1. What is PCB Controlled Impedance?

Definition: Controlled impedance is the matching of PCB trace dimensions and locations with dielectric materials to create a specific electrical impedance.

Purpose: Critical for high-speed digital circuits and RF applications where signal integrity depends on consistent impedance throughout the transmission path.

2. How Does the Calculator Work?

The calculator uses the fundamental transmission line equation:

Where:

• \( Z \) — Characteristic impedance (ohms)
• \( L \) — Inductance per unit length (Henrys/meter)
• \( C \) — Capacitance per unit length (Farads/meter)

Explanation: The square root of the inductance to capacitance ratio determines the characteristic impedance of the transmission line.

3. Importance of Controlled Impedance

Details: Proper impedance control prevents signal reflections, maintains signal integrity, and ensures proper power transfer in high-frequency circuits.

4. Using the Calculator

Tips: Enter the inductance and capacitance per unit length values. Typical values range from 200-400 nH/m for L and 50-150 pF/m for C.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical impedance value for PCBs?
A: Common values are 50Ω (RF systems), 75Ω (video), and 90-100Ω (differential pairs in digital circuits).

Q2: How do I determine L and C values?
A: Use field solvers, empirical formulas, or manufacturer's specifications based on trace geometry and dielectric properties.

Q3: What affects PCB trace impedance?
A: Trace width, thickness, dielectric constant, height above ground plane, and copper roughness.

Q4: When is controlled impedance critical?
A: For signal frequencies where the trace length approaches 1/10 of the signal wavelength (typically >50MHz).

Q5: How accurate is this calculator?
A: It provides theoretical values; actual PCBs require consideration of manufacturing tolerances (±10% typically).