Fixtures and Probes Aid RF Measurements

Oct. 14, 2010
Those seeking to connect small packages or circuit boards to coaxial test equipment will find the solution in the form of universal microwave test fixtures and low-loss microwave probes.

Test fixtures are often overlooked until needed. But for test equipment with coaxial connectors, test fixtures can be an essential part of a measurement system, helping to pass signals to and from circuits and components that otherwise won't allow simple connections to coaxial- or waveguide-equipped test gear.

Like any mechanical fixture, a microwave test fixture is designed to hold a device under test (DUT) in place and provide electrical transitions to test equipment. Rather than epoxy an integrated circuit (IC) or printed circuit board (PCB) in place, and then make soldered electrical connections, a well conceived test fixture should allow a DUT to be positioned and connected without having to make permanent attachments. Test fixtures often consist of different sections, and may be fabricated with any number of transmission-line technologies, including microstrip, stripline, and coplanar waveguide transmission lines.

Because of the many different component and IC packages and pin configurations, flexible RF and microwave test fixtures often use inserts that adapt different package and their pin layouts to the input/output (I/O) configuration of the main fixture body. Rather than purchase a custom test fixture for each device package, these inserts allow a user to buy a universal test fixture and add inserts to match a package to the test fixture and the test system. Many manufacturers also provide calibration standards as inserts, for performing calibration procedures with a microwave vector network analyzer (VNA) to remove the electrical contributions of the test fixture from a measurement.

For active components, a welldesigned test fixture must also provide one or more bias points on the appropriate pin connections. Depending upon the device, some ICs may require three or more voltage supplies, as well as digital control connections, such as TTL-level control signals.

Maury Microwave Corp., for example, has developed its MT950 transistor test fixture (TTF) for characterizing packaged microwave transistors from DC to 18 GHz. The unit consists of the base test fixture with its coaxial connectors for simple hookup with a microwave VNA, and MT951 transistor package inserts to match a transistor's pin configuration to the input and output ports on the test fixture. Accessories include an adjustable height stand to support the fixture when it is connected to a VNA, tools for assembling the inserts to the fixture body, tweezers for handling the devices, and spare parts. The operators' manuals for the TTF and its inserts include circuit models that can be used to develop de-embedding parameters for the VNA. These de-embedding parameters represent the electrical contributions of the fixture and insert by itself, allowing the VNA to subtract these electrical contributions from the measured total of the transistor under test in the test fixture, leaving only the electrical performance of the transistor under test.

Inter-Continental Microwave, acquired in 2007 by cable-manufacturer C. E. Precision Assemblies, operates as a wholly-owned affiliate of CEPA. Inter-Continental's model UTP-3000B Universal Test Platform includes a basic test fixture with transition assemblies to accommodate a wide range of package styles from DC to 26.5 GHz. Depending upon the test instrument, the fixture can be used for measuring capacitance, inductance, impedance, crosstalk, insertion loss, and return loss. Dual transitions are available for measuring forward and reverse crosstalk. The test platform is supported by a variety of calibration standards for performing such methods as thru-reflect-line (TRL) and linereflect- match (LRM) calibrations with a microwave VNA.

GGB Industries has developed its PicoProbe test fixture for applications from DC to 18, 26, or 40 GHz. It features individually springloaded, low-inductance contacts, in a user-defined mix of RF and DC contacts. For circuits that may not conveniently fit into a test fixture, the company also offers extensive lines of high-speed, high-frequency probes, including its model 35 active probe (see figure). Ideal for use with high-speed oscilloscopes, the probe requires contact with only one measurement point on a circuit. Signal attenuation is 10:1 with a 50-O oscilloscope input. It has a nominal loading input impedance of 1.25 MO shunted by a capacitance of 0.05 pF for DC-coupled measurement capability through 26 GHz. It achieves 2 percent linearity over its operating bandwidth. The probe is designed to be mounted on any multipositioner for manual or automatic measurements.

The company also supplies the model 40A microwave probe for applications from DC to 40 GHz. The probe controls insertion loss to less than 0.8 dB through 40 GHz with return loss of greater than 18 dB through 40 GHz.

Focus Microwave offers lines of microstrip and coaxial transistor test fixtures specially designed for load-pull applications. In both series of fixtures, a DUT is clamped in place. The firm, well known for its accurate load-pull and source-pull measurement systems, supports its impedance tuners with fixtures based on extremely short electrical signal paths with low loss and low VSWR. Both series of text fixtures can be used through 18 GHz.

Many test-equipment suppliers also offer high-frequency test fixtures. Anritsu, for example, offers the 3680 series of universal test fixtures for applications from DC to 60 GHz. The fixtures feature spring-loaded jaws to ensure 0.1-dB measurement repeatability on circuit substrates with thicknesses of 5 to 75 mils.

Suppliers of fixtures and probes include Accuprobe, Inc., J Micro Technology, KEYCOM, Microtest and its production-ready model MW 101 W test fixture, and Southern Microwave. Additional suppliers can be found in the Microwaves & RF Product Data Directory.

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