More Cost-Effective RF Power Measurements

Blog Post created by benz on Dec 29, 2017

  In praise of the humble power sensor

It’s always nice to get a reality check from a fellow RF engineer, and Keysight’s Eric Breakenridge recently delivered one in the form of an explanation of the capabilities of modern RF power sensors.

I guess I’ve become a bit of a measurement snob, having spent many years working with vector signal analyzers (VSAs). When we developed and introduced these products 25 years ago, we were really enthusiastic about a new tool that would tell us virtually anything about the most complex RF signals.

Measurements with the VSAs weren’t just comprehensive, they also had unprecedented accuracy, including RF power. They were significantly more precise than the spectrum analyzers of the time, especially on time-varying signals or those with complex modulation.

Looking back, however, I remember the RF engineers who were developing VSAs also had power meters and power sensors on their benches, and used them frequently. Those power meters and sensors were the benchmarks for our nascent VSAs, and the new analyzers would never have achieved such exceptional accuracy without them.

Power sensors—whether they’re connected to power meters or to PCs via USB or LAN—are relatively inexpensive and have advantages that ensure the ultimate in power accuracy. For one, you can attach the sensors directly to the DUT, eliminating cabling and adapters. Also, many sensors are designed for specific frequency ranges, letting them cover the frequencies in question with excellent impedance match and accuracy—and that accuracy is highly traceable.

The sensors, as I learned from Eric, can also make great measurements of power versus time. Here’s his example, a measurement of the time to switch the gain state of an amplifier.

RF power vs. time measurements using power sensor.  Power shown on log (dBm) and linear (Watts) scales

Two measurements from the U2042XA X-Series power sensor show the time to switch the gain state of an amplifier. Power is shown in watts (top) and decibels (bottom). The default 10 percent and 90 percent reference points have been adjusted to better reflect the time for the gain to reach its final value.

The USB and LAN power sensors can be connected to PCs and used with the power meter application in Keysight's BenchVue software. That application provides both graphical results and compiled tabular data such as this pulse analysis table.

Power meter application program on PC creates pulse statistics table from data acquired from RF power sensor via LAN or USB

When connected to an X-Series power sensor, the Power Meter Application assembles a series of measurements and creates a complete summary of pulse characteristics.

In addition to benchtop configurations, the LAN models (via power-over-Ethernet) are useful for remote monitoring, placing the sensor right at the DUT. Multiple sensors can be used with a single PC.

The power sensors have a wide dynamic range of –70 dBm to +26 dBm and sample rates as short as 50 ns. While they can’t match the speed, sensitivity, or selectivity of signal analyzers, their performance is a good fit for many applications, and the combination of low cost and measurement accuracy can help you make better use of the more-expensive signal analyzers in your lab. A power sensor demonstration guide shows some example measurements and configurations.

I don’t suppose anything will dull my esteem for VSAs, but my recent exposure to power sensors and the sophisticated power tools in my previous post have made me a little less of a measurement elitist. Whatever gets the job done the best!