Use Case: Impedance Matching

Matching 50Ω to 75Ω Systems

The most common mismatch in RF engineering is connecting 75Ω video/TV equipment to 50Ω lab instruments. Here is the most reliable way to solve it.

Go to Minimum Loss Pad Calculator

The Problem: Reflection

Directly connecting a 75Ω cable to a 50Ω spectrum analyzer creates an impedance mismatch. This results in a reflection coefficient (Γ) of:

Γ = (75 - 50) / (75 + 50) = 0.2

This corresponds to a **VSWR of 1.5:1** and a Return Loss of **14 dB**. While this might seem "okay" for rough measurements, it introduces significant amplitude uncertainty and ghosting in video signals.

The Solution: Minimum Loss Pad (L-Pad)

A resistive matching pad (L-Pad) can perfectly match the two impedances in both directions. The trade-off is that it introduces a fixed **Insertion Loss** (attenuation). You cannot purely match unequal impedances with resistors without loss.

Pros

• Broadband (DC to GHz)
• Very cheap (2 resistors)
• Perfect VSWR (1.0:1)

Cons

• Fixed Loss (5.7 dB)
• Cannot block DC voltage

Key Values

For a 50Ω to 75Ω match, the theoretical minimum loss is **5.72 dB**. The resistor values are standard:

Series Resistor (Rs): 43.3Ω (Connect to 75Ω side) ... Wait, connect to High Z side? Let's check the calculator.

Wait, actually, you should check our calculator to be sure!

Pro Tip: If you cannot tolerate the 5.7 dB loss (e.g., measuring very weak signals), you must use a matching transformer (Unun) instead of a resistive pad. Transformers have very low loss (< 1 dB) but limited bandwidth.