“High-speed signal” is a widely used term, somehow rarely explained. Development of electronic boards with high-speed circuits requires specific knowledge and experience. Design of such PCBs means the necessity to follow the strict implicit size and spacing constraints and technology limitations that make possible to transmit accurate information on very high speed of switching a useful signal.
Let’s try to find out how to determine a signal which is high-speed. And how it is getting different from a “regular” electrical signal from the point of view of a Signal Integrity specialist.
General ways of defining high-speed
The general rule of thumb helping to define if signals can be referred to as high-speed is to check their frequencies. Oftentimes engineers can classify the boards with signals above 100 MHz are propagating as high-speed. However, even 25 MHz can already become a reason for concern.
Another common approach is to check out the standard-specific technologies in your design. For instance, having DDR, HDMI, PCI Express, USB, or SATA, makes it absolutely clear that high-speed signals will be presented in the given device.
Low-speed signals vs. High-speed signals from the Signal Integrity point of view
The table below gives a brief overview of the differences between high-speed and low-speed signals from the point of view of electronics as a science:
|At low speeds, all available receivers see the same signal coming at the same time due to insignificance of a transmission line electrical effects in relation to a signal period (which is bigger for low-speed signals).||At high speeds, the electrical effects of a transmission line, due to their significance raise in comparison to a short signal period (typical for high-speed signals), cause receivers to see the same signal coming to receivers not simultaneously, causing miscommunication between devices.|
|The delay caused by passing the PCB trace can be considered as a single (lumped) delay on the output of the driver (a device, generating the signal under consideration).||The delays caused by a PCB trace must be considered as a set of different path-dependent delays.|
|No additional simulation is required here.||An analysis of the transmission lines should be conducted to make sure that Signal Integrity is in place.|
We define the signal a high-speed whenever it’s driver’s rise/fall time is less than the round-trip delay of the circuit.
This is exactly the case when transmission line effects may cause unwanted problems like ringing, oscillation, overshoot or undershoot and Signal Integrity analysis becomes a mandatory part of the PCB design process.
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