**Notebook**

Calculating signal speed. According to physics, electromagnetic signals travel in a vacuum or through the air at the same speed as light, which is:

^{8}M/sec = 186,000 miles/sec = 11.8 in/ns

There are two general PCB trace structures*: stripline and microstrip (FIGURE 1).

The formulas for calculating the signal speed on a PCB are given below:

*V*c is the velocity of light in a vacuum or through the air*E*r is the dielectric constant of the PCB material*E*r_{eff}is the effective dielectric constant for microstrips; its value lies between 1 and*E*r and is approximately given by:

*Er*

_{eff}≈(0.64

*Er*+ 0.36) (1c)

*Er*≈4 (as with FR-4 material types), then the speed of signals on a stripline is half that of the speed through the air; i.e., it is about 6 in/ns.

*V*is the signal speed in the transmission line.

On PCB transmission lines, the propagation delay is given by:

In the example below, the normal FR-4 material with Dk of 4 is used on the PCB with a differential clock rate of 1.2GHz (i.e., 833ps clock period).

Question: What is the maximum skew of the trace length for all the signals?

_{eff}≈2.92.

So, for striplines, the maximum skew should be less than +/-(20.825/(85*SQT(4)) = +/-0.1225 in = +/-122.5 mil.

For microstrips, the maximum skew should be less than +/-(20.825/(85*SQT(2.92)) = +/-0.1433 in = +/-143.3 mil.

*Note: Different microstrip and stripline structures will affect the signal speed, but only slightly.

Keep this in mind the next time you calculate trace lengths. It should make the job a little easier.

- Atar Mittal, Signal Speed and Propagation Delay in a PCB Transmission Line, protoexpress.com/blog/signal-speed-propagation-delay-pcb-transmission-line/.
- JEDEC, jedec.org.