Dealing with Radiated Noise (EMI) in Electronic Circuits

All-sky mollweide map of the Cosmic Microwave Background, created from Wilkinson Microwave Anisotropy Probe data. Seen initially as annoying noise in an experimental communication antenna, it became one of the most important RF ‘signals’ from Space yet discovered.
Image credit: NASA

In my previous article on the subject of unwanted signals (electrical noise) in electronic systems, Dealing with Conducted Noise in Electronic Circuits, I gave some examples of how poor component interconnection design leads to erratic operation. Now let’s take a look at sources of radiated noise; how to avoid creating interference, and if that’s not possible, shielding sensitive components from its effects.

Electromagnetic Interference (EMI)

We’re surrounded by electromagnetic radiation (EMR), and have been since the dawn of time. It’s tempting to think that ‘radio waves’ are a recent phenomenon, only appearing with the first radio transmitter created by Marconi. In fact, EMR includes microwaves, heat, infrared, visible light and X-rays; the categories distinguished from each other in theoretical terms by their wave frequencies, and by their affect on biological materials. The key feature these waves have in common is that they do not require a physical medium through which to propagate, unlike sound which does. A consequence of this is that RF electromagnetic wave energy can ‘flow’ through a vacuum from a transmitting antenna to a very distant receiving antenna without the need for a physical circuit. Good job really: without radiated heat and light from the Sun we wouldn’t exist, let alone communicating with each other over long distances using ‘wireless’ technology. Ironically, the downside is that high-frequency electrical/electronic circuits are prone to radiating RF energy via an unintended ‘antenna’ built into the design. The signal can then be ‘received’ by other circuits as interference or noise.

Another way in which radiated energy differs from the conducted kind becomes apparent when a circuit is broken (a switch opened), and a radiated energy source is turned off. Assuming there are no reactance effects, the switch opening instantly stops current flowing in the circuit. The circuit is dead. When the conducted power feeding an antenna is cut off, the energy in the radiated wave generated up to that point continues to travel outwards – at the speed of light. So, for example, a burst of radio waves from a space communications antenna will be 200,000 miles away just over a second after the transmission ends!

Near and Far

The electromagnetic field set up by a transmitting antenna is divided into two zones known as the Near Field and the Far Field. The Near field only extends for a short distance from the antenna and disappears as soon as transmission ceases. The Far field extends outwards at the speed of light for, in theory, an infinite distance. The burst of RF electromagnetic energy isn’t converted to heat, like electrical energy in a wire, as it passes through Space; it just spreads out, so that at a distance d from the transmitter, the amount of energy at a given point is reduced by a factor proportional to 1/d². In the context of EM interference in a somewhat smaller situation than the whole Universe, most is likely to be Near Field, produced by the system itself. Far Field interference could come from remote communication signals such as WiFi and Bluetooth.

Incidental Radiators

An incidental radiator is a device that generates unwanted radio frequency electrical ‘noise’ while performing its designed function. A common example is the brushed DC motor. This type of motor consists of an electromagnet rotating in a static magnetic field produced by two permanent magnets. The electromagnet consists of at least two coils wound on a shaft called the rotor or armature. Current is fed to the coils via brushes that rub on contacts fixed to the shaft. In order for the armature to rotate, the polarity of the electromagnetic field has to reverse every half-cycle and that’s achieved by that shaft contact assembly being wired as a commutator. This minimum motor with its two coils works fine as a technology demonstrator, but is not really practical because of what happens as the brushes pass over the narrow gaps between the contacts at the same time. Being wider than the gap each brush shorts the two contacts together and in doing so short-circuits both the coils and the power supply:

• Heavy current through the brushes and commutator contacts will cause damage by overheating.
• Slight misalignment of brushes and contacts could cause the short-circuits to briefly turn into open-circuits. Sudden breaking of the inductive circuit leads to the generation of a high voltage and a spark between brush and commutator contact. The spark contains a great deal of high-frequency electromagnetic including visible light and RFI. The RFI is conducted away by the brush connections and radiated directly. The spark is a very hot conducting plasma which will eventually erode the contact surface making the sparking worse.

A two-pole motor will have a short life! Fortunately, a three-pole motor with three coils and a three-segment commutator is a much better proposition. The gaps between the contact segments are separated by 120° as opposed to 180° for the two-pole version. However, there are still only two brushes arranged 180° apart. This means that when one brush runs over the gap between two segments, the other brush is in full contact with the third. Short-circuiting of the brushes is impossible, and the current through the coils is never completely cut off, resulting in a big reduction in the EMI generated. But some still is, which may require the use of extra passive components to deal with it (Fig.1). Generally, a spark that’s no more than a tiny point of blue light is acceptable; arcing right around the commutator while it’s in motion is a sure sign of extensive damage and worn out brushes.

Working out how much suppression is needed usually comes down to experience/guesswork/trial-and-error. The photo shows a 3-pole RE-280 type motor frequently used in small mobile robot projects. It has a minimum of EMI suppression: a single 0.1μF ceramic capacitor across the brush terminals to suppress high-voltage ‘spikes’, and the power wires twisted together to minimise their behaviour as a loop antenna. Note that the capacitor is soldered as close to the motor brushes as possible. The sparking at the commutator generates both radiated RFI directly, and conducted RFI back down the power cables which, if spaced apart, can radiate energy too. For most applications, the capacitor across the brushes and the twisted power wires will be enough to prevent any ‘glitching’ of a controlling MCU. To reduce EMI production at the commutator still further, capacitors C2 and C3 can be added. Ferrite rings or ‘beads’ slipped over the power wires will increase their inductive impedance to conducted RF.

Unintentional Radiators

An unintentional radiator is a device that generates RF signals for conduction to other circuits, but radiates them as well. A classic example of this is the local RF oscillator in a radio transmitter. The output from this oscillator is conducted to the modulator circuit before passing to the antenna. It’s the modulated signal that needs to be transmitted, but without some form of shielding, the unmodulated carrier is radiated too. Parts of the circuit prone to emitting unwanted signals can be enclosed in a miniature Faraday cage made with solid or perforated metal. Sensitive circuits can be shielded in the same way.

A number of shielding cans in a mobile phone. For maximum effect, the cans should be made from solid sheet metal, but perforated shielding can be used for ventilation purposes providing the holes are at most 1/20^th the size of the signal wavelength.
Image credit: Wikipedia

A microwave oven is a good example of a Faraday cage designed to contain the microwave radiation so that food gets heated and not the cook! Most of the shield is made from solid metal, except for the door; this is perforated to keep the microwaves in, but allows much higher frequency EM radiation – visible light – to get through.

Stray Signals

Given that we depend so much on far-field ‘wireless’ communication in the modern world it’s tempting to think that all useful EM transmissions consist of precisely modulated RF carrier frequencies, and that RFI is just ‘noise’. It’s not always like that…..

Generating radio waves with sparks

In the late 19^th century a lot of research was going into wireless telegraphy to eliminate the need for long and expensive cabling – particularly undersea cables. Things started to move once it was discovered that an alternating electrical spark generated radio waves that could be detected by a remote receiver. The Spark-Gap Transmitter was born. However, very-wideband bursts of EM ‘noise’ making up a Morse code transmission made it impossible to operate two or more telegraph stations in the same vicinity. The development of narrowband modulated carrier transmitters based on thermionic valves relegated the spark to the level of interference source, rather than signal generator.

When noise became the signal

In 1965 two researchers at Bell Labs were struggling with their attempts to show that long range radio communication was possible by bouncing the radio waves off a metallic ‘balloon’ satellite in Earth orbit. No matter where in the sky they pointed their huge horn antenna, a constant level of microwave noise was registered. A great deal of time was spent fruitlessly searching for faults in the receiver. It turned out they had stumbled upon the Cosmic Microwave Background, whose presence was first postulated back in 1948. This was ground-breaking stuff because it was hard evidence in support of the Big Bang Theory of creation. On this occasion, seemingly random noise turned out to be a deeply meaningful signal!

Very short-range communication

Research in radio communication technology normally focuses on increasing the range and data-rate of the signal for a given amount of transmitter power. A technology that appears to work in the opposite direction has come into widespread use for contactless payment systems: Near-Field Communication or NFC. Every time you ‘tap and pay’ with a credit card, a sophisticated but short-range (5cm) wireless communication circuit springs into action. The RF energy emitted by the card reader is transferred to the card by induction as this is a near-field system. That energy is used to power the card’s electronic chip which then sends data back to the reader. Ideally, the card soaks up nearly all the RF energy leaving little to escape into the far-field where data-thieves lurk.

A photo-sensitive Raspberry Pi

Finally, something weird. When the Raspberry Pi 2 was first put on sale in 2015, one of the first users took a picture of it while it was powered up and running. The xenon flash of his camera appeared to cause a hard reset or turn the power off, giving a whole new meaning to the expression ‘flashing the chip’. It was eventually determined that one of the small power supply chips was sensitive to light even though it was fully encapsulated in black plastic. A blob of paint or a strip of insulating tape fixed the problem of the not-so-opaque plastic that was supposed to shield the chip from electromagnetic radiation in the form of light. It serves as a reminder that any exposed semiconductor chip is photosensitive to some degree.

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