Transmission Line Reflection Calculator

Sierra Circuits’ Transmission Line Reflection Calculator is a signal integrity simulator that utilizes a SPICE-based engine to model the dynamic behavior of high-speed signals as they propagate through PCB traces, cables, and interconnects.

You need to input trace length, characteristic impedance, termination strategy, parasitic inductance, and parasitic capacitance. The tool visualizes your design’s overshoot, ringing, propagation delay, and signal attenuation.

This predictive capability enables you to identify and resolve signal integrity issues early in the design phase, before prototyping.

The tool allows you to simulate and analyze:

1. Signal reflection behavior due to impedance mismatches
2. Lossless vs. lossy transmission line performance
3. Impact of driver and receiver parasitics on signal quality
4. Timing margins and signal degradation across interconnects

Key features of the Transmission Line Reflection Calculator

• Visualizes signal reflections caused by impedance mismatches along the transmission path.
• Supports multiple transmission line models such as ideal delay line, segmented lossless (LC), and segmented lossy (RLGC) models.
• Accounts for inductance and capacitance at the source and load ends.
• Simulates step inputs for transient analysis or clock signals for continuous high-speed operation.
• Calculates rise time degradation, ringing frequency, and propagation delay at the receiver.
• Enables precise control of driver impedance, termination resistance, and load characteristics.
• Displays multiple signal waveforms to analyze signal behavior across the transmission path.

How to use the Transmission Line Reflection Calculator

The tool supports the following conductor models:

1. Ideal delay line
2. Segmented lossless (LC)
3. Segmented lossy (RLGC)

First, choose the appropriate transmission line model for your design and follow the steps in that section. You can ignore the other models.

Transmission line model 1: Ideal delay line

It is a purely mathematical model that requires only line impedance (Z0) and propagation delay (TD).

Step 1: Enter Line Impedance (Z0) (Ω) and Propagation Delay (TD) (ns).

Step 2: Select the source type under the Source Parameters drop-down: Step (Signal Pulse) or Clock (Period).

• If you choose Step (Signal Pulse), enter Launch Voltage/Amplitude (V) and Rise Time (ns).

• If you select Clock (Period), enter Launch Voltage/Amplitude (V), Frequency (MHz), Duty Cycle (%), and Rise/Fall Time (ns).

Step 3: Provide Source Resistance (Rs) (Ω) and Load Resistance (Rl) (Ω) under the Input/Output Paramaters section.

Step 4: Enter Stop Time (ns) in the Analysis Parameters block and click Simulate.

The output waveform will appear as shown below.

Transmission line model 2: Segmented Lossless (LC)

This structure models the trace as a physical chain of inductors and capacitors. It accurately captures signal ripple, dispersion, and the impact of track length.

Step 1: Enter Inductance (nH/inches), Capacitance (pF/inches), and Total Length (inches).

Step 2: Select the source type under the Source Parameters drop-down: Step (Signal Pulse) or Clock (Period).

• If you choose Step (Signal Pulse), enter Launch Voltage/Amplitude (V) and Rise Time (ns).

• If you select Clock (Period), enter Launch Voltage/Amplitude (V), Frequency (MHz), Duty Cycle (%), and Rise/Fall Time (ns).

Step 3: Provide Source Resistance (Rs) (Ω), Source Inductance (Ls) (nH), Source Capacitance (cs) (pF), Load Resistance (RI) (Ω), Load Inductance (LI) (nH), and Load Capacitance (Cl) (pF).

Step 4: Enter Stop Time (ns) in the Analysis Parameters block and click Simulate.

The tool will display the results as shown below.

Transmission line model 3: Segmented Lossy (RLGC)

It is the most realistic model.  This structure adds resistance and conductance to simulate signal attenuation (loss) and leakage over long trace lengths.

Step 1: Enter Resistance (Ω/inches), Inductance (nH/inches), Conductance (S/inches), Capacitance (pF/inches), and Total Length (inches).

Step 2: Select the source type under the Source Parameters drop-down: Step (Signal Pulse) or Clock (Period).

• If you choose Step (Signal Pulse), enter Launch Voltage/Amplitude (V) and Rise Time (ns).

• If you select Clock (Period), enter Launch Voltage/Amplitude (V), Frequency (MHz), Duty Cycle (%), and Rise/Fall Time (ns).

Step 3: Provide Source Resistance (Rs) (Ω), Source Inductance (Ls) (nH), Source Capacitance (cs) (pF), Load Resistance (RI) (Ω), Load Inductance (LI) (nH), and Load Capacitance (Cl) (pF).

Step 4: Enter Stop Time (ns) in the Analysis Parameters block and click Simulate.

The tool generates waveform plots and calculates key signal integrity metrics as shown below.

How to interpret the simulation outputs

The tool displays three key waveforms:

• Source V(s): The perfect signal generated inside the driver with no losses.
• Input V(1): The signal entering the transmission line. This reflects source impedance effects.
• Output V(2): The signal at the receiver, including all reflections, delays, and attenuation.

A clean output waveform indicates uniform impedance and effective termination. A distorted waveform (overshoot, undershoot, ringing, or slow edges) reflects signal integrity issues.

In addition to the waveforms, the tool automatically computes:

• Rise/fall time at v(2): Indicates signal degradation due to loading and parasitics.
• Ringing frequency at v(2): Identifies oscillations caused by impedance mismatch.
• Actual propagation delay: Measures signal travel time from source to load.

These metrics help validate timing and signal integrity requirements.

The Transmission Line Reflection Calculator enables accurate prediction of signal behavior in high-speed PCB designs. By combining SPICE-based simulation with flexible parameter control, the tool helps you identify reflection issues, optimize termination, and ensure reliable signal transmission. Use this tool to reduce design iterations, improve signal integrity, and build robust high-speed systems.