Design of surface acoustic wave oscillator circuit for gas detection

1 Introduction

In recent years, surface acoustic wave gas sensors have developed rapidly and have been applied in many fields, with good development prospects. Since SAW sensors have the advantages of small size, low price, high precision, high sensitivity and high resolution, SAW gas sensors consisting of surface acoustic wave devices, sensitive films and related detection circuits have been produced. In the process of gas adsorption by the sensitive film, the stress caused by the measured gas acts on the medium in the surface acoustic wave transmission path, causing its dynamic characteristics to change, thereby changing the resonant frequency of the surface acoustic wave device. The matching detection circuit tests this change to obtain the relevant characteristics of the gas to be measured, and the relevant performance of the detection circuit directly affects the technical indicators such as the precision and accuracy of the entire sensor. Therefore, the design of the circuit is a crucial link in the SAW gas sensor.

Surface acoustic wave (SAW) oscillators are constructed using surface acoustic wave resonators or delay lines as frequency stabilization elements. Surface acoustic wave devices are usually made of single crystal quartz materials with stable performance, and the output frequency is very stable. The fundamental frequency of the surface acoustic wave oscillator is high, and the use range can be as high as 2000MHz. Unlike the traditional bulk acoustic wave oscillator with a lower frequency, it has the advantages of light weight, small size, low power consumption, low cost, and increased reliability.

As the sensitive element of the SAW sensor, the frequency stability of the SAW oscillator directly affects the resolution, stability and test accuracy of the sensor. Its oscillation frequency depends on the condition when the feedback loop phase is zero. Therefore, improving the frequency stability of the SAW oscillator helps to further improve the performance indicators of the sensor.

2 Surface Acoustic Wave Resonator

Surface acoustic wave (SAW) devices are devices with good frequency selectivity using an interdigital structure. They are made by precisely designing the number of interdigital pairs and spacing of the interdigital transducers on both sides, and then by processes such as evaporation and photolithography. Surface acoustic wave resonators (SAWR) are high-Q resonators that are similar to quartz crystal resonators in many ways.

Compared with the surface acoustic wave delay line (SAWDL) oscillator, the transmission characteristics of the two-terminal resonator are similar to those of a high-Q delay line, but it has the following significant features: first, the size of the resonator is very small; second, the insertion loss of the resonator is much smaller; third, the frequency modulation range of the resonator is narrower than that of the delay line, but as the frequency modulation range increases, the stability decreases.

For sensors based on resonant SAW devices, baseline frequency stability is one of the most important technical indicators. For surface acoustic wave gas sensors, a sensitive film needs to be added to the surface acoustic wave device, which will increase the insertion loss of the device. Too much insertion loss will reduce stability, so the insertion loss of the SAW device itself cannot be large.

SAW resonators can also be divided into single-ended resonators and double-ended resonators according to the logarithm of the interdigital transducer. These two types of resonators have their own characteristics. The single-ended SAW resonator has a simple oscillation circuit, fewer components, good frequency stability, low phase noise, and can achieve low voltage and low power consumption. However, its band coverage factor is small, and it will not oscillate when it exceeds this range. The advantage of the double-ended SAW resonator is that it is easy to oscillate. If the phase shift network is selected appropriately, frequency adjustment can be achieved in a wide range. Its disadvantage is that this circuit is relatively complex and the cost is relatively high.

3. Design of oscillation circuit

This design is based on a two-terminal surface acoustic wave resonator circuit design. In order to make the circuit easily meet the oscillation conditions and achieve good performance, a closed-loop positive feedback amplification oscillation circuit is adopted, which includes an amplifier circuit and a feedback loop. The circuit principle block diagram is shown in Figure 1. The surface acoustic wave device model used in this design is RP1308, a two-terminal resonant device with a center frequency of 433.92MHz and a phase shift of 180.

Figure 1 Schematic diagram of oscillation circuit

The conditions for this circuit to form a single-mode oscillation at the resonance point are similar to those of other types of resonant sensors, and the conditions are as follows:

(1) Phase condition, that is, the total phase shift caused by the resonator and other loop components is an integer multiple of 2.

(2) Amplitude condition, that is, the loop gain is 1.

(3) Suppression of other resonant modes, that is, at frequency points other than the required resonant mode, the phase and amplitude conditions required for resonance are not met at the same time.

Based on the above conditions, the first step of this design is to select a suitable radio frequency integrated circuit (RF IC) amplifier so that its parameters meet the relevant requirements of the surface acoustic wave oscillation circuit. These parameters include amplification gain, bandwidth, maximum input power, etc. By selecting a suitable amplifier, surface acoustic wave devices of different frequencies can use the same oscillation circuit within a certain frequency range to achieve oscillation. The second step is to design a passive LC filter to limit the open-loop gain to a narrow frequency range near the fundamental frequency of the two-terminal surface acoustic wave device. At this time, the two-terminal surface acoustic wave device is placed in the feedback circuit of the amplifier as a component.

Since the oscillation established in the closed loop should meet the phase balance condition, the phase change of the amplifier itself will inevitably cause the output frequency to change. This phase change mainly comes from the change of power supply voltage, temperature change and aging of components. Therefore, the closed-loop amplifier should be selected to keep its phase-frequency characteristics flat in a large range near the center frequency of the oscillator, so that the power supply voltage and other factors have the least effect on the frequency change. In addition, it is particularly important to choose a low-noise, high-gain amplifier. In addition, in order to simplify the design of the matching circuit, a monolithic amplifier with an input and output impedance of 50 can be selected. Considering the above, the amplifier selected for this design is UPC2748T. The operating voltage of this model of amplifier is 3V, the center operating frequency is 900MH z, it has excellent performance, small size, and low price.

The schematic diagram of the oscillation circuit of this design is shown in Figure 2. The oscillation circuit is composed of amplifier U1, L2, L3, R1 and surface acoustic wave resonator, where L1 and C1 are the processing parts of the amplifier power supply voltage. The signal starts to oscillate in the oscillation circuit and reaches a stable oscillation frequency. In the oscillation loop, the adjustable components are L2, L3, R1 and C2, so the debugging of the oscillation circuit is simple and fast, and the oscillation circuit can be designed quickly.

When L2 and L3 are 39nH, R1 is 0Ω, and C2 is 1.5pF, the circuit oscillation conditions are met and the circuit achieves stable oscillation.

Figure 2 Schematic diagram of positive feedback amplifier oscillation circuit.

The oscillation circuit has a simple structure and is easy to start oscillation. Since there are fewer active devices, it will not introduce too much noise, making the oscillation circuit more stable. The oscillation frequency of the oscillation circuit is determined by the resonant frequency of the SAW device. The closed-loop components are adjusted so that the oscillation circuit meets the oscillation conditions and obtains the ideal oscillation frequency. By selecting a suitable matching circuit, the oscillation circuit can achieve frequency adjustment in a wide range. This circuit uses fewer components, which is convenient for the design and manufacture of multi-channel detection channels and can be well applied to SAW sensor array systems.

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4. Test results and analysis

After the whole circuit design and debugging is completed, the test data is recorded by an oscilloscope. When the surface acoustic wave device is the RP1308 type selected for the initial design and debugging, the measured frequency graph is shown in Figure 3. When the double-terminal resonant type device with model B433, center frequency of 433.9MHz and 180.. phase shift is selected, good performance can also be obtained, and its test frequency graph is shown in Figure 4. It can be seen that their resonant frequencies are all 433.9MHz, achieving the expected goal.

Figure 3 Circuit test diagram based on RP1308 resonator.

Figure 4 Circuit test diagram based on B433 resonator.

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The circuit designed in this paper can not only be applied to 433MHz double-terminal resonant devices, but also to similar devices of other frequencies.

This circuit also has good stability. The debugging results show that the frequency jump of the designed circuit can be stabilized within 30 Hz.

Figure 5 Circuit test diagram based on Q284 resonator.

5 Conclusion

The circuit designed in this paper has a simple structure, small size, low power consumption, good performance, and can be widely used in a variety of surface acoustic wave gas sensors based on two-terminal resonant devices. Tests show that the circuit has high stability and can be applied to two-terminal surface acoustic wave resonant devices with different resonant frequencies, greatly reducing the cost of making SAW sensors.