This is a reasonable assumption for PCBs because trace resistance and conductance at high signal frequencies are negligible compared to their inductance and capacitance.Īn impedance discontinuity is anything that affects the ratio between the inductance of the trace and its capacitance. The characteristic impedance of a line is the square root of the inductance divided by the capacitance. Selection process of high-speed PCB materials.Understanding transmission lines and controlled impedance.Explanations of signal integrity issues.Read the difference between microstrip and stripline in PCBs. You can characterize a transmission line formed by PCB traces by the following parameters: resistance, conductance, and trace resistance. ![]() The upshot of this is that, at high switching frequencies, the distortion this causes to digital signals by impedance discontinuities can be so severe that signal sampling errors may occur. These lead to ringing and signal distortion. Therefore, high-frequency digital signals should follow the signal integrity discipline associated with high-frequency signals.Ĭonsequently, any change in the impedance of a PCB trace will cause signal reflections. A very short signal rise time implies very high frequencies contained in the digital signal. High-frequency digital circuits would have a shorter pulse width. In practice, these frequencies will be an order of magnitude higher than the circuit’s clock frequency. 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. Impedance Discontinuities Affect Signal IntegrityĪ digital signal is, in theory, a square wave pulse that switches in a very short time. Read: why controlled impedance really matters. As such, try and keep impedance discontinuities as minimal as possible. This means the signal distortion is higher as well. The greater the discontinuity in the characteristic impedance, the higher the signal reflection. This presents a considerable challenge that requires careful PCB design to mitigate the effects of signal degradation caused by impedance discontinuity. You must then load impedance at the source and destination ends. In order to avoid signal distortion at the source and destination, you must match the PCB trace impedance to source. Signal trace discontinuities or non-uniform discontinuities form signal integrity discontinuities. You must even treat shorter traces like transmission lines. The higher the frequency, the shorter the wavelength. For that reason, PCB traces exhibit the same characteristics. The reason for this is that the physical wavelength of a high-frequency signal is very short. This phenomenon is equally true for PCB traces and transmission lines. Any variation or discontinuities to the impedance causes signal reflection and distortion. The first thing to remember is that signal reflection in PCB transmission lines occurs due to impedance discontinuity.Ī transmission line should have a uniform characteristic impedance. This is an in-depth article about this very important topic that PCB designers should master. ![]() We previously quickly discussed signal reflection in our PCB transmission line article. RLC Resonant Frequency and Impedance Calculator. ![]() Bandwidth Rise Time and Critical Length Calculator.Transmission Line Reflection Calculator.Trace Width and Current Capacity Calculator.
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