High Speed PCB Design Guide

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As electronic system design complexity and integration continue to improve, designers are facing new challenges. Circuit design needs to go beyond a certain scale, referred to here as plate_number_1. Bus operating frequencies have also increased, with some exceeding plate_number_1.

Nowadays, about 50% of clock frequencies exceed 50MHz, and almost 20% exceed 120MHz. When the system operates at 50MHz, transmission line effects and signal integrity issues may occur. When the clock reaches 120MHz, traditional PCB-based methods won't work without high-speed circuit design knowledge. As a result, high-speed circuit design technology has become a vital tool for electronic system designers. The design process's controllability can only be achieved by adopting high-speed circuit design techniques.

But what is a high-speed circuit? Generally, if the frequency of the digital logic circuit reaches or exceeds 45-50 MHz, and the circuit operating above this frequency already accounts for a certain amount (for example, 1/3) of the entire electronic system, it is called a high-speed circuit.

The harmonic frequency of the signal edge is higher than the frequency of the signal itself. This means that the rising edge and falling edge of the signal (or the signal transition) can cause unintended consequences of signal transmission. Therefore, if the line propagation delay is greater than the rise time of the 1/2 digital signal drive terminal, signals are considered to be high-speed signals and produce transmission line effects. The transmission of the signal occurs at the moment the signal state changes, such as the rise or fall time. The signal passes from the driver to the receiver for a fixed time. If the transmission time is less than 1/2 of the rise or fall time, the reflected signal from the receiver will reach the driver before the signal changes state. Conversely, the reflected signal will arrive at the drive after the signal changes state. If the reflected signal is strong, the superimposed waveform may change the logic state.

High Speed PCB Design Guide

Determining the High-Speed Signals

In order to understand the transmission line effect, we need to identify the conditions that need to be met. But how can we determine if the line delay is greater than the signal rise time of the 1/2 drive?

Usually, the device manual specifies the typical value of the signal rise time, and the propagation time of the signal depends on the actual wiring length in the PCB design. The figure below shows the correlation between the signal rise time and the allowed wiring length (delay).

The delay per inch on the PCB is 0.167 ns. However, if there are many vias, device pins, and constraints on the network cable, the delay will increase. Typically, high-speed logic devices have a signal rise time of approximately 0.2 ns, and if there is a GaAs chip on the board, the maximum wiring length is 7.62 mm.

The signal rise time is denoted by Tr, and the signal line propagation delay is denoted by Tpd. If Tr ≥ 4Tpd, the signal falls in the safe area. If 2Tpd ≥ Tr ≥ 4Tpd, the signal falls in the uncertainty region. If Tr ≤ 2Tpd, the signal falls in the problem area. For signals that fall in the uncertainty and problem areas, the high-speed wiring method should be used.

What is Transmission Line?

The traces on the PCB can be modeled as series, parallel, capacitor, resistor, and inductor structures shown below. The typical values for series resistors are 0.25-0.55 ohms/foot, which are high due to the insulating layer. Once the parasitic resistance, capacitance, and inductance are added to the actual PCB trace, the final impedance on the trace is called the characteristic impedance Zo. The wider the wire diameter, the closer to the power/ground, or the higher the dielectric constant of the isolation layer, the smaller the characteristic impedance.

If the impedance of the transmission line and the receiving end do not match, the output current signal and the final steady state of the signal will be different, which causes the signal to reflect at the receiving end. The reflected signal is then transmitted back to the signal transmitting end and reflected again. As the energy decreases, the amplitude of the reflected signal will decrease until the voltage and current of the signal stabilize. This effect is called oscillation, and the oscillation of the signal is often seen on the rising and falling edges of the signal.

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