During impedance matching, a series termination resistor can be placed at the signal source. However, due to space constraints, the resistor may be placed slightly farther away. This raises a common question: if the resistor is placed far from the transmitter or at the receiver, can it still eliminate reflections on the transmission line? The following simulation results verify this.
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Receiver waveform without series termination resistor (transmission line: 2000mil):
Severe signal reflection is observed without the resistor, resulting in significant waveform distortion.
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Receiver waveform with series termination resistor placed close to the source (transmission line: 2000mil, resistor: 33ohm):
Compared with the non-resistor condition, signal reflection is greatly reduced, with no obvious overshoot or undershoot.
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Receiver waveform with resistor placed 100mil away from the source (transmission line: 2000mil, resistor: 33ohm; 100mil trace from source to resistor, 1900mil remaining):
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Receiver waveform with resistor placed 300mil away from the source (transmission line: 2000mil, resistor: 33ohm; 300mil trace from source to resistor, 1700mil remaining):
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Receiver waveform with resistor placed 500mil away from the source (transmission line: 2000mil, resistor: 33ohm; 500mil trace from source to resistor, 1500mil remaining):
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Receiver waveform with resistor placed 1000mil away from the source (middle of transmission line, 2000mil total; resistor: 33ohm):
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Receiver waveform with resistor placed close to the receiver (2000mil transmission line, resistor: 33ohm):
From the above simulation results, the impedance matching effect degrades as the series termination resistor is placed farther from the transmitter. Obvious signal fluctuations start to appear at a distance of 500mil. Placing the resistor at the midpoint of the transmission line causes significant signal reflection. Placing the resistor at the receiver end completely invalidates impedance matching, leading to severe signal reflection.
Therefore, in PCB layout, series termination resistors must be placed close to the transmitter to effectively suppress signal reflections and improve signal integrity.
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Severe signal reflection is observed without the resistor, resulting in significant waveform distortion.
Compared with the non-resistor condition, signal reflection is greatly reduced, with no obvious overshoot or undershoot.
