Here, we want to create a differential-pair impedance profile set to 90 Ohms differential impedance with 15% tolerance. If you open the Layer Stack Manager in the PCB Editor window, you can go to the Impedance tab to create a new impedance profile. This template is accessible when you create a new project in Altium Designer. Here, I’m using a 64-bit PCI Universal Short Card board with 10 layers it's a bit overkill but it's a quickly accessible template in Altium Designer. Once you’ve created your schematic and decided on a layer stack, you’ll need to create an impedance profile. Getting Started: Defining an Impedance Profile After you do this, you can get started with the steps required for differential pair tuning in a controlled impedance application. When creating your schematic, make sure to define the appropriate nets as positive and negative ends of the differential pairs, otherwise the following steps will not work for length tuning. These design rules (differential impedance and length/delay tolerance) are the two inputs In the USB 2.0 PHY standard, the length mismatch shouldn’t exceed 2 mils, and total length should not exceed 4 inches. If the distances are long, and depending on how the pairs are arranged, the pairs may not meet the precise length matching requirements for USB connections. Here, the size of the connectors I’m using are much wider than the size of the hub, so we have a situation in which we almost “fanout” (not in the BGA sense) the differential pairs from the hub’s 48-pin QFP and to the downstream PHY/connector interfaces. Since we’re using USB 2.0, we have a generous 15% tolerance on the impedance. According to the USB standards, the differential impedance of the pairs leading to the USB connectors must be set to 90 Ohms, with tolerance depending on the USB generation. If you’re not familiar with USB, the data connections to the USB ports are differential pairs, which must be impedance controlled and length matched to ensure connections have low noise and reflections are suppressed. In a simple example I’ll show below, I’m routing differential pairs between a USB 2.0/Ethernet hub chip which I want to connect to multiple USB PHY interfaces, which then connect to multiple USB ports. Here’s how this works in Altium Designer. You can encode permissible length mismatches as design rules as part of controlled impedance routing, or you can manually perform differential pair tuning using a variety of meandering styles.

Instead of eyeing out your different pair lengths, the interactive routing tools in Altium Designer make differential pair length matching easy. Today's CAD tools make it easy to come close to all three objectives, but only if you have the right design tools. In order to properly suppress common-mode noise, differential pairs should be routed in parallel with symmetry and matched lengths.

Altium designer 20