Advantages and Disadvantages of RF Power Meters

This article explores the benefits and drawbacks of RF Power Meters, covering their basic principles and different sensor types. We’ll look at both diode detector-based and heat-based (thermistor and thermocouple) power meter sensors.

What is Power?

Power, in its simplest form, is the product of current and voltage, measured in watts (joules per second). We often measure power in two ways: relative power, expressed in decibels (dB), and absolute power, expressed in dBm.

The conversion to dBm is:

$$ P_{\mathrm{dBm}} = 10 \log_{10}\left(\frac{P_{\mathrm{mW}}}{1~\mathrm{mW}}\right) $$

For example, 30 dBm equals 1 watt.

Power can also be expressed in dBW:

$$ P_{\mathrm{dBW}} = P_{\mathrm{dBm}} - 30 $$

Therefore, 0 dBW = +30 dBm, and -30 dBW = 0 dBm.

What is a RF Power Meter?

RF power measurement is crucial across different frequencies, from DC to very high frequencies. We typically measure three types of power: average power, pulse power, and peak envelope power.

To protect the power meter sensor from damage, a small portion of the RF power is extracted using a coupler, divider, or attenuator, limiting the RF power within the sensor’s operating range.

Image alt: RF Power Meter

Power sensors, the core of an RF power meter, fall into two main categories:

• Diode Detector Power meter
• Thermal Energy Meter (TEM)
  • Thermistor RF power sensors
  • Thermocouple RF power sensors

Diode Detector RF Power Sensor

As shown in the diagram, a diode detector-based RF power meter uses a diode in the sensor head. The power meter acts as a load, dissipating RF power in the sensor head. A diode rectifier generates an output which is then processed using advanced Digital Signal Processing (DSP) techniques. Microprocessors and Analog-to-Digital/Digital-to-Analog converters (ADC/DAC) are used for monitoring and display purposes.

Advantages of Diode Detector RF Power Meters

• High Sensitivity: Can measure very low power levels, down to -70 dBm, unlike heat-based sensors.
• Fast Response: Reacts quickly during measurements compared to heat-based versions.
• Advanced Processing: The diode’s output is processed using DSP, offering greater capabilities and the ability to detect a variety of waveforms.

Disadvantages of Diode Detector RF Power Meters

• Limited by Stored Charge: The stored charge effects of typical diodes limit their operating range. Schottky diodes are used to mitigate this due to their smaller stored charge and low forward conduction turn-on point.
• Turn-on Voltage Limit: Although Schottky diodes have a low turn-on voltage (around 0.3 volts for silicon), it limits the lowest signal levels that can be measured.
• Cost: For superior performance, Gallium Arsenide (GaAs) diodes are used, which are more expensive and fabricated using planar doped barrier technology.

Thermal Energy Meter

Heat-based sensors measure true average power by dissipating power as heat. They measure the integral of the input power over a period of time. This method is independent of the signal’s waveform, whether it’s Continuous Wave (CW), Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM), Quadrature Amplitude Modulation (QAM), or pulsed signals.

Advantages of Thermistor RF power sensors

• High-Quality Measurements: Offers highly accurate RF power measurements.
• Calibration Facilitation: Allows for the substitution of DC power to calibrate the system, making it a preferred type of sensor.

Disadvantages of Thermistor RF power sensors

• Temperature Sensitivity: Changes in ambient temperature can affect the accuracy of readings.
• Balanced Bridge Requirement: Requires a balanced bridge technique, using a DC bias, to maintain the thermistor’s resistance.

Advantages of Thermocouple RF power sensors

• High Sensitivity: More sensitive than thermistor sensors, enabling the measurement of low power levels down to microwatts.
• Square Law Detection: Has a square-law detection characteristic, meaning the input RF power is proportional to the DC output voltage.
• Rugged Design: Can be manufactured as very rugged power sensors compared to thermistors.
• True Average Power: Thermocouples are true heat-based power sensors, providing true average power measurements, suitable for all types of signals.

Disadvantages of Thermocouple RF power sensors

• Not for Instantaneous Measurements: Not suitable for measuring instantaneous values due to the time constant of heat-based RF power sensors.