RF & Microwave Power Measurement

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

Microwave and RF power measurements are an important type of measurement that is made within the RF environment. Making accurate RF power measurements not only depends upon having an accurate RF power meter, but also on knowing how to make the measurements in a real environment.

There are a number of pointers that can be used to ensure that the RF power measurements that are made are as accurate as possible.

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.

By Ian Poole

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