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High-speed designs are more commonplace and the impact on the performance is becoming greater and greater. As a PCB designer, you need to know what nets require controlled impedance, how to avoid common mistakes when designing, what to specify in the fabrication drawing, how to design controlled impedance traces, how to get a stack-up from your manufacturer, as well as make sure they can meet your requirements.
This design guide empowers designers: the purpose is to get you on the advanced side of controlled impedance.
Electronic products are transforming into more complex devices due to the increase in component counts on PCBs. This is making room for high-density and high-frequency of circuit operation. Typically, signal frequencies from 50MHz to as high as 3GHz are considered to be high-speed signals. Soon, the demand for high performance from devices would require even higher speeds of operation and higher component counts.
These requirements demand challenging PCB design skills. This guide covers the performance requirements, such as signal integrity, and related issues of controlled impedance, power integrity, and EMI/EMC. To achieve these performance requirements, we cover how component placement, routing, stack-up design, and material selection is done for high-speed boards.
To execute a successful design for a circuit board, one has to first determine if it is in fact a high-speed design. If it is a high-speed design, then the PCB designer would need to take special care during the design process; and we would call them as high-speed design considerations – the main subject of this guide.
In most cases, based on their past experience, the design engineering team may determine the nature of the design.
In order to determine if the PCB requires high-speed design, we follow a two-step process:
First, the designer must state the values of one or more of the following parameters:
Second, you need to determine the wavelength (λm) on the PCB of the electrical (electromagnetic to be precise) signals for a given frequency, in our case Fm.
Typically, you will need controlled impedance for PCBs used in high-speed digital applications, such as RF communication, telecommunications, computing using signal frequencies above 100MHz high-speed signal processing, and high-quality video, such as DDR, HDMI, Gigabit Ethernet, etc.
At high-frequency, the signal traces on a PCB act like transmission lines, which have impedance at each point on the signal trace trajectory. If this impedance varies from one point to the next one, there will be a signal reflection whose magnitude will depend on the difference between the two impedances. The larger the difference is, the greater the reflection will be. This reflection will travel in the opposite direction of the signal, which means that the reflected signal will superimpose on the main signal. As a result, the original signal will be distorted: the signal intended to be sent from the transmitter side would have changed once it got to the receiver side. The distortion may be so much that the signal may not be able to perform the desired function. Therefore, to have an undistorted signal travel, it is essential that the PCB signal traces have a uniform controlled impedance to minimize signal distortions caused by reflections. This is the first step to improve the integrity of the signals on the PCB traces.
Perhaps the most important cause of signal integrity issues in a PCB is faster signal rise times. When circuits and devices are operating at low-to-moderate frequencies with moderate rise and fall times, signal integrity problems due to PCB design are rarely an issue. However, when we are operating at high (RF & higher) frequencies, with much shorter signal rise times, it becomes a big issue.
For analytical purposes, we can divide various signal integrity issues into the following categories:
High-speed signals must be routed over a solid ground reference plane. Traces should not be routed over a split plane or a void in the reference plane. Routing high-speed signals across split planes can produce the following issues:
The designer can use stitching capacitors across split planes if the signals have to be routed over them. The capacitors contribute a return path for the high-frequency current and minimize the current loop area along with any impedance discontinuity created by traversing the split plane. In the image shown below, the signals are routed around the void in the plane rather than across the split/void
It is good practice to minimize the use of sockets since they can introduce inductance. The designer should select the correct package, and accompanying component footprint while designing for high-speed. Components like op-amps are available in different packages. One type of an op-amp may support shorter trace lengths in a circuit than the other. Finally, the component footprint shapes should be optimized for thermal considerations. The best way to dissipate heat is to place power pads right under the IC footprints that are connected to an internal plane. Even the smallest change in the pad shapes can tighten up the routing and decrease the connection lengths. This will result in a compact design.
A minimum distance should be maintained between traces to minimize crosstalk. The crosstalk level depends on the length and the distance between two traces. In some areas, the routing of traces reaches a bottleneck where the traces are closer than the allowed distance between them. In such situations, the distance between the signals outside the bottleneck should be increased. Even if the minimum requirement is met, the spacing can be increased a little further.
Ralph Morrison said, ‘It’s all about the space!’
