A New Frequency Detection Method Based on FFT in the Application of SAW Resonator Sensor

Recognition of wireless and non-battery measurement systems makes passive surface acoustic wave (SAW) sensors valuable. The interrogation unit coordinates the SAW sensor system and provides the sensor's energy by transmitting pulse signals. The SAW sensor also transmits its response to the interrogation unit after being charged. The response signal of the SAW sensor is a noisy damped sine wave that should be received by the interrogation unit antenna. The received signal resonates at the SAW intrinsic frequency which changes with sensing characteristics. So the interrogation unit has the responsibility of detecting this damped sine wave frequency accurately and finding the exact value of the sensing parameter. Fast Fourier transform (FFT), zero crossing, and peak detection methods are used in most papers. However, after studying these methods, it is concluded that all of them have fundamental disadvantages namely inaccuracy and low speed. So by using estimation formulae, a new frequency detection method is designed based on FFT and used to compensate the drawbacks. Finally, the proposed method can help researchers to reduce the sampling frequency of the analog-to-digital converter (ADC) used in the interrogation unit. Also, the proposed method detects the initial phase and the amplitude of the response signal.