RF & Microwave Power Measurement Techniques

- an overview of microwave & RF power measurement techniques - how to make the measurements and the precautions to take.

RF & microwave power meter Includes:
RF power meter PEP, average, & pulse power Power measurement techniques In-line power meter Absorption-power meters & sensors

Making RF measurements is normally quite straightforward, but by implementing few techniques and taking a few precautions it is possible to ensure accurate measurements are made.

Ensure the RF power measurement type suits the meter

There are many different types of waveform that may need to be measured. Some waveforms have a varying envelope, while others will remain constant. Different power meters sensors operate in different ways and as a result different power sensors are required for different types of RF power measurement.

In general the following types of power sensor are available for absorptive power meters:

Thermistor power sensor: The thermistor RF power sensor is a heat based power sensor and it is used for situations where RF power measurements are required. Being a heat based sensor, it will detect the heat being dissipated over a period of time - typically over tenths of a second. Thermistor power sensors have the advantage that the RF power can be substituted by DC which can be accurately measured using a digital multimeter to give a very accurate calibration of this type of sensor.
Thermocouple : Thermocouple RF power sensors are also heat based. They are similar in many respects to thermistor types but are able to make RF power measurements down to lower power levels.
Diode based power sensors: These power sensors act as rectifiers and provide an output of the envelope of the waveform to the meter processor. Using signal processing, these sensors are able to make a variety of RF power measurements including average power, pulse power and peak envelope power. Accordingly these sensors are ideal for making a wide variety of RF power measurements.

It is necessary to ensure that the power meter and sensor are able to make the type of RF power measurement that is required. Choosing the meter carefully along with its specification, will ensure that the correct meter is chosen for the RF power measurement in mind.

Ensure the RF power measurement range suits the meter / sensor

As with any item of test equipment, it is necessary to ensure that the expected readings for the RF power measurements will fall within the range of the meter. It is also best to leave some margin at both top and bottom ends.

Often at the bottom end of the range there will be some noise and this adds uncertainty to the RF power measurement. It is often best to leave a margin of around 10 dB at the bottom end.

Also at the top end of the range a margin is useful. Typically leave 3 to 5 dB at the top end of the RF power measurement range. This will leave a margin for power measurement readings coming out larger than expected and it will ensure that the RF power measurements are within the optimum range of the meter.

Ensure the power handling capability cannot be exceeded

Although it really comes under the heading above, it is necessary to ensure that there is no way that excessive power can be applied to the meter while making an RF power measurement. Although the power sensor will be able to tolerate some level of overload when making RF power measurements, it is easy to damage them if too much power is applied.

To prevent any possibility of damage, power attenuators may be added to the input of the sensor to ensure that the signal falls within the acceptable range of the sensor.

Ensure connectors are correctly connected

It is important to ensure that the power sensor is properly connected to the power source being measured. There are two key pints for this:

Connectors tightly screwed on: It is necessary to ensure that the connectors are attached firmly and tightened, if necessary to the required torque. In this way a good connection can be assured. Loosely tightened connections can introduce serious losses, especially at high frequencies.
Keep leads short: The ideal situation is to connect the power sensor or power meter directly onto the power source. However if a length of coaxial cable is needed, this should be kept as short as possible and it should also be of a low loss variety.

By adopting a few precautions and adopting the right measurement techniques, accurate measurements can be taken. With power meters able to read quite accurately, adopting the right techniques ensures their accuracy can be used correctly.