Controlled impedance is the characteristic impedance of a transmission line formed by PCB traces and its associated reference planes. It is relevant when high-frequency signals are propagating on the PCB transmission line. In this article, we will demonstrate controlled impedance routing using Altium Designer.
Controlled impedance is important for solving signal integrity issues, which is the propagation of signals in PCB traces without distortion.
Let’s take you through the following steps to achieve the desired impedances for single-ended and differential traces using Altium Designer:
In this demo, we will show you how to route the differential pair with 100 ohms and a single-ended line with 50 ohms. Here, this is the ethernet section with ethernet IC and RJ45 connector.
Ethernet section with ethernet IC and RJ45 connector.
You can see the net with the receiver (RX) and transmitter (TX) section, RX clock, RX control, and RXD0, D1, D2, and D3. These all are single-ended 50-ohm traces.
A net with the receiver (RX) and transmitter (TX) section.
On another side of the IC, you can see an MDI0, one, two, and three. All these connections are 100-ohm differential pairs.
To achieve the impedances, we need certain trace widths; those will be provided by the manufacturer in the form of a stack-up. The image given below is depicting an example of a stack-up. In this stack-up, the layers are given, the required impedances are given, and the trace width and spacing between the traces are also given.
Classes for 50 and 100-ohm traces.
Note: In the same way we will create a 50-ohm class.
To set the rules for the classes we created above, we will follow these steps:
Note: The values in the top layer and the bottom layer will be the same. The ground plane and the power planes are not to be used.
Layers contain controlled impedance that is why we need to specify these impedances in the fab notes since there can be more than one value of impedance traces per layer. Separate aperture codes are defined for controlled impedance traces. Trace impedance is a critical factor in the effort to transmit signals without distortion over a trace. Impedance must match the drivers and the load. We hope that our tutorial on controlled impedance routing using Altium Designer helps the designer to understand and follow the minute details while routing their PCB.
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