EMI can be either conducted or radiated emissions.
The most commonly-used RFI filter is the three-pole filter, which prevents radiated emissions within the 150kHz-30MHz range.
Another standard method of limiting RFI is EMI shielding.
Many of the devices in this photo radiate EMI
Today’s electronic equipment is equipped with embedded components, which makes them more vulnerable to harmful EMI. When an electronic device is a source of EMI, it conducts and emits unwanted electromagnetic signals. The EMI can be either in the form of conducted or radiated emissions.
Fortunately, designers can use EMI reduction techniques to protect electronic circuits from both being affected by and being a source of EMI. The mitigation methods used to reduce radiated emissions are different from the techniques used to minimize conducted emissions. In this article, we will focus on how to reduce radiated emissions.
The Basics of Radiated Emissions
EMI can be coupled from the generating point to the receiver in a variety of ways. When the source and receiver or victim of EMI are physically connected via cables or wires, coupling happens through the conductive path; this is called conducted EMI. When the source and receiver are physically separated, yet unwanted electromagnetic signals are still interfering with the proper functioning of a device, the electromagnetic noises are called radiated emissions, or radio frequency interference (RFI).
Typically, RFI is coupled via induction. For emitting or absorbing unwanted radiations, any one component of the circuit needs to operate as the antenna. The component acting as the antenna should be at a length at least equal to 1/20 th of the signal wavelength. Cables, tall components, and apertures in the enclosures are often seen as the antennas emitting RFI in circuits. Considering this component length-wavelength relationship, high-frequency signals are more likely to radiate RFI than low-frequency signals.
Causes of Radiated Emissions
When electronic circuits or devices are present in an environment that is crowded with several other radiating devices (cellphones, radio systems, wifi routers, microwaves, etc.), RFI can be created. All these devices send out signals, which produce unwanted noise for the equipment under test. The equipment under test also emits RFI, which interferes with nearby devices.
Other causes of RFI include:
- Multiple devices allocated to the same frequency band of operation or spectrum.
- Miniaturized and compact designs of electronic circuits.
- Low reflection loss and absorption loss of the shielding or enclosure used.
- Flaws encountered in the circuit design.
Significant Sources of Radiated Emission
Some devices are known to cause RFI more often than others. The following devices or systems are significant sources of RFI.
- Switched-mode power supplies
- Power converters
- DC-DC converters
- Electrical motors
- Communication systems
- Modulated radio waves
- Radar systems
- Wireless devices
How to Reduce Radiated Emissions
Designers must figure out how to reduce radiated emissions in their device designs. The two major techniques used to mitigate RFI from affecting sensitive circuits are filtering and shielding.
Through radiation tests, the emissions from devices can be evaluated. Accordingly, EMI filters can be designed for specified frequencies or bands of frequencies. EMI filters protect devices from being an RFI source or RFI victim. One commonly-used RFI filter is the three-pole filter which prevents radiated emissions within the 150kHz-30MHz range.
Another way to reduce radiated emissions is to use EMI shields. An EMI shield is an enclosure made of metal that completely encloses the sensitive circuit inside it. Depending on the radiated emission to be mitigated, the material properties of the enclosure—such as permeability and conductivity—change. The shape and thickness of the metal shield are also influenced by the type of circuit and its emissions. A Faraday’s cage is an example of an EMI shield used in electronic circuits.
In summary, if you are wondering how to reduce radiated emissions, filters and shields are the two best solutions. Luckily, Cadence software offers EM simulation tools to help optimize EMI filter designs.