PCB Tech - Five problems affecting signal quality in high-speed PCB design

In high-speed PCB design, "signal" is always a knowledge point that engineers cannot bypass. Whether in the design link, or in the test link, signal quality is worth paying attention to. In this article, we will mainly understand the five major problems affecting signal quality.

According to the conclusion of current work, the common problems of signal quality are mainly manifested in five aspects: overshoot, backwash, burr, edge and level.

1) the overshoot

Bring a figure

The problem of overshoot is easy to cause device damage, overshoot is also easy to cause crosstalk around the signal. The cause of overshoot is mismatch, which can be eliminated by starting series resistance or terminal parallel resistance (or resistance).

2) the burr

Bring burr figure

When burrs act on high-speed devices, they are prone to mistrigger, control signal control error, or clock signal phase error. The problems caused by burrs usually occur when the board is unstable or the device is replaced. There are many reasons for burrs, such as logic risk, crosstalk, ground bounce, etc., and their elimination methods are not the same.

3) the edge

Bring the edge of the figure

When the edge speed is slow on the signal line, data sampling errors may occur. It is usually caused by capacitive load at the output terminal (too many loads) and low charging (discharging) current at the output in three states.

4) rushed back

Bring back the figure

Backflush is caused by mismatching of signal lines or multiple loads, which can be eliminated by adding matching resistors or adjusting the topology of the bus.

5) level

Bring about level figure

If the input level amplitude does not meet the requirements, device output errors may occur. The main causes of abnormal level are output overload, level mismatch, three-state bus, bus conflict and so on.

Extension:

Engineers should have the following knowledge when conducting signal quality tests:

1) Have a clear understanding of the measuring tool (oscilloscope), to understand the performance of oscilloscope, master the use of oscilloscope and its probe, and understand the relationship between the test of abnormal signal quality and the setting of oscilloscope menu.

2) Have a comprehensive and clear understanding of abnormal signal forms and abnormal indicators of abnormal signals.

3) Have certain knowledge and understanding of the principle circuit of the board under test, and need to be able to classify signals, understand the key components on the board, key bus, signal quality requirements of key signals and related timing parameters.