Effects Of Impedance Matching And Switch Quality On RF Test System Performance

By National Instruments

This document is part 4 of the Guide to Selecting an RF Switch. This 5-part guide is an accumulation of insightful content that will arm you with the necessary knowledge to design your RF switch network. This section will highlight the importance of impedance matching in RF systems using graphs and circuit diagram analysis.

Click here for Understanding Key RF Switch Specifications, part 1 of this 5-part series.

Introduction

Switch systems can be used to increase channel count and automate signal routing in test systems ranging from precision DC measurements to RF. However, the characteristics and design of a switch module that will work for DC measurements versus those of a product best suited for RF systems are vastly different. Switches designed for higher frequency systems typically have lower channel count than their low-frequency counterparts due to increased relay size and larger isolation (spacing) requirements and can be considerably more expensive. What properties do switches marketed as ‘RF' or ‘High Frequency' have that are not present in those used for lower frequency applications? The answer lies in the careful attention to characteristic impedance throughout the switch design.

This chapter will begin by defining characteristic impedance and the importance of impedance matching in RF switch systems. Impedance matching is designing an RF system so that all the source, load, and characteristic impedances match, in order to ensure maximum power transfer and minimum reflected power. It will then proceed to elaborate on design considerations for switch products and networks to improve impedance matching in switch systems and minimize signal reflections and power loss.