How to Test RF Characteristics of a Switching System
Testing the characteristics of switch modules is not always as simple as it seems. Some are easy because they have RF connectors; others use connectors not usually used for RF applications, particularly those with bandwidths less than 100MHz.
Some switch systems are designed to handle signals in 50-ohm transmission lines, others in 75 ohms. Some modules are designed to handle differential signals using 2-pole switching; in this case, the critical parameter is the differential impedance. This is particularly true when handling signals like RS232 (and derivatives), telecoms signals and serial interfaces such as Ethernet and USB.
In a system, of course, none of these conditions may apply. Still, the only way reproducible specification can be generated is by specifying a condition that can easily be checked. For the most part, that means measuring in a single-ended 50-ohm environment using either a ground pin on the user connector or the connector ground to terminate the coaxial connection screen.
To help users when verifying performance, take a look at our informal guide:
Radio Frequency Testing Procedures for Pickering Interfaces Switch Cards
The purpose of this guide is to define the methods for performing radio frequency (RF) measurements in 50Ω & 75Ω unbalanced and 100Ω-120Ω balanced test systems using a 50Ω vector network analyzer (referred to as “VNA” from this point forward).
To perform 75Ω tests with a 50Ω VNA, it is necessary to use minimum loss pads (referred to as “min loss pads” from this point forward). A set of two pads will be required to perform the tests detailed within this document.
To perform balanced tests, it is necessary to use a purpose-built impedance converter and min loss pads under certain circumstances.
Please note that it is not possible to detail every test permutation, i.e., multiple test cards etc., that may be encountered. For this reason, the following sections will refer to testing a single card; the same principles can be applied to multiple cards.
This document assumes that the operator has been trained to safely operate and perform measurements on the test equipment and the device under test (referred to as “DUT” from this point forward). If in any doubt regarding this procedure or equipment, consult a suitably qualified engineer.
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