High Speed ​​PCB Design Guide

1. The challenges faced by electronic system design

With the large-scale improvement of system design complexity and integration, electronic system designers are working on circuit design above 100MHZ. The bus operating frequency has reached or exceeded 50MHZ, and some even exceed 100MHZ. At present, about 50% of the designed clock frequency exceeds 50MHz, and nearly 20% of the design frequency exceeds 120MHz.

When the system is operating at 50MHz, transmission line effects and signal integrity issues will occur; when the system clock reaches 120MHz, PCBs based on traditional methods will not work unless high speed circuit design knowledge is used. Therefore, high-speed circuit design technology has become a design tool that electronic system designers must adopt. The controllability of the design process can only be achieved by using the design techniques of high-speed circuit designers.

2. What is a high speed circuit

It is generally considered that if the frequency of the digital logic circuit reaches or exceeds 45 MHz to 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.

In fact, the harmonic frequency of the signal edge is higher than the frequency of the signal itself, and the rising edge and falling edge of the signal (or the transition of the signal) cause unintended consequences of signal transmission. Therefore, it is generally agreed that if the line propagation delay is greater than the rise time of the 1/2 digital signal drive terminal, then such 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 period of 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.

3. The determination of high-speed signals

Above we define the preconditions for the transmission line effect, but how do we know if the line delay is greater than the signal rise time of the 1/2 drive? In general, the typical value of the signal rise time can be given in the device manual, and the propagation time of the signal is determined by the actual wiring length in the PCB design. The figure below shows the correspondence between the signal rise time and the allowed wiring length (delay).

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

Let Tr be the signal rise time and Tpd be the signal line propagation delay. 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 falling in uncertain areas and problem areas, the high-speed wiring method should be used.

4. What is the transmission line?

The traces on the PCB can be equivalent to the series, parallel, capacitor, resistor and inductor structures shown below. Typical values for series resistors are 0.25-0.55 ohms/foot, which are typically high due to the insulating layer. After 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, and the reflected signal will be transmitted back to the signal transmitting end and reflected back 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.