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Is my design a high speed digital board design?

by Mitali Mistry on October 12, 2016
Is my design a high speed digital board design?

With increasing demands for higher data rates and bandwidths of the signals over wired and wireless communications, more and more digital boards are now falling in to the category of “HIGH SPEED DIGITAL BOARDS”. There are critical issues associated with high speed digital boards if those boards are not handled appropriately during design and development phases of the product development. This can lead to iterations in the product design cycle and affect the bottom lines of the product development projects. Therefore the first step in order to avoid this situations is to answer following question: “Is my design High Speed Digital board design?”

Here is how you can get an idea about your category of digital board design: Low speed digital board design or high speed digital board design, during your board design activity. Following three parameters of the design will define the category of digital signal speed: Propagation delay of a medium, Rise time of a signal and trace length of a signal on board.

Let’s understand these three parameters first and then we will try to understand the correlation to find out the speed of the signal.

Propagation Delay:

Propagation delay is the time taken by digital signal to reach to point-B from point-A in a particular transmission medium. The propagation delay can be described in the “sec/inch” units. E.g. digital signals travelling on a FR4 PCB, in internal layers, shall experience a propagation delay of 180 psec/inch considering 4.5 as the dielectric constant of FR4. Propagation delay value changes based on the type of medium and its dielectric constants. You may find out information about propagation delay of common materials / mediums from internet. You may also get this information from material manufacturer.

Rise Time:

The time required by a digital signal to reach to 90% of its high voltage level value from 10% of its high voltage level value when signal is transitioning from low voltage level to high voltage level. The rise time is generally expressed in terms of “milli-second”/ “micro-second” / “nano-second” etc. The rise time of the signals generally depends on the characteristics of the transmitter and the capacitive elements along the transmission path. Rise time details are generally specified in the chipset datasheets.

Trace length of a signal:

Trace length of a signal is the actual physical trace length from transmitter of the signal to receiver of the signal on the board. Trace length can be described in terms of inches or mills. If you have not started with layout phase, you can approximate the trace length distance by analyzing your top level placement strategy.

Correlation & speed of signal:

Based on various experimentations and from available literatures, following equation is derived. It can be used for categorizing the type of board.

6 x Trace length > (Rise time of a signal) / (Propagation delay)

If above mentioned condition is true, we can approximate that the digital signal is a high speed digital signal and all precautions for component selection, schematic and routing should be taken care to handle on the board.

For example,

The digital signal of 50MHz has a rise time of 2 ns and runs on a FR4 PCB in internal layer. The trace length of this signal is 2 inch on PCB.

So,

B = Rise time / Propagation delay = (2 ns) / (180 ps / inch) = 11.11 inch

And

A = 6 x Trace length = 6 x 2 = 12 inch

Here, A > B. Therefore this signal can act as a High speed digital signal on the board and all the necessary precautions are required to be taken care during design phase.

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