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Considerations in Remote Power Sensing Applications

Parent Category: 2015 HFE

By Orwill Hawkins

Modern communication systems, military applications, satellite testing and other applications have increased the need for remote and autonomous testing. Because accurate power measurements are best made with the sensor as close as possible to the RF measurement point, sensor-to-user connectivity is very important. Many power measurement applications can be satisfied using small portable USB power sensors. Longer distances require the use of USB extenders, Ethernet, or Autonomous functionality. Power sensor manufacturers have responded with new products that are well suited to these applications.

Meters vs. Sensors
As new advanced power sensors with remote capability are introduced, it has become important to understand the differences between power meters, power sensor and the various sensor technologies that are available. Modern power sensor systems employ a variety of interfaces allowing users to place sensors in inaccessible locations. Further, some sensors such as the LB5918A, can operate autonomously. In remote, unattended, and autonomous applications, understanding your power sensor’s temperature stability, zero and calibration functionality can be of great significance. Other power sensor systems such as measurement technology and path selection requirements can be extremely important as well. We will review how these important factors come into play when power sensors are used in remote applications.
In early power measurement technology, all user zero and calibration functions were manual; zeroing and calibration was required any time conditions such as temperature changed. After a short period of time passed, zeroing and calibration would again be required. The analog connection between the meter and sensor further complicated the process. Of course this would not work well for remote or unattended power sensing applications.

Zeroing and Calibration
Today, RF power sensors using patented measurement technology such as the LB5918A, do not require any user zeroing or calibration. Other power sensor technology contains internal zeroing and calibration systems that work well for moderate power levels where measurement interruption for zero and calibrate functions do not create any issues. Both of these sensor technologies could be well suited for remote, unattended, and autonomous applications.

Recognized primary metrology laboratories such as NIST continue to use thermal mounts and Wheatstone Bridge circuitry for extremely stable CW power calibration. Power sensor manufacturers utilize and transfer this recognized accuracy to their power sensors. Well-designed manufacturing practices, increasing component stability, and new engineering ideas have allowed much of the calibration and zeroing drudgery to be pushed from the user to the power sensor manufacturer.

Modern power sensors apply these precision standards to other types of power measurements, adding capabilities such as fast accurate average power, pulse profiling, plus peak and pulse measurements. In addition to the myriad measurement types, modern power sensors may be capable of unattended operation or at the very least, remote control capability.

1504 HFE powerSensors01

Figure 1 • Sensor with Internal Zero and Cal.

No-Zero No-Cal or Internal Zero and Cal, what’s the difference?
Sensors with low level measurement capability function into the sensor’s noise floor. Quality power sensors have the ability to reach down in power level to a point at which the noise overwhelms the signal and requires the sensor to make continuous zero calculations. In fact, random noise peaks will be well above the average noise level and well over a quality power sensor’s minimum calibrated power measurement level. Zeroing a sensor under these conditions can be difficult, largely because environmental conditions cause continuous movement of the zero level. In most conditions zero is a moving target.


Internal Zero and Cal Sensors. Sensors that employ internal zero and cal have an electronic switch that moves the detection circuitry from the user port to calibration circuitry which is then used to zero and calibrate the sensor. Temperature sensors and timing controls are employed to determine when the sensor needs to be zeroed and calibrated. This can occur often if environmental conditions change and can easily disrupt measurements. Normally the process requires several seconds to complete and can be an issue in automated, remote or unattended measurement systems.

Figure 1 depicts a sensor with internal zero and calibration capability. Switch A is used to disconnect the source and connect the calibration circuitry. Unfortunately, electronic switches are not perfect. The switch adds a small amount of noise, plus its insertion loss affects calibration and therefore your measurement. Further, switch isolation can be an issue; some portion of the input signal may leak through the switch potentially requiring external zeroing for low power level calibration. Therefore, manufacturers of sensors with this type of calibration technology often recommend removal of the measured input signal to improve low level calibration accuracy.
While some of the issues can be characterized and accounted for, uncertainty is added to the system. It should be further noted that the calibration reference signal is normally at a single frequency that may not me close to the user’s measurement frequency. For measurements below -40dBm, careful consideration is recommended, particularly in remote and unattended applications where environmental conditions may vary.

No-Zero No-Cal sensors. Patented No-Zero sensors such as LadyBug’s LB5918A, undergo factory calibration across the full operational temperature range. During use, the sensor continuously measures the temperature and employs a patented technique to ensure that the correct factory temperature calibration factor is always applied to the measurements. These sensors make highly accurate measurements without the need for user zeroing or calibration, either internal or external. The measurements are never interrupted to calibrate, and source and switch uncertainty are not a factor.

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Figure 2 • Diode Sensor Temperature Stability.


Figure 2 details measurement data taken from various sensors in the marketplace and shows the degree that temperature can cause measurements to vary. Note that as power levels drop, temperature variation can be particularly significant. Patented No-Zero No-Cal sensors such as the LadyBug sensor shown in green, compensate for wide variations in temperature to stabilize your measurements, and do not ever requiring user zeroing or calibration. These sensor features are particularly useful where operating conditions are unpredictable and accuracy is important.

Sensitivity and Measurement Paths
A basic simple power sensor is depicted in Figure 3. The system consists of input impedance matching circuitry, a detector, analog to digital converter and finally the processor with power level correction and communication engine.

1504 HFE powerSensors03

Figure 3 • Basic Power Sensor.

With this simple arrangement and square law detection techniques, detection range is only about 40 - 50 dB of dynamic range. This limits its value in test instrumentation and remote power sensing applications.
To increase the measurement range of power sensors, multiple detectors are utilized. Each detector circuit is referred to as a path. Two or three path sensors are common in high quality modern power sensors. Certain aspects of multiple path sensors should be carefully considered when selecting power sensors for remote or autonomous applications. A common two-path detector scheme is depicted in Figure 4.

1504 HFE powerSensors04

Figure 4 • Basic Two-Path Power Sensor.


The detected analog signal from each path is switched to a single analog-to-digital converter. With this method, the user might choose to select the path manually based on an expected signal level. The sensor will most likely have an auto-ranging feature to determine the best path automatically; however the process can be time consuming because the sensor must check both paths to make the determination. This solution is problematic if a signal is in or near the crossover region or if the signal makes level changes wherein both paths must be utilized to make a fast measurement. For example, if the high sensitivity path is selected to measure the base level of a pulse stream, path change may be required to measure pulse top amplitude. The process can be made faster by utilizing two analog to digital converters, as shown in Figure 5. This allows the processor to select the path digitally from data that is already present from both detectors.

1504 HFE powerSensors05

Figure 5 • Improved Two Path Sensor.


This scheme improves the sensor, however measurements are still subject to issues involving the crossover region wherein both paths are near the end of their range and either will work. In this case user intervention and manual path selection may be desirable.

To create an effective auto-ranging path selection system, as in both analog switching and digital selection systems, hysteresis must be employed to prevent unnecessary rapid path switching due to minor level changes. It is important that all paths follow similar level-to-temperature change characteristics so that measurement stability is maintained when paths are switched. With modern digital communication signals’ rapid level changes the user may need to avoid operating at certain levels or select a sensor that employs best path technology. The ideal sensor for remote and unattended applications would isolate all of these issues and deliver accurate measurements with no concerns involving path selection.


The power sensor system depicted in Figure 6 resolves the path selection issues. This two-path sensor measures both paths at once and splices them into a single measurement, using a path weighting scheme. No user path selection is ever required. Utilizing careful designs and modern components, the LB5918A sensor splices the measurement paths, ignoring the out-of-range data from each path, producing a seamless, accurate measurement that does not require user zeroing or calibration.

1504 HFE powerSensors06

Figure 6 • LadyBug Two Path Sensor.


Depending on accuracy requirements, these modern power sensors allow measurements to be made with little effort placed into any of these complexities. Simply make your measurements, even if the sensor is somewhere else.

Summary
Advanced electronic components including efficient, high-speed, low-power microcontrollers with flexible interfaces, along with new engineering ideas, have allowed the creation of a new generation of RF power sensors. These sensors have fully self-contained measurement and user calibration capability. In addition to USB operation, some sensors such as the LB5918A are capable of fully autonomous measurements and do not require any connection other than power. They use their internal real-time backed-up clock to trigger highly accurate measurements; advanced, patented zero and calibration technologies complement the measurement.

Optical and cabled USB extenders, Ethernet connectivity and SPI/I2C direct control interfaces further expand capability of these new power sensors. These options allow a tremendous flexibility in making remote, unattended, and autonomous power measurements in applications such as radio astronomy, isolated military applications, and testing in dangerous locations. For example, an LB5918A sensor operating in unattended mode can monitor a transmitter with an intermittent issue, logging 1,000 measurements per second into its flash memory to be reviewed at a later time. Along with the vast array of connectivity choices, it is also important to review these zero, calibration and measurement capabilities when choosing a power sensor for remote, unattended and autonomous 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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