R&S®ESSENTIALS | Power supplies fundamentals
Paul Denisowski, Product Management Engineer
Accurate energy consumption measurements are crucial for assessing device performance and ensuring efficient operation. Modern benchtop DC power supplies, such as the Rohde & Schwarz R&S®NGM series, provide advanced functions for power measurements and analyzing power consumption.
Readback is a feature that enables a power supply to measure and display its actual output voltage and current. Readback values are updated continuously during operation, and they are often displayed on the screen when the instrument is in use. Unlike the configured values, which are user defined, readback values represent real-time data that reflect the power supply’s output.
Readback resolution defines the granularity with which the actual voltage and current can be measured, and readback accuracy refers to how accurately a power supply can measure the actual output voltage or current. Both specifications differ between power supply models.
Along with providing the operator with accurate, real time information, readback allows for:
In particular, logging is necessary to record readback values for future analysis.
Logging involves recording power over time. This function is essential for detailed energy consumption measurement. It requires voltage and current to be measured simultaneously, so one possible approach is using two external meters: one for voltage and one for current. However, it is often easier and more accurate to use a power supply’s integrated logging function, which is non invasive (in that it doesn’t require the circuit to be broken) and ensures that the voltage and current values are time synchronized.
Common parameters for logging include:
The standard logging rate in most benchtop power supplies is generally limited to about 10 samples per second. Although this is sufficient for many power measurements, it may not be fast enough to capture transients or other very short duration events. As such, some power supplies have a high speed or fast logging option. This mode allows the user to specify voltage and current sample rate, which can be up to 500,000 samples per second, enabling transitions in the microsecond range to be captured.
Logged data is typically stored in plain text files in CSV (comma separated values) format. This format is convenient for processing and analyzing data. Each CSV file includes a header with information such as timestamp, voltage, current and power. Subsequent lines provide the recorded values at each measurement interval.
For fast logging, data is stored in a binary format, which can later be converted to CSV. Fast log files do not contain timestamps, but timestamps can still be derived from the start time and logging interval.
Logged data can be stored on the power supply itself or saved to removeable media like USB drives. In some cases, the data can also be streamed to an external device or computer over a LAN interface using SCPI commands.
Analyzing logged data often involves plotting and performing statistical or trend analysis to gain insights into power consumption patterns and system performance. This can be done with a variety of software tools, such as Microsoft Excel or MATLAB.
Now, it’s time go over the steps of power supply measurement. The precise details would depend on the model of your power supply, but the basic steps should remain more or less the same. Today, we’ll look at how to make and record power measurements using the Rohde & Schwarz R&S®NGM power supply.
Let’s start with standard logging:
We’ll go through each of these steps on the R&S®NGM.
To access the logging menu, press the Menu button and go to the “Device” tab. Logging parameters can be found by scrolling to the “Logging” menu item and pressing the arrow. The logging function must be turned off (or disabled) before the logging configuration can be changed.
The next step is to define where the log file will be saved. If “Save Location” is set to “Auto,” the power supply will use an attached USB drive as the save location if available. If there no USB drive, it will use its own internal storage folder. “Manual” can be used to manually set both the target location as well as the path or directory where the log files will be stored.
The parameter “Interval” defines how often data is written to the log; this is also the time between measurements. Clicking on “Interval” will bring up a dialog box where the logging interval can be entered.
The R&S®NGM supports four different logging modes:
Logging can be manually started or stopped from the “Logging” menu. Another way to start or stop logging is by using a trigger, which can be enabled using the toggle next to “Triggered.”
You can easily check whether logging is ongoing or inactive by looking at the “LOG” icon on the status bar.
Now let’s talk about how to modify the format of the CSV log files. This can be done by clicking on the “CSV Setting” menu item. From here, you can configure the following parameters:
If this sounds confusing, don’t worry about it. The default CSV settings are the most common ones, so in most cases, these parameters will not need to be set or modified.
Summary
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