Since the birth of the first radio transmitter, engineers have been concerned about RF power measurement, which is still a hot topic today. Whether in the laboratory, production line, or teaching, power measurement is essential.

In the early stage of radio development, most of the signals faced by test engineers are continuous wave, amplitude modulation, frequency modulation, phase modulation, or pulse signals, which have rules to follow. For example, the power measurement of continuous-wave frequency modulation or phase modulation signal is very simple, and only its average power needs to be measured; The power of AM signal is related to its modulation depth, and the characteristics of pulse signal are expressed by pulse width and duty cycle. For the above analog or analog modulation signals, RF power measurement is basically concerned with average power and peak power.

Now, especially after the 1990s, digital communication began to develop rapidly, and the focus of RF power measurement began to change. Because the envelope of digital modulation signal has no rules to follow, its maximum and minimum levels will change randomly, and the amount of change is large. In order to describe the characteristics of this kind of signal, some new description methods are introduced, such as lead power, burst power, channel power, and so on. Many traditional power meters have been unable to meet the requirements of digital signal power measurement, and some power measurement tasks have been completed by the spectrum analyzers.

Next, we introduce several common RF power measurement methods. Before that, we need to clarify one thing – why is it customary to use power to describe signal strength in frequency domain test and measurement, rather than voltage and current commonly used in time domain test and measurement? That is because in the RF circuit, due to the standing wave on the transmission line, the voltage and current lose their uniqueness. Therefore, the size of the RF signal is generally expressed in power, and the internationally used power units are w, MW, and DBM.

Both spectrum analyzer and power meter can measure RF power. Among them, the power meter is divided into an absorption power meter and a pass-through power meter.

The same is power measurement. Different testing instruments and methods pay different attention.

There are three common methods to measure RF power:

  • Spectrum analyzer measurement;
  • Absorption power measurement;
  • Through power measurement.

1, Spectrum analyzer measurement

Spectrum analyzer is a basic frequency domain testing and measuring instrument. Figure 4 shows the basic working principle of spectrum analyzer using digital if technology. The measured signal enters the mixer after passing through the low-pass filter and mixes with the local oscillator signal entering the mixer at the same time. As the mixer is a nonlinear device, it will generate intermodulation signals. The signals falling into the filter will pass through the ADC, and then enter the IF filter, envelope detector, video filter and video detector in turn. Finally, the track will be displayed on the screen.

When measuring RF power parameters, the spectrometer has the following characteristics:

1) The spectrometer can measure very small amplitude RF signals, which depends on a key indicator of the spectrometer – DANL (displayed average noise level), Chinese “display average noise level”. Figure 5 shows the result of measuring a signal with a frequency of 999mhz and power of – 130dbm. The signal is clearly visible, which is beyond the reach of any power meter.

2) The spectrometer has a large amplitude measurement range, from DANL to safe input level + 20 DBM or even + 30 DBM, and the dynamic range can reach 190 DB! At present, the maximum dynamic range of power meters on the market is basically within 100 dB, such as:

Nrp8s of a German company: – 70 DBM to + 23 DBM;

U2041xa of an American company: – 70 DBM to + 26 DBM.

3) The spectrometer can measure the frequency component of the signal and can carry out narrowband measurement.

4) The spectrum analyzer can measure multi-carrier frequency signals at the same time and observe the distribution of the signal spectrum analyzer.

2. Absorption power measurement

An absorption power meter is a common microwave and RF power measurement equipment. Its working principle is shown in Figure 8 (diode detector power meter). The measured signal first enters the power meter. The power meter circuit can be composed of a thermistor, thermocouple, or diode detector. The power meter is composed of three measurement channels to measure the signals of different power sizes respectively. After digital processing, the power value is displayed in the power meter host or computer software. Now more and more display parts are realized by software.

The absorption power meter has the following characteristics:

1) Among the common microwave and RF power measuring instruments, the amplitude measurement accuracy of the absorption power meter is the highest;

2) Generally, the dynamic range will not exceed 100 dB;

3) High power cannot be measured. Usually, the upper limit of measurement is about + 30 DBM (1 W). If the measurement range needs to be expanded, an external attenuator is required;

4) It can measure the average power, peak power, burst power, pulse width and rise/fall time of various modulated signals;

5) The frequency component of the signal cannot be measured like a spectrometer;

6) VSWR cannot be measured.

In view of these characteristics of the absorption power meter, it is widely used as laboratory calibration equipment to calibrate signal source and spectrometer.

3, Through power measurement

The limitation of the absorption type of power measurement is solved by the application of the absorption type of power measurement. The greatest significance of pass through power measurement is to measure the matching between amplifier or transmitter and load under a high power state. When it comes to pass through power meter, many people will think of the product bird 43, which was invented by the Bird company in 1952 and is still being generated and applied.

The core device of pass through power meter is directional coupler. VSWR is calculated by measuring the forward power and reflected power of pass through power meter. This measurement method has the following characteristics:

1) The passing power meter has the ability of high-power measurement;

2) Power with small amplitude cannot be measured;

3) The through power meter is limited by the bandwidth of the directional coupler, and the measurement bandwidth is much smaller than that of spectrometer and absorption power meter;

4) The high-power matching between the transmitter and the load (antenna) can be measured by the power meter.

Conclusions

Through the introduction of this paper, it can be seen that in RF power measurement, the spectrometer has inherent advantages in flexibility and scope of application. The accuracy of absorption power meter is the highest, while the through power meter is more inclined to high-power signal measurement.