PCB Technical - Learn the good method of PCB circuit board

High-speed PCB circuits work on relatively long lines without serious waveform distortion. TTL adopts Schottky diode clamping method for fast falling edges, so that the overshoot is clamped to a level lower than the ground potential by one diode drop. This reduces the magnitude of the backlash. The slower rising edge allows overshoot, but it is attenuated by the relatively high output impedance (50-80Ω) of the circuit in the level "H" state. In addition, due to the greater immunity of the level "H" state, the kickback problem is not very prominent. For HCT series devices, if the Schottky diode clamp and series resistance termination method are combined, it will improve The effect will be more obvious.

When there is fan-out along the PCB signal line, the TTL shaping method introduced above appears to be somewhat insufficient at a higher bit rate and a faster edge rate. Because there are reflected waves in the line, they will tend to be synthesized at a high bit rate, which will cause serious distortion of the PCB signal and reduced anti-interference ability. Therefore, in order to solve the reflection problem, another method is usually used in the ECL system: the line impedance matching method. In this way, the reflection can be controlled and the integrity of the signal can be guaranteed.

Strictly speaking, for conventional TTL and CMOS devices with slower edge speeds, transmission lines are not very necessary. For high-speed ECL devices with faster edge speeds, transmission lines are not always needed. But when using transmission lines, they have the advantages of predicting the connection delay and controlling reflection and oscillation through impedance matching.

1. There are five basic factors in deciding whether to use a transmission line:

They are: (1) the edge rate of the system signal, (2) the connection distance (3) the capacitive load (how much fan out), (4) the resistive load (the line termination method); (5) allowable The percentage of backlash and overshoot (the degree of reduction in AC immunity).

2. Several types of PCB transmission lines

(1) PCB coaxial cable and twisted pair: they are often used in the connection between the system and the system. The characteristic impedance of coaxial cable is usually 50Ω and 75Ω, and twisted pair is usually 110Ω.

(2) Microstrip line on PCB

The microstrip line is a strip conductor (signal line), separated from the ground plane by a dielectric. If the thickness, width, and distance between the line and the ground plane are controllable, its characteristic impedance can also be controlled. The characteristic impedance Z0 of the microstrip line is:

(3) Strip line in PCB printed board

A stripline is a copper strip line placed in the middle of a dielectric between two conductive planes. If the thickness and width of the line, the dielectric constant of the medium, and the distance between the two conductive planes are controllable, then the characteristic impedance of the line is also controllable. The characteristic impedance of the strip line is:

3. Terminate the transmission line

At the receiving end of a line, a resistance equal to the characteristic impedance of the line is used to terminate, then the transmission line is called a parallel terminal connection. It is mainly used to obtain the best electrical performance, including driving distributed loads.

Sometimes in order to save power consumption, a 104 capacitor is connected in series to the terminating resistor to form an AC termination circuit, which can effectively reduce DC loss.

A resistor is connected in series between the driver and the transmission line, and the terminal of the line is no longer connected to the termination resistor. This termination method is called series termination. The overshoot and ringing on the longer line can be controlled by series damping or series termination technology. Series damping is achieved by using a small resistance (generally 10 to 75Ω) connected in series with the output of the drive gate. This damping method is suitable Used in conjunction with lines whose characteristic impedance is controlled (such as backplane wiring, circuit boards without ground planes, and most winding wires, etc.).

In series termination, the sum of the value of the series resistance and the output impedance of the circuit (driving gate) is equal to the characteristic impedance of the transmission line. The series connection end wiring has the disadvantages that it can only use the lumped load at the terminal and the transmission delay time is longer. However, this This can be overcome by using redundant serially terminated transmission lines.

Whether to choose a PCB double-sided board or a PCB multi-layer board when making a PCB depends on the highest operating frequency, the complexity of the circuit system, and the requirements for assembly density. It is best to choose a multi-layer board when the clock frequency exceeds 200MHZ. If the operating frequency exceeds 350MHz, it is best to choose a printed circuit board with PTFE as the dielectric layer, because its high-frequency attenuation is smaller, the parasitic capacitance is smaller, and the transmission speed is faster. Large and low power consumption, the following principles are required for the wiring of the printed circuit board

(1) Keep as much space as possible between all parallel signal lines to reduce crosstalk. If there are two signal wires that are close together, it is best to run a ground wire between the two wires, which can play a shielding role.

(2) When designing signal transmission lines, avoid sharp turns to prevent reflections due to sudden changes in the characteristic impedance of the transmission line. Try to design a uniform arc line with a certain size.

(3) The width of the printed line can be calculated according to the above-mentioned characteristic impedance calculation formula of the microstrip line and the strip line. The characteristic impedance of the microstrip line on the printed circuit board is generally between 50 and 120Ω. To get a large characteristic impedance, the line width must be very narrow. But very thin lines are not easy to make. Considering various factors, it is generally appropriate to choose an impedance value of about 68Ω, because the characteristic impedance of 68Ω can achieve the best balance between delay time and power consumption. A 50Ω transmission line will consume more power; of course, a larger impedance can reduce the power consumption, but it will increase the transmission delay time. The negative line capacitance will increase the transmission delay time and decrease the characteristic impedance. However, the intrinsic capacitance per unit length of the line segment with very low characteristic impedance is relatively large, so the transmission delay time and characteristic impedance are less affected by the load capacitance. An important feature of a properly terminated transmission line is that the short branch line should have no effect on the line delay time. When Z0 is 50Ω. The length of the branch stub must be limited to 2.5cm or less. In order to avoid loud ringing.

(4) For PCB double-sided boards (or four-layer lines in a six-layer board). The lines on both sides of the circuit board should be perpendicular to each other to prevent crosstalk caused by mutual induction.

(5) If there are high-current devices on the PCB printed board, such as relays, indicator lights, speakers, etc., their ground wires should be separated separately to reduce the noise on the ground wire. The ground wires of these high-current devices It should be connected to an independent ground bus on the plug-in board and the backplane, and these independent ground wires should also be connected to the ground point of the entire system.

(6) If there is a small signal amplifier on the board, the weak signal line before amplification should be far away from the strong signal line, and the trace should be as short as possible, and if possible, shield it with a ground wire.