# Test and Measurement World

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## Advantages of RF Power Meter | disadvantages of RF Power Meter

This page covers advantages and disadvantages of RF Power Meter and its basics. It mentions benefits or advantages of RF Power Meter and drawbacks or disadvantages of RF Power Meter. Both diode detector based and Heat based (Thermistor, Thermocouple) power meter sensors are described.

### What is RF Power Meter?

Introduction:
As we know Power is the product of current and voltage. It is measured in watt which is equal to 1 joule/sec. The relative power is measured in dB where as absolute power is measured in dBm.
P in dBm = 10 Log ((Power in mwatt)/(1 mwatt))
30dBm = 1watt

Power can also be expressed in dBW which is defined as follows.
Power in dBW = Power in dBm - 30
Hence as per above equation, 0 dBW = +30 dBm or -30 dBW = 0 dBm

Power can be measured at different frequencies viz. DC, low or high frequency. The different power measurement types are average power, pulse power and peak envelope power.

Power can be measured using power sensor heads which can be categorized into two types.
Diode detector based
Heat based (Further categorized into thermistor RF power sensor and Thermocouple RF power sensor)

Small part of RF power is extracted using coupler or divider or atenuator to prevent damage to the power meter sensor by limiting RF power in its range.

Diode detector based RF Power sensor :
The figure depicts block diagram of RF Power meter. It uses diode detector in the sensor head. Here power meter acts as load which dissipates RF power in sensor head. It employs diode rectifier to produce output. The output of the diode rectifier is processed using advanced DSP techniques. It uses microprocessor, ADC/DAC for monitoring and display purpose.

Heat based RF Power Sensor:
It measures true average power as power is dissipated as heat in this power sensor type. It measures integral of input power over period of time. It measures power which is independent of signal waveform type used such as CW, AM, FM, PM, QAM, Pulsed etc.

The advantages and disadvantages of RF Power Meter depends on power sensor type (diode or heat based) used which are described below.

### Benefits or advantages of RF Power Meter (Diode detector based)

The benefits or advantages of RF Power Meter based on diode detector are as follows.
➨It can measure very low power as low as -70 dBm compare to heat based RF power sensor.
➨It can respond quickly during measurement than heat based version.
➨The output from diode is processed using DSP techniques which provide large variety of capabilities than heat based power sensor. Moreover variety of waveforms can be detected using diode as detector.

### Challenges or disadvantages of RF Power Meter (Diode detector based)

Accurate test equipments based on diode as detectors face some challenges. The challenges or disadvantages of RF Power Meter based on diode detector are as follows.
➨Stored charge effects of ordinary diodes limit operating range of the diodes. Hence schottky diodes are used due to its much smaller stored charge level and low forward conduction turn on point.
➨Though schottky diode has low turn on voltage (about 0.3 volts in silicon), it limits the lowest signal levels which can be measured.
➨GaAs semiconductor diodes are used for superior performance compare to silicon diode. The GaAs diodes are expensive and fabricated using planar doped barrier technology.

### Benefits or advantages of RF Power Meter (Heat based thermistor RF power sensor)

The benefits or advantages of RF Power Meter using heat based thermistor RF power sensor are as follows.
➨It offers high quality RF power measurement.
➨It is used as preferred RF power sensor as it enables DC power to be substituted to enable calibration of the system.

### Drawbacks or disadvantages of RF Power Meter (Heat based thermistor RF power sensor)

The drawbacks or disadvantages of RF Power Meter using heat based thermistor RF power sensor are as follows.
➨Changes in ambient temperature affects the readings.
➨It requires balanced bridge technique to maintain resistance of thermistor using DC bias.

### Benefits or advantages of RF Power Meter (Heat based thermocouple RF power sensor)

The benefits or advantages of RF Power Meter using heat based thermocouple RF power sensor are as follows.
➨It offers higher sensitivity than thermistor RF power sensor. Hence low power levels on the order of microwatts can be measured using it.
➨Thermocouple has square law detection characteristic and hence input RF power is proportional to DC output voltage.
➨It can be used to produce very rugged power sensor than thermistor.
➨Thermocouples are used as true heat based power sensors as they provide true average power. Hence they are used to measure signals of all the formats.

### Drawbacks or disadvantages of RF Power Meter (Heat based thermocouple RF power sensor)

The drawbacks or disadvantages of RF Power Meter using heat based thermocouple RF power sensor are as follows.
➨It is not suitable for measuring instantaneous values due to time constant of heat based RF power sensors.