how noise is controlled through EMI Filters

[goddessworship]

Hi friends
I am basically from Mechanical stream,
could you tell me,how Filters are helpful in reducing noise,

thanx in before

Jhon

 

[Skogsgurra]

Since you say that you are basically mechanical, I will use a mechanical analogy:

You want to do take a precise measurement - say the height of an object. But you have to do it in a high-speed boat bouncing on the waves. Not easy. Vibrations and shocks disturbs the measurement.

If you could install a heavy platform and cushion it on vibration dampers, the task would be a lot easier. No more bumbs and shocks.

An EMI filter works the same way. It filters the bumbs and shocks (transients and high-frequency noise) away and lets the useful signal through.

The useful signal can be something like a TV signal, an analogue temperature measurement - or plain old mains voltage.

In the TV case, your filter has to let rather high frequencies through, at least 5 megahertz (MHz). In the temperature case, a filter with a cut-off frequency around 1 hertz is OK. And in the mains voltage case, everything above 50 or 60 Hz should be filtered away (for practical reasons and cost reasons, a mains filter lets up to a few thousand Hz through).

So, an EMI filter can have many shapes. From a simple capacitor or ferrite bead to multi-section filters with reactors (coils, with or without magnetic core), capacitors and resistors. They are built to let useful frequencies through and block off noise frequencies.

To get back to the mechanical analogy; they are the shock absorbers and mufflers in the electric world.

 

[Barry1961]

Capacitors can also be compared to hydraulic hat seal accumulators and coils can be compared to long runs of hydraulic tubing. One other component is the resistor which is like a flow control.

The hyraulic accumulator will try to maintain a constant pressure in the system. The size of accumlator you need depends on the depends on the surges it must over come. In the electrical world pressure equals voltage. The capacitor trys to maintain a constant voltage.

For the coil comparison I am going to ignore the effects of the magnetic field. When you have fluid flowing through a long run tubing you must take care when stopping the flow to avoid a "water hammer". The moving mass of hydraulic fluid will resist a sudden change in flow rate or "current". The electrical coil will do roughly the same thing which is try to maintain a constant current.

With these three basic components you can make circuits that will allow low frequency surges to pass, "low pass", high frequency surges to pass, "high pass", or only one frequency to pass, "notch pass". The notch pass filter is what your car radio uses to just tune in one station.

Another common method of reducing EMI is by sheilding. This is basically done by placing a antenna such as aluminum foil around what you want to protect and then grounding your sheilding antenna.

In your field of aerospace though things are not so simple. Reducing EMI on an airborne platform with a mix high power/high voltage circuits and low power/low voltage circuits with no real "ground" is almost witchcraft. Much of the EMI reduction is done by changing the components physically to make a filter of sorts out of the component itself. The simplest example I can think of would be changing the tuning of a wire which is picking up harmfull EMI.

Just as you have probably played with a radio antenna by moving it, adding wire, shaping a clothes hanger you added to it or holding on to it and standing on one foot to get better reception you can do basically the same thing on an aircraft. One common temporary method is to get the problem cable and start wrapping it around a ferrite bar. This has the effect of tunning the cable to pick up a different frequency which hopefully is not as harmfull as the frequency you tuned out.

This filtering by changing the physical properties of the components is used all the way from deciding the path of traces in circuit boards to placement of the antenna on the platform.

Barry1961