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There are two ways to make 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 diode voltage lower than the ground potential. At the falling level, this reduces the magnitude of the backlash. The slower rising edge allows overshoot, but it is affected by the relatively high output impedance (50～80Ω) of the circuit in the level "H" state. attenuation. 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 signal line, the TTL shaping method introduced above appears to be somewhat inadequate 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, causing serious signal distortion 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) allow The percentage of backlash and overshoot (the degree of reduction in AC immunity).
2. Several types of transmission lines
(1) 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 the printed circuit board
The microstrip line is a strip conductor (signal line). A dielectric is used to isolate it from the ground plane. 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:
In the formula: [Er is the relative permittivity of the printed board dielectric material
6 is the thickness of the dielectric layer
W is the width of the line
t is the thickness of the line
The transmission delay time of a microstrip line per unit length depends only on the dielectric constant and has nothing to do with the line width or spacing.
(3) Strip lines in printed boards
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 B of the strip line is:
Where: b is the distance between two ground boards
W is the width of the line
t is the thickness of the line
Similarly, the transmission delay time of a strip line per unit length has nothing to do with the width or spacing of the line; it only depends on the relative permittivity of the medium used.
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.
4. Unterminated transmission line
If the line delay time is much shorter than the signal rise time, the transmission line can be used without series termination or parallel termination. If a non-terminated wire has a round trip delay (the time it takes for a signal to travel on the transmission line once) than pulse The rise time of the signal is short, so the kickback caused by non-termination is about 15% of the logical swing.
PCB wiring technology
Whether to choose a double-sided board or a 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 multilayer 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 lines close to each other, it is best to (2) avoid sharp turns when designing the signal transmission line to prevent reflections caused by sudden changes in the characteristic impedance of the transmission line, and 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.
(4) For double-sided boards (or four-layer lines in six-layer boards). 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 printed circuit board, such as relays, indicator lights, speakers, etc., their ground wires should be separated to reduce noise on the ground wire. The ground wires of these high-current devices should Connect to an independent ground bus on the plug-in board and 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.
