ECG 497/498
Homework #3
Due Wed, Oct. 13

When dealing with high-speed signals it is important to match the impedance of the transmitter and receiver to minimize reflections. This is usually done with a terminating resistor located as close as possible to the driver or receiver IC. When the I/O connector cannot be located close to the driver or receiver IC transmission lines should be used.

For example: You are designing a video distribution amplifier and think it would look cool if the video input was on the left hand side of the box and the video output was on the right hand side of the box. To save money, and for reliability, you've decided against using a small circuit board and running wires from the IC to the connectors. Instead, you use a single PC board with video connectors attached to the outer edges. You now need to run traces from the I/O connectors to the driver & receiver ICs.

You decide to use the PC boards available to you (for free) in Mark shop (type FR4).
They have the following specifications:

T (conductor thickness) = 0.00134" (for 1oz copper)
H (spacing between top & bottom layer) = 0.062"
Er (dielectric permittivity) = 4.8

The following datasheet displays a diagram of what a microstrip trace looks like and has a calculator to determine trace width.
• microstrip trace impedance calculator

What is the trace width for 50 ohm microstrip on FR4?

What is the trace width for 75 ohm microstrip on FR4?

Note: You can't fit many 110mil wide traces on a PCB. How do they route the many high-speed connections needed for memory on a PC motherboard? Answer: They use a different PCB material. By making H smaller you can make the traces much thinner. Er can also change. They also us 15 layer motherboards with multiple ground and power planes.