This article talks about the essential characteristics, types and performance factors of power sensors and power meters in RF power measurement.
Radio frequency (RF) and microwave power measurements are important for measuring the performance of a device under test (DUT). Selection of measuring instruments and techniques is based on type of measurement. Understanding the different types of RF power meters, and the ways in which errors in RF and microwave power measurements can be minimised are key in making accurate RF power readings.
Power sensors and spectrum analysers are two key instruments for RF power measurements. Selection of equipment is done on the basis of its dynamic range, accuracy, frequency range, durability and portability.
Thermal or diode-based detectors are used for RF power sensing. The complete setup of a power sensor and a power meter consists of a sensor element on the front, followed by amplifier, filter, analogue-to-digital converter (ADC), microprocessor and display unit.
Incoming RF signal is converted to DC or low-frequency voltage waveform by this sensor element. Waveform is then amplified and filtered.
Some setups have increased filter bandwidth to improve measurement speed, or reduced to improve measurement sensitivity. An ADC converts analogue signals into digital ones. A microprocessor handles additional filtering and time averaging of the sampled waveform, as well as interfacing to the display.
Effects of uncertainties on RF power measurement
Despite their features, equipment for RF power measurement are sometimes subjected to different factors that may affect the quality of results. Some examples of sources of uncertainty are mismatches, linearity errors, interference and temperature effects. To evaluate total uncertainty, it is important to determine and combine all sources of uncertainty to obtain the combined uncertainty.
Generally, accurate RF power measurement is difficult to do as frequency is high (1GHz to 100GHz), apart from complex modulation types and formats. Highest measurement accuracy for absolute power can be obtained through a power sensor. Error sources of the sensor include linearity uncertainty, zero offset and drift, calibration uncertainty, temperature effects and display noise.
These sources of errors should be taken into account while RF power measurement. For absolute power measurement, uncertainty in reference measurement with the power sensor is the main source of error. Two crucial factors affecting this are sensor specification and voltage standing-wave ratio (VSWR) occurring due to a mismatch between the sensor and source.
John Hamilton, vice president – marketing, RFMW Ltd, says “The higher the VSWR, the greater the uncertainty! Metrology-grade components have very low VSWR, which minimises unwanted reflections. A minimum of 10dB signal-to-noise ratio (SNR) is necessary to make accurate measurements with these devices. Inherent errors can be minimised through a calibration process. It is essential to calibrate on a regular basis, since temperature, test cable flexure and even connection quality affect the calibration of test equipment.”
Types of RF power meters
Different types of RF power meters are used under different circumstances. Absorptive RF power meter is the most widely-used for accurate laboratory measurements, while through-line power meter is used mostly for field applications. Hamilton adds, “Continuous wave is the most common. It provides an average power measurement that is highly accurate and traceable. With certain applications, such as radar, pulsed RF measurements are required and provide pulse power information. With modulation (FM, AM, QAM, etc), special considerations are required to provide instantaneous peak-power analyses.”
Absorptive RF power meters
These utilise a power sensor to generate a signal proportional to the power level entering the sensor. Sensor signal is coupled with the main instrument to process the results and display the readings.
These meters measure total input power, regardless of frequency. These have digital readouts and significant levels of digital signal processing. This enables a variety of signal types to be measured.
Through-line RF power meters
These meter take a sample of the power flowing along a feed line and use it to indicate power level. These are used on live systems, such as radio transmitters, as a check for outgoing power. These meters can be used to check the power travelling in either direction. Total input power is measured regardless of frequency.
These instruments can measure RF power levels on a particular frequency but not the total power entering on all frequencies. These make measurements of signals in the frequency domain.
Types of RF power sensors
Maximum input is the signal power level that can be fed to the power sensor without damaging it. If signal level beyond maximum input is fed, the sensor may get permanently damaged or become non-linear. Frequency range of measurement and sensitivity depend on the power sensor used. A single sensor may not cover the entire range of frequencies or power levels.
For higher power-level measurements and very-low power-level measurements, separate sensors may be required. So, its is advisable to select one depending on application requirements.
Main factors for consideration while selecting a power sensor are frequency, signal and power range to be measured, and connector type.
There are two types of sensors used for RF power measurement, as given below.
These measure power purely based on heat dissipation, and can be used to measure any waveform, including CW, AM, FM, PM and pulsed. Thermal sensors are further divided into two types:
Thermistor RF power sensor
This sensor works on the fact that temperature in an RF load increases due to incident input RF signal.
Thermocouple power sensor
This is based on the thermocouple principle that says, at different temperatures, small potential is induced at metal junctions. It can be used to measure average power levels.
Diode-based power sensors
These can measure signals at very low levels (below -70dBm). These have a wider dynamic range, but their non-linear characteristics need compensation to achieve accurate average and peak power measurements.
These sensors can measure the average power of CW signals and peak power of modulated signals. Schottky barrier diodes are often used as RF power sensors, as these have a much smaller level of stored charge and a low forward conduction turn on point.
Criteria for selecting the right equipment
Different types of power sensors are suited for different types of RF power measurements. Peak power of a signal is not the same as its average power. This is because of the phase relationship between tones. Some types of RF power sensors support average power measurement, whereas others support pulse power or peak-envelope power measurements.
Take care to select a power sensor that covers the required frequency range. When there is a need for extra frequency ranges, a different power sensor can be used along with the same basic power meter. If there is a need for extended frequency range in the next five years, it is better to include those frequencies now instead of purchasing additional instruments after five years. An instrument that can be augmented easily for future requirements by installing plug-in modules can also be chosen.
The most important and essential factor in selecting test instruments is their reliability, accuracy, sensitivity and cost.
The service office should be in close vicinity of the setup.
If the instrument is procured from abroad, the manufacturer should have local service centres.
Specifications should take care of current requirements as well as possible future requirements to some extent.
Taking measurements requires an informed choice of technique and equipment.
Development of radio systems requires consistent measurements of RF or microwave power levels. There are many other applications that need to measure output power from a power amplifier, or received power to the system, as given below.
- Measuring output power of IC amplifier
- Verifying compliance to ensure public safety and that RF signals do not overpower devices on adjacent frequencies
- Measuring signals between devices
To sum up
The key determinant of performance of an RF system is its output power. This requires measurement of signal power at every stage, from design to prototyping.
Calibration can provide greater assurance of RF instrument accuracy. ISO 9001-compliant services have been conducted within ISO 17025-accredited laboratories to check RF power sensor testing capabilities.