High-Speed Reader for Wireless Resonant SAW Sensors V. Kalinin, J. Beckley, I. Makeev Transense Technologies plc Bicester, Oxfordshire, UK victor.kalinin@transense.co.uk Abstract—A new reader is presented that is based on a narrowband pulsed excitation of two SAW resonators contained in a differential wireless resonant sensor such as a torque senor. Simultaneous excitation of the resonators working at 433-437 MHz allows achieving a flat system frequency response up to Nyquist frequency. An interrogation algorithm and a structure of the reader ensure a torque sampling rate of at least 16 kHz and the system bandwidth of at least 8 kHz achievable at the processor clock of 150 MHz. Systematic errors caused by mutual interference of the two SAW responses, intermodulation products and aliasing are discussed. Random errors caused by the phase noise of the local oscillators are analyzed theoretically. Experimental results are also presented. I. INTRODUCTION Wireless resonant SAW sensors have already found a number of applications in industry to measure temperature in harsh environment, pressure and temperature in car tires and torque in various automotive systems. A number of wireless readers were developed to interrogate the resonant SAW sensing elements. The first designs [1] relied on a close coupling between the sensor antenna and the reader antenna that allowed using the SAW devices as frequency selective elements in the feedback loop of the oscillators contained in the reader. The main problem with this approach was a high variability of the RF coupling between the sensor and the reader that made it difficult to achieve a stable oscillation. More reliable readers were built using CWFM signals and automatic frequency tracking loops [2, 3]. Their interrogation bandwidth was limited by the time constant of the loop. The achieved figures were around 0.6 ms ensuring the sensor bandwidth just over 1.5 kHz. The use of the continuous interrogation signals in wireless readers either dictates implementation of the front-end circuit in the form of a high-quality directional coupler or a circulator, or requires close coupling between the reader and the sensor antennas. Only in this case a sufficient separation between the interrogation signal and the resonator response is achieved. Pulsed signals are much better suited for wireless interrogation at larger distances between the resonant sensor and the reader antennas. One of the proposed designs was based on the use of a gated PLL in the receiver to track the resonant frequency [4]. This relatively cheap reader had one serious disadvantage – it required the use of several gated PLLs in the case of several resonators per sensor. Even relatively simple temperature sensors usually contain two resonators since differential measurement helps cancelling frequency shifts caused by a variable impedance of the sensor antenna [5] and reduces influence of aging. The number of resonators in a temperature compensated sensor for other physical quantities can be larger than two, up to five as in the case of a torque sensor [6]. Emergence of fast DDS synthesizer chips with a small frequency increment has simplified pulsed interrogation of several resonators in a sequential way, one after another. The usual approach is to launch the RF pulse sufficiently long to reach the steady-state amplitude of oscillation in the resonator and measure free oscillations radiated by the sensor antenna after the interrogation pulse is over. This measurement is repeated at several different frequencies around the resonant frequency and, based on the measured amplitude/phase of the SAW response, the resonant frequency is estimated [7, 8]. The speed of interrogation is limited by the time constant of the resonator loaded with the antenna, τ = Q/πf = 7…15 μs at f = 434 MHz for the Q-factor Q = 5000…10000. Using the RF pulse width of around 3τ to reach the steady state and the same time to let the free oscillations decay before the next pulse is launched, it takes around 50…100 μs to obtain a reading at one interrogation frequency. The number of the interrogations required for determining one resonant frequency depends on the tracking algorithm implemented in the reader. It was suggested to use three interrogations in [9] and two interrogations in [10] so the minimum time required for measuring one resonant frequency can be as small as T 1 = 100…200 μs in this case. The problem with the above approach is that the tracking bandwidth Δf tr is limited by the frequency step and the number of interrogation points needed to update the frequency reading. For two or three points it is of the order of f/2Q = 20…40 kHz. Quite often, the peak variation f m of the resonant frequency in SAW strain sensors is an order of magnitude higher, 200...400 kHz. This means that the highest frequency