1. This is an in-depth article on a crucial topic that PCB designers must master.
2. The first point to remember is that signal reflections occur in the PCB transmission line due to discontinuous impedance.
3. The transmission line should maintain uniform characteristic impedance. Any changes or discontinuities in impedance will result in signal reflection and distortion.
4. This phenomenon also applies to PCB traces and transmission lines. The reason is that high-frequency signals have very short physical wavelengths. Therefore, PCB traces exhibit similar behavior. The higher the frequency, the shorter the wavelength. Even shorter traces should be treated as transmission lines.
5. Discontinuities or non-uniformities in signal trajectory create signal integrity issues. To prevent signal distortion at both the source and target, match the PCB trace impedance to the source impedance. Additionally, load impedances at the source and target ends. This presents a significant challenge, requiring meticulous PCB design to minimize signal attenuation caused by impedance discontinuities.
6. The greater the discontinuity in characteristic impedance, the higher the signal reflection and resulting distortion. Thus, aim to minimize impedance discontinuities in both amplitude and time.
7. Read: Why controlled impedance is crucial for signal integrity.
In theory, a digital signal is a square wave pulse that switches within a short period of time. Naturally, the short rise times required by high-frequency digital circuits result in extremely high frequencies associated with these fast rise times. In fact, these frequencies are an order of magnitude higher than the circuit’s clock frequency. High-frequency digital circuits have shorter pulse widths and thus shorter rise times. A very short rise time implies that the digital signal contains very high frequencies. Therefore, high-frequency digital signals must adhere to signal integrity rules relevant to high-frequency signals.
Consequently, any change in the impedance of PCB traces will lead to signal reflections, which can cause ringing and distortion. At high switching frequencies, impedance discontinuities can significantly distort the digital signal, potentially resulting in signal sampling errors. You can characterize the transmission line formed by PCB traces using parameters such as resistance, conductance, and trace resistance. It is important to understand the differences between microstrip lines and striplines in PCB design.
**Typical Impedance Discontinuities**
The characteristic impedance of a transmission line is the square root of the inductance divided by the capacitance. This assumption is reasonable for PCBs because trace resistance and conductance at high signal frequencies are negligible compared to their inductance and capacitance.
Impedance discontinuity refers to any factor that affects the ratio of trace inductance to capacitance. Typical examples include:
1. **Impedance Changes in the Line:** If the impedance of the line changes due to alterations in the copper cross-section or routing path, the mutual inductance will change, leading to impedance discontinuity.
2. **Branches in the Line:** Routing signals to multiple devices may require branches and line stubs, which can alter line impedance and introduce discontinuities.
3. **Return Signal Splitting:** High-frequency signals travel along the path with the lowest impedance, typically found directly beneath the signal trace in the ground plane. Any physical feature that causes the return signal to deviate from this path will create discontinuities.
4. **Vias:** Vias are used to transmit signals between PCB layers. Although this is a basic feature of PCB design, the shape and size of vias can affect trace inductance and capacitance, creating discontinuities. For more information, refer to how to reduce parasitic capacitance in PCB layout.
**How to Limit the Impact of Impedance Discontinuity**
To mitigate the negative effects of impedance discontinuities, it is crucial to treat all PCB signal traces as transmission lines. Ensure that the characteristic impedance is consistent along the entire signal path.
Follow these guidelines:
1. **Match Source and Load Impedance:** Ensure that the source impedance and load impedance match the trace impedance. Achieve this by using series or parallel resistors to set the correct impedance. Additionally, terminate any open traces with resistors of the appropriate value.
2. **Avoid Branching:** If the signal needs to be shared among multiple chips, connect the lines in a daisy chain rather than using branches. Alternatively, use a matching buffer device to drive the signal to the branch.
3. **Signal Return Path:** Ensure the signal return path follows the same route as the signal line. If using a ground plane, confirm that there are no interruptions or splits in the return signal path. Maintain a solid plane throughout the trace’s length with no cracks or cuts. If a solid plane is not feasible, use a thicker return trace that covers three times the length of the trace and the height of the dielectric.
4. **Via Design:** Position high-frequency traces on layers as far apart as possible. When vias are necessary, use micro vias instead of traditional ones. Through-holes have significantly different capacitance and inductance characteristics, so minimize their use on signal traces. Where needed, use micro vias with lower capacitance and inductance than standard vias. Micro vias also help keep stub lengths as short as possible. Alternatively, consider high-density interconnection (HDI) PCB technology.
**Impedance Discontinuity and Signal Reflection**
On a uniform transmission line, the signal encounters a constant impedance “Zc (V/I)” throughout its length, allowing it to propagate as required. However, any impedance discontinuity will affect signal propagation and cause signal reflection, similar to how light reflects when encountering a medium discontinuity.
**Types of Impedance Discontinuities and Possible Causes:**
Since transmission line impedance depends on conductor geometry and PCB material characteristics, any change in these parameters will alter impedance. Examples include:
– **At the Source or Destination:** The source or receiver impedance may differ from the line impedance.
– **Changes in Line Width or Height:** Variations in copper thickness or changes in the height and/or dielectric constant between the signal line and return path can cause impedance changes.
2. The first point to remember is that signal reflections occur in the PCB transmission line due to discontinuous impedance.
3. The transmission line should maintain uniform characteristic impedance. Any changes or discontinuities in impedance will result in signal reflection and distortion.
4. This phenomenon also applies to PCB traces and transmission lines. The reason is that high-frequency signals have very short physical wavelengths. Therefore, PCB traces exhibit similar behavior. The higher the frequency, the shorter the wavelength. Even shorter traces should be treated as transmission lines.
5. Discontinuities or non-uniformities in signal trajectory create signal integrity issues. To prevent signal distortion at both the source and target, match the PCB trace impedance to the source impedance. Additionally, load impedances at the source and target ends. This presents a significant challenge, requiring meticulous PCB design to minimize signal attenuation caused by impedance discontinuities.
6. The greater the discontinuity in characteristic impedance, the higher the signal reflection and resulting distortion. Thus, aim to minimize impedance discontinuities in both amplitude and time.
7. Read: Why controlled impedance is crucial for signal integrity.
In theory, a digital signal is a square wave pulse that switches within a short period of time. Naturally, the short rise times required by high-frequency digital circuits result in extremely high frequencies associated with these fast rise times. In fact, these frequencies are an order of magnitude higher than the circuit’s clock frequency. High-frequency digital circuits have shorter pulse widths and thus shorter rise times. A very short rise time implies that the digital signal contains very high frequencies. Therefore, high-frequency digital signals must adhere to signal integrity rules relevant to high-frequency signals.
Consequently, any change in the impedance of PCB traces will lead to signal reflections, which can cause ringing and distortion. At high switching frequencies, impedance discontinuities can significantly distort the digital signal, potentially resulting in signal sampling errors. You can characterize the transmission line formed by PCB traces using parameters such as resistance, conductance, and trace resistance. It is important to understand the differences between microstrip lines and striplines in PCB design.
**Typical Impedance Discontinuities**
The characteristic impedance of a transmission line is the square root of the inductance divided by the capacitance. This assumption is reasonable for PCBs because trace resistance and conductance at high signal frequencies are negligible compared to their inductance and capacitance.
Impedance discontinuity refers to any factor that affects the ratio of trace inductance to capacitance. Typical examples include:
1. **Impedance Changes in the Line:** If the impedance of the line changes due to alterations in the copper cross-section or routing path, the mutual inductance will change, leading to impedance discontinuity.
2. **Branches in the Line:** Routing signals to multiple devices may require branches and line stubs, which can alter line impedance and introduce discontinuities.
3. **Return Signal Splitting:** High-frequency signals travel along the path with the lowest impedance, typically found directly beneath the signal trace in the ground plane. Any physical feature that causes the return signal to deviate from this path will create discontinuities.
4. **Vias:** Vias are used to transmit signals between PCB layers. Although this is a basic feature of PCB design, the shape and size of vias can affect trace inductance and capacitance, creating discontinuities. For more information, refer to how to reduce parasitic capacitance in PCB layout.
**How to Limit the Impact of Impedance Discontinuity**
To mitigate the negative effects of impedance discontinuities, it is crucial to treat all PCB signal traces as transmission lines. Ensure that the characteristic impedance is consistent along the entire signal path.
Follow these guidelines:
1. **Match Source and Load Impedance:** Ensure that the source impedance and load impedance match the trace impedance. Achieve this by using series or parallel resistors to set the correct impedance. Additionally, terminate any open traces with resistors of the appropriate value.
2. **Avoid Branching:** If the signal needs to be shared among multiple chips, connect the lines in a daisy chain rather than using branches. Alternatively, use a matching buffer device to drive the signal to the branch.
3. **Signal Return Path:** Ensure the signal return path follows the same route as the signal line. If using a ground plane, confirm that there are no interruptions or splits in the return signal path. Maintain a solid plane throughout the trace’s length with no cracks or cuts. If a solid plane is not feasible, use a thicker return trace that covers three times the length of the trace and the height of the dielectric.
4. **Via Design:** Position high-frequency traces on layers as far apart as possible. When vias are necessary, use micro vias instead of traditional ones. Through-holes have significantly different capacitance and inductance characteristics, so minimize their use on signal traces. Where needed, use micro vias with lower capacitance and inductance than standard vias. Micro vias also help keep stub lengths as short as possible. Alternatively, consider high-density interconnection (HDI) PCB technology.
**Impedance Discontinuity and Signal Reflection**
On a uniform transmission line, the signal encounters a constant impedance “Zc (V/I)” throughout its length, allowing it to propagate as required. However, any impedance discontinuity will affect signal propagation and cause signal reflection, similar to how light reflects when encountering a medium discontinuity.
**Types of Impedance Discontinuities and Possible Causes:**
Since transmission line impedance depends on conductor geometry and PCB material characteristics, any change in these parameters will alter impedance. Examples include:
– **At the Source or Destination:** The source or receiver impedance may differ from the line impedance.
– **Changes in Line Width or Height:** Variations in copper thickness or changes in the height and/or dielectric constant between the signal line and return path can cause impedance changes.
