Contactless liquid level measurement using reflectometer chips

The RF impedance can be detected by attaching an air dielectric transmission line to the outer wall of a non-metallic water tank to accurately measure its liquid level. This article provides an empirical design example showing how a reflectometer device (such as the ADL5920 from Analog Devices) can help simplify the design.

Reflectometer-based solutions offer several advantages over traditional mechanical float level sensing methods, including:

● Capable of rapid, real-time measurement of liquid levels

● Supports extensive electronic post-processing

● Non-contact design (will not contaminate liquid)

● No moving parts

● Minimum RF radiated field (far field cancellation)

● No need to drill holes in the water tank for installing external sensors (reduced risk of leakage)

● It is safer because there are no wires or parts on the water tank

Level Measurement Overview

Figure 1 shows a block diagram of the entire system, including an RF signal source driving a balanced and terminated air-dielectric transmission line with a reflectometer in the line.

Figure 1. Liquid level measurement system block diagram.

working principle

A transmission line suspended in air can be used to accurately measure impedance characteristics and reduce RF losses because it uses low-loss conductors and no solid dielectric materials. Classic E and H vector diagrams show that the electric and magnetic fields are concentrated around the conductor and their magnitude decreases rapidly with distance, which is measured relative to the size and spacing of the transmission line structure itself. Nearby dielectric materials (such as the walls of a water tank and the liquid inside the tank) change the electrical characteristics of the transmission line1, which can be roughly measured by a reflectometer (such as the ADL5920).

Detailed description

Consider an air dielectric low loss transmission line designed to determine a specific characteristic impedance ZO in air. Any added dielectric substance, such as a liquid in the near field of the transmission line, will:

● Reduce the characteristic impedance of the transmission line;

● reduce the speed of propagation, thereby increasing the effective electrical length of the line; and

● Increase line attenuation.

These three effects combine to reduce return loss, which can be measured directly using a reflectometer device or instrument. With careful design and calibration, return loss can be correlated to fluid level.

To simplify the analysis, consider setting the impedance of the air-dielectric transmission line in Figure 1 equal to ZO before connecting the transmission line to the water tank. Because the line is terminated at ZO, theoretically there is no reflected energy in the line, so the return loss is infinite.

After attaching the transmission line to the side of the tank, what was once a single transmission line is now two separate transmission lines, cascaded in series:

● Above the liquid level, the transmission line uses air as the dielectric, excluding the tank wall material. The impedance of the transmission line, ZOA, does not change much compared to the air dielectric value, ZO. The same is true for the propagation speed of the transmission line.

● Below the liquid level, the transmission line impedance ZOF is lower than ZOA. Because there is additional dielectric material in the near field of the transmission line, the electrical length is effectively increased, and so is the attenuation.

The impedance of the termination ZO at the far end of the transmission line will transform when measured by a reflectometer at the source end of the transmission line. The transformation is depicted graphically, approximately as shown in Figure 2. Since ZOF is lower than ZO, the Smith chart rotates clockwise, as shown by the arrow.

Figure 2. An expanded standard Smith chart showing the input impedance of a transmission line. The line endpoints illustrate how the fluid level translates into a return loss measurement.

The transmission line causes no impedance transformation when the transmission line impedance is exactly matched to the resistive tip at the end of the line. This situation corresponds to the center of the Smith chart shown in Figure 2, which shows the normalized impedance as 1 + j0 Ω. The return loss should be at least 26 dB before connecting the transmission line to the water tank.

After connecting the transmission line to the empty water tank, the material of the water tank wall increases the dielectric material of the transmission line, lowering the line impedance to ZOA and slightly increasing the effective electrical length of the transmission line Trace 1, as shown in Figure 2. The measured value of the return loss remains almost unchanged at about 20 dB.

As the water level in the tank rises, the impedance of the transmission line decreases because the liquid occupies part of the air that was originally used as the transmission medium. The impedance of the transmission line was originally ZOA, but now it becomes ZOF. Therefore, the center point of the Smith chart rotation decreases. At the same time, because the effective electrical length of the transmission line increases, the amount of Smith chart rotation increases. This is specifically shown by Trace 2 and Trace 3 in Figure 2. Therefore, the reflectometer measures that the return loss at the RF generator end decreases.

Because the ADL5920 measures the magnitude of the reflection, not the phase, the impedance transformation should be limited to the lower half of the Smith chart, where the reactive component is negative. Otherwise, the impedance is transferred back to the center of the Smith chart, resulting in inaccurate measurements. This means that the electrical length of the transmission line connected to the entire water tank should be 90° or less. If the electrical length exceeds 90°, the measured return loss will appear to be folded back.

A bidirectional RF detector, such as the ADL5920, can measure the incident power and the reflected power in dBm with the characteristic impedance of the transmission line, ZO = 50 Ω. The ADL5920 can also subtract these two readings and directly measure the return loss in dB.

What is return loss?

In simple terms, when an RF source is connected to a load, some of the power is transferred to the load and the remainder is reflected back to the source. The difference between the two power levels is the return loss. This is generally used to measure how well the load is matched to the source.

Purpose of Balun

The balun is used to drive conductors with equal voltage but opposite polarity, so it has two main functions:

● Reduce stray RF at the input/output of the transmission line. This is very important for controlling compliant EMI. Far-field EMI in all directions is also reduced due to cancellation.

● Transformed impedance. Higher impedance means greater spacing between transmission line elements, which means the electric field penetrates deeper into the container. The result is a greater change in return loss and liquid level, which means a more sensitive level measurement.

The balun should provide excellent common-mode rejection ratio (CMRR) across the entire passband of the bandpass filter .

Is a bandpass filter necessary?

Where stray RF may couple onto the transmission line, it is recommended to use the optional bandpass filter shown in Figure 1. The bandpass filter helps reduce or eliminate interference from Wi-Fi, cellular, PCS services, land mobile radio, and all other external signals that are not in the same frequency band as the wanted source.

For best results, it is recommended that the bandpass filter design have low insertion loss and a return loss comparable to the measured value of return loss; that is, approximately 30 dB or better.

Basic design steps

The design steps are as follows:

● Select the operating frequency based on the length of the transmission line. Generally, the length of the transmission line is about the height of the water tank, or slightly longer. When selecting the operating frequency, ensure that the length of the transmission line is generally 1/10 to 1/4 of the RF wavelength in air. Figure 3 shows the approximate frequency range. At lower frequencies, better return loss linearity and liquid level are achieved, and at higher frequencies, a larger return loss signal range is achieved, but linearity may be poor and measurement foldback may occur (Figure 2). If electromagnetic emission compliance is required, the frequency can be selected from the list of applicable ISM frequencies. 2

● Design or select a balun based on the selected frequency or frequency band. The balun can be based on a lumped element LC or a transformer. The balun should have excellent return loss when connected to the balanced end L.

● Calculate the conductor width and spacing of the transmission line. You can use a transmission line impedance calculator, such as the Arbitrary Transmission Line Calculator (ATLC).

Figure 3. Recommended operating frequency and transmission line length.

Simple design example

For demonstration purposes, a level monitor was designed for a car's windshield washer tank. The test setup had water flowing between two identical tanks, one of which was connected to a transmission line to measure the level.

According to the previous plan:

● Because the water tank height is approximately 6 inches (0.15 meters), a target RF excitation of approximately 300 MHz is reasonable (see Figure 3).

● Next, design and build the LC balun for this frequency range. A slight boost impedance shift to ZO is needed to increase sensitivity to fluid level changes4 (see Figure 4). Use a network analyzer or reflectometer to verify that the return loss on the single-ended port is approximately 30 dB or better, with the fixed resistor termination connected directly to the balun before connecting to the transmission line.

● We design and build parallel transmission lines with ZO equal to the resistor value used previously. The transmission lines are connected in the circuit and the resistor terminations are moved to the ends of the lines. See Figures 4 and 5. Again, a network analyzer or reflectometer is used to verify that the return loss remains excellent—approximately 25 dB or better.