A transmission line should have a uniform characteristic impedance. Any variation or discontinuities to the impedance causes signal reflection and distortion. This phenomenon is equally true for PCB traces and transmission lines. The reason for this is that the physical wavelength of a high frequency signal is very short and, for that reason, PCB traces exhibit the same characteristics. The higher the frequency, the shorter the wavelength. Even shorter traces must be treated like transmission lines.
Signal trace discontinuities or non-uniform discontinuities form signal integrity discontinuities. In order to avoid signal distortion at the source and destination, the PCB trace impedance must be matched to source and load impedance at the source and destination ends. This presents a considerable challenge that requires careful PCB design to mitigate the effects of signal degradation caused by impedance discontinuity. The greater the discontinuity in the characteristic impedance, the higher the signal reflection, meaning the signal distortion is higher as well. As such, try and keep impedance discontinuities as minimal as possible—both in terms of amplitude and time.
The Effects of Impedance Discontinuities
A digital signal is, in theory, a square wave pulse that switches in a very short time. It naturally follows that the short signal rise times required by high frequency digital circuits will result in extremely high frequencies associated with fast signal rise times. In practice, these frequencies will be an order of magnitude higher than the circuit’s clock frequency. High frequency digital circuits would have a shorter pulse width, and thereby even shorter rise times. A very short signal rise time implies very high frequencies contained in the digital signal. Therefore, high frequency digital signals should follow the signal integrity discipline associated with high frequency signals.
Consequently, any change in the impedance of a PCB trace will cause signal reflections that lead to ringing and signal distortion. The upshot of this is that, at high switching frequencies, the distortion caused to digital signals by impedance discontinuities can be so severe that signal sampling errors may occur. A transmission line formed by PCB traces can be characterized by the following parameters: resistance, conductance, and trace resistance.
Typical Impedance Discontinuities
The characteristic impedance of a line is the square root of the inductance divided by the capacitance. This is a reasonable assumption for PCBs because trace resistance and conductance at high signal frequencies are negligible compared to their inductance and capacitance.
An impedance discontinuity is anything that affects the ratio between the inductance of the trace and its capacitance. Here are some typical examples:
- Impedance variation in the line: If the impedance of a line changes for any reason, such as due to a change in copper section or a variation in the trace path, the mutual inductance changes, and impedance discontinuity will occur.
- Branches in traces: Although it may be necessary to route a signal to more than one device, the use of branches and line stubs change the line impedance, causing a discontinuity.
- Splits in return signals: A high frequency signal follows the path of lowest impedance, which is directly under the signal trace, often in a ground plane. Any physical feature in the return line or ground plane that forces the return signal to deviate from this route will create a discontinuity.
- Vias: A via is used to transfer a signal from one layer in a PCB to another. Although an essential feature of PCB design, the shape and size of a via changes the inductance and capacitance of the trace, creating another discontinuity.
Ways To Limit the Effects of Discontinuities
The key to controlling the negative effects of impedance discontinuities is to treat all PCB signal traces as transmission lines and ensure that the characteristic impedance is the same at all points of the signal path. Here are a few guidelines for achieving this:
- Match source and load impedances: Ensure the source and load impedances are the same as the trace impedance. This can be achieved through the use of series or shunt resistors to realize the correct impedance. Additionally, any open traces must be terminated using a resistor of the correct value.
- Avoid branches: If signals have to be shared by several chips, daisy-chain the traces to connect the circuits in preference to using branches. Alternatively, a matched buffer device could be used to transfer the signal to a branch.
- Signal return paths: Make sure that signal returns follow the same route as the signal line. If using a ground plane, ensure there are no splits that interrupt the return signal path. Ensure there is a solid plane underneath the trace through the entire length with no splits or cuts. If there is no solid plane, use a thick return trace, which should cover the trace lengthwise and the height of the dielectric three times over.
- Via design: Lay out the high frequency traces on one layer as far apart as possible. If vias are required, use microvias instead of traditional vias. Because vias have significantly different capacitance and inductance characteristics, minimize their use on signal traces, and where they are required, use microvias that have a much smaller capacitance and inductance of standard vias. The microvia also help keep the stub length as short as possible. An alternative is to use high-density interconnect (HDI) PCB technology.
The detrimental effects of impedance discontinuities are severe and cannot be ignored in digital PCB circuits operating at high clock frequencies. Careful attention needs to be given to controlling these effects by adhering to sound design practices and, where possible, using advanced PCB design software to assess and reduce impedance discontinuities.