Introduction
Today’s board design is not as simple as it was in the past. Current
high-speed designs demand a board designer’s extensive knowledge
of these issues: transmission line effect, EMI, and crosstalk.
The designer also needs to be an expert concerning board material,
signal and power stacking, connectors, cables, vias, and trace
dimensions.
Pericom offers an extensive line of clock products for desktop,
notebook, set-top boxes, information devices, servers, and workstations.
This application note will help a designer ensure proper
termination, placement, routing, and stacking of the board to ensure
proper data transfers between the microprocessor, cache, main
memory, and expansion busses.
Q. Is this a transmission line or a two wire interconnect?
This is a very important question that we need to answer before
starting a PCB design. It is crucial because transmission lines act
differently than a simple wire.
If we have a 100 Hz circuit that is driving a 1 Meg Ohm load through
a one meter wire, then the resistivity of the line is negligible
compared to the load. The equation V = V0 (1-e )
is used to determine the characteristic of the signal. The step
voltage input will be delayed by the RC constant.
If, however, this circuit is running at 100MHz, then the analysis
shown above does not work and we need to treat this as a
transmission line. We have to use Transmission Line Theory to
determine the response of this circuit. If this transmission line is not
terminated properly, you will have the following to contend with:
ringing delays, overshoot, and undershoot. Other conditions that
could affect your circuit are crosstalk, driver overload, and reduced
noise margins. Any of which will ensure that your board is not
working properly.
Q. How do we determine if this is a transmission line?
The length of the interconnection and the frequency of the circuit
is the answer. If the length of the interconnection is larger than a
tenth of the signal’s sinusoidal wavelength, you will have to rely on