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In order to ensure that power measurements are as accurate as possible, it is necessary to choose the right power sensor. Different signals have different waveforms and factors like average power, peak power, peak envelope power,- PEP and the like have a major impact on the type of power sensor needed.
Each power sensor, or power meter will state the type of waveforms that it can measure. Some are able to measure carrier waveforms that are steady, whereas others are able measure peak, RMS, peak envelope and pulsed waveforms. These typically require much faster levels of response and often the ability to compute the value within the power meter.
In view of the options available for measuring different formats for the power, it is important to select the correct power meter or test instrument and its sensor or sensors if applicable.
RF average power
The most obvious way to measure power is to look at the average power. This is defined as the energy transfer rate average over many periods of the RF waveform.
The simplest waveform to measure is a continuous wave (CW). As the signal is a single frequency steady state waveform, the average power is obvious.
For other waveforms the averaging parameters may be of greater importance. Take the example of an amplitude modulated waveform. This varies in amplitude over many RF cycles, and the RF power must be averaged over many periods of the modulating waveform to achieve a meaningful result.
To achieve the required results, the averaging period for RF power meters may range from several hundredths of a second up to several seconds. In this way the RF or microwave power meter is able to cater for the majority of waveforms encountered.
RF pulse power or peak power
In a number of applications, it is necessary to measure the power of a pulse of energy. If this were averaged over a long period of time, it would not represent the power of the pulse. In order to measure the power of the pulse itself, a method of defined exactly what must be measured.
As the name pulse power implies, the power of the actual pulse itself is measured. For this the pulse width is considered to be the point from which the pulse rises above 50% of its amplitude to the point where it falls below 50% of its amplitude.
As the pulse is likely to include some overshoot and ringing, the most accurate term for the power is the pulse power. Peak power would imply that the value of any overshoot would need to be taken, whereas the actual power measurement required is that of the overall pulse.
Peak envelope power, PEP
For some applications another form of RF power measurement is required. Called peak envelope power, PEP, it is used to measure the power of some varying waveforms.
There are many instances where a power measurement that takes the peak of the envelope is needed. Many digitally modulated waveforms may require this, and also transmissions such as AM and Single Sideband may also need this type of RF power measurement.
The envelope power is measured by making the averaging time greater than the period of the modulating waveform, i.e. 1/fm where fm is the maximum frequency component of the modulation waveform.
This means that the averaging time of the RF power measurement must fall within a window:
- It must be large when compared to the period of the highest modulation frequency.
- It must be small compared to the period of the carrier waveform
The peak envelope power is therefore the peak value obtained using this method.
Of all the forms of RF power measurement, the average power is the most widely used. It is the most convenient to make, and often expressed the value that needs to be known. However pulse power, sometimes referred to as peak power, and also the peak envelope power also need to be known on many occasions. However the techniques and equipment needed to make peak envelope power and pulse power are different to those needed for average power. Accordingly it is necessary to understand the differences between the different types of RF power measurement and the equipment needed.