Filters are vital in many areas from the design of communication and radar circuits and systems to EMC protection. The filter is a circuit element that filters out unwanted signals in another frequency region while ineffective and does not touch useful signs in a certain frequency region.
Filters used to troubleshoot EMC problems can be classified as mains power source filters, DC power supply filters and communication filters. The mains power supply filters are in a linear structure and are placed inside the screen and the mains power supply input to prevent electromagnetic noise from entering the system. DC power supply filters have the same structure as mains power supply filters. However, the cutting frequencies are selected as low as possible. Communication filters can be any kind.
Operating characteristics (voltage, current and temperature), reliability (maximum leakage current, overload current), electrical properties (high voltage characteristics, insulation resistance) and mechanical properties (dimensions, installation problems) are the most important selection parameters in filter selection. EMC characteristics expected from the selected filter are interference loss, cut frequency, capacitive or inductive load, determination of device impedance at high frequencies and common mode and / or differential mode filter requirement are important EMC features.
Another factor that determines the filter performance is the way the filter is mounted on the screen.Therefore, for a good performance, the filter is mounted on the screened body at the point where the power cable enters. The input cables must be well coupled from the output cables. Bodymounted filters are preferred. Otherwise, it is necessary to screen the filter and isolate the inputs and outputs from each other. Good electrical contact must be maintained between the filter body and the ground. Connection wire should not be used. Attention should be paid to galvanic compatibility.
EMC FILTER DESIGN
The EMC filter design is critical to the electromagnetic compatibility, EMC performance. The EMC filter must be capable of providing the required level of attenuation of the unwanted signals while allowing through the wanted signals. In addition to this the EMC filter design must match both the source and load impedances.
Typically for a high impedance circuit, a capacitor connected between the line and ground provides better results, while for low impedance circuits a series inductor placed within the line provides the best results. Often a single component like this designed to have a reactance with little effect at frequencies appropriate to the wanted signals, but a much higher effect at the higher frequencies of the unwanted signal can provide levels of attenuation of up to 30 dB or 40dB in some cases. To improve the performance of one of these basic filters, further components can be added to make multi-component EMC filters. However, to give the required performance they must be configured correctly. One precaution to ensure that inductors face a low impedance sink or source and capacitors face a high impedance.
Functionally, there are basically four types of filters: Low-pass filter, High-pass filter, Band-pass filter and Band-stop filter.
Low Pass Filter
A Low Pass Filter is a circuit that can be designed to modify, reshape or reject all unwanted high frequencies of an electrical signal and accept or pass only those signals wanted by the circuits designer.
High Pass Filter
A High Pass Filter is the exact opposite to the low pass filter circuit as the two components have been interchanged with the filters output signal now being taken from across the resistor.
Band Pass Filter
Band Pass Filters can be used to isolate or filter out certain frequencies that lie within a particular band or range of frequencies.
Band Stop Filter
A band Stop Filter known also as a Notch Filter, blocks and rejects frequencies that lie between its two cut-off frequency points passes all those frequencies either side of this range.