Power Measurement Overview
Power is a necessary parameter to measure across a wide range of modern applications. Power is defined as work per unit of time. It includes units of measure such as horsepower, watts, calories per second and BTU per second, depending on the domain of interest. Electrical power is typically measured in watts, defined as one Joule per second. With RF circuits, dBm, a logarithmic representation of power level relative to the milliwatt, is often used.
An assortment of instruments and methods are used for measuring power. In this article, we will limit the discussion to electrical power, defined as voltage multiplied by current. We will further narrow the discussion to RF power, which implies higher frequency, say signals greater than 10 MHz, requiring more sophisticated instrumentation than a voltmeter due to the behavior of high-frequency electrical signals.
It is important to select the appropriate technique for measuring RF power in a given scenario. If the signal level is too high, the instrument input will become saturated; distortion, harmonics, spurious and other non-linear signals could occur. If the signal level is too low, the signal will be buried in the noise floor, making it difficult to retrieve.
Types of RF Power Measurements
Modern test equipment performs a variety of RF power measurements. The differences in RF waveforms require different RF power measurement methodologies.
- The most straightforward waveform is the continuous wave (CW) RF sinusoidal signal, whose amplitude, frequency and phase are constant over some time frame. CW RF power measurements provide an economical solution for measuring single-tone waveforms.
- RMS RF power measurements are ideal for signals that are time variant, such as modulated or multitone signals, capturing data for RF average power over time.
- Peak and average RF power are high-performance measurements targeted at modulated and fast pulse signals requiring advanced features such as pulse profiles, rise/fall time, peak values and wide bandwidths.
- Additionally, for pulse modulated RF signals we would like to know the Pulse Repetition Interval (PRI or period) or Pulse Repetition Frequency (PRF) as well as the Pulse Width (PW). For a pulsed RF signal the pulse power is the average power divided by the duty cycle of the waveform. Since many pulse modulated signals are not as well behaved as an ideal square wave, peak envelope power is often measured to determine the maximum RF power present during the time when the pulse is present.
Other modulated RF signals may have additional power measurements to describe specific power characteristics, such as Complementary Cumulative Distribution Function or CCDF. This is a measure of how far the peak power lies above the average power values. It is a useful measure in RF communications to reduce power consumption and lower the stress on the RF power amplifier. A similar measure is peak to average ratio also know as crest factor. AM and FM signals have modulation index values to indicate the efficiency of the modulation which in turn provides a measure of how efficiently RF power is being utilized.
For electrical circuits, RF power is most often expressed in dBm or decibels referenced to one milliwatt. Since decibels use a logarithmic scale, zero dBm is equivalent to one milliwatt, +10 dBm is equivalent to 0.01 watts, etc. Because dBm is referenced to one milliwatt, it is an absolute measure of power. When working with RF power, it is often convenient to represent RF power in relative terms, such as decibels referenced to the carrier power (dBc), or decibels referenced to the noise floor (Signal to Noise ratio, SNR). Since decibels are logarithmic, wide ranges of linear power, typically Watts, can be measured on a relatively small scale. For example, 0 dBm is one milliwatt, and 10x10x10 milliwatts = 1 watt = 30 dBm. So, a linear factor of 1000 can be represented by only three divisions on a logarithmic scale.
RF Power Measuring Instruments
What type of instrument is best for measuring RF power? That depends on the requirements of the measurement. A spectrum analyzer is a well-rounded instrument that is capable of many measurements. Many modern spectrum analyzers also have specific measurement personalities. They are frequency selective which makes it especially useful when narrowband RF power measurements are needed. One of the tradeoffs is fair – but not exceptional – average power accuracy. Depending on the model, most spectrum analyzers have RF power measurement accuracies in the order of ±2.0 dB to ±0.25 dB.
A Vector Network Analyzer (VNA) can measure RF power as well. VNAs are very precise because they measure both phase and magnitude, generally as ratioed parameters such as gain and group delay. Most VNAs are not quite as capable at measuring absolute parameters such as the output power of an RF amplifier. Nevertheless, most modern VNAs can provide acceptable absolute accuracies better than a spectrum analyzer by utilizing calibration techniques and specific applications.
One of the most precise ways to measure RF power is with an RF power meter or RF power sensor. Power meters and power sensors have a sensing module that measures the RF power present at the input and converts that to a voltage proportional to the RF power. The analog voltage is then sent down a cable where it is read and combined with calibration constants specific to the sensor resulting in a precise RF power measurement. In modern power meters and power sensors, these calibration constants are stored in an EEPROM that is typically updated during the annual calibration of the instrument. Older instruments will have the calibration values on a label on the instrument that need to be entered manually.
Modern power meters and power sensors detect RF power in one of two ways. In one approach, diodes are used to convert the RF power into a voltage directly while operating the linear region of the diode. Multiple diodes are often used to extend the measurement range. Diode sensors are fast and respond quickly to changes in the RF signal.However, they have a limited dynamic range.
RF power can also be detected with a thermistor. Thermistors are heated by the RF power present and produce a resistance proportional to the RF power. Thermistors are very broadband and are especially good at measuring average RF power. However, they are generally slower than diodes. Whether diodes or thermistors, they are located near or in the RF connector on the RF sensing head.
RF Power Meters vs. RF Power Sensors
Power meters and power sensors are closely related with a few key differences. Power meters are stand alone instruments that read and display the measured RF power. In general, they have been around much longer than power sensors. Most RF power meters have interchangeable RF heads to accommodate different RF power ranges, measurement types and frequencies. RF power meters usually have an RF reference signal to calibrate the meter between traceable factory calibrations (typically every year).
A modern approach to a power meter is the power sensor. In this case, the external meter is replaced by software running on a PC connected to the power sensor via a USB or LAN cable. The interface also supplies DC power to the power sensor. Annual calibration ensures the power sensor remains accurate within its specified values. Power sensors come in a variety of frequency and power ranges. The software provides a Graphical User Interface (GUI) and other functions, such as specialized measurements, and can typically support multiple USB power sensors from a single PC.
Mini-Circuits’ PWR series is an example of a broad family of USB and Ethernet controlled power sensors that provide exceptional performance at a competitive price point. Wideband sensors are available in a series of common frequency ranges up to 40 GHz, with fast measurement speeds, excellent accuracy and up to 80 dB of dynamic range. The included GUI software, shown in Figure 1, provides full power measurement capabilities from any Windows PC, and the API (Application Programming Interface) allows power measurements to be automated through most common programming environments (including Python, MatLab, LabVIEW and any more).
Figure 1: A Mini-Circuits USB and Ethernet RF power sensor and GUI interface.
Sources of Measurement Error
With any RF power meter or USB RF power sensor, care should be taken to ensure the best possible match between the DUT and instrument. As mentioned previously, these instruments do not measure phase, so vector error correction is not possible. This is true for any scalar instrument, such as the previously mentioned spectrum analyzers.
For power meters and USB power sensors, there are two primary sources of measurement errors. First is mismatch loss. This is the power loss produced internally within the sensor due to heating, calibration and other factors intrinsic to the sensor layout and design. Second is mismatch uncertainty. The imperfect impedance match between the power sensor and signal source produces a standing wave in the RF path. This interaction of the incident and reflected power results in uncertainty in the measurement. This is illustrated in Figure 2 below.
Figure 2: Mismatch loss and uncertainty contribute to measurement error in RF power meters and power sensors.
Best practices such as high-quality connectors and properly torqued components can minimize these errors. Other sources of error include temperature stability, drift, calibration uncertainty, poor quality/damaged components, and quantization. With any high-performance test system, the user should take care to understand and minimize any significant measurement errors.
Summary
Many modern applications require the measurement of RF power. The user has many choices for the instrument most suitable to their requirements. USB or LAN power sensors are an excellent choice for many applications. When properly implemented, they offer excellent measurement accuracy, a competitive price point, and simple connectivity combined with a PC-based GUI. Mini-Circuits has a variety of power sensors to meet a broad range of current applications. A current list of Mini-Circuits power sensors and their capabilities can be found at this link.
As a leader in RF measurements, Minicircuits continues to enhance and broaden its line of power sensors. We invite you to periodically visit the Minicircuits web site as we continue to innovate our RF test solutions.
