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Electrical Noise part 2.

A farewell lunch for yours truely -- This is not the way to do shielding

A farewell lunch for yours truly — This is not the way to do shielding.

In part 1 of electric noise I talked about simple but effective ways to keep the bad out while allowing the good to flow. Those methods are effective when they can be used, but often it is impossible.  For example, the only way to catch radio waves is to put up an antenna and with it you will get the bad along with the good.  The only answer at this point is to try to filter out the bad.   For example, a directional antenna could be used so the signals in the direction it is aimed will be much stronger than those in the other directions.

To filter out the noise means you must know something about the thing you want to filter out as well as the signal you want to keep.  Obviously, in the directional antenna example the direction of the signal is a very important thing to know.  Normally in electronics the important thing to know is the frequencies involved.  Basically there is 4 broad types of filters: a low pass filter; a high pass filter; a band pass filter; and a notch filter.  As always there are details in filters, but for now we are just going to talk about the broad classes.

A low pass filter as the name says, will allow low frequencies to pass, but will reject the high frequencies.  An inductor in series and/or a capacitor in parallel will exactly do that.   We saw an example of a low pass filter in the post: “Bigger is not always Better –  Filter Capacitors for Power Supplies“.  Sometimes it is necessary to install smaller capacitors from power to ground throughout circuits where the power is fed to individual I.C. circuits.  This is called a decoupling capacitor and is often used in digital circuits where power is switched off and on in the individual chips.   It may be a problem for me in my analog thermistor circuit because of the routing of the power leads and I could end up with inductor loops and capacitor feedback forming oscillators.  (More about oscillators in a future post.)

Another example of a filter being installed was the problem with alternator whine in a C.B. radio installed in a car.  There was an aftermarket filter we installed on the power feed from the alternator.  This filter consisted of a few turns of wire to form an inductor coil and a capacitor afterwards going to ground.   The generator whine was caused by the diodes inside the alternator switching on when the voltage being generated was higher than the battery voltage.

The next kind of filter to consider is a high pass filter.  This can be a capacitor in series and/or an inductor in parallel. Normally noise is a higher frequency than the desired signal, so I have never seen one of these used except in loud speakers.  Often there is a capacitor in series with the tweeters to keep the lower frequencies from reaching the tweeters.

A band pass filter is simply a high pass filter followed by a low pass filter.   The “cut off” frequency of the low pass filter is higher than the cut off frequency of the high pass filter.  This means only a small “pass band” of frequencies makes it on to the processing electronics.  All wireless communications is very dependent upon band pass filters.  A good example is short wave radio.  The band is full of noise, both man made and natural occurring “static”.  Of all those 1000’s of signals there is only one you are interested in at a time.  A tuner is used to select the frequency of interest and additional filters are used to narrow the pass band down to only a few frequencies.  The type of signal determines how wide the pass band needs to be.   For example, the human ear can hear from approximately 20 Hz to 20KHz.  However, for a voice to be understandable only 3KHz needs to be the top end.  Normal AM communication signals would require a band pass of +/- 3KHz or 6KHz wide for this signal.  However, Single Side Band (SSB) only requires only one-half of this band width.  This means only one-half as much noise is received with SSB.  People that enjoy Morse Code are able to use a much less bandwidth.

The final type of a filter is called a notch filter.  A notch filter is the exact opposite of a band pass filter and is used to remove a specific band of frequencies.  This is often used to “notch out” a specific man-made source.

Another way of removing objectionable signal is to inject that signal into a summing amplifier, but to make it exactly the same amplitude and 180 degrees out of phase with the original signal.  An example is a special kind of muffler that was designed to muffle audio sounds of machines by reproducing those sounds 180 degrees out of phase.  Similar things are used in radar to filter out unwanted signals.

In the world of military electronics this whole filtering and creating bogus signals it carried to the extreme in radar and missile controls.   An aircraft may want to jam the enemy radar to disable it.   This is called Electronic Counter Measures, (ECM).  The enemy doesn’t want his radar installation disabled, so he installed Electronic Counter-Counter Measures (ECCM)…. obviously this Countering of the Countering can go on forever.  Luckily the naming stopped at ECCM.

This has been a very general description of noise elimination.  The math becomes very involved very quickly and I have not had to actually deal with it.  However, it is an interesting subject and there are cookbook approaches to designing filters.  Most often these use active circuits as well as capacitors and inductors.  We will be touching upon it soon with LCR circuits where we have both and inductor and a capacitor in the circuit to form a tuned circuit.  A book that is dedicated to filters is called “The Active Filter Cookbook” by Don Lancaster.   I have not read that book, but I have read other work by Don Lancaster and if I was needing to build a filter, it would be a book I would consider.

I apologize for being kind of slow on posting lately.  Not only am I trying to fabricate the projects, but my day-job just became a lot more intense.   Things will get back to normal very soon.

Gary

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