Differential Microstrip Traces Require Controlled Impedance
Coming up with the correct trace widths and spacing between the pairs is important when controlling the impedance, and calculating those values is the key to success. There are many calculators available online, as well as built into your PCB design software. Controlled differential impedance starts with characteristic impedance. The characteristic impedance of your microstrips is determined by the trace width for a given layer stackup.
Once you know the characteristic impedance, the differential impedance is determined by setting the spacing between each side of the differential pair. By setting the right spacing between your microstrip traces, you’ll set the differential microstrip impedance to a specific value. Some empirical formulas have been determined in the past, but the most accurate results for both types of impedance are obtained when you have access to a differential microstrip impedance calculator in your PCB design software.
Differential Microstrip Propagation Delay and Skew
Coming up with the correct trace widths and spacing between the pairs is important when controlling the impedance, and calculating those values is the key to success. Differential pair routing establishes a balanced transmission system that carries the equal and opposite differential signals across the PCB. The objective in PCB routing for differential signals is to ensure that the pair of signals arrive at the target at the same time.
The time required for signals to reach the end of a differential pair is determined by the differential impedance, which is related to the propagation delay. The signal speed in a stripline is simple; it’s just determined by the dielectric constant of the substrate. In a differential microstrip, the signal velocity depends on an effective dielectric constant, which then depends on the geometry of the differential microstrip pair. Accurately determining the propagation delay in a differential microstrip takes an integrated field solver in your PCB routing tools.
Using Differential Microstrip Propagation Delay Calculations in Your PCB
Altium Designer has automatic functions that speed and simply differential microstrip propagation delay calculations. The unified design environment provided by Altium Designer eliminates the need for shifting out of the design, loading a third-party calculator, and manually feeding the calculations back into the design.
By determining the propagation delay for your differential microstrip traces, you can reduce skew between each signal in a differential pair and ensure your signals arrive at the receiver on time.