Optimizing RF/MW power measurement speed on a power meter and a power sensor is often a subject of concern, especially in a manufacturing environment. This article describes some useful tips on how to effectively minimize test times while obtaining power measurements.
Having a detailed understanding of some of the SCPI commands and settings of the Keysight Technologies power meter/sensor is one means of improving measurement speed without compromising measurement accuracy. Selecting the measurement speed settings best-suited to the application's need is another method. For example, this application note explains how the Fast measurement setting and buffer size can be best leveraged based on the power level of interest. To increase speed even further, it is important to choose the right format and units in which the results are to be returned. To eliminate as much wasted test time as possible, such as waiting for processes like sensor zeroing and calibration, a basic understanding of the complete operation is required.
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Power specifications are often the critical factor in the design, and ultimately the performance, of almost all RF and microwave equipment. Power meters and power sensors are commonly used to capture these power measurements. Understanding the capabilities of the power meter/sensor helps ensure the best power meter/sensor test methodology for capturing power measurement is correctly applied.
That knowledge can prevent implementing choices that may cause inaccurate power measurement or needlessly lengthen test time in manufacturing. The following practices explain how to leverage some of the commands, settings, and output selections in order to obtain accurate power measurement and shorten test times.
Practice 1: Measurement Query Method
There are three different methods to query a measurement from the power meter. It is beneficial to understand the basic differences between these three query commands in order to fully optimize measurement speed.
This command is a 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.
This 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? The command allows users 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? 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.
Practice 2: Measurement Averaging
Measurement average count is basically used to reduce signal noise in order to obtain better measurement accuracy, especially for a lower power signal. The power meter uses a digital filter to average the power readings. The number of readings averaged can range from 1 to 1,024. While increasing the number of measurements averaged reduces the measurement noise, the measurement time is increased. Users can manually configure the measurement average or keep the measurement average in auto mode (by default). When the auto measurement average mode is enabled, the power meter automatically sets the number of readings averaged based on the power level currently being measured as defined by the power meter.
For example, a high power signal of +10 dBm requires a smaller average count for accurate measurement. The average count might be set to “1” by default from the power meter. Using the FETCH? command to query +10 dBm power gives an accurate measurement upon completion of the average count process. The FETCH? command produces results faster than READ? and MEAS?.
Conversely, at low power, a noisy signal of –40 dBm requires a higher average count to reduce the noise. In this case, the power meter could, for example, select 128 for the average count. Using the FETCH? Command, if insufficient WAIT time is specified for the meter, all 128 readings may not be obtained before the results are averaged and the final value returned. The FETCH? query is faster because it returns measurements with or without completion of the average count process.
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