Understanding Key RF Switch Specifications

This article is part 1 of the Guide to Selecting an RF Switch. This 5-part series is an accumulation of insightful content that will arm you with the necessary knowledge to design your RF switch network. This section will explain basic RF switch specifications such as insertion loss, VSWR, characteristic impedance, and rise time.

Introduction to RF Switching

With the recent surge in availability of RF switching products for test system development, choosing the right product for your application has become increasingly difficult. Most RF vendors describe their RF switching products using two main specifications – topology and bandwidth (such as the NI PXI-2594 2.5 GHz 4x1 multiplexer). While these specifications are indeed important and vital during the evaluation phase, they do not provide the buyer with enough information to make an informed purchasing decision. The goal of this tutorial is to introduce to you the following seven important specifications that must be considered when designing your RF switch network:

1. Characteristic impedance
2. Bandwidth
3. Topology
4. Insertion Loss
5. Return loss and voltage standing-wave ratio (VSWR)
6. Isolation and crosstalk
7. Rise time

Before discussing characteristic impedance and other RF switch specifications, it is first important to understand the difference between how signals propagate in DC circuits versus RF systems. In DC circuits or circuits where the propagating signal has low frequencies, the voltage of the signal at different points on a cable in the signal path varies minimally. This is not so in the case of RF or high-frequency signals where wavelength of the signal is considerably small in comparison to the length of the cable allowing multiple cycles of the signal to propagate through the cable at the same time.