Critique Components For Test Applications

Although often overlooked, it is these additional active and passive components that make many measurements possible in even the most elaborate of test systems.

Measurements evaluating the performance of RF/microwave components usually involve racks of instruments. Amidst the test gear, however, often lie more humble microwave components that are "instrumental" in making many high-frequency measurements, including various types of amplifiers, attenuators, detectors, directional couplers, filters, and switches. Take away the attenuators, for example, and the dynamic range of most signal analyzers is greatly diminished, at least for higher-level signals. Although a review of components used for RF/microwave measurements is a topic worthy of more extended coverage, this brief review will point out some of the key components available and what to consider when comparing different units for a test system.

Components for test are often no different than components engineered for commercial and military systems, although they typically must provide instrument-grade performance with extremely tight tolerances. Attenuators, for example, must provide performance that is extremely flat and predictable with frequency, with excellent impedance match in terms of voltage standing wave ratio (VSWR) when installed between the test gear and the device under test (DUT).

Accessory components for use with RF/microwave test equipment and systems include amplifiers, such as to boost test signal levels in electromagnetic-interference (EMI) and electromagneticcompatibility (EMC) testing; adapters, to mate different types of coaxial connectors or to convert a waveguide flange to a coaxial connector; attenuators, to decrease the levels of high-power test signals when feeding sensitive test receivers and analyzers; power dividers, to split test signals for analysis on multiple receiver channels simultaneously; and switches, to route signals to different parts of a test system. Additional components include DC blocks, directional couplers, limiters, matching pads, phase shifters, and terminations. Components for test should not be confused with test fixtures, test sets, or switch matrices, which are more like instruments that work with other instruments.

Often, well-known names in component manufacturing also supply components designed for test, held to tight manufacturing and performance tolerances. For example, ARRA supplies components to a wide range of military electronics systems, but also offers components suitable for test applications. For example, the company offers extensive lines of coaxial and waveguide attenuators, including continuously variable units capable of as much as 100 dB attenuation in bands from DC to 18 GHz. For example, model 9684-20FS is a directreading continuously variable attenuator designed for applications from 12.4 to 18.0 GHz. It provides maximum attenuation of 20 dB with an attenuation-versusfrequency curve of 1 dB. With 0.1-dB or better resettability, these attenuators can handle 10 W CW input power and as much as 5 kW peak power.

Mini-Circuits, which offers extensive lines of components for test, including bias tees, cable assemblies, and matching pads, recently introduced the model BW-N20W5+ fixed coaxial attenuator for applications from DC to 18 GHz. It provides nominal attenuation of 20 dB and can handle input power levels to 5 W CW.

For automatic-test-equipment (ATE) applications, JFW Industries supplies programmable 50- and 75-Ω attenuators that can be controlled by TTL commands. The firm's model 50P-1501-N, for example, operates from 200 to 3000 MHz with Type N connectors. It achieves total attenuation of 127 dB, which can be selected in 1-dB steps with 2s switching speed. The company also offers fixed attenuators for manual test systems. The company also offers a brief but concise guide to selecting and using matrix switches in test applications, simply titled "Application Note for Matrix Switches" and available in PDF form for free download from the JFW Industries web site. It discusses the differences between blocking and nonblocking matrix switches and how to assemble a handover test system.

Krytar, well known for its directional couplers and detectors, also offers the components for various test setups. The firm recently introduced the models 100318010 and 100318030 directional couplers for applications from 300 MHz to 18 GHz. The former is a 10-dB coupler and the latter a 30-dB unit, both with frequency sensitivity of 1.2 dB and better than 12-dB directivity. The VSWR at any port is 1.40:1 while the insertion loss is less than 1.6 dB for either model. The couplers, which handle 20 W CW power and 3 kW peak power, are supplied with SMA female connectors but available with Type N female connectors as an option. The company has long enhanced its directional couplers with the addition of detectors, such as the models 1820S and 1821S directional detectors. The former offers frequency sensitivity of 0.5 dB from 1.0 to 12.4 GHz while the latter has frequency sensitivity of 0.7 dB from 1 to 18 GHz. Both exhibit maximum VSWR of 1.35:1 with insertion loss of less than 1.3 dB. The low-level sensitivity for the lower-frequency model, which is the amount of voltage supplied for an applied input power level, is better than 40 V/W. For the 1821S, the low-level sensitivity is better than 10 V/W through 18 GHz.

In some cases, component manufacturers leverage their expertise in a certain area to produce components useful as test accessories. Micro Lambda Wireless, known for its YIG-based oscillators and filters, recently announced a line of instrument-grade filters that include filter components and drivers within a compact benchtop housing. The company's P-series bandpass filters operate to 50 GHz while the R-series band-reject filters operate to 20 GHz. (For more on these benchtop instrument filters, click here).

Jmicro Technology supplies a line of thin-film components designed to simplify measurements of microstrip circuits with coplanar-waveguide test probes. Called the ProbePoint Test Interface Circuits, the products include test adapter and interface circuits for transforming a signal path from the coplanar waveguide of many test probes to a microstrip transmission mode for testing microwave circuits.

On the active side, amplifiers are needed to generate high power levels for EMI/EMC testing, and companies such as AR RF/Microwave Instrumentation and Instruments For Industry have long supplied high-power amplifiers. AR's "T" series of microwave amplifiers, for example, incorporate traveling-wave tubes (TWTs) to achieve power levels from 10 to 10,000 W at frequencies from 800 MHz to 45 GHz. Both companies also offer solid-state power amplifiers, or combinations of the two technologies, as in IFI's SMX and CMX amplifiers, with as much as 1 kW through 1 GHz.

Many manufacturers of measurement equipment offer accessory components for test. For example, the model 8480G fixed attenuator from Agilent Technologies covers an impressive bandwidth of DC to 67 GHz with a 1.85-mm connector (see figure). It is available with attenuation values of 3, 6, 10, 20, 30, and 40 dB and can handle power levels to 1 W.

The 70 and 75 series Schottky diode detectors from Anritsu provide coverage as wide as 10 MHz to 50 GHz in a single unit using coaxial V or K connectors. The response is flat within 0.5 dB to 20 GHz and within 1.5 dB to 40 GHz.

For evaluating receiver performance or making noise-figure measurements, Noisecom offers its NC 3000 series of calibrated coaxial noise sources for use with noise-figure meters. These noise sources provide precise excess noise ratios (ENRs), such as 15.5 or 30 to 35 dB, with noise flatness of 1 dB or better across frequency ranges as wide as 0.5 to 18.0 GHz.

Test-equipment manufacturers often develop components or assemblies customized to work with their own instruments. Recently, Rohde & Schwarz introduced its ZVA-Z500 frequency converters for use with the company's VNAs. The frequency converters connect to the ZVA analyzers to extend frequency coverage as high as 500 GHz. The converters are designed specifically for use with a Rohde & Schwarz fourport VNA and are recognized by the analyzer's operating system software upon connection. Of course, working at these higher frequencies points out the importance of having a decent collection of transmission-line adapters on hand, including coaxial-to-waveguide and waveguide-to-coaxial adapters.

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