RF TEST FIXTURE BASICS

摘要

When the RF test system and the device-under-test (DUT) both have coaxial connectors, hooking them together to make a measurement is easy. However, as radio-frequency integrated circuits (RFIC) have shrunk in size, traditional coaxial connectors are no longer useful. With the multitude of RF component sizes and shapes found on the market, no single connector type fits them all. A test fixture is needed to act as an electrical and mechanical interface from the RF test system to the DUT. RFICs are commonly marketed not as die but as packaged components. Although an essential part of the component, the package is easily overlooked, usually an afterthought to designing the die. A high performance die in an inferior package results in a mediocre performing component. An RF package can be characterized either empty or with a die inside. Many packages cannot be directly probed, in which case a test fixture provides the solution. Test fixtures are less expensive than wafer probe stations and offer more flexibility to the RF port location. With a test fixture, the matching and bias circuitry can be located near the DUT. Yet test fixtures add parasitics to the characterization scheme such as ground loops and RF reflections at the interfaces. Since the calibration plane is not usually at the package's pads, accurate de-embedding becomes essential. This article covers RF package characterization principles using test fixtures, and will help the reader to understand the package's effect on the die, especially parasitic inductances (self, mutual and ground) and capacitances (both self and mutual).