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RF Power meter basics

In simple terms,power is the quantity of energy dissipated or stored per unit time. For measurement simplification, microwave power is categorized in low power, medium power and high power. Low power is considered to be less than 10mWatt, medium power from 10mWatt to 10Watt and high power is considered to be greater than 10Watt.

The convenient unit of power measurement is dBm and dBw.
P in dBm = 10 Log ((Power in mwatt)/(1 mwatt))
30dBm = 1watt
-30dBm = 1 microwatt

microwave power meter

As shown in the figure, simple power meter is made using balanced bridge circuit. One of the arm of the bridge is bolometer to which microwave power is applied. When the power is applied it will change the resistance in the bolometer arm, which causes the imbalance in the bridge from its initial balance condition(when no power is applied). The non zero voltage is recorded using volt meter, which is calibrated to read the microwave/RF power.

Single bridge circuit does false reading under ambient temperature. Also change in resistance due to mismatch at the input port results in wrong reading.Due to these drawbacks double bridge circuit is employed in the power meter design. In this type of RF power meter, upper bridge measures RF power and lower bridge compensates effect of ambient temperature variation.

Power meter sensors

As mentioned above power meter consists of power sensors which helps measure microwave power. Based on the sensors different types of power meters are manufactured by manufacturers as per applications which include CW, average, peak and average power meters. we will see type of sensors used in power measurements viz. schottky barrier diode,bolometer and thermocouple.

schottky barrier diode sensor:
It is used as square law detector whose output is proportional to the input power.

bolometer sensor:
It is a device whose resistance changes with temperature as it absorbs microwave power. Common types of bolometers are barretter and thermistor. Barretter is a thin metallic wire of platinum which ahs positive temperature coefficient of resistance. Thermistor is a semiconductor which has negative temperature coefficient of resistance.

Thermocouple sensor:
It is a juncion of two dissimilar metals or semiconductors. It generates emf which is proportional to the incident microwave power when two ends are heated up differently by absorption of microwaves in a load deposited on substrate.

Measurements using RF power meter

In this section we will see how power can be measured using power meter.
Agilent E4418B series power meter is one of the popular one. Power meter is supplied with power heads or sensors which is used for power measurement. The power heads are sensors that convert microwave signals to analog voltages.

•  First calibrate the power meter as per instructions outlined in the power meter manual using power head provided.
•  Now connect the power sensor with the signal port of which you would like to measure the power. LCD display provides teh power in various units supported by the device. To convert power from one unit to the other refer links mentioned below.

RELATED LINKS

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RF and Wireless Terminologies


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