RF Component & Device Test Series: How Are Active RF Devices Tested
By: Peter McNeil
RF devices are active elements in the RF signal chain and are used in almost all RF systems with exception of purely passive systems. Active RF devices are distinguished from passive RF devices in that electrical power or electrical control signals are used to energize the device and change its performance. In many cases, active RF devices require direct energizing to function, in the case of biasing for amplifiers and mixers.
Common RF Active Devices
- Power amplifiers (PA)
- Low noise amplifiers (LNA)
- Gain block amplifiers (GBA)
- Bias Tees/DC Blocks
- Variable Attenuators/Equalizers
- Variable or switched Filters
- Electronic, mechanical, or semiconductor-based switches
- Noise sources
- Phase shifters/Trimmers
Testing active devices is typically more complex than testing passive devices. For some types of testing, an active device may have signals at the inputs and outputs that exceed the performance thresholds of common testing devices, such as signal generators, spectrum/signal analyzers, vector network analyzers, and power meters. In these cases, amplifiers or attenuators may be used in addition to the basic test equipment to either cut signal powers down to safe levels for the equipment or boost the RF signals to the devices under test (DUTs) to reach levels necessary for practical testing.
If very high power levels are used, attenuators may be inadequate to fully protect downstream sensitive test equipment, and directional couplers may be used to provide power reduction (isolation) from the high power signal path, which may then end in a high power termination.
Common RF Active Device Test Equipment & Accessories
- Vector Network Analyzer (VNA)
- Spectrum/Signal Analyzer (SA)
- Signal Generator (SG) and Frequency Synthesizer
- Noise Generator
- Power Meter
- Switch and Switch Matrices
- Attenuator and Attenuator Matrices
- Directional Coupler
There are many active devices that are also nonlinear. This poses additional considerations when testing, especially at high power and high frequency regimes where the nonlinear aspect of a device’s performance may be significant compared to the linear behavior. S-parameter measurements made by VNAs are only capable of describing the behavior of a device in response to small-signals where the behavior of a device is likely approximated as a linear component operating at a static operating point (typically a DC bias voltage(s)). In these cases a VNA with capability of measuring X-parameters may be used. X-Parameter capable VNAs are able to measure linear and nonlinear aspects of a device’s performance and include nonlinear information in the X-Parameter files, such as magnitude and phase distortion generated by a device when subjected to large-signal conditions.
In other cases, the actual impedance of a device may be unknown throughout all operating conditions of the device. In the case of high power amplifiers, ensuring efficient transmission and optimum linearity requires knowing this impedance to best match the downstream electronics and antenna. This is especially true with high power amplifiers and high power electronics where the signals are of such a level that even minor reflections could induce damage when sent back to a device’s output or develop a standing wave that presents a high enough voltage to change the device’s behavior. This is when load-pull testing is employed. Load-pull is a measurement technique where the performance parameters of a device are observed while the impedance at the input or output of the device is variable. The end-goal of load pull testing is often to characterize a device, and also to find the optimum operating point for a given output impedance.
Learn more about Pasternack’s expansive line of RF/Microwave test hardware and systems by following these links:
- Pasternack VNA Calibration Kits
- Pasternack Phase Stable (VNA) Coaxial Cables
- Pasternack Cable Generic Test Cable Assemblies
- Pasternack Skew Matched Pair Coaxial Cable
- Pasternack Precision Coaxial Adapters
- Pasternack In-series and Between-series Adapters
- Pasternack RF Coaxial Probe
Courtesy of Pasternack