Decoupling Capacitors and RF Networks
Many electronic circuits use components that may be a source of Electromagnetic Interference or EMI. Often the electromagnetic noise produced by these components or circuits can cause a product to fail EMC emissions tests. But sometimes the EMI can be filtered or de-coupled using a combination of decoupling capacitors and other components. EMI can be caused by many switching events, including:
- Switching of resistive, capacitive or inductive loads
- Integrated Circuit (IC) clocks
- Power converter circuits (such as switch-mode power supplies)
- Data traffic (such as Ethernet traffic)
- Electrical motors
One way of dealing with electromagnetic interference is by using a capacitor to de-couple the unwanted RF signal from a given circuit. This capacitor is referred to as a decoupling capacitor. These components are connected between power conductors and return paths. To act as a bypass path of low impedance for RF energy at a given frequency of concern. For this reason, decoupling capacitors are sometimes referred to as “bypass” or “shunt” capacitors.
RF noise occurring on power lines and power circuits tend to be lower in frequency. Power-line de-coupling capacitors are most commonly either:
- X capacitors (connected from power line to power line) or
- Y capacitors (connected from each power line to ground/earth)
Decoupling Capacitors for common-mode or differential mode EMI
Depending on whether the EMI is Common-Mode or Differential-Mode in nature. It is best practice for the Y capacitors to be placed nearer the equipment or load side of the circuit. This helps to provide a capacitive divider with the common-mode source capacitance. It is also important to consider the return paths of unwanted signals. In general, these should be kept as small as possible to minimise the potential source of radiation. When adding a capacitor to a circuit it is important to consider the “interconnect inductance” or “Equivalent Series Inductance” (ESL). This is essentially the inherent inductance that is present in the current loop. Essentially, the longer the loop or path, the greater the ESL which may counteract the effect of the capacitor and thus require a larger capacitor value.
Capacitors can also be used to reduce high-frequency noise produced by I/O circuits and clock frequency edges in noisy Integrated Circuits. By increasing the rise and fall time of signal edges i.e. the slew-rate. The RF noise produced by the fundamental frequency and later harmonics can be greatly reduced. The slower rise/fall times are a result of the capacitors charging/discharging time. It is important to consider that by doing so, signal integrity may be compromised. So using a capacitor in this way is a balancing act between EMC performance and functional operational performance.
Inductors and Chokes
In EMC an inductor can be used to block higher-frequency AC currents or RF noise. While allowing lower-frequency AC currents and DC currents to pass. The act of blocking higher frequencies is sometimes referred to as “choking” and thus inductors used in this way are often referred to as chokes. This is due to a chokes impedance increases with frequency while having a low electrical resistance.
The basic construction of a choke consists of a coil of insulated wire wound around a magnetic core or circularly shaped bead of ferrite material. It is important to ensure the windings and core material that can support the current of the entire line without saturating under full load.
Chokes can be divided into either Common-Mode or Differential-Mode. The EMI choke type depends on the structure of the component’s windings and the nature of the emissions needing to be blocked or choked.
Radio-frequency (RF) Filter Networks
Inductors and capacitors can be combined to create filter networks that can filter out unwanted frequencies from signals.
The simplest filters are either RL network which is a resistor and inductor. RC network is a resistor and capacitor network. Typically including only one reactive component.
More complex filters can consist of several components with resistivity and reactive elements. Common forms of these kinds of filters can include “T” networks and “п” or “Pi” networks. Which are thusly named based on the shape of the configuration of the components!
By combining several components and filter configuration, complex multi-stage EMI filters can be achieved.
There are many technical resources that provide an in-depth analysis of different components and filter types for different applications. Some filters and components work better for certain applications.
Above is a basic inline EMI AC mains filter. Typically used to help with conducted emissions or conducted RF immunity EMC compliance issues.
The response characteristics of any filter should be understood. Some basic parameters are as follows:
- Pass-band: This is the frequency band in which signals are not attenuated.
- Cut-off frequency: This is usually the point at which the response of the filter has fallen by 3dB.
- Ripple band: The band between the pass-band and the cut-off frequency which may not have a stable response.
- Stop-band: The point at which the signals are fully attenuated.
- Transition band: This is the band between the pass-band and the stop-band.
- Impedance: The characteristic impedance of a filter.