High-speed PCB circuits operate on relatively long traces without significant waveform distortion. TTL employs the Schottky diode clamping method to fasten the falling edges, which limits the overshoot by clamping it to a level lower than the ground potential by a single diode drop. This technique helps reduce the magnitude of the backlash. The slower rising edge permits overshoot, but this is attenuated by the relatively high output impedance (50–80Ω) of the circuit when in the “H” state. Furthermore, due to the higher immunity in the “H” state, the kickback issue is less pronounced. For HCT series devices, combining Schottky diode clamps with series resistance termination provides a more noticeable improvement in performance.
When there is fan-out along the PCB signal line, the TTL shaping method mentioned earlier may be inadequate at higher bit rates and faster edge rates. Reflected waves on the line can combine at high bit rates, causing significant distortion to the PCB signal and reducing its immunity to interference. To address the reflection issue, a common solution in the ECL system is to use the line impedance matching method. This approach helps control reflections and ensures signal integrity.
Strictly speaking, for conventional TTL and CMOS devices with slower edge rates, transmission lines are not always necessary. However, for high-speed ECL devices with faster edge rates, transmission lines may not always be essential either. Nevertheless, when transmission lines are used, they offer the advantage of predicting connection delays and managing reflections and oscillations through impedance matching.
1. **Five Basic Factors in Deciding Whether to Use a Transmission Line:**
These factors include:
(1) the edge rate of the system signal,
(2) the connection distance,
(3) the capacitive load (fan-out),
(4) the resistive load (termination method),
(5) the allowable percentage of backlash and overshoot (which determines the AC immunity level).
2. **Types of PCB Transmission Lines**
(1) **PCB Coaxial Cable and Twisted Pair:** These are often used in inter-system connections. The characteristic impedance of coaxial cables is typically 50Ω or 75Ω, while twisted pairs generally have an impedance of 110Ω.
(2) **Microstrip Line on PCB:** A microstrip line consists of a strip conductor (signal line) separated from the ground plane by a dielectric material. When the thickness, width, and distance between the line and ground plane are controlled, the characteristic impedance can be adjusted. The characteristic impedance ( Z_0 ) of the microstrip line is:
(3) **Strip Line on PCB:**
1. **Stripline Overview**
A stripline is a copper conductor placed in the center of a dielectric material, sandwiched between two conductive planes. If the thickness and width of the conductor, the dielectric constant of the material, and the distance between the conductive planes can be controlled, then the characteristic impedance of the stripline can also be controlled. The characteristic impedance of the stripline is:
2. **Terminating the Transmission Line**
At the receiving end of a transmission line, a resistance equal to the characteristic impedance of the line is used to terminate it; this configuration is called a parallel termination. This method is primarily used to achieve optimal electrical performance, including driving distributed loads.
In some cases, to reduce power consumption, a 104 capacitor is connected in series with the termination resistor to create an AC termination circuit, which can significantly reduce DC loss.
In other situations, a resistor is placed in series between the driver and the transmission line, and the line’s termination resistor is omitted. This is known as series termination. Series damping or series termination techniques are effective in controlling overshoot and ringing on longer lines. Series damping is achieved by placing a small resistor (typically 10 to 75Ω) in series with the output of the driver. This method is particularly useful for lines with controlled characteristic impedance, such as backplane wiring, circuit boards without ground planes, and many types of wound wires.
For series termination, the sum of the series resistance and the output impedance of the driving circuit should equal the characteristic impedance of the transmission line. However, series termination can have some disadvantages, such as being limited to lumped load usage at the terminal and potentially causing longer transmission delays. These issues can be mitigated by using multiple serially terminated transmission lines.
3. **Choosing Between Double-Sided and Multi-Layer PCBs**
The decision to use a double-sided PCB or a multi-layer PCB depends on factors such as the highest operating frequency, circuit complexity, and assembly density requirements. A multi-layer PCB is generally preferred when the clock frequency exceeds 200 MHz. For frequencies above 350 MHz, a PCB with PTFE as the dielectric material is recommended due to its lower high-frequency attenuation, smaller parasitic capacitance, and faster signal transmission speed with lower power consumption. The following guidelines should be considered when routing the PCB:
(1) **Signal Line Spacing**
Maximize the spacing between parallel signal lines to minimize crosstalk. If two signal lines must be placed close together, it is advisable to route a ground trace between them to provide shielding.
(2) **Minimize Sharp Turns**
Avoid sharp bends in signal transmission lines, as these can cause reflections due to sudden changes in the characteristic impedance. Instead, design smooth arcs of appropriate size.
(3) **Line Width and Impedance Calculation**
The width of the printed trace can be determined using the characteristic impedance formulas for microstrip or stripline configurations. The characteristic impedance of a microstrip line on a PCB typically ranges from 50 to 120Ω. To achieve a higher characteristic impedance, the trace width must be very narrow, although excessively thin traces are difficult to manufacture. Considering various factors, a value around 68Ω is often ideal, as it provides a good balance between delay time and power consumption. A 50Ω transmission line typically consumes more power, while higher impedance lines reduce power consumption but increase transmission delay. Lines with very low characteristic impedance tend to have higher parasitic capacitance per unit length, which increases transmission delay and decreases the characteristic impedance. A well-terminated transmission line should ensure that short branch lines do not affect delay time. When the characteristic impedance (Z₀) is 50Ω, the length of any branch stub should be limited to 2.5 cm or less to avoid significant ringing.
(4) **PCB Layer Orientation**
On double-sided (or four-layer within a six-layer) PCBs, the traces on opposite sides of the board should be perpendicular to one another to prevent crosstalk due to mutual inductance.
(5) **Managing High-Current Devices**
For PCBs containing high-current devices (e.g., relays, indicators, speakers), separate ground traces should be used to minimize noise. The ground connections of these devices should be routed to an independent ground bus on the PCB and the backplane. These separate ground paths should then be connected to the main system ground.
(6) **Amplifier Signal Line Placement**
If a small signal amplifier is present on the PCB, the weak signal lines before amplification should be kept away from strong signal lines. These traces should be as short as possible, and if feasible, they should be shielded by a ground trace.
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When there is fan-out along the PCB signal line, the TTL shaping method mentioned earlier may be inadequate at higher bit rates and faster edge rates. Reflected waves on the line can combine at high bit rates, causing significant distortion to the PCB signal and reducing its immunity to interference. To address the reflection issue, a common solution in the ECL system is to use the line impedance matching method. This approach helps control reflections and ensures signal integrity.
Strictly speaking, for conventional TTL and CMOS devices with slower edge rates, transmission lines are not always necessary. However, for high-speed ECL devices with faster edge rates, transmission lines may not always be essential either. Nevertheless, when transmission lines are used, they offer the advantage of predicting connection delays and managing reflections and oscillations through impedance matching.
1. **Five Basic Factors in Deciding Whether to Use a Transmission Line:**
These factors include:
(1) the edge rate of the system signal,
(2) the connection distance,
(3) the capacitive load (fan-out),
(4) the resistive load (termination method),
(5) the allowable percentage of backlash and overshoot (which determines the AC immunity level).
2. **Types of PCB Transmission Lines**
(1) **PCB Coaxial Cable and Twisted Pair:** These are often used in inter-system connections. The characteristic impedance of coaxial cables is typically 50Ω or 75Ω, while twisted pairs generally have an impedance of 110Ω.
(2) **Microstrip Line on PCB:** A microstrip line consists of a strip conductor (signal line) separated from the ground plane by a dielectric material. When the thickness, width, and distance between the line and ground plane are controlled, the characteristic impedance can be adjusted. The characteristic impedance ( Z_0 ) of the microstrip line is:
(3) **Strip Line on PCB:**
1. **Stripline Overview**
A stripline is a copper conductor placed in the center of a dielectric material, sandwiched between two conductive planes. If the thickness and width of the conductor, the dielectric constant of the material, and the distance between the conductive planes can be controlled, then the characteristic impedance of the stripline can also be controlled. The characteristic impedance of the stripline is:
2. **Terminating the Transmission Line**
At the receiving end of a transmission line, a resistance equal to the characteristic impedance of the line is used to terminate it; this configuration is called a parallel termination. This method is primarily used to achieve optimal electrical performance, including driving distributed loads.
In some cases, to reduce power consumption, a 104 capacitor is connected in series with the termination resistor to create an AC termination circuit, which can significantly reduce DC loss.
In other situations, a resistor is placed in series between the driver and the transmission line, and the line’s termination resistor is omitted. This is known as series termination. Series damping or series termination techniques are effective in controlling overshoot and ringing on longer lines. Series damping is achieved by placing a small resistor (typically 10 to 75Ω) in series with the output of the driver. This method is particularly useful for lines with controlled characteristic impedance, such as backplane wiring, circuit boards without ground planes, and many types of wound wires.
For series termination, the sum of the series resistance and the output impedance of the driving circuit should equal the characteristic impedance of the transmission line. However, series termination can have some disadvantages, such as being limited to lumped load usage at the terminal and potentially causing longer transmission delays. These issues can be mitigated by using multiple serially terminated transmission lines.
3. **Choosing Between Double-Sided and Multi-Layer PCBs**
The decision to use a double-sided PCB or a multi-layer PCB depends on factors such as the highest operating frequency, circuit complexity, and assembly density requirements. A multi-layer PCB is generally preferred when the clock frequency exceeds 200 MHz. For frequencies above 350 MHz, a PCB with PTFE as the dielectric material is recommended due to its lower high-frequency attenuation, smaller parasitic capacitance, and faster signal transmission speed with lower power consumption. The following guidelines should be considered when routing the PCB:
(1) **Signal Line Spacing**
Maximize the spacing between parallel signal lines to minimize crosstalk. If two signal lines must be placed close together, it is advisable to route a ground trace between them to provide shielding.
(2) **Minimize Sharp Turns**
Avoid sharp bends in signal transmission lines, as these can cause reflections due to sudden changes in the characteristic impedance. Instead, design smooth arcs of appropriate size.
(3) **Line Width and Impedance Calculation**
The width of the printed trace can be determined using the characteristic impedance formulas for microstrip or stripline configurations. The characteristic impedance of a microstrip line on a PCB typically ranges from 50 to 120Ω. To achieve a higher characteristic impedance, the trace width must be very narrow, although excessively thin traces are difficult to manufacture. Considering various factors, a value around 68Ω is often ideal, as it provides a good balance between delay time and power consumption. A 50Ω transmission line typically consumes more power, while higher impedance lines reduce power consumption but increase transmission delay. Lines with very low characteristic impedance tend to have higher parasitic capacitance per unit length, which increases transmission delay and decreases the characteristic impedance. A well-terminated transmission line should ensure that short branch lines do not affect delay time. When the characteristic impedance (Z₀) is 50Ω, the length of any branch stub should be limited to 2.5 cm or less to avoid significant ringing.
(4) **PCB Layer Orientation**
On double-sided (or four-layer within a six-layer) PCBs, the traces on opposite sides of the board should be perpendicular to one another to prevent crosstalk due to mutual inductance.
(5) **Managing High-Current Devices**
For PCBs containing high-current devices (e.g., relays, indicators, speakers), separate ground traces should be used to minimize noise. The ground connections of these devices should be routed to an independent ground bus on the PCB and the backplane. These separate ground paths should then be connected to the main system ground.
(6) **Amplifier Signal Line Placement**
If a small signal amplifier is present on the PCB, the weak signal lines before amplification should be kept away from strong signal lines. These traces should be as short as possible, and if feasible, they should be shielded by a ground trace.
If you have any PCB manufacturing needs, please do not hesitate to contact me.Contact me
