RF Power and Voltage Measurement

RF system’s output power level is often the key factor in the design and ultimately the performance of virtually all radio frequency and microwave equipment. For the measurement of average power, a sensor along with a calibrated power meter is connected with the RF transmitter. Initially, if the output of the sensor is switched OFF or not allowed to enter the power meter, the pointer of the power meter is set to zero. Afterward, the sensor is switched ON and the indication on the power meter is observed which indicates the average power of the transmitter.

It is essential to determine the power for the design and application of RF and Microwave systems. The average power is widely used in specifying almost all of RF and Microwave systems.

RF & Microwave Power Measurement

Generally, there are three methods for measuring power at RF and Microwave frequencies. Each of these methods employs different kinds of devices to convert RF power to measurable dc or low frequency signal. These methods are:

1. Power measurement using a Thermistor.
2. Power measurement using a Thermocouple.
3. Power measurement using a diode detector

Power Measurement using a Thermistor

This is a method that was previously used to measure RF and Microwave power. Currently thermocouple and diode technologies are preferred in most applications because of their increased sensitivities, wider dynamic ranges, and higher power capabilities. Nevertheless, a thermistor is still the sensor of choice in certain applications due to its power substitution capability.

Here, a bolometer which is a temperature-sensitive resistive element whose resistance varies due to change in temperature is used. The change in temperature results from converting RF or Microwave energy into heat within the bolometric element. Principally, two types of bolometers are used – one is the barretter and the other is the thermistor. A barretter is thin wire that has a positive temperature coefficient of resistance, which is not frequently used now. Thermistors are semiconductors with negative temperature coefficient.

Power Measurement using a Thermocouple

Thermocouples work on the principle based on dissimilar metals generating a voltage due to temperature differences at hot and a cold junction of the two metals.

The two main reasons for the development of thermocouples are:

• They exhibit higher sensitivity than the previous thermistor technology.
• They feature inherent square-law detection characteristic (input RF power is proportional to dc voltage out).

Since thermocouples are heat-based sensors, they are true ‘’averaging detectors’’. This is why they are recommended for all types of signal formats from continuous wave to complex digital-phase modulations.  Thermocouples are also more rugged than thermistors, make useable power measurements down to 0.3 mW (-30 dBm, full scale) and have lower measurement uncertainty because of better SWR.

Power Measurement Using a Diode Detector

Diodes convert high-frequency energy to dc by way of their rectification properties, which arise from their nonlinear current-voltage (I-V) characteristics. Rectifying diodes have been used as detectors and for relative power measurements at Microwave frequencies. However, for absolute power measurement, diode technology had been limited mainly to RF and low Microwave frequencies.

You can also read: Basics of Radio Frequency (RF) and Wireless Communication Systems

The Measurement of RF Voltages with a Voltmeter

To measure RF voltages, ranging from few hundred millivolts to several hundred millivolts, a voltmeter uses diode detectors. Usually diode detectors follow inverse square law below 100 V. So by taking advantage of this inverse square law, power detectors or voltage detectors are designed.

To achieve best sensitivity, a diode should be matched as closely as possible to source impedance. Simple diode detectors are used for designing RF voltmeter to measure voltages from 100 mV to several hundred mV. In the design of voltmeter, two types of diode detectors are used:

1. Series detector
2. Shunt detector

Out of these two, shunt detectors are mostly suitable for measuring RF and Microwave voltages. In this case, diodes are directly connected to the ground as illustrated in Figure 1(a).

The design components should have short leads, to avoid any error readings and a termination resistor is connected a cross the input to measure the output of amplifier and signal sources. A small silicon diode can work well here, but germanium diodes will give low offset voltages. In order to remove these offset voltages, a battery and resistor of high value is connected in the circuit.