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It has been several decades since vacuum tubes, cat’s whisker detectors, and water-flow calorimeters were the dominant options for measuring RF/microwave power. In that bygone era, power-measurement technology could only describe the average amount of radiation absorbed by a simple detector. With complex modulation techniques and the need to measure powerful signals on integrated substrates, power measurements have obviously become much more sophisticated. At the same time, spectrum congestion has increased the monitoring and design precision of high-power amplifiers to prevent interference with other transmitters and telecommunications cells.

There are several methods of measuring RF/microwave power signals. Direct sensing methods—such as thermal measurements from the heating effect upon a sensing area or detecting a signal with diodes—create a direct-current (DC) voltage proportional to the signal amplitude. These methods are common in scenarios where the absolute signal power is needed and the signal power is extremely high. Indirect methods are used where the high-precision characterization of RF/microwave devices and knowledge of the modulated signal power are necessary.

An indirect receiver method consists of a tuned circuit that measures the signal’s amplitude component. Another indirect method is the digitization of the signal, known as sampling. Indirect methods use advanced test equipment, which can bring a captured signal into a processing domain for analysis after detection. These test typologies enhance the test accuracy of power signals with advanced modulation.

Multiplexing methods, such as time-domain multiplexing, are designed to enable multiple devices to communicate on the same upstream channel simultaneously. Real-time measurements of the signal path are necessary to properly monitor the multiplexed signals. If one of the multiplexed signals, for example, were to significantly exceed the others in power, it could prevent the reception of the other signals. Several integrated-circuit (IC) detectors designed to solve this problem are receive signal strength indicators (RSSIs), root-mean-square (RMS) voltage detectors, analog multipliers, and demodulating logarithmic amplifiers (Fig. 1).

Test High-Power RF/Microwave From Tower To Tabletop, Fig. 1

These devices convert the signal amplitude into a proportional DC signal. Generally, such devices have nonlinear response curves and require calibration. In addition, they fail to provide true power measurements in lieu of voltage detection. Other methods that provide information based on the signal amplitude involve more complex electronics. These devices and techniques are used to gain knowledge of the devices in the signal stream before broadcasting to ensure proper operation.

If a signal’s carrier frequency is slow enough to allow for direct digitization, the signal can be re-created digitally and analyzed. This is the process by which direct storage oscilloscopes operate. For wideband modulated signals, the ability to sample a signal needs to be around 10 times the carrier frequency. For modern telecommunications and radar systems, this method would require extremely fast sampling devices—which could, in turn, mean an expensive solution. With phased-array radars pulsing at higher frequencies with narrower pulses—and with the telecommunications spectrum density increasing—there is demand for more precise methods of characterizing and measuring the power response of electronic devices.

Hiro Maehara, product marketing engineer/application expert, Component Test Division of Agilent, shares his insights into devices in high-power RF/microwave signal streams: “All components in transmitters used in high-power applications, such as wireless base stations, point-to-point communications, satellite communications both in ground stations and payloads, transmit/receive modules used in radar applications, and more require testing. These components include amplifiers, mixers, converters, circulators/isolators, attenuators, connectors, cables, etc.”

To conduct the testing and measurement of power signals and power devices in a closed-loop and open-loop environment, tools like vector network analyzers (VNAs), vector signal analyzers (VSAs), and signal analyzers (SAs) are used to better understand the dynamic parameters of powerful RF/microwave devices. VNAs are predominantly used for characterizing a device’s S-parameter, stability, power leveling, gain compression, pulsed-RF, and intermodulation distortion (IMD) measurements on devices like power amplifiers (PAs; Fig. 2).

Test High-Power RF/Microwave From Tower To Tabletop, Fig. 2

Generally, VSAs are used to measure a known signal’s error vector magnitude, spectral flatness, and code domain power to ascertain the quality of the modulated signals. Spectrum analyzers sweep their full frequency range for the power level of signals and produce a response that measures dominant frequency, power, distortion, harmonics, bandwidth, and other frequency-domain components.

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