Non-contact measurement & inspection A bidirectional RF detector such

as the ADL5920 can measure both incident and reflected power in units of dBm, along an RF transmission

line of characteristic impedance ZO = 50 Ω. The ADL5920 is also able to subtract these two readings, directly measuring return loss in dB.

Figure 3. Recommended operating frequency vs. transmission line length. When the transmission line impedance is

precisely matched to the resistive termination at the end of the line, there shall be no impedance transformation due to the transmission line. This condition corresponds to the center of the Smith chart, Figure 2, which shows a normalised

impedance of 1 + j0 Ω. Return loss should be at least 26 dB before the transmission line is attached to the tank. After attaching the transmission line to an empty

tank, the wall material of the tank will contribute some extra dielectric material to the transmission line, thus lowering the impedance of the line to

ZOA, and slightly increasing effective electrical length of the transmission line, Trace 1, as exemplified in Figure 2. Return loss should still measure quite well at approximately 20 dB. As the fluid level rises in the tank, transmission line impedance becomes reduced due to fluid displacing a portion of the air as the dielectric transmission. Transmission line impedance that was

ZOA now becomes ZOF. Hence, the center of rotation on the Smith chart moves lower. Simultaneously, the amount of Smith chart rotation increases, because the effective electrical length of the transmission line is increasing. This is depicted by Trace 2 and Trace 3 in Figure 2. Consequently, the reflectometer measures reduced return loss at the generator end of the line. Because the ADL5920 measures reflection magnitude, not phase, the impedance transformation should be constrained to the bottom half of the Smith chart where the reactive component is negative. Otherwise, impedance is being transformed back toward the center of the Smith chart, causing a magnitude measurement ambiguity. This means the electrical length of the transmission line attached to a full tank should be 90° or less. If electrical length exceeds 90°, the measured return loss will appear to foldback.

WHAT IS RETURN LOSS? Simply stated, when an RF source is connected to a load, some of the power will be transferred to the load, and the remainder will be reflected back toward the source. The difference between these two power levels is the return loss. It’s essentially a measure of how well- matched the load is to the source.

PURPOSE OF THE BALUN The balun serves to drive each conductor with equal but opposite polarity ac voltage, and thus serves two primary purposes:

Reducing stray RF coupling to and from the transmission line. This is important for regulatory emissions and susceptibility compliance. Far-field EMI in either direction is reduced by cancellation.

Transforming impedance. Higher impedance means wider spacing of the transmission line elements, which means deeper electric field penetration into the container. The result is more change in return loss vs. fluid level, which means a more sensitive fluid- level measurement.

The balun should be designed to provide good common-mode rejection ratio (CMRR) over the entire pass band of the band-pass filter.

IS A BAND-PASS FILTER NECESSARY? The optional band-pass filter of Figure 1 is recommended whenever stray RF could couple into the transmission line. A band-pass filter will be very helpful for reducing or eliminating interference from Wi-Fi, cellular, and PCS services, land mobile radio, and all other outside signals that are not in the same frequency band as the desired source. For best results, it is recommended that the band-pass filter design features low insertion loss, with return loss commensurate with that of the return loss measurement; that is, approximately 30 dB or better if possible.

Figure 4. Balun and transmission line used for fluid level sensing example.

BASIC DESIGN PROCEDURE The design procedure outline is approximately as follows:

Choose an operating frequency based on the length of the transmission line. Normally, the transmission line length will be about the same as the tank height or slightly longer. Operating frequency should be chosen such that transmission line length is typically one-tenth to one-fourth of the RF wave-length in the air. Figure 3 illustrates this approximate frequency range. A lower frequency will give the best linearity of return loss vs. fluid level, while a higher frequency will give a larger range of return loss signals, but linearity may not be as good, and measurement foldback may occur (Figure 2). If radiated emissions compliance is required, the frequency may be chosen from the list of applicable ISM frequencies.

Design or choose a balun for the chosen frequency, or frequency band. The balun can be a lumped-element LC or transformer based. The balun should exhibit excellent return loss when terminated at the balanced end.

Calculate the conductor width and spacing dimensions of the transmission line. A transmission line impedance calculator such as an arbitrary transmission line calculator (ATLC) is useful for this purpose.

A SIMPLE DESIGN EXAMPLE For demonstration purpose, a fluid-level monitor for an automotive windshield washer tank was devised. The test setup moves water between two identical tanks, one of which is to have a transmission line attached, for fluid-level measurement. In accordance with the previous outline:

Because tank height is approximately 6” (0.15m), a target RF excitation of about 300 MHz is appropriate (see Figure 3).

Next, an LC balun is designed and constructed for this frequency range. A slight step-up

impedance transformation to ZO is desired to increase sensitivity to the fluid-level variation (see Figure 4). A network analyser or reflectometer is used to verify approximately 30dB or better return loss on the single-ended port, with the fixed resistive termination connected directly to the balun, before connecting the transmission line.

A parallel transmission line is designed and

fabricated with ZO equal to the resistor value previously used. The transmission line is connected in-circuit, and the resistor termination moves to the end of the line. See Figure 4 and Figure 5. The network analyser or reflectometer is again used to verify that return loss remains good—approximately 25 dB or better.

Now the transmission line may be attached to the side of the tank, as shown in Figure 6. It is normal to observe return loss drop slightly when

Instrumentation Monthly June 2020

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