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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 packages 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).
THE BASIC TEST FIXTURE
An RF test fixture has three basic components - the RF launcher (usually a coaxial connector), RF interconnecting lines leading to the DUT (usually microstrip) and a block housing for the test fixture's body. Figure 1 shows a test fixture designed for evaluating leaded packages. Also known as a split block fixture, it consists of separate end sections (A-C and C-A) and a mid-section (C-C). Each end section holds an RF launcher and an interconnecting RF line. The interconnecting lines can take any number of forms (microstrip, stripline, CPW, etc.). The width of the mid-section is sized to fit the DUT.
Each section of the test fixture can be individually characterized by its S-parameters and...