Frequency drifts mixing

A dual SAW delay line oscillator is illustrated in Figure 3. It consists of two SAW delay lines fabricated on the same ST-quartz substrate. One delay line is coated with the chemically sensitive film and the other is left uncoated. The frequencies of the two delay line oscillators are mixed and filtered to provide a frequency equal to the difference of the two oscillator frequencies. This scheme has several advantages when used in chemical vapor sensor applications. First, it helps to compensate for SAW frequency drift caused by ambient temperature and pressure fluctuations. Additionally, the difference frequency is much lower than the frequency of the oscillators themselves. This permits SAW vapor response measurements using inexpensive digital circuitry. 

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