Imagine trying to have a crucial discussion, but nearby people talk loud enough to interrupt. Not only is it annoying, but it might be so disruptive as to leave all participants confused. Speaking to someone in a rabble is a recipe for miscommunication similar to what occurs in a circuit with appreciable background noise.
Circuits with excessive noise require some form of correction – typically through filtering, although other design options can rid it at the source – to prevent the electrical equivalent of a shouted-over signal. Electromagnetic interference (EMI) can affect signal transmissions, like common mode vs. differential mode: depending on the direction of current flow through two parallel conduction paths, noise can couple differently with separate alleviation methods. Both forms of noise are troublesome, and designers will have to examine signal pathways and component selection to negate their effects.
Relationship between Coupled Mode Parameters |
|||
|---|---|---|---|
| Voltage Lines | Current Lines | Characteristic Impedance (Z0) | |
| Even Mode | V1 = V2 = V | I1 = I2 = I | Z0,E = 2Z0,C |
| Common Mode | I1 + I2 = IC | Z0,C = (½)Z0,E | |
| Odd Mode | |V1| = |V2| = |V| | |I1| = |I2| = |I| | Z0,O = (½)Z0,D |
| Differential Mode | V1 – V2 = VD | Z0,D = 2Z0,O | |
Determining Common Mode vs. Differential Mode Noise
Common mode and differential mode noise refers to the signal transmission method. Both methods use two lines of equal magnitude voltage/current, but common mode lines are in phase while differential lines are maximally out of phase. The phase difference accounts for the return path and, therefore, the current loop area, with the signal and return loop confined to the differential lines. In contrast, the common mode signaling uses the shared ground between source and load for return current. It’s crucial to note that common and differential mode noise can manifest outside defined transmission lines, whether that is unintentional coupling between nearby traces or through reference planes.
Not only is detecting the noise source difficult, but test engineers must also determine its type. Since these types of noise are usually present in transmission lines, the most straightforward diagnostic check is attaching a clamp-on or snap ferrite to a noisy cable. If the noise falls, it is common mode in origin as signal and ground are within the cable. Conversely, no change to the noise readings indicates differential mode noise.
The cause of this response is the creation or cancellation of magnetic flux within the core of the choke. Magnetic flux is the surface integral of the normal component of a magnetic field passing through a surface. However, more important than its definition is the description of its function: devices resist changes in magnetic flux (this is the primary functionality of the inductor). The signal and return path of common mode transmission will travel through wires wrapped around a magnetic core, with each wire generating a magnetic field. These magnetic fields sum to form a total magnetic field within the core of the choke that offers high impedance to the common mode signals to oppose the resultant magnetic flux.
Differential Mode: Common Mode Chokes and Filtering
The wires in a choke wrap around a core so that the two magnetic fields produced by the differential mode transmission will cancel out. Since there is no resultant magnetic flux, the choke does not offer a high impedance, and the transmission passes through the choke with little opposition besides the material’s resistivity and parasitics. Effectively, the differential mode signals see the choke as conducting wires or a low-value resistor.
If the common mode choke filters the common mode noise but passes the differential mode noise, what’s the solution for the latter? Ferrite beads help to filter differential signals by acting as a low-pass filter – low-frequencies face no opposition, while high-frequency signals experience significant attenuation. The difference may be surprising – a choke and ferrite bead are all inductors on some level – but the difference arises from the performance characteristics of the two devices. While replacing a choke with two ferrite beads can reasonably approximate the noise-filtering response of the choke, the signals undergo distortion, making this approach less suitable for applications where perpetuating the waveform is critical to the circuit performance (think audio or video signals).
Additional Components for Filter Design
Inductance is not the sole method for filtering out common mode or differential mode noise; a particular class of capacitors also plays a part. X and Y capacitors, also known as safety capacitors, are used in AC power lines to suppress noise:
- X capacitors sit across the line with one leg tied to the live wire and the other to return. These are rated less capable than Y capacitors: X capacitors are only deployable where the failure will not result in shock but could result in fire. They filter differential mode noise.
- Y capacitors typically bypass the return and ground lines. However, they can take the position of an X capacitor. Their installation prevents shock because they have higher ratings and dielectric breakdown parameters than X capacitors. They filter common mode noise.
Provided space and cost are not an issue, designers can combine the filtering effects of chokes, ferrite beads, and capacitors for a superior transmission line signal-to-noise ratio. For low-voltage differential signal lines (USB, HDMI, etc.), the selection of component values will primarily depend on the operating frequency range of the signal: the cutoff frequency will need to offer a high impedance to common mode transmission and a relatively low impedance to differential mode transmission that is carrying the signal.
Transmission Mode Noise Stifling Design? An Experienced CM Can Help
Common mode vs. differential mode noise can detract from device performance. Simultaneously, certain operations can benefit from the two transmission styles. Designers must realize when the transmission styles are advantageous, where noise will undermine signal integrity, and how to identify and remedy noise depending on the generating source. At VSE, our team of engineers is committed to building electronics for our customers, and that extends to a comprehensive analysis of signal integrity issues to ensure electromagnetic compatibility and testing compliance.
