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These high-precision instruments cannot handle significant power levels. As a result, components like attenuators, directional couplers, and loads are necessary to weaken the signals prior to reception. This process adds additional error, inaccuracies, nonlinearities, and potential distortion factors into the test signal paths. For high-power test, directional couplers are sometimes preferred over attenuators, as fixed attenuators for high power change in performance as power levels and case temperatures change. When primarily using directional-coupler test systems,  a load termination is necessary to deplete the remaining signal energy.

This can enable highly accurate measurements, as directional couplers exhibit low insertion loss, high directivity, and high stability. But the termination load could introduce intermodulation distortion and add to the voltage standing wave ratio (VSWR). Both components use coaxial connectors at the interface between devices. It is important to remember that the connectors’ power-handling capability changes as a function of frequency (Fig. 3).

Test High-Power RF/Microwave From Tower To Tabletop, Fig. 3

In some high-power systems, both attenuators and directional couplers are necessary to create a test setup like a telecommunications transmitter. To avoid compounding errors and distortion, the configuration of the test setup is critical. Maehara explains, “It is best to place test couplers right at the DUT input and output for network analysis. Attenuators are placed between the coupler’s output and the instrument input port.” Using this approach will prevent the attenuator from hiding the device response while it protects the test equipment from high power levels.

Going into examples, Maehara notes that a reference coupler for measuring the stimulus in network analysis is best placed after a boost amplifier in a transceiver configuration. Another tip is that the use of attenuators at the outputs of a boost amplifier will reduce mismatch in the measurement path. These methods work for larger systems with high power. But testing high power levels on integrated components requires an advanced set of tools that can interface with small-scale devices.

Test High-Power RF/Microwave From Tower To Tabletop, Fig. 4Compounding all of today’s test requirements is the fact that RF/microwave devices are being combined and compacted into ever-smaller footprints. Material technologies and advanced processes are enabling higher-frequency and higher-current operation on ICs. High-power and high-frequency test instruments must therefore be adapted to the surface of a wafer. Companies like Cascade and Picoprobe offer specialized probe heads for measuring high-density, power, and frequency signals from the wafer surface. These companies, among others, offer sophisticated probe platforms for proper temperature dissipation, vibration reduction, and backside connections.

Such probing devices require high power handling, high current handling, low/stable contact resistance, and low insertion loss. Among the challenges they face are maintaining reliable contact with oxide development on probe pads, the bending of very thin wafers, thermal shifts, and enabling millimeter-wave frequencies with reasonable power in compact probes. The solutions currently offered in the marketplace include probes to 110 GHz, vacuum chucks, thermally regulated chucks, and probes with built-in waveguide converters (Fig. 4).

Clearly, when applying RF, microwave, and millimeter-wave signals, significant care is needed when planning the devices and interconnects of a system. This process—called power budget analysis—involves a detailed breakdown of every device’s and connection’s power-handling capability. The maximum expected power levels throughout the signal path must be known for effective power-budget analysis. Ensuring that each individual component rating complies with the signal power levels is critical in avoiding ineffective testing, device failure, and even health hazards.

Thermal management of attenuating and amplifying components is another necessary aspect of high-power system design. As most electronics properties drift with temperature, regulating the temperature within operating norms is a critical aspect of system reliability. This problem becomes more complex as these previously discrete components are integrated into a monolithic or even stacked-IC typology. Examples of software packages that simulate thermal dissipation and generation for modern RF/microwave ICs are Tecnode’s Symmic software, ANSYS’s RF & Microwave software, and Silvaco’s Thermal 3D.

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