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 into low power, medium power, and high power.

• Low power: Less than 10mWatt
• Medium power: From 10mWatt to 10Watt
• High power: Greater than 10Watt

The convenient units for power measurement are dBm and dBw.

$P_{dBm} = 10 \cdot Log_{10} \left(\frac{Power_{mWatt}}{1 \, mWatt}\right)$

• 30dBm = 1watt
• -30dBm = 1 microwatt

As shown in the figure, a simple power meter is made using a balanced bridge circuit. One arm of the bridge contains a bolometer, to which microwave power is applied. When power is applied, it changes the resistance in the bolometer arm, causing an imbalance in the bridge from its initial balanced condition (when no power is applied). The non-zero voltage is recorded using a voltmeter, which is calibrated to read the microwave/RF power.

A single bridge circuit gives false readings under ambient temperature changes. Also, changes in resistance due to mismatch at the input port result in wrong readings. Due to these drawbacks, a double bridge circuit is employed in the power meter design. In this type of RF power meter, the upper bridge measures RF power, and the lower bridge compensates for the effect of ambient temperature variation.

Power Meter Sensors

As mentioned above, a 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 explore the types of sensors used in power measurements: Schottky barrier diode, bolometer, and thermocouple.

Schottky Barrier Diode Sensor

It is used as a 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 barretters and thermistors.

• Barretter: A thin metallic wire of platinum that has a positive temperature coefficient of resistance.
• Thermistor: A semiconductor that has a negative temperature coefficient of resistance.

Thermocouple Sensor

It is a junction 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 a substrate.

Measurements Using RF Power Meter

In this section, we will see how power can be measured using a power meter. The Agilent E4418B series power meter is a popular choice.

Power meters are supplied with power heads or sensors, which are used for power measurement. The power heads are sensors that convert microwave signals to analog voltages.

• First, calibrate the power meter as per the instructions outlined in the power meter manual using the provided power head.
• Now, connect the power sensor with the signal port of which you would like to measure the power. The LCD display provides the power in various units supported by the device.

To convert power from one unit to another, refer to the links mentioned below.