Tips for Querying CW and Average Power

Tips for Querying CW and Average Power

Application Notes

Introduction

This application note applies to the following products from Keysight Technologies

  • N1913/14A EPM power meter
  • E4416/17A EPM-P power meter
  • N1911/12A P-Series power meter
  • U2020 X-Series USB Peak and average power sensor
  • U2000A Series USB average power sensor
  • U8480A USB Thermocouple USB power sensor
  • Keysight power sensors

This application brief demonstrates the capabilities Keysight Technologies, Inc. power meters and power sensors have for measuring CW and average power signals. It also explains the advantages and disadvantages of selecting SCPI commands and settings for Keysight power meters and power sensors, illustrating how to these items to improve power measurement speed without compromising measurement accuracy.

Table of Content

  • Power measurements
  • Querying tips
  • Power sensor measurement speed
  • Test configuration and SCPI commands

Power measurements

There are many ways to analyze a modulated signal. The power-versus-time measurement is a very useful method for examining power level changes due to pulsed or burst carriers.

Average, pulse, and peak envelope power measurements provide different types of information about the signal.

Pulse power is determined by measuring the average power of the pulse and then dividing the result by the pulse duty cycle. This is a mathematical representation of the pulse power rather than an actual measurement and assumes constant pulse power. Pulse power measurement averages out any aberrations in the pulse, such as overshoot or ringing. For this reason, it is called pulse power and not peak power or peak envelope power. To ensure accurate pulse power readings, the modulating signal must be a rectangular pulse with a constant duty cycle. Other pulse shapes such as triangular or Gaussian will cause erroneous results. This technique is not applicable for digital modulation systems, where the duty cycle is not constant, and the pulse amplitude and shape vary.

When the power pulse becomes non-rectangular and the pulse-power measurement equations become inaccurate, using peak envelop power measurements is a better measurement method. This technique is the most suitable for modern digital communication systems with variable duty cycles and pulse widths.

Unlike measuring a pulsed signal that has a pulse repetition period and a constant duty cycle, burst signal measurement is considerably more challenging. Measuring a burst signal with an unpredictable burst length that lacks a constant duty cycle requires time-gated functionality (independent measurement gates). This can be accomplished using high-performance power meters and power sensors.

Keysight power meters and power sensors offer the ability to measure CW and average power. The following example provides some tips for using power meters and power sensors to acquire accurate power without increasing the test time.

Querying tips

Measurement query method

There are three different methods that can be used to query a measurement from the power meter or power sensor. It is beneficial to understand the basic differences between these three commands in order to fully optimize measurement speed.

MEASure?

This command is compound command consisting of ABORt, CONFigure, and READ?. It is the simplest of the query commands because it relies on the power meter to select the best settings for the requested configuration and immediately perform the measurement. One drawback to this command is that its use results in a longer test time and overrides some of the power meter’s settings, such as switching the meter from Free Run to Single Shot mode or changing the average count set to the ON state.

READ?

The READ? command is another compound command that is equivalent to an ABORt followed by an INITiate and FETCH?. The READ? query is similar to MEAS? in that it causes the meter to perform initializations and auto-configuration. However, READ? gives users the flexibility to change certain settings such as the average count. The READ? command allows the user to manipulate the settings in order to optimize the measurement speed, resulting in a shorter test time than that realized using the MEAS? command.

FETCH?

This command retrieves a reading upon measurement completion and puts it in the output buffer. The FETCH? command allows users to manipulate settings such as the average count. However, this command requires additional settling time to complete the average count process, or else FETCH? will return invalid data.