Working principle of FMCW microwave level meter

FMCW is an abbreviation of Frequency Modulated Continuous Wave. FMCW technology is the earliest technology used in radar level measurement equipment.

FMCW microwave level meter uses linear modulated high frequency signal, generally 10GHz or 24GHz microwave signal. It is an indirect measurement method based on complex mathematical formulas, and the level distance is calculated from the spectrum. The antenna transmits a linearly modulated continuous high frequency microwave signal and scans it, while receiving the return signal. The frequency difference between the transmitted microwave signal and the returned microwave signal is proportional to the distance to the surface of the medium.

If we consider that the slope of the linearly modulated microwave signal is K, the frequencies of the transmitted signal and the reflected signal are rf, the lag time difference is rt, the distance from the transmitting antenna to the medium surface is R, and C is the speed of light.

Then we can get: rt = 2R/C

Since a frequency-modulated microwave signal is used, we can obtain: rf = K×rt;

After combining the two formulas, we get the formula:

R = C × rf/2K (Formula 2)

According to formula 2, we can see that the distance R from the antenna to the medium surface is proportional to the difference between the transmission frequency and the reflection frequency rf.

The signal processing part mixes the transmitted signal and the echo signal to obtain a mixed signal spectrum, and distinguishes different frequency signals through independent fast Fourier transform (FFT) changes. Finally, an accurate digital echo signal is obtained to calculate the distance from the antenna to the medium surface.

In fact, the FMCW signal cycles between two different frequencies. Currently, the FMCW microwave level meters on the market are mainly based on two frequencies: 9 to 10 GHz and 24.5 to 25.5 GHz.

Microwave level meters using the FMCW principle have a continuous self-calibration processing function. The processed signal is compared with an internal reference signal representing a known fixed distance. Any difference is automatically compensated, thus eliminating possible measurement drift caused by temperature fluctuations or aging of electronic components inside the transmitter.

2.2 Pulse

Pulse radar level meters, like ultrasonic technology, use the time difference principle to calculate the distance to the surface of the medium. The device transmits pulses of a fixed frequency, then receives and builds up an echo pattern. The propagation time of the signal is directly proportional to the distance to the medium. But unlike ultrasound, which uses sound waves, radar uses electromagnetic waves. It uses tens of thousands of pulses to "scan" the container and get a complete echo pattern.

Generally, the accuracy and reliability of pulse microwave level meters are not as good as those of FMCW microwave level meters, but pulse level meters are the most widely used microwave level meters in the market because their prices are much lower than those of FMCW. Of course, many manufacturers have greatly improved the reliability of pulse radars by enhancing echo processing functions.

2.3 Guided Wave Radar

It is also called time domain reflectometry radar, or TDR. Guided wave radar is a combination of non-contact radar and waveguide antenna. It transmits microwave signals to the waveguide rod or cable, so that the microwave energy is concentrated around the waveguide rod or cable without "spreading". It generally uses pulse waves, but there are also continuous waves.

This working principle enables it to measure media with lower dielectric constants, effectively avoid the influence of interferences in the container, not be affected by water vapor, and can be used to measure solids. However, like all contact level measurement devices, it has the disadvantages of easy adhesion, easy wear, and even cable breakage, and is greatly affected by dust.

2.4 Technical Review

I once heard a statement from a user that the use of pulse technology is much better than the radar based on the principle of continuous frequency modulation. I think this statement is unscientific. Of course, radars using FMCW technology have defects such as relatively high cost and high power consumption, but its working method ensures that it is more reliable and the distortion of the signal will be minimized. Therefore, in some applications with more complex working conditions, its advantages can still be reflected. Of course, after the vigorous development in recent years, pulse radar technology has also made a huge leap, overcoming many technical defects and greatly improving reliability.

Some time ago, one of our agents told me that a salesperson from a certain radar supplier mentioned to the customer that pulse radar can achieve multiple radars installed in the same warehouse without mutual influence of signals; while radars with FMCW technology will have problems. But no explanation was given as to why. After listening to this, I was very surprised. Has the market competition reached this level? In fact, the mutual influence of radar signals installed in the same warehouse is precisely a shortcoming of pulse radar. Since its working state is equivalent to the "simplex" mode in communication, it is easy to receive signals from other radars of the same specification and generate erroneous measurement signals. Therefore, it is necessary to correct this problem by setting the "synchronization" function, and most pulse radars have this function, which is equivalent to coordinating the working states of multiple radars and sending or receiving them at the same time. However, continuous frequency modulation radars do not have this problem because they use a working mode similar to "full-duplex" in communication.

Different technologies have value if they exist, otherwise the market will eliminate them. So, my opinion is that all technologies and brands that can survive in the market have their value. There is no such thing as good or bad in a simple sense. It depends on where and how they are used. The most suitable one is the best. Just like mahjong tiles, any tile can be a good one or a bad one, it all depends on how you play it.