Engineers involved in design and manufacturing have long sought RF/microwave measurement solutions with greater simplicity, cost effectiveness, speed, and accuracy. High on that list are simple but accurate power measurements. Although RF/microwave testing has become quicker and easier in recent times, RF power measurements remain time consuming. Thanks to personal-computer (PC) connectivity via the Universal Serial Bus (USB), however, test instrument suppliers are making significant headway in supplying power meters and sensors that fill a wide range of requirements while being more compact than traditional power measurement solutions.
National Instruments' Raajit Lall, Product Manager for RF and Wireless Test, notes that USB-controlled power meters are popular for both characterization and production environments, thanks to the portability of the instruments. Nor have there been any performance issues due to USB implementation, despite some earlier doubts. "Because power meters and sensors do not have large amounts of data to transfer to a PC, like analyzers or generators do, the USB is not really a bottleneck," notes Lall.
While capitalizing on the flexibility and ease of use provided by USB, manufacturers of power meters and sensors have optimized their products to fulfill various performance needs. "With fast access to acquisition data (2000 readings/s), the PSM series power/sensor meters from Tektronix not only provide basic average and duty-cycle-corrected pulse power, but PC analysis software can extend the functionality for power measurements," states Darren McCarthy, Technical Marketing Manager, RF Test (Fig. 1).
"Software for the top-of-the-line PSM5000 series power-sensor meters adds peak and pulse-power and pulse-profiling measurements, such as pulse width, rise time, fall time, overshoot droop, time gating, and statistical measurements," McCarthy continues. As a result, this series of sensors and meters can both evaluate and test products ranging from radios to radars. With total (RSS) instrument uncertainty as low as 2.6%, he notes that the USB connectivity also enables impressive utility via a PC. The meters are delivered with Microsoft Windows-based, power-meter application software for controlling the meter, displaying readings, and recording data.
A high-speed logging application enables this data to be brought into a host PC for analysis.
With the R&S NRP-Z family of USB power sensors, Rohde & Schwarz has moved the complete power-measurement functionincluding signal-processing and calibration factorsinto the measurement head (Fig. 2). According to Justin Stallings, Product Marketing Manager, Signal Generators and Power Meters, "Not only did this eliminate noise inherent in the traditional technique of sending the analog voltage signal back to a base unit for processing, but it also eliminated the need for a power-meter base unit. To facilitate data acquisition, we included a USB interface for communicating between the R&S NRP-Z sensor and a controller. The controller may be a computer running a Windows, Linux, or Mac operating system or a Rohde & Schwarz signal generator, spectrum analyzer, or network analyzer." Because it was challenging to integrate the signal processing and calibration factors into the measurement head while maintaining high performance, Rohde & Schwarz invented a diode topology called Smart Sensor Technology. According to Stallings, it enables the R&S NRP-Z power sensors to offer superior measurement accuracy, speed, and dynamic range, regardless of signal modulation.
The latest power sensor from Anritsu Co., dubbed the MA24105A, can be used with a PC running Microsoft Windows via USB. It comes with PowerXpert application (version 2.1 or greater) for data display, analysis, and sensor control. This in-line peak power sensor covers 350 MHz to 4 GHz. It boasts a measurement range of 2 mW to 150 W and a combination of forward and reverse measurement functionality. With this wide measurement range, it eliminates the need for lower-power sensors. To ensure accuracy, calibration data is stored directly in the sensor. In addition, all necessary corrections are made inside the sensor's microprocessor. With its "dual-path" architecture, the MA24105A also enables a wide measurement range for bi-directional True-RMS measurements over the entire frequency.
Despite their impressive performance feats, however, these cutting-edge power meters and sensors may simply be overkill for some applications. For this reason, explains CEO Craig Walsh, Telemakus offers a lower-price sensor with lower precision. "Power accuracy is specified at 0.3 dB from 50 MHz to 6 GHz," notes Walsh. "The sensor is also offered with a signal source which, when used with the proprietary software, combines to create a scalar analyzer." This USB-focused company has plans for future sensors with greater accuracy, bandwidth, and dynamic range.
