A power meter is a modular or handheld electronic device used to measure the strength of radio frequency (RF) or microwave signals. These types of signals are crucial to many electronic designs, so measuring their output power is a necessity in testing, repairing and developing electronic equipment. Two basic types of power meters in use today are through-line power meters and absorptive power meters. Through-line power meters measure signal power as it travels through a feed. Absorptive meters measure signal power by rerouting the signals into the meter.
The sensors of a power meter use electrical circuits called bridges to detect changes in signal strength. These changes are detected by measuring the flow of the signals through four resistors. When bridge resistance is balanced, the output voltage is zero. When a signal passes through the resistors, they become unbalanced and produce voltage that is converted into a digital signal and sent to a display or other type of readout on the power meter.
The calibration coefficient, also known as the calibration factor, of a power meter is the known distortion between the actual strength of a signal and the measured strength of a signal. The calibration coefficient is in the form of a percentage, and it is specific to each model of power meter. When calibrating the power meter, it is necessary to enter the calibration factor to account for the distortion, thus calibrating the device to produce an accurate result.
An oscillator is a chip or other small electronic component that uses an input signal to generate a sustained output signal. In a power meter, a calibration oscillator is a component that generates a known signal. The power head of the power meter is connected to the oscillator and the calibration factor is entered. The power meter then adjusts its measurement to match the known output of the calibration oscillator.
Compensation Bridge Voltage
Power meter sensors are capable of adjusting for signal distortions such as those caused by the temperature of a circuit. Distortions are detected and an appropriate voltage is sent through the bridge to adjust for it. The compensation bridge voltage is of equal magnitude to the distortion but of the opposite polarity.
CW power is more properly known as continuous wave or carrier wave energy. CW power was the first type of transmission wave signal to be used. It is still used in many electronic applications. For a power meter to measure CW power, it must have CW power sensors. When CW power is measured by a power meter, peak power and average power are equal.
DC Substitution Measurement
DC substitution is considered the most accurate method of measuring power. Power meters using 100 percent DC substitution use corrective voltage to account for temperature variations. In DC substitution, the RF or microwave power that is measured is substituted with DC power so voltage adjustments can be made.
Power meters are able to measure power transmitted in various frequencies. The frequencies that fall within the maximum and minimum frequency make up the frequency range of a power meter. Power meter calibration is only effective within a specific frequency range.
Linearity error is the deviation of a power meter’s actual measurements from the theoretically ideal range of measurement. Errors of higher value represent greater deviations.
Peak Power Meter
Peak power meters are designed to measure only the highest levels of pulsed signal power. The performance of pulsed signals is dependent upon peak power. Peak power measurement of pulsed signals is in contrast with CW power, where the peak power is equivalent to the average power.
The power range of a power meter is the minimum level of power that can be measured by the meter to the maximum level of power that can be measured. A power meter is unable to measure power levels outside its power range.
RF blanking is a feature of many power meters that is used to turn off the output signal of the power meter. Some power meters allow RF blanking to be automatically activated when an input signal is measured in a specified range.
RF Power Meter
An RF power meter is a type of power meter made specifically to measure the power of radio frequency (RF) signals. An RF power meter is one of two standard types of power meter. The other type is a microwave power meter.
Sensor Type, Diode
Diode sensors are one of the three types of sensors used in power meters. Diodes differ from the other two types because they are not heat-based. Diode sensors can measure lower levels of power than heat-based sensors, and they respond faster than heat-based sensors.
Sensor Type, Thermistor
Thermistor sensors are one of two types of heat-based sensors that can be used in power meters. Thermistor sensors have fallen in popularity, but they are known for their high-quality measurements. They are also capable of DC substitution.
Sensor Type, Thermocouple
Thermocouple sensors are one of two types of heat-based sensors used in power meters. Thermocouple sensors are more sensitive than thermistor sensors, and at the same time, more durable.
Thermoelectric error is the variation of a power signal due to changes in temperature. Many power meters correct for thermoelectric error through the use of DC substitution.