cleaning up a EMI/RF mess

When you’re on the last steps of completing a project such as a AVR based Oscilloscope you attend to run into issues such as interference from AC mains voltage or even your WiFi network. Reason being is all electronics produce a sound signal. For a example a typical home in the U.S. will push out 60Hz. Since the device I am building will use a small power supply from AC mains I have to take some extra steps on making sure I don’t have trash noise going into the device. This trash I speak of is called “Ripple”. If you use batteries then you do not need to worry about the Ripple. Ripple is leftover noise from AC rectified to DC.

When looking on a Oscilloscope AC looks like a sine wave and DC is a straight clean line. A full bridge rectifier will swap the sine wave of AC and turn it into a DC signal but a very bumpy ride so to speak. This is called pulsing DC and for something like this to power a AVR let alone a DIY scope this is a bad thing. To smooth out the bumpy mess of pulsing DC we need to filter it. There are a few ways to do this but the easiest is with capacitors. Now this is with a Linear prospective. With switch mode power supplies it goes in more depth and can really make your head hurt.

When using one or two capacitors to filter out ripple this is when a scope is handy so you can properly get the correct mixture so to speak. The capacitor charges when the pulse hits around the peak and carries the charge for the next peak of the signal.

Example:
120 volt 60Hz mains with a transformer dropping the voltage to 6 volts 60Hz. Add Four 1n4001 diodes to make a full wave bridge rectifier to convert to DC. This will turn the DC output to 6 volts 120Hz.

Look at the bottom left sine wave, that is typical AC. Look at the bottom right. That is typical pulse DC. Since the rectifier has turned the AC down stroke signal and pulls it up. Now when we add the filter capacitor it turns the pulse DC into more of a clean signal.

The red line is the DC straighting out becoming a cleaner signal. Now you can only get so far and you’ll need a regulator to make it a true clean signal. So you will end up with something like this.

Yes, this is a typical linear power supply with a regulator. A lot of novices don’t know this. It’s the fact you get a clean signal in you’ll get a clean signal out. In this case since this is for a AVR it will be harder for a messy signal bleeding into the input voltage of the device.

Now for EMI/RF on other parts of a device. I had to look around some HAM radio sites for this info. All electronics not only produce sound but also produce radiation and also receive interference from sounds and radiation. This is why most home consumer products have metal cases or have metal shielding all over the place. This helps the device to block interference. There are several ways to shield a project. You can use a metal shielded case/enclosure, EMI/RF paint, smae stuff that is found inside phones, tablets and even laptops to reduce weight. If you have time on your hands and a decent scope you can use ferrite beads. I never know that that ferrite cores had a special color coding. Different colors are form different grades of material the core is made from and used for certain things. The typical dark gray type are for suppressing signals typically found on computer cables. I use them for video signals and long data lines and will come in handy for my DIY AVR scope. To use a metal shield it’s nothing more then conducive sheet metal and attached to chassis ground. HAM radio users prefer copper and the paint stuff is nickel or copper conductive ink. You could get away with foil but I wouldn’t use aluminum/tin, it’s better to use copper foil that is mostly found in arts and craft stores.

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