RF & Microwave Power Meter Tutorial
- a summary or tutorial description of RF & microwave power meter technology and measurement.
RF and microwave power measurement and RF and microwave power meters are important elements within the RF and microwave design arena. The output power level of many RF and microwave amplifiers is a critical parameter in the design. As a result it is often necessary to measure the RF power level to ascertain the performance of the item under test.
As a result RF power meters are widely used and RF power measurement techniques are important for any RF design engineer. Different types of RF and microwave power meter are available and some are more suited than others to different types of measurement.
Along with this, it is necessary to know and understand the ways in which power measurements are made, and the errors and methods for minimising them.
While RF power measurement may be thought is as simply using an RF power meter or a microwave power meter, this is not just the case. Knowing the different measurement techniques, understanding the different types of RF power meter, and ways in which the errors in RF and microwave power measurements can be minimised are all key to making good RF power readings.
RF and microwave power meter basics
There are a number of ways in which RF power (including microwave power) can be measured. There are two main types of RF power meter that are used:
- Through-line RF power meters: These RF power meters take a sample of the power flowing along a feed-line and use this to indicate the power level. These through-line RF power meters are used on live systems, such as radio transmitters as a check of the outgoing power. They are normally directional and can be used to check the power travelling in either direction. Measurements made by these RF power meters are frequency insensitive - they measure the total power entering them regardless of frequency (within the overall frequency limitations of the instrument).
- Absorptive RF power meters: As their name implies, these RF power meters absorb the power they measure. Typically they utilise a power sensor that may be one of a variety of types. This generates a signal proportional to the power level entering the sensor. The sensor signal is coupled to the main instrument within the overall RF power meter to process the results and display the reading. Measurements made by these RF power meters are frequency insensitive - they measure the total power entering them regardless of frequency (within the overall frequency limitations of the instrument).
The absorptive RF power meters generally have digital readouts these days. An analogue voltage is generated within the power sensor or power head and this is fed into the main RF power meter unit. With high levels of digital signal processing available these days, many RF power meters contain significant levels of processing and this can enable a variety of signal types to be measured.
When selecting an RF power meter or a microwave power meter, it is important to select the correct type of power sensor. There are a number of different types of power sensor, and these are suited to different types of RF power measurement. Some types of RF power sensor are suited to make measurements of average power, whereas others can make measurements of pulse power or peak envelope power. Further pages of this tutorial address the power measurements - average, pulse power (often termed peak power), and peak envelope power, as well as the different types of sensor that can be used with RF power meters.
- Spectrum analysers and other instruments: Instruments such as spectrum analysers have power measurement capabilities within them. These instruments are able to measure the RF power level on a particular frequency, but cannot measure the total power entering on all frequencies. Spectrum analyser RF power measurements used not to be accurate, but with the improvements in their technology, the RF power measurements have far greater levels of accuracy.
Each type of RF power meter is used under different circumstances. However the absorptive RF power meter is the most widely used for accurate laboratory measurements. The through-line power meters tend to be used more for field applications.
Units for RF and microwave power measurements
Power is a measure of energy per unit time and it is typically measured in watts - this is a energy transfer at the rate of one Joule per second.
Although the watt is the base measure, often this is preceded by a multiplier as power levels can extend over a vast range. Levels of kilowatts (103 watts), or even megawatts (106 watts) are used in some large power installations, whereas other applications have much lower levels - milliwatts (10-3 watts), or microwatts (10-6 watts) may be found.
In some instances power may be specified in terms of dBW or dBm. These use a the logarithmic decibel scale but related to a given power level.
In itself a decibel is not an absolute level. It is purely a comparison between two levels, and on its own it cannot be used to measure an absolute level. The quantities of dBm and dBW are the most commonly used.
- dBm - This is a power expressed in decibels relative to one milliwatt.
- dBW - This is a power expressed in decibels relative to one watt.
From this it can be seen that a level of 10 dBm is ten dB above one milliwatt, i.e. 10 mW. Similarly a power level of 20 dBW is 100 times that of one watt, i.e. 100 watts.
A more extensive table of dBm, dBW and power is given below:
|+60||+30||1 000||1 kilowatt|
It can be seen that RF power meters or microwave power meters are crucial for the development engineer, service engineer and field engineer in many areas. Wherever RF signals are present it is necessary to be able to measure the power levels and dedicated RF power meters are often the best way of achieving this. In some instances spectrum analysers may be used, but to gain the best view of the power levels an RF power meter is often the best method.
By Ian Poole
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