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Why does impedance discontinuity in transmission lines really matter?
The greater the discontinuity in the characteristic impedance, the higher the signal reflection. This means 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. Read: why controlled impedance really matters.
Which is an example of an impedance discontinuity?
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:
What happens when the impedance of a PCB change?
Consequently, any change in the impedance of a PCB trace will cause signal reflections. These lead to ringing and signal distortion. 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.
What causes a discontinuity in a PCB transmission line?
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.
What causes a discontinuity in a return signal?
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.
Why do reflections occur in a transmission line?
The first thing to remember is that reflections in transmission lines occur due to impedance discontinuity. A signal on a uniform transmission line encounters a constant impedance ‘Zc (V/I)’ at all locations on the line and the signal travels as desired along it.
When does the impedance of a track begin to matter?
General rule of thumb: the impedance of the track begins to matter when its length is greater than 1/10th the wavelength.