High speed Designs - Part 8

Propagation delay is one of the important parameters in the high speed designs. 

Definition 1:

Propagation delay is defined as the time taken for the signal to travel from source to destination. This is calculated as,

length of the channel/speed of the signal

The length of the channel is nothing but distance from source to destination.

Definition 2:

In the case of an IC, propagation delay is the time taken for the signal output to arrive after the input is provided. 

Definition 3:

It can also be defined as the measure of the IC of it's speed of response to the input provided.

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Propagation delay depends on several factors:

• Switching of digital circuit, Example: Transistor switching inside an IC
• Charging of the components like inductors, capacitors
• Trace Delay on the PCB (150ps/inch, depending on dielectric material)

The cumulative effect of this propagation delay is that signals which need to arrive together at the same time might arrive at a different times if proper care is not taken. This leads to requirement of length matching in high speed designs.

Take the example of buffer MC74HVC1G50 which is used for buffering in circuits. From the datasheet of this, we can see the below propagation delay from Input 'A' to output 'Y'..