PCB Tech - Signals in high-speed digital PCB board design

With the increase of the output switching speed of integrated circuits and the increase of PCB density, signal integrity has become one of the issues that must be concerned in the design of high-speed digital PCBs. The parameters of components and PCBs, the layout of components on the PCB, and the wiring of high-speed signal lines, etc. Factors will cause signal integrity problems.

For PCB layout, signal integrity requires the provision of a circuit board layout that does not affect signal timing or voltage, while for circuit layout, signal integrity requires the provision of termination components, layout strategies, and routing information.

High signal speed on the PCB, incorrect layout of termination components, or incorrect wiring of high-speed signals can cause signal integrity problems, which may cause the system to output incorrect data, the circuit does not work properly or even does not work at all. How to design the PCB Full consideration of signal integrity factors in the process and effective control measures have become a hot topic in the PCB design industry today

1. Signal integrity issues

Good signal integrity means that the signal can respond with correct timing and voltage level values when needed. Conversely, when the signal cannot respond normally, a signal integrity problem occurs.

Signal integrity problems can cause or directly lead to signal distortion, timing errors, incorrect data, address and control lines, system malfunctions, and even system crashes. Signal integrity problems are not caused by a single factor, but in board-level design. Caused by a variety of factors.

IC switching speed, incorrect layout of termination components, or incorrect wiring of high-speed signals can all cause signal integrity problems. The main signal integrity problems include: delay, reflection, synchronous switching noise, oscillation, ground bounce, crosstalk, etc.

2. Definition of signal integrity

Signal integrity refers to the ability of a signal to respond with the correct timing and voltage in the circuit. It is a state in which the signal is not damaged, and it represents the quality of the signal on the signal line.

2.1 Delay

Delay means that the signal is transmitted at a limited speed on the wires of the PCB, and the signal is sent from the sending end to the receiving end, during which there is a transmission delay. The delay of the signal will affect the timing of the system, and the transmission delay mainly depends on the length of the wire and the dielectric constant of the medium around the wire.

In high-speed digital systems, the length of the signal transmission line is the most direct factor that affects the phase difference of the clock pulse. The phase difference of the clock pulse refers to the two clock signals that are generated at the same time, and the time when they arrive at the receiving end is not synchronized.

The clock pulse phase difference reduces the predictability of the signal edge arrival. If the clock pulse phase difference is too large, an error signal will be generated at the receiving end. As shown in Figure 1, the transmission line delay has become an important part of the clock pulse cycle.

2.2 Reflection

The reflection is the echo on the sub-transmission line. When the signal delay time (Delay) is much greater than the signal transition time (Transition Time), the signal line must be used as a transmission line. When the characteristic impedance of the transmission line does not match the load impedance, part of the signal power (voltage or current) is transmitted to the line and reaches the load, but part of it is reflected.

If the load impedance is less than the original impedance, the reflection is negative; otherwise, the reflection is positive. Variations in wiring geometry, incorrect wire termination, transmission through connectors, and discontinuities in the power plane can all cause such reflections.

2.3 Synchronous switching noise (SSN)

When the many digital signals on the PCB are switched synchronously (such as the data bus of the CPU, the address bus, etc.), due to the impedance of the power line and the ground line, synchronous switching noise will be generated, and ground plane bounce noise will also appear on the ground line (Ground bomb).

The strength of SSN and ground bounce also depends on the I/O characteristics of the integrated circuit, the impedance of the PCB power layer and plane layer, and the layout and wiring of high-speed devices on the PCB.

2.4 Crosstalk (Crosstalk)

Crosstalk is the coupling between two signal lines, and the mutual inductance and mutual capacitance between the signal lines cause noise on the line. Capacitive coupling induces coupling current, and inductive coupling induces coupling voltage. Crosstalk noise originates from electromagnetic coupling between signal wire networks, between signal systems and power distribution systems, and between vias.

Cross-winding may cause false clocks, intermittent data errors, etc., which may affect the transmission quality of adjacent signals. In fact, we do not need to completely eliminate the crosstalk, as long as it is controlled within the range that the system can withstand to achieve the goal.

The parameters of the PCB layer, the signal line spacing, the electrical characteristics of the driving end and the receiving end, and the baseline termination method all have a certain impact on the crosstalk.

2.5 Overshoot and Undershoot

Overshoot is when the first peak or valley exceeds the set voltage. For a rising edge, it refers to the highest voltage, and for a falling edge, it refers to the lowest voltage. Undershoot means that the next valley or peak value exceeds the set voltage.

Excessive overshoot can cause the protection diode to work, leading to its premature failure. Excessive undershoot can cause false clock or data errors (misoperation).

2.6 Ringing and Rounding

The oscillation phenomenon is repeated overshoot and undershoot. The oscillation of the signal is the oscillation caused by the inductance and capacitance of the line transition, which belongs to the under-damped state, and the surrounding oscillation belongs to the over-damped state.

Oscillation and surround oscillation are also caused by many factors like reflection. Oscillation can be reduced by proper termination, but it is impossible to completely eliminate it.

2.7 Ground bounce noise and return noise

When there is a large current surge in the circuit, it will cause ground plane bounce noise. For example, when a large number of chip outputs are turned on at the same time, a large transient current will flow through the power plane of the chip and the board. The chip package and the power supply The inductance and resistance of the plane will cause power noise, which will produce voltage fluctuations and changes on the true ground plane (OV). This noise will affect the actions of other components.

The increase of load capacitance, the decrease of load resistance, the increase of ground inductance, and the increase of the number of switching devices at the same time will all lead to the increase of ground bounce.

Due to the division of the ground plane (including power and ground), for example, the ground plane is divided into digital ground, analog ground, shielding ground, etc., when the digital signal goes to the analog ground area, ground plane return noise will be generated.

Similarly, the power plane may also be divided into 2.5V, 3.3V, 5V, etc. Therefore, in the multi-voltage PCB design, special attention should be paid to the rebound noise and return noise of the ground plane.