A Method to Reduce Noise in Electronic Circuits

A very annoying and difficult problem when designing and building electronic circuits is getting rid of noise. This instructable goes over a simple but little known method of cutting down on noise in circuits.

1) Oscilloscope

2) Assortment of Plastic capacitors (also known as mylar or film capacitors).

In circuits, noise is any undesirable signal that you don't want to be in your circuit. It can be powerline hum 60 Hz in the Americas and 50 Hz in the rest of the world. It can be "shot noise" which is produced in active devices from the electron flow through the device, atmospheric noise, EMI from switching power supplies, radio transmitters and other electronic devices. There are many ways to cut down on noise in circuits which involve using decoupling capacitors, filters and shielding. In this instructable, I will show a method in which the orientation of the capacitor can be used to cut down on noise.

In the photo above, an old fashioned paper capacitor from the 40's or 50's is shown. Note the black mark on the top of the capacitor. This is to denote the outer foil or shield side of the capacitor. This side should be connected to ground if possible or the lowest impedance point if used as a coupling capacitor. It was convenient for technicians as the shield side was marked. With modern plastic capacitors the shield side isn't normally marked. Here is a simple method of determining which side is the shield.

The type of capacitor above is very commonly used in bypassing, coupling and filtering. Other capacitors types are electrolytic, ceramic, mica and polystyrene. I will stick with plastic capacitors as they seem to be the most like the old paper capacitors in construction and they seem to work best with this testing methadology.

In the first picture you take the capacitor and connect it to the leads of the oscilloscope while holding it between the fingers. Be careful not to touch the leads as it would interfere with the reading. The whole purpose of holding the capacitor is to allow your body to pick up the stray electric fields from your house or building wiring. Your body does it much more efficiently than the capacitor itself does. In one picture, the electric field measurement is 13 mV peak to peak. In the next picture the electric field measurement is only 2 mV peak to peak. The black lead is connected to the outer foil of the capacitor. The oscilloscope is in AC mode and is set to 5 mV per division vertical and 20 mS per division horizontal. Mark the foil side of the capacitor with a felt pen mark for future reference. This method should work with a sensitive meter put in AC mode also. The meter will give higher readings because it's got a higher input impedance of 11 megohms as opposed to the 1 megohm input impedance of most scopes.