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This paper describes a new method for the selection of an appropriate signal line Electromagnetic Interference (EMI) filter. To date, EMI filter selection has been based on the measurement of the radiation of the entire device. The new selection method based on the signal's Fast Fourier Transform (FFT) measurement has proved to be efficient. The EMI filter is optimized separately for each line. The method described in this paper involving a Central Processor Unit (CPU) module demonstrates that the proposed FFT-based selection method is better than the radiation-based one. The radiation level in the frequency range 30 MHz to 1 GHz is lower for approximate 2 – 6 dBμV/m.

When we develop small devices such as sensors we are often confronted with Electromagnetic Compatibility (EMC) problems because components are placed very close to each other. To some extent these EMC problems can be solved with suitable component selection method and printing circuit board design. When this does not solve the problems EMI filters must be used. They must be placed near noise sources such as microcomputers, digital drivers, oscillators, input-output lines, etc.

There are three EMI filter selection methods. The first method uses ground plane for EMI filter construction. It is described in Chapter 2 of this paper. The second one is based on radiation measurements. The radiation of the entire device is measured, which provides a discrete frequency component with the maximum amplitude. The EMI filter with maximum insertion loss close to this discrete frequency component is selected. The resulting filters have the same frequency characteristics within the entire device. The third method has been developed and tested in the EMC Laboratory of the ISKRAEMECO d.d. Company. It is based on the FFT measurements on a single signal line.

The main advantage of the new method is that a filter is selected for each single line separately. This gives better results in terms of EMC problem solving than the other two methods. It is very important that this new EMI filter selection method and all other recommendations for good EMC design are used in the early development phase of a product such as sensor.

Correct grounding is critical to EMC problem solving. Every signal line must be surrounded by ground (

The expression “critical line length” is known from high-speed transmission line theory [

The spectrum consists of discrete frequency components f_{n} = nf_{T}, where f_{T} = 1/T. By drawing asymptotes of the spectrum, we get a horizontal line up to the first corner frequency (

For higher frequencies, the rate of decrease is 40 dB/dec. (

Frequency f_{knee} is a practical maximum which is about 1.5 times f_{2}.

The critical line length is determined by the frequency f_{knee}. It is well known that electromagnetic emission increases with frequency until half the wavelength of the signal exactly fits the length of the trace.

The propagation of the line is

T_{pd} is the propagation delay. The wavelength in

By using the

At the critical line length the rise time T_{r} exactly matches the two-way propagation delay time T_{pd} (source – load – source). This means that the transient phenomenon formed by the low-to-high signal transition precisely fits the line length. For that reason, this distance (the critical line length) is referred to as the length of the rising edge [

To simplify _{r}=4.6). The latter varies with signal frequency within the material. Most engineers generally assume ε_{r} to be in the range of 4.5 to 4.7. These are the values that are published in various technical references [

A line length equal to or longer than the critical length definitively behaves as a transmission line. Characteristic impedance, delay and reflections should not be ignored. At the same time, it also behaves as an efficient antenna and is reflected in considerable electromagnetic radiation and susceptibility problem. Corrupted signals are usually rich on higher frequency components.

Typical frequency – observed as EMI – is a frequency at which most EMI-related problems can be expected [

These typical frequencies – observed as EMI – are very important when designing an electronic circuit.

To check the accuracy of the

The measured signal rise time t_{r} was 12 ns. The result of the calculation of the typical frequency – observed as EMI (11) – is 265.258 MHz (12). The calculated value is the same as the measured one.

The calculation of the typical frequency – observed as EMI – is relatively simple. Its accuracy was also confirmed by the measurement.

So far no attention has been paid to the filter adjustments [

EMI filters have been placed near the noise source on all signal lines exceeding the critical line length (l_{max}). The adjustments can also be further improved by a proper EMI filter selection. In addition, attention has to be paid to the EMI filter structure.

EMI filter insertion losses are handled at 50Ω input and output impedance [

The procedure for a proper EMI filter selection consists of the following steps:

Measurement of the signal rise time t_{r};

Calculation (or measurement) of the typical frequency – observed as EMI;

Selection of a suitable EMI filter family with regard to the application requirements;

Selection of an EMI filter from the family which has the maximum insertion loss at the approximate typical frequency.

The need for the use of EMI filters depends upon the critical line length which has been verified in practice [_{max} (critical line length) and if there are no via connections on the line, the use of an EMI filter is not necessary.

The described EMI filter selection procedure is shown in

Generally, a selection of suitable EMI filters is very important for designing. The tests performed on the CPU module clearly demonstrate that the proposed FFT-based EMI filter selection method is better than the radiation-based one. We used CPU module with 24 MHz, 32 bits microcontroller (Motorola MC 68332) on the eight layer printed circuit board.

First, CPU module was equipped with EMI filters selected with the radiation-based method and the radiated emission was measured (

The main advantage of this method is that it solves EMC emission problems separately for each single signal line, which is why we believe it produces better results.

EMI filter selection method described in this paper was developed in the EMC Laboratory of the ISKRAEMECO d.d. Company and in the Electrical Measurements Laboratory, Faculty of Electrical Engineering and Computer Science, University of Maribor, and I thank them both.

Correct grounding.

Spectrum of digital periodic signal.

FFT of an output signal from 74HC245 logic circuit.

Selection of an appropriate EMI filter structure.

FFT of a signal that has passed through the Murata EMI filter NFW31SP506X1E4 – an output from 74HC245 logic.

EMI filter selection flow chart.

Radiated emission (radiation-based method).

Radiated emission (new method).