode of emission field can be minimized by reducing the current level and also loop area. The common mode emission level is minimized by reducing line length and current level.
• Clock pulse implementation: In the mixed circuit design consisting of both digital and analog parts, clock pulse implementation is very important. As we know clock signals are periodic and deterministic signals. As we know pure square wave will have many harmonics of the sine wave. This square wave consists of high frequency spectral components. By changing the shape of square to the trapezoidal pulse, these high frequency emissions can be greatly reduced. This is done by introducing rise and fall time to the square wave pulse.
• Crosstalk reduction: This can be done by providing shielding ground lines adjacent to the active interconnections.
• Substrate material selection: A substrate of MMIC is a piece of material on which all the RF circuit layout can be etched. On this layout different RF components are soldered. The ideal substrate material should have high dielectric constant, low dissipation factor and should have no variation of DC voltage over the entire frequency range of the operation.
• Conductor material selection: The ideal conductor material should have high conductivity, low temperature coefficient and good adhesion with the substrate.
• Impedance mismatch: Line should be perfectly terminated with proper value of characteristic impedance. If not it will results into reflections and damped oscillations.
• Power supply: Variations and transients should be minimized at the supply source using ferrite bead and/or capacitor.
With the above considerations one can able to design electromagnetic compatible system.
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