1. What is PCB Propagation Delay?

Definition: Propagation delay is the time it takes for a signal to travel from one point to another through a transmission line on a PCB.

Purpose: Understanding propagation delay is crucial for high-speed PCB design to ensure signal integrity and proper timing.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• Delay — Signal propagation time (seconds)
• length — Trace length (meters)
• L — Inductance per unit length (Henrys/meter)
• C — Capacitance per unit length (Farads/meter)

Explanation: The delay is proportional to the square root of the product of inductance and capacitance per unit length, multiplied by the trace length.

3. Importance of Propagation Delay Calculation

Details: Accurate delay calculation helps in designing proper trace lengths for clock distribution, matched lengths for differential pairs, and meeting timing requirements in high-speed digital circuits.

4. Using the Calculator

Tips: Enter the trace length in meters, inductance per unit length (default 250nH/m), and capacitance per unit length (default 100pF/m). All values must be > 0.

5. Frequently Asked Questions (FAQ)

Q1: What are typical values for L and C?
A: Typical values range from 200-300nH/m for L and 80-120pF/m for C, depending on trace geometry and PCB material.

Q2: How does dielectric constant affect delay?
A: Higher dielectric constant increases capacitance, which increases delay. The delay is proportional to the square root of the dielectric constant.

Q3: What's the speed of signal propagation in PCBs?
A: Typically about 6in/ns (15cm/ns) in FR4 material, which corresponds to about 66ps/inch of delay.

Q4: When is propagation delay critical?
A: For signals above 50MHz, matched differential pairs, and high-speed interfaces like DDR, PCIe, USB, etc.

Q5: How can I reduce propagation delay?
A: Use shorter traces, lower dielectric constant materials, or adjust trace geometry to reduce L and C.