However, despite some reported success stories, there still exist

However, despite some reported success stories, there still exists the problem of the extraction of optimal sensor parameters theoretically due to a lack of precise extraction of mechanical parameters like shear modulis. In this paper, the Martin model was used to deal with the response analysis of a SXFA-coated chemical sensor, its shear modulus was determined experimentally by the inversion method. Then, referring to the Wang model [15], the response mechanism of the SXFA-coated chemical sensor was depicted in detail, including the polymer thickness and frequency effect on vapor adsorption, allowing optimal design parameter extraction.

Also, it is well known that the sensor performance, especially the threshold detection limit, depend, mainly on the frequency stability of the oscillator.

Thus, improvement of the frequency stability is another main topic of SAW chemical sensor research. Many groups have reported some success with SAW oscillators of different design for gas sensing. Schickfus et al. analyzed the effect of temperature changes, ageing of the transducer and the layer material on the frequency stability of the oscillator [18]. Hoyt et al. presented a way to improve the frequency stability of the oscillator, in which both of the dual delay line oscillators were modified with chemically sensitive interface materials to compensate for the noise and drift in SAW oscillator frequency and frequency associated with temperature effects on wave velocity in coating films [19].

Schmit et al. reported a rapid design of SAW oscillator electronics for sensor application with high frequency stability [20].

Jasel et al. reported the detailed design of oscillation circuits to improve the frequency stability of the oscillator Anacetrapib [21]. Also, in our previous work, a new SAW device for chemical sensing Batimastat design was done, in which electrode width controlled single-phase unidirectional transducers (EWC/SPUDT) and combed transducers were used to structure the SAW delay line, resulting in lower insertion loss and single oscillation frequency mode [13,14]. The frequency stability was improved effectively, however, to maintain the stable oscillator status, further work should be done for the oscillator including the SAW devices and oscillation circuit.

This is the second topic in this paper. Here, a delay line with frequency of 300 MHz and Al/Au metallization was fabricated; lower insertion loss less than 10 dB and single frequency oscillation were realized from the measured data. A new approach of phase modulation was also demonstrated, which make the oscillation occur at the lowest insertion loss point. Superior short-term freqency stability was obtained experimentally.

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