1. **The Basic Concept of PCB Reflow**
In a digital PCB schematic, the propagation of digital signals occurs from one logic gate to another. The signal is transmitted from the output to the receiving end through a wire, typically in a unidirectional flow. Many digital engineers often assume that the path of the circuit is irrelevant, since both the driver and receiver are voltage-mode devices, leading them to overlook the importance of current flow.
However, basic circuit theory tells us that signals are actually propagated by electric current, specifically through the movement of electrons. A key characteristic of electron flow is that electrons never remain in one place; they must always return to their starting point. As a result, current always flows in a loop, and any signal in a circuit exists as part of a closed loop. In high-frequency signal transmission, this process involves charging the dielectric capacitors between the transmission line and the DC layer.
2. **The Influence of PCB Reflow**
Digital circuits generally rely on ground and power planes to facilitate signal return. The return paths for high-frequency and low-frequency signals differ. For low-frequency signals, the return path with the lowest impedance is preferred, while high-frequency signals require the path with the lowest inductance.
When current starts from the signal driver, flows through the signal line, and reaches the receiving end, a return current flows in the opposite direction. This return current begins at the ground pin of the load, travels through the copper plane, and returns to the signal source. The current flowing through the signal line and the copper plane forms a closed loop. The noise frequency generated by the current in the copper plane corresponds to the signal frequency, meaning that higher signal frequencies result in higher noise frequencies. Logic gates do not respond to the absolute input signal, but rather to the difference between the input signal and the reference pin. A single-point termination circuit responds to the difference between the incoming signal and its reference ground plane, so disturbances in the ground reference plane and interference along the signal path are both critical factors to consider.
1. The logic gate responds to the input pin and the designated reference pin, but it is unclear which one is the designated reference pin. For TTL, it is usually a negative power supply, while for ECL, it is typically a positive power supply, though this is not always the case. In this context, the differential signal’s ability to resist interference can effectively reduce ground bounce noise and power plane sliding.
2. When multiple digital signals on the PCB are switched synchronously (such as CPU data bus, address bus, etc.), transient load currents flow from the power supply into the circuit, or from the circuit to the ground. Due to the impedance of the power and ground traces, synchronous switching noise (SSN) is generated, and ground plane bounce noise (commonly referred to as ground bounce) also appears on the ground trace. Moreover, when the area around the power and ground traces is larger, the radiation energy increases. Therefore, we analyze the switching behavior of the digital chip and implement measures to control the return path, aiming to minimize the surrounding area and reduce radiation.
3. IC1 is the signal output terminal, IC2 is the signal input terminal (with the assumption that the receiving terminal includes a downstream resistor for simplicity). The third layer is the ground layer, and both IC1 and IC2 share the same ground plane. The power plane in the upper-right corner of the TOP layer connects to the positive terminal of the power supply. C1 and C2 are the decoupling capacitors for IC1 and IC2, respectively. The power and ground pins of the chips shown in the diagram correspond to the power and ground of the signal transmission and reception ends.
4. At low frequencies, if the S1 terminal outputs a high level, the entire current loop flows as follows: the power supply connects to the VCC power plane via a trace, then enters IC1 through the orange path, exits from the S1 terminal, and enters IC2 via the second layer trace through the R1 terminal. It then flows into the GND layer and returns to the negative terminal of the power supply via the red path.
5. At high frequencies, the PCB’s layout significantly affects the signal integrity. The “ground return” problem is especially important for high-frequency signals. When there is an increase in current on the signal line from S1 to R1, the rapidly changing external magnetic field induces a reverse current in nearby conductors. If the third-layer ground plane is continuous, a current, shown by the blue dashed line, will be generated in the ground plane. If the TOP layer has a complete power plane, the return current will flow along the blue dashed line on the TOP layer as well. In this configuration, the signal loop has the smallest current path, radiates the least energy, and is less prone to external signal coupling. (The skin effect at high frequencies also minimizes outward radiation, following the same principle.)
6. Since the high-frequency signal level and current change rapidly but the duration of these changes is brief, the energy required is relatively small. As a result, the chip is powered by the decoupling capacitor closest to it. When C1 is sufficiently large and responsive (with a very low ESR value—ceramic capacitors are typically used, as their ESR is much lower than that of tantalum capacitors), the orange path on the TOP layer and the red path on the GND layer can be considered negligible.
7. Therefore, in the designed PCB environment, the current path is as follows: from the positive terminal of C1, through the VCC of IC1, the signal line from S1, L2, R1, the GND of IC2, the yellow path on the GND layer, the via, and finally to the negative terminal of the capacitor. It can be seen that there is a brown equivalent current flowing vertically through the PCB, inducing a magnetic field in the middle. This loop can easily couple with external interference. If the signal is a clock, as shown in the diagram, with a set of 8-bit data lines in parallel, all powered by the same power supply, and the return path is the same, a large reverse current will be induced on the clock signal if the data lines switch simultaneously. If the clock line is not properly matched, this crosstalk can significantly disrupt the clock signal. The intensity of this crosstalk is not proportional to the absolute values of the high and low levels of the interference source, but rather to the rate of current change in the interference source.
If your have any questions about PCB ,please contact me info@wellcircuits.com
In a digital PCB schematic, the propagation of digital signals occurs from one logic gate to another. The signal is transmitted from the output to the receiving end through a wire, typically in a unidirectional flow. Many digital engineers often assume that the path of the circuit is irrelevant, since both the driver and receiver are voltage-mode devices, leading them to overlook the importance of current flow.
However, basic circuit theory tells us that signals are actually propagated by electric current, specifically through the movement of electrons. A key characteristic of electron flow is that electrons never remain in one place; they must always return to their starting point. As a result, current always flows in a loop, and any signal in a circuit exists as part of a closed loop. In high-frequency signal transmission, this process involves charging the dielectric capacitors between the transmission line and the DC layer.
2. **The Influence of PCB Reflow**
Digital circuits generally rely on ground and power planes to facilitate signal return. The return paths for high-frequency and low-frequency signals differ. For low-frequency signals, the return path with the lowest impedance is preferred, while high-frequency signals require the path with the lowest inductance.
When current starts from the signal driver, flows through the signal line, and reaches the receiving end, a return current flows in the opposite direction. This return current begins at the ground pin of the load, travels through the copper plane, and returns to the signal source. The current flowing through the signal line and the copper plane forms a closed loop. The noise frequency generated by the current in the copper plane corresponds to the signal frequency, meaning that higher signal frequencies result in higher noise frequencies. Logic gates do not respond to the absolute input signal, but rather to the difference between the input signal and the reference pin. A single-point termination circuit responds to the difference between the incoming signal and its reference ground plane, so disturbances in the ground reference plane and interference along the signal path are both critical factors to consider.
1. The logic gate responds to the input pin and the designated reference pin, but it is unclear which one is the designated reference pin. For TTL, it is usually a negative power supply, while for ECL, it is typically a positive power supply, though this is not always the case. In this context, the differential signal’s ability to resist interference can effectively reduce ground bounce noise and power plane sliding.
2. When multiple digital signals on the PCB are switched synchronously (such as CPU data bus, address bus, etc.), transient load currents flow from the power supply into the circuit, or from the circuit to the ground. Due to the impedance of the power and ground traces, synchronous switching noise (SSN) is generated, and ground plane bounce noise (commonly referred to as ground bounce) also appears on the ground trace. Moreover, when the area around the power and ground traces is larger, the radiation energy increases. Therefore, we analyze the switching behavior of the digital chip and implement measures to control the return path, aiming to minimize the surrounding area and reduce radiation.
3. IC1 is the signal output terminal, IC2 is the signal input terminal (with the assumption that the receiving terminal includes a downstream resistor for simplicity). The third layer is the ground layer, and both IC1 and IC2 share the same ground plane. The power plane in the upper-right corner of the TOP layer connects to the positive terminal of the power supply. C1 and C2 are the decoupling capacitors for IC1 and IC2, respectively. The power and ground pins of the chips shown in the diagram correspond to the power and ground of the signal transmission and reception ends.
4. At low frequencies, if the S1 terminal outputs a high level, the entire current loop flows as follows: the power supply connects to the VCC power plane via a trace, then enters IC1 through the orange path, exits from the S1 terminal, and enters IC2 via the second layer trace through the R1 terminal. It then flows into the GND layer and returns to the negative terminal of the power supply via the red path.
5. At high frequencies, the PCB’s layout significantly affects the signal integrity. The “ground return” problem is especially important for high-frequency signals. When there is an increase in current on the signal line from S1 to R1, the rapidly changing external magnetic field induces a reverse current in nearby conductors. If the third-layer ground plane is continuous, a current, shown by the blue dashed line, will be generated in the ground plane. If the TOP layer has a complete power plane, the return current will flow along the blue dashed line on the TOP layer as well. In this configuration, the signal loop has the smallest current path, radiates the least energy, and is less prone to external signal coupling. (The skin effect at high frequencies also minimizes outward radiation, following the same principle.)
6. Since the high-frequency signal level and current change rapidly but the duration of these changes is brief, the energy required is relatively small. As a result, the chip is powered by the decoupling capacitor closest to it. When C1 is sufficiently large and responsive (with a very low ESR value—ceramic capacitors are typically used, as their ESR is much lower than that of tantalum capacitors), the orange path on the TOP layer and the red path on the GND layer can be considered negligible.
7. Therefore, in the designed PCB environment, the current path is as follows: from the positive terminal of C1, through the VCC of IC1, the signal line from S1, L2, R1, the GND of IC2, the yellow path on the GND layer, the via, and finally to the negative terminal of the capacitor. It can be seen that there is a brown equivalent current flowing vertically through the PCB, inducing a magnetic field in the middle. This loop can easily couple with external interference. If the signal is a clock, as shown in the diagram, with a set of 8-bit data lines in parallel, all powered by the same power supply, and the return path is the same, a large reverse current will be induced on the clock signal if the data lines switch simultaneously. If the clock line is not properly matched, this crosstalk can significantly disrupt the clock signal. The intensity of this crosstalk is not proportional to the absolute values of the high and low levels of the interference source, but rather to the rate of current change in the interference source.
If your have any questions about PCB ,please contact me info@wellcircuits.com
