Power meter three classifications

A power meter is an instrument that measures electrical power. It is no stranger to the RF microwaves. The power meter is basically one of the big ones in parallel with the signal source, spectrum analyzer and network analyzer. Of course, it is not as expensive as the previous big brothers.

Figure 1 The composition of the power measuring instrument

The RF or microwave power meter can be divided into two types according to the connection mode in the test system: terminal type and pass type.

The terminal type power meter uses the power meter probe as the terminal load of the test system, and the power meter absorbs all the power to be tested, and the power indicator directly reads the power value. Terminal power meters are often used to test small signals due to the need to absorb the full incident power.

The terminal power meter has the following features:

(1) In the common RF and microwave power measuring instruments, the terminal meter power meter has the highest amplitude measurement accuracy, which exceeds the spectrum analyzer or signal analyzer. The typical measurement accuracy can reach ±1.6%.

(2) Cannot measure high power. The upper limit is usually +20 dBm and the lower limit is about -60 dBm.

(3) The average power, peak power, burst power, etc. of various modulated signals can be measured.

The pass power meter uses a coupling device such as a directional coupler, a coupling loop, a probe, etc. to couple a part of the power from the transmitted power by a certain ratio, and sends it to the power meter to measure the total power transmitted equal to the power. The indicator value is multiplied by the scale factor. The industry pioneer of the pass-through power meter is Bird, and the elderly of RF microwave should know. The following figure is a block diagram of a typical pass-through power meter:

Figure 2. Block diagram of a pass-through power meter

The main features of the pass-through power meter;

(1) Pass-through power meter has high power measurement capability. In theory, as long as the transmission line can pass the power, the pass-through power meter can measure. Therefore, the power of kilowatts on the radio and television is measured by a pass-through power meter.

(2) It is difficult to achieve broadband by a power meter, which is due to the limitation of the directional coupler inside.

(3) Pass-through power meters cannot be used for too small power measurements due to the degree of coupling of the directional coupler. This and the terminal power meter have their own strengths.

RF or microwave power meters are classified according to sensitivity and measurement range, and can be classified into a thermal resistance type power meter, a thermocouple type power meter, a calorimetric power meter, and a crystal detection type power meter.

Thermistor type power meter uses a thermal resistor as a power sensing element. The temperature coefficient of the thermal resistance value is large. The power of the signal to be measured is absorbed by the thermal resistance to generate heat, so that its own temperature rises, the resistance value changes significantly, and the resistance value is measured by the resistance bridge to display the power value.

The thermocouple junction of the thermocouple type power meter thermocouple type power meter directly absorbs the high-frequency signal power, and the junction temperature rises, generating a temperature difference potential, and the magnitude of the potential is proportional to the absorbed high-frequency power value. This kind of power meter has high measurement accuracy and is generally used for more accurate power measurement.

Figure 3 Schematic diagram of the thermocouple power meter

The typical thermal effect power meter of the calorimetric power meter absorbs the high-frequency signal power by the heat-insulating load to increase the temperature of the load, and then uses the thermocouple element to measure the temperature variation of the load, and calculates the high-frequency power value based on the generated heat. This is basically not seen in our lab, and it is mostly used for calibration level power benchmarking.

Crystal Detector Power Meter The crystal diode detector converts high frequency signals into low frequency or direct current signals. The operating point is chosen such that the amplitude of the detector output signal is proportional to the power of the high frequency signal. Transistor detection power meter has been widely used in RF microwave measurement because of its fast measurement speed and moderate accuracy.