Why a Series Termination Resistor Must Sit Next to the Driver
For source-end impedance matching, a series termination resistor is placed between the driver and the transmission line. In practical PCB layouts, however, limited board space or component placement constraints may force the resistor away from the driver. This raises an important signal-integrity question: if the resistor is moved some distance from the driver, or even placed near the receiver, can it still effectively reduce reflection on the transmission line?
The following cases compare different resistor positions along the same transmission line. The simulation keeps the line length and resistor value consistent while changing only the distance between the driver and the series termination resistor.
Receiver Waveform with No Series Termination
The first case uses a 2000 mil transmission line without a series termination resistor. Without source termination, the impedance mismatch at the driver causes significant reflection. The reflected signal returns along the line and distorts the waveform observed at the receiver.
Figure 1 | Receiver waveform with no series termination resistor
The resulting waveform shows severe reflection and noticeable distortion. This provides the reference case for evaluating how resistor placement changes signal behavior.
Resistor Placed Next to the Source
Next, a 33 Ω series resistor is placed directly next to the source while the transmission line remains 2000 mil long. In this configuration, the resistor is part of the source-end impedance from the moment the signal enters the transmission line.
Figure 2 | Receiver waveform with 33 Ω series resistor at the source
Compared with the unmatched case, the reflection observed at the receiver is significantly reduced. The overshoot and undershoot are no longer obvious, and the waveform is much more stable.
This is the intended operating condition for source termination. The resistor increases the effective source impedance and helps it approach the impedance of the transmission line, reducing the amount of signal energy reflected back toward the driver.
Resistor Offset 100 mil from the Source
In the next case, the same 33 Ω resistor is moved 100 mil away from the source. The transmission line remains 2000 mil long, but the signal now travels 100 mil before reaching the series resistor.
Figure 3 | Receiver waveform with 33 Ω resistor 100 mil from the source
Moving the resistor creates a short transmission-line section between the driver and the termination point. Electrically, the resistor is no longer located directly at the source. As the resistor is moved farther away, this additional section becomes increasingly relevant to the signal path and can affect how effectively the source is terminated.
Resistor Offset 300 mil from the Source
The resistor is then moved to 300 mil from the source. The transmission line remains 2000 mil long and the resistor value remains 33 Ω.
Figure 4 | Receiver waveform with 33 Ω resistor 300 mil from the source
At this point, the section between the driver and resistor is long enough to become an increasingly significant part of the transmission path. The resistor still adds series resistance, but it cannot provide exactly the same source-end termination behavior as when it is placed immediately next to the driver.
The key issue is not simply the physical distance itself. The trace between the driver and resistor has its own transmission-line behavior. The resistor therefore terminates the signal only after the signal has already propagated through this additional section.
Resistor Offset 500 mil from the Source
With the resistor moved to 500 mil from the source, the signal travels a substantially longer distance before reaching the 33 Ω termination resistor.
Figure 5 | Receiver waveform with 33 Ω resistor 500 mil from the source
The waveform now shows visible ripple. The farther the resistor moves from the driver, the less effectively it behaves as a source termination. The additional trace section between the driver and resistor changes the electrical structure of the link, allowing signal behavior in that section to contribute to the observed waveform.
This demonstrates why simply selecting the correct resistance value is not enough. The physical location of the component is part of the termination design. A resistor value that works well when placed at the driver cannot be assumed to provide the same result after being moved a significant distance along the trace.
Resistor Offset 1000 mil from the Source
The resistor is then moved to 1000 mil from the source, halfway along the 2000 mil transmission line. The resistor remains 33 Ω.
Figure 6 | Receiver waveform with 33 Ω resistor 1000 mil from the source
With the termination resistor in the middle of the line, reflection becomes much more apparent. The first section of the transmission path between the driver and resistor is effectively left without the intended source termination at the point where the signal is launched.
The resistor still affects the signal after the signal reaches it, but its position prevents it from providing the same source-matching function as a resistor located directly at the driver. This is why a series termination resistor should not be treated as a component that can simply be placed anywhere in series with the signal.
Resistor Placed Near the Receiver
Finally, the 33 Ω resistor is moved near the receiver. The signal travels approximately 1000 mil before reaching the resistor, while the total transmission-line length remains 2000 mil.
Figure 7 | Receiver waveform with 33 Ω series resistor near the receiver
In this configuration, the resistor no longer provides source-end matching. Most of the signal path from the driver to the resistor remains without the intended series termination, so reflection becomes severe again.
This also illustrates the difference between source termination and simply adding resistance to a signal path. A resistor placed near the receiver may still change the waveform, but it does not perform the same function as a resistor placed immediately at the driver. Its physical location determines which part of the transmission-line behavior it can control.
Why Series Termination Must Be Close to the Driver
The simulation results show a consistent relationship: as the series termination resistor moves farther away from the driver, the signal waveform becomes less stable. With the resistor directly at the source, reflection at the receiver is greatly reduced. As the resistor is offset by 100 mil, 300 mil, and 500 mil, the termination becomes progressively less effective. When the resistor is moved to the middle of the line or near the receiver, reflection becomes much more pronounced.
The reason is that source termination works by controlling the impedance at the point where the signal is launched into the transmission line. If the resistor is separated from the driver by a trace section, that section becomes part of the signal path before the termination takes effect. The resistor therefore cannot provide the same source impedance that it would provide when placed directly at the driver.
For PCB placement, the practical rule is straightforward: when a series resistor is being used for source-end impedance matching, place it as close to the driver as the layout allows. Keeping the connection between the driver and resistor short preserves the intended termination structure and helps reduce signal reflection.
Series Termination Placement Is Part of the Design
A series termination resistor should be evaluated as part of the complete transmission-line structure rather than as an isolated component. Its resistance, the driver output impedance, the PCB trace impedance, and its physical position all contribute to the final waveform.
The 33 Ω resistor used in these cases demonstrates the effect of placement under the stated simulation conditions. The result should not be interpreted as a universal requirement that every high-speed PCB use a 33 Ω resistor. Instead, the comparison shows why a resistor selected for source termination must be placed at the intended source location if it is to provide the expected signal-integrity benefit.
When board space is limited, moving the resistor farther along the trace may appear to be a simple placement adjustment, but it changes the electrical behavior of the interconnect. For high-speed signals, component placement is therefore part of impedance control, not merely a mechanical layout decision.
