Peak vs. Average RF Power Sensors
Filed under Sensors
Choosing between peak and average RF power sensors is a critical decision when characterizing modern wireless, radar, and modulated signals. Average power sensors measure the integrated power of a signal over time, making them ideal for continuous wave (CW) signals or simple modulation schemes where envelope tracking is unnecessary. Peak (or wideband) power sensors, conversely, track the fast-changing envelope of modulated and pulsed waveforms. They require sufficient video bandwidth (VBW) and fast rise times to accurately capture peak power and peak-to-average power ratios. This guide explains how these sensor types differ, the role of video bandwidth, and how to select the appropriate instrument for your application using standard hardware examples.
- Average-Only Dynamic Range
- -60 dBm to +20 dBm
- Maximum Video Bandwidth
- Up to 30 MHz
- Fastest Rise/Fall Time
- ≤ 13 ns
- Standard RF Impedance
- 50 Ω
Average Power Sensors and CW Measurements
Average power sensors are designed to measure the total continuous power of an RF signal by integrating the power over a specified time window. They do not track the rapid envelope variations of modulated signals but provide high accuracy and wide dynamic ranges. For example, the Keysight U2002A is a USB-based average power sensor operating from 50 MHz to 24 GHz with an 80 dB dynamic range spanning -60 dBm to +20 dBm. Because they do not require high-speed envelope tracking circuitry, average sensors like the U2002A can detect extremely low signal levels, making them excellent for continuous wave calibration, transmission path testing, and simple non-pulsed systems.
Peak Sensors and Video Bandwidth Requirements
When working with pulsed or complex modulated waveforms, a peak power sensor is required to capture the peak envelope power and peak-to-average ratio. These sensors rely on wide video bandwidth (VBW) to follow the fast amplitude changes of the signal. If the modulation bandwidth exceeds the sensor's VBW, the output will be filtered, leading to inaccurate peak readings. The Keysight N1921A wideband power sensor provides a video bandwidth of 30 MHz and an extremely fast rise and fall time of 13 ns or less, enabling accurate characterization of high-speed pulsed signals. Other sensors like the Keysight E9327A offer a 5 MHz VBW with a rise/fall time under 0.2 µs, showing how VBW directly influences a sensor's ability to track rapid signal transitions.
Selectable Bandwidth and Multimode Functionality
Many peak power sensors feature adjustable VBW settings to optimize the trade-off between tracking speed and noise. For example, the Keysight E9322A peak and average sensor offers three selectable video bandwidth settings: High (1.5 MHz), Medium (300 kHz), and Low (100 kHz). Additionally, peak sensors often feature dual measurement modes. In normal mode, the E9322A measures power from -45 dBm to +20 dBm, but switching to average-only mode disables the wideband path and extends the lower measurement range down to -60 dBm. This versatility allows engineers to perform high-fidelity time-gated peak measurements and high-sensitivity average measurements with a single sensor.
Example instruments
Frequently asked questions
- What is video bandwidth (VBW) in an RF power sensor?
- Video bandwidth represents the maximum modulation frequency or envelope speed that the sensor's internal detection and amplification circuitry can accurately track without distorting the peak profile.
- Can a peak power sensor measure average power?
- Yes, many peak sensors feature dual-mode operation. The Keysight E9322A and E9327A can switch from normal peak-tracking mode to an average-only mode, which also extends their lower sensitivity range down to -60 dBm.