Noise is an unwanted disturbance that interferes with the desired signal in an electronic circuit, introducing error into the system. The countermeasures that we take to suppress the noise component depend on whether it’s common-mode or differential-mode. In this article, we’ll learn how common-mode noise can interfere with differential signals. We’ll then discuss sources of common-mode noise in differential interconnections, placing an emphasis on timing skew.
Before we talk about noise, let’s review how differential transmission works.
Differential Transmission: Core Concepts
In a differential configuration, a pair of lines transmit signals of equal amplitude but opposite polarity, as we see in Figure 1. The signal level represented by a differential pair is the difference between the voltages of the two signal lines.
Figure 1. Waveforms propagating through a differential link. Image used courtesy of Steve Arar
High-speed data transmission interfaces—USB, HDMI, Ethernet, and DDR, to name a few examples—typically use differential signaling. This method of data transmission has multiple advantages over single-ended transmission, particularly at high frequencies.
Noise Immunity
The major advantage of the differential configuration is its higher immunity to external noise. If an external noise couples equally to both lines, it appears as a common-mode signal that doesn’t change the voltage difference between the two lines. The common-mode noise is therefore inherently removed at the receiver.
Real-world differential lines can still be affected by common-mode noise, however. There are two reasons for this, the first being that external noise doesn’t necessarily couple equally to both lines.
Consider a single-ended PCB trace routed in parallel with a differential path. The single-ended trace introduces greater noise to the closer signal line. In this case, some of the noise is converted to differential mode due to the unequal coupling. At that point, the noise can no longer be eliminated by taking the voltage difference of the two signal lines.
We also know from basic electronics courses that our circuits have a limited ability to detect differential signals while rejecting common-mode signals. This performance dimension is characterized by the common-mode rejection ratio (CMRR) specification of the circuit. Therefore, while the receiver can suppress the common-mode component, it can’t completely eliminate it.
Despite these issues, differential signaling is still a very effective method for reducing the impact of common-mode noise on system performance. Furthermore, differential transmission has advantages that go beyond noise immunity.
Other Advantages: Less Radiation and Ground Bounce
A differential link uses two matched lines carrying equal-magnitude but opposite-polarity signals. In fact, the two signal lines emit equal but opposite magnetic fields, which interfere destructively to produce a much lower spurious radiation than a single-ended signal (Figure 2).
Figure 2. The magnetic fields of the two matched lines interfere destructively. Image used courtesy of Altium
Differential configurations are also less susceptible to ground bounce. To understand this, note that the two signal lines act as return current paths for each other. Therefore, unlike the single-ended configuration, an ideal differential link has no return current through the board’s ground plane. Figure 3 illustrates the difference.
Figure 3. With a differential link, no current flows through the ground plane. Image used courtesy of STMicroelectronics
Ideally, we intend to transmit a purely differential signal along the interconnect. In practice, this signal will be corrupted by common-mode noise. Before we examine where this noise comes from, let’s take a look at the potential paths that common-mode signals might take.
Common-Mode Propagation in Differential Interconnections
A two-conductor connection can’t carry a common-mode signal. Such interconnections can support only differential-mode signals. However, because our PCBs also include a ground connection, our interconnections are actually multi-conductor rather than two-conductor.
The ground plane, together with the two differential signal lines, creates a multiconductor interconnect that can transmit both common-mode and differential-mode signals. Figure 4 shows the general pattern of electric fields for common-mode and differential-mode signals of such a setup.
Figure 4. Cross-section of a two-conductor interconnection above a ground plane. Image used courtesy of D. Jackson
We can see that a three-conductor structure can support both differential-mode and common-mode signals. For a common-mode signal, the two signal lines effectively act as a single wire with the return current flowing through the ground plane back to the source.
Figure 5 shows a more general example. Here, a circuit board is placed close to a metal chassis.
Figure 5. Common-mode noise in a circuit board with a metal frame. Image used courtesy of TDK
In this case, the common-mode noise current flowing through the signal lines travels back toward the source through the chassis. It then completes the loop by flowing through the stray capacitances between the signal lines and the chassis. Common-mode noise is introduced through the stray capacitances and magnetic coupling between nearby signal lines. Since capacitive coupling increases with frequency, higher-frequency signals are more likely to generate common-mode noise.
Skew As a Source of Common-Mode Noise
Another source of common-mode noise is the skew between the two lines of a differential link. Skew is the difference in the timing of two waveforms that should be aligned. As Figure 6 shows, it causes the waveforms to lose their symmetry and creates a common-mode component.
Figure 6. Skew between the signal lines D+ and D- creates common-mode noise. Image used courtesy of Steve Arar
A length mismatch between the two traces and/or a difference in the rise and fall times of the D+ and D– signals leads to a skew between the two signals. In addition, skew can be produced by a variety of sources, including:
- Thermal noise.
- Ground bounce.
- Crosstalk.
- PCB construction.
Though we can reduce skew, we can’t completely eliminate it. Let’s consider how variations in the fiber weave construction of a PCB substrate create skew, for example.
The Fiber Weave Effect
PCB laminates and cores are made from woven glass impregnated with resin. This woven structure can lead to timing skew in a high-speed board. To understand why, consider the woven-glass PCB material in Figure 7.
Figure 7. High-magnification image of woven-glass PCB material. Image used courtesy of Isola
Traces (1) and (2) in the figure experience different effective dielectric constants and hence different signal velocities. At slow rise times (> 1 ns) and low frequencies (< 1 GHz), the fiber weave effect might be negligible. However, the fiber weave style can significantly impact system performance in fast interconnects and high-frequency RF systems that require phase matching.
For a summary of different methods of mitigating the fiber weave effect, please refer to “How the Fiber Weave Effect Influences High Frequency Signal Integrity” on the Altium website.
Common-Mode Filters
Ideally, we should seek to avoid transmitting a common-mode signal through our differential links—the common-mode component increases the noise at the receiver, the radiation from the link, and the ground bounce effect. In reality, common-mode noise can occur despite our best intentions, whether it results from the mismatch between the traces or is coupled to a differential line from an external noise source.
To address the issue of common-mode noise, we use common-mode filters. These devices create a high-impedance path for common-mode currents while allowing differential-mode signals to pass largely unaffected. We’ll discuss common-mode filters at greater length in a future article.
