If you wanted a message delivered quickly between Missouri and California in 1860, your best option was to use the Pony Express. For a hefty fee, this service promised to deliver your letter in just ten days, which was extremely fast for the time. The safe and reliable delivery of messages was the highest priority, sometimes at the expense of the horse and rider. Today, the requirement for reliable delivery of information is just as essential, especially in the transmission of high-speed signals in electronics.
Electronic devices and systems rely on the clear and uninterrupted high-speed transmission of data within their circuitry. However, the signal transmission integrity can be degraded by both internal and external sources of electronic interference. It is vital to lay out the PCB to mitigate such degrading effects and ensuring the signal’s clear delivery. Here are some rules and techniques for signal integrity in PCB design that can help.
Signal Integrity Basics
The faster signal speeds get in electronics, the more they are affected by crosstalk, impedance miss-matches, ringing, and ground bounce. Eventually, these effects will degrade the fidelity of the signals, or the signal integrity, to the point where transmission errors occur, causing failures in how the system operates. These failures may manifest themselves as intermittent problems, or they may completely shut down the system. To further complicate the matter, issues from poor signal integrity may not show up during prototyping but cause multiple failures when the circuit boards are in regular production.
The following effects characterize a signal that has lost its integrity:
- The signal is degraded by the impact of unwanted noise superimposing itself and changing its signal-to-noise ratio.
- The signal’s shape is changed or distorted from its initial profile.
- The signal creates noise that adversely affects other circuitry on the PCB.
Instead, the goal is to protect the circuit board from susceptibility to EMI from other electronics and prevent it from radiating EMI that will affect other circuitry. The way to achieve this is by designing the layout to avoid board signals distorting. Some key layout techniques are essential to understand for ensuring signal integrity.
Techniques for Ensuring Signal Integrity in PCB Layout
To maintain good signal integrity on a circuit board, identify potential problems and layout the board to address those issues specifically. Here are four signal integrity issues and the layout techniques that will allow PCB designers to control them:
Impedance Miss-Match
The quality of a signal depends on the trace and return path that it uses for transmission. With sensitive high-speed signals, any change to the uniformity of the transmission line can cause reflections resulting in signal distortion. Controlling this requires that high-speed transmission lines be laid out with uniformly controlled impedances. This process will require routing lines on layers adjacent to a reference plane for a clear return path and using specific trace widths and spaces calculated for the board layer configuration.
Crosstalk
When circuit board traces are too close together, the signal pulse in one may overpower the other. This crosstalk is also referred to as unintentional electromagnetic coupling and can result in the victim signal mimicking the characteristics of the aggressor signal. Crosstalk is especially problematic for high-speed sensitive traces, even though they may be routed at the minimum spacing requirements for manufacturing. The key here is to maintain a greater spacing between sensitive traces like clock lines, usually three times the trace width rule. Also, avoid routing traces parallel for great distances, both on the same layer or adjacent signal layers. To prevent coupling broadside, it is always better to alternate adjacent signal layers’ horizontal and vertical routing.
Electromagnetic Interference (EMI)
Traces with higher frequency signals can behave like antennas and radiate EMI if they aren’t routed carefully. These traces include signal return paths on the reference plane, which must be kept clear of splits or other obstructions resulting in a noisy signal. Additionally, sensitive high-speed signals such as clock lines should be kept separated from other traces and routed cleanly on a single layer as much as possible without 90° corners. Differential pairs need to be routed together tightly, maintaining their required spacing and not splitting around vias or other obstructions. Another good habit is to avoid traces or via configurations that are stubs which can create signal reflections and act as an antenna.
Ground Bounce
With the high-speed switching of the components on the board, the voltages may not return to the ground level and instead “bounce” above it. This bouncing leads to signal pulses being miss-interpreted by receivers and producing false results. To avoid this, make sure to place the decoupling capacitors as close as possible to their associated power supply pin on their assigned ICs. This practice will reduce the current spikes during the switching. Also, make sure to connect each ground pin to the ground plane individually. Daisy-chaining ground connections may help to reduce the number of vias and simplify routing, but they also increase the return current loop and inductance.
Signal Integrity in PCB Manufacturing
Signal integrity is heavily tied to how high-speed transmission lines are routed, and good routing techniques start with well-developed component placement. First, the board needs to be partitioned into its functional areas and components placed to provide the shortest and most direct signal paths for high-speed circuits. But before component placement can begin, it is critical to develop a board layer stackup that will best facilitate the impedance routing for the design.
Here is where it is prudent to work ahead with your PCB contract manufacturer to determine the circuit board’s layer count and configuration. Not only does your board need to be configured for the best signal integrity, but designed for the operating environment and its manufacturability. At VSE, we help customers develop circuit boards that are cost-efficient to manufacture and have the most optimum high-speed performance.
