Simplify RF Power Measurements in the Field with USB Power Sensors

Key takeaways:

• USB power sensors greatly improve the convenience and cost-effectiveness of power tests.
• They can turn any computer or compatible instrument into accurate power meters.
• They are heavily deployed for field use in many critical industries.

Normally, engineers use power meters to accurately measure the power of radio frequency (RF) signals. However, the form factors of power meters are more suited to lab environments.

For field use, they need something compact and lightweight that's easier to use but also accurate. Preferably, it should be something that can easily plug into laptops or tablets over the universal serial bus (USB). Find out how USB sensors satisfy this need.

What is a USB power sensor?

A USB power sensor is a device for measuring the power of RF and microwave signals in real-time but with two unique features:

1. It can turn any computer or instrument that has a USB interface into a precision power meter with the help of suitable software or firmware.
2. It can measure signal power with high accuracy on the order of ± 0.1 decibels (dB) compared to ± 1-2 dB for typical spectrum analyzers.

These devices have female mini-USB connectors through which they connect to computers. At the other end, they have typical N-type RF connectors with standard 50-ohm impedance as shown below.

Fig 1. Power sensor connected to a computer over USB

USB sensors can also plug into compatible instruments like spectrum analyzers and signal generators and turn them into precision power meters as shown here.

Fig 2. USB power sensor can work with a range of compatible Keysight instruments

USB power sensors use different sensing methods to convert signal power into proportional voltages, digitize them using analog-to-digital converters (ADCs), and send back the digital data to the host computer or instrument over USB.

Additionally, USB power sensors are convenient for field use and long-distance monitoring. Their portable form factors and USB cable lengths of up to 5 meters (~16 feet) enable measurements away from the host computer or instrument.

What are the types of USB power sensors?

Based on the sensing method used to convert signal power to voltage, there are two main types of USB power sensors.

1. Thermocouple power sensors

• 
• Fig 3. Thermocouple design
• The thermocouple is a junction of two dissimilar, heat-sensitive metals whose junction temperature rises in proportion to the incident signal power. Due to the Seebeck effect, the temperature difference between the hot and cold junctions generates a voltage that is proportional to the signal power.

2. Diode power sensors

• 
• Fig 4. Diode power sensor design

Diode sensors are characterized by wide dynamic ranges of power levels. They achieve this using an assembly of diodes operating in their square-law regions.

Their wide dynamic ranges are because of using two diode networks, one for low-power sensing and the other for high-power sensing attenuated by a resistor divider. The diodes effectively rectify the input signal into a voltage that's proportional to the square root of the power. The diodes and their positions inside the housing undergo careful calibration for high accuracy. The design of the housing is optimized for maximum power transfer.

Fig 5. Comparison of the two types of power sensors

Diode sensors tend to have the widest dynamic ranges and fastest measurement speeds. However, they are easily damaged if the incident power goes even slightly above their limit.

Thermocouple sensors are a good mix of robustness against damage with good dynamic range and measurement speeds for many use cases.

What are the ranges of the different types of power sensors?

Fig 6. Typical power ranges of different types of power sensors

Thermocouples have better dynamic ranges of 50 dBm that can go up to 75 dBm using suitable attenuators.

Diode sensors have the widest dynamic ranges of 80 to 90 dBm and, with the use of attenuators, can go as high as 105 dBm.

What are the benefits of USB power sensors in data acquisition and monitoring?

The advantages of USB power sensors include:

• Simplicity: USB is widely available in computers and even consumer devices like tablets and smartphones. You can connect power sensors directly to them without additional power supplies or adapter cards. This simplifies the setup and reduces the equipment needed for power measurements, making them cost-effective.
• Real-time monitoring: USB power sensors enable real-time monitoring of power levels. The data acquired by the sensor can be instantly transferred to a connected computer, allowing for immediate analysis and decision-making.
• Automated measurements and data analysis: The associated software allow automated measurements, powerful data loggers, and advanced data analysis. They capture rapid changes in power levels, which is crucial for detailed analysis of power fluctuations and transient events. Visualize power trends, perform statistical analysis, and generate detailed reports directly from the software. Set up custom algorithms for data processing, noise filtering, or correlating power measurements with other phenomena.
• Versatility: USB power sensors have a wide variety of uses, like calibrating RF devices, testing wireless communications equipment, and monitoring radar systems.
• Portability: The compact size and USB-powered nature of these sensors make them highly portable. This is particularly useful for field use, on-site diagnostics, and applications where space is limited.
• Scalability: In high-volume testing environments, power measurement accuracy, test-time efficiency, and test system cost are the key factors for investment consideration. High-performance power meters and power sensors are costly investments. Instead, just add more USB power sensors for simultaneous monitoring of multiple channels or devices without a significant increase in cost or complexity.
• Capability additions to other instruments: A key benefit of power sensors is their ability to enhance the capabilities of other instruments. For example, a spectrum analyzer may natively only support power measurements up to 6 gigahertz (GHz). But by connecting a microwave power sensor capable of up to 24 GHz, the analyzer can display power readings up to 24 GHz.
• Built-in data storage: USB power sensors have adequate internal memory to store 1 million data points and can act as good enough alternatives to more complex data acquisition setups.

How do USB power sensors connect to and communicate with host devices?

When connected to computers or consumer devices over USB, the host device must run suitable software that's typically provided by the vendor of the sensor and can drive the configuration and measurements.

For example, Keysight offers the BenchVue Power Sensors App for computers running Windows. For other operating systems like Linux, vendor libraries and third-party tools are available.

For driving measurements from tablets and smartphones, Keysight publishes the BenchVue Mobile app for Android and iOS.

What are some key specifications of USB power sensors?

Some key specifications of USB power sensors to keep in mind include:

• Frequency range: What's the highest frequency it can measure?
• Frequency selectivity: All power sensors measure total power over their entire bandwidth and typically have poor selectivity. Ask yourself if you need selectivity, in which case other instruments like spectrum analyzers may be better.
• Power dynamic range: What's the power range of the signal? Is there a risk of damaging the device due to high signal power?
• Accuracy: How accurate is the sensor at different power levels? Does it support auto-uploading of its calibration parameters via USB?
• Measurement speed: How many measurements per second can the sensor take? Typically, diode sensors are the fastest, followed by thermocouples, while thermistors are the slowest.
• Aperture time: Aperture is the time window during which the sensor measures power. Does it support continuous apertures, or are there gaps between apertures?
• Average count: How many apertures does it use to obtain the average power? More apertures mean high accuracy but slow measurement time.

What power parameters can USB power sensors measure?

The key parameters that USB sensors capture include:

• Average power: This is the average power of a continuous wave (CW) or modulated signal over a specified period. It's crucial for assessing the performance of RF systems and ensuring compliance with regulatory standards.
• Peak power: This is the maximum power level of pulsed signals, such as those used in radar and digital communication systems. It helps ensure that devices can handle the highest power levels without distortion or damage.
• Pulse power: For pulsed RF signals, USB power sensors can measure parameters like pulse width, repetition interval, and repetition frequency.
• Pulse shape analysis: Some USB power sensors can also measure the rise time, fall time, and any irregularities or distortions.
• Time-gated power: This measures power during specific segments of a modulated or pulsed signal. It's useful for measuring signals in complex, multi-signal environments and analyzing transient events.
• Modulation analysis: USB power sensors can provide power distribution across different modulation components.
• Channel power: USB power sensors can measure the power associated with specific channels or frequency bands. This is useful for assessing channel performance and avoiding interference.
• Spectral analysis: Some sensors and software support limited spectral analysis capabilities to analyze the distribution of power across different frequencies.

What are the calibration considerations when using USB power sensors for precise measurements?

Most USB power sensors feature internal calibration capabilities like a reference source and switching circuits, facilitating calibration without disconnecting from the device under test. This speeds up the testing process and minimizes connector wear.

Zeroing, which is essential for accurate RF power measurements, is performed to minimize the zero offset and noise. It can be done in two ways:

• Internal zeroing: This allows the sensor to remain connected to an active RF source by isolating the RF input from the measurement circuitry, enabling calibration without turning off or disconnecting the RF source.
• External zeroing: This requires the removal of RF power from the input port, either by turning off the source or by physical disconnection.

Which industries use USB power sensors?

Let's look at some industry applications where USB power sensors are particularly useful for measuring power.

Wireless technologies

USB power sensors measure the signal power for various wireless protocols — Wi-Fi, Bluetooth, Zigbee, and others — and ensure compliance with their standards and regulatory limits.

Telecommunications

USB power sensors are extensively used for testing mobile network components according to technical and regulatory standards of 5G, 6G, LTE, and GSM.

USB power sensors are also frequently used for time-gated measurements of mobile signals. Measuring the signal power during specific timeslots is crucial for ensuring regulatory compliance, optimizing network performance, capacity planning, and diagnosing issues.

Internet of Things (IoT) device testing

USB power sensors are used for measuring many aspects of IoT devices like:

• their transmission and reception power levels, which affect their battery lifetimes
• the signal quality of their wireless transmissions
• interference and noise in their signals
• their electromagnetic emissions

These measurements facilitate better network planning, optimized placement of IoT devices, and compliance with maximum limits on the radiated power.

High-volume manufacturing

Some power sensors offer high measurement speeds of hundreds of readings per second. This feature is especially important in high-volume manufacturing, where fast measurement speed is essential to maximize production throughput and increase productivity. By increasing the speed of measurements, test engineers can shorten the design cycle and accelerate the product time-to-market, as they would require less time to carry out the same number of tests.

Defense applications

USB power sensors are used for defense applications like radar pulse measurements that require sophisticated characterizing of pulsed RF power, such as peak power, peak-to-average power, rise time, and fall time.

Scalar Network Analysis of RF Communications

Fig 7. USB power sensors used for scalar network analysis

USB power sensors are used for scalar network analysis to understand the response of an entire RF device to signals by measuring incident power, transmitted power, reflected power, gain, insertion loss, return loss, and other parameters.

What are some limitations and future trends of USB power sensors?

Below are some limitations and possible improvements in the USB power sensor ecosystem:

• Poor frequency selectivity: Generally, power sensors have poor frequency selectivity. This may improve with future designs.
• USB limitations: Most USB sensors still use mini-USB 2.0 connectors, which limit data transfer speeds to 480 megabits per second and in turn limit aperture times. Upgrading to the newer USB 3 and USB 3.2 standards can increase speeds to 10 gigabits per second, allowing far narrower aperture times.
• Cable length limitations: USB cables are limited to 5 meters. For more long-distance measurements, use LAN power sensors, which work over ethernet and support cable lengths up to 90 meters.

Get precision on the go with USB power sensors

In this article, we learned how portable USB power sensors simplify power measurements in the field by adopting the widely used USB interface.

Contact us to find out more about Keysight's USB and LAN power sensors and related products. We’ll help you select the right sensor for your project.

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