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Using Power Sensors in Unattended Applications

Parent Category: 2015 HFE

By Orwill Hawkins

Power sensors are one of the most universal test devices in the microwave industry. They are utilized in a broad range of applications such as laboratory, manufacturing, and in the field testing and verification. Power sensors are the “Gold Standard” when comes to calibrating other equipment or determining or verifying actual power.

Recent advances in Power Sensors include broadened connectivity including Ethernet and TTL interfaces such as I2C/SPI TTL along with added capability such as unattended, autonomous operation, the subject of this article.

These new sensors are capable of unattended or autonomous operation including storing the measurements to internal flash. Also a calibrated DC voltage is available that is proportional to measured power. This is often referred to a Recorder Out. It is important to note that in unattended operation, these capabilities are available with no computer connected. 

No Computer or Power Meter Connected

One such example is the LadyBug LB5918A Sensor with option UOP (Unattended Autonomous Operation) employed for this paper. This sensor is capable of making and storing measurements with no computer or power meter connected. Only 5 Volt power is required. The self-contained, high-accuracy, fully calibrated sensor includes a user programmable internal real-time clock with backup, a substantial non-volatile memory for measurement storage, and a programmable measurement control system. In addition to non-volatile storage, the sensor’s Recorder Output can be utilized in unattended mode, providing accurately calibrated analog output for various uses. A LB5918A with option UOP installed is shown in Figure 1, along with the sensor’s included full featured application operating in the background. The UOP features are complemented by Just Measure—LadyBug’s patented No-Zero No-Cal system that eliminates user calibration requirements. The no-zero, no-cal feature is an essential to usability in autonomous applications. This sensor is capable of highly accurate measurements under varying environmental conditions without user intervention. 

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Figure 1 • LB5918A with Unattended Operation Capability.

In Figure 2 the sensor is logging measurements in unattended mode. It is not connected or controlled by a PC. Instead it is powered by a 5 volt USB power adaptor. After a period of time the sensor can be connected to an external PC and the logged measurements can be retrieved. Each measurement is accompanied by a time and date stamp.

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Figure 2 • LB5918A in  Unattended Mode.

The LB5918A Sensor’s Option UOP has four modes of operation, Off, Basic, Advanced and Reset. The sensor is fully self-contained and the settings are stored in the sensor. When UOP mode is set to Off; the sensor functions as a normal power sensor. 

Basic mode is designed to assure maximum functionality with minimum setup. Once UOP is set to Basic and powered up, the sensor will collect measurements until its memory is full. If power is lost and then restored the sensor will resume unattended operation. Setting Basic when using the sensor’s factory default settings (Preset), will cause the sensor to immediately begin making and storing measurements. Adjusting the measurement setup is easy. Basic Unattended mode utilizes the sensor’s current measurement setup and parameters. Simply set up the measurement parameters with UOP set to Off. Then set UOP mode to Basic. Once this is done, the sensor begins making and storing measurements. No computer or power meter is required. The sensor only requires 5VDC to operate. Again, if power is lost, then restored, the sensor will automatically resume unattended operation. 

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Figure 3 • Set Frequency to 1.8 GHz & Averages to 10.

UOP mode must be set to Off to use the sensor for normal operation. As with all power sensors, frequency and averaging are important parameters. These should be set to achieve high accuracy, refer to Figure 3. To start unattended operation, set the Measure button to Single then click Start UOP as shown in Figure 4.

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Figure 4 • Sensor Settings.

Measurements and storage commence as soon as Start UOP has been clicked. If power is removed and restored at a later time, the sensor will simply start appending additional measurements to the sensors non-volatile memory. 

All measurements are stored sequentially in memory on a real-time basis. Measurements are time stamped using the internal RTC (real-time clock). The RTC system includes a backup that will maintain the clock for at least a day when the sensor is not powered. The real-time clock’s crystal is disciplined by the sensor’s high accuracy time base when the sensor is powered. This system provides the user very accurate time stamp unattended measurements. 

Demonstration

To demonstrate unattended operation, a PLL demonstration board was used as shown in Figure 5. The board was set to generate 1.8 GHz at various power levels. Prior to running the demonstration, the sensors unattended memory was cleared by selecting UOP then Clear UOP Memory in LadyBug’s Precision Power Meter Application. Frequency and Averaging were set as shown in Figure 3. 

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Figure 5 • Test Setup.

A separate source was connected to the sensor when the computer was used to initiate UOP. Since the sensor begins storing measurements as soon is UOP is set to Basic, measurements were stored while the computer was still connected. The source level was toggled before the sensor was disconnected from the computer. The initial signals were logged as shown below in Figure 6, which details a portion of the 231 stored measurements that were made during the entire demonstration. When UOP memory is retrieved, the sensor returns an index number; date & time stamp; measured power and flags, if present, for each measurement. 

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Figure 6 • Table of Collected Data.

Upon completion of the unattended PLL measurements, the sensor was reconnected to the computer and the UOP mode returned to Off. Prior the setting the condition to Off, a few measurements were automatically logged at the noise floor when the sensor was connected. These are shown and noted on the right of the graph in Figure 7.

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Figure 7 • Graph of Logged Measurements.

Data was retrieved from the sensor using the PMA-12 application by selecting the UOP dropdown then Retrieve Data. A window opens that lists the available measurements. Maximum was selected and all available data was downloaded. The list of 241 stored measurements is partially displayed in Figure 6, the entire list was several pages long.

Demonstration Graph

The graph in Figure 7 was created in EXCEL with data entered using the copy button visible in Figure 6 and described above. The entire demonstration used only 231 measurements of the sensor’s 50,000,000+ measurement storage capability while using Basic mode. The data marked Initial UOP Start up was collected after starting UOP and before the sensor was disconnected from the computer. Data marked PLL Output Measurements began at measurement index 21 and is the data collected from the board. Power was set to three different levels measuring -38.9 dBm, - 9.7 dBm and 0.4 dBm. Finally, the data marked Measurements made while connected to stop UOP were made when the sensor was reconnected to the computer just prior to turning off UOP. These data are all shown in the graph shown in Figure 7. 

At the demonstrated averaging and storage rate, the sensor could log measurements without interruption for over 250 days provided power was supplied.  

Other Information 

Specific measurement data can be downloaded by specifying a beginning and ending index number. Large blocks of data can be downloaded from the sensor by selecting two or more ranges of measurement indexes. Using the copy button shown in Figure 6, very large or small blocks of data can be pasted into EXCEL or various other programs that use Windows’ copy and paste system.

Alternatively, data can be saved to a CSV (comma separated value) text file that can be opened by various programs including EXCEL. This can be particularly useful for large amounts of data and for users that wish to share data by email etc.

Advanced measurements can also be made while the sensor is operating in unattended modes. This includes internal and externally triggered measurements. For example, internal or external triggering functions can be setup while the sensor is not in UOP. Once the measurement is setup and confirmed, UOP Basic can be started and the measurements will be made based on the triggering that was just setup and confirmed.

Recorder Output can be activated and used while in unattended autonomous mode. Recorder Output and Trigger Output share the same physical connection, and are controlled in the Output section in the sensors left control pane. Trigger Out is disabled when Recorder Out is in use.

The sensor can be powered by a USB power only cable, or if it is equipped with option SPI it can be powered through the SPI connection. A USB battery pack of sufficient capacity may be utilized (similar to the USB power supply shown in Figure 2). The sensor requires 5 volts at 500 Ma.

Unattended Autonomous applications include remote monitoring, unattended analog output applications, transmitter monitoring and triggered warning systems, portable equipment, and defense applications. The example sensor is designed for use in automated test equipment as well as isolated usage. It’s very stable and accurate time base makes it suitable for long term monitoring as well as short term applications.

About the Author

Orwill Hawkins serves as Vice-President of Marketing at LadyBug Technologies, Santa Rosa, Calif. He has over three decades of management, marketing, engineering and manufacturing experience, and extensive hands-on design and manufacturing experience in the RF, analog, and digital fields. Among the many products he has designed and marketed are a self-contained RF field disturbance burglar alarm system, a sailboat speedometer, and various robotic servo systems. Additional inventions include a prototype oscilloscope, a CNC cutting system, and various other analog, digital and RF projects.

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