High-Speed Resonant Surface Acoustic Wave Instrumentation Based on Instantaneous Frequency Measurement

Reader Architectures for Wireless Surface Acoustic Wave Sensors

“…Recently, a further reader concept for resonant SAW sensors was proposed: it is based on time domain sampling with an (analog) instantaneous frequency measurement of the sensor’s response signal using a six-port microwave interferometer and a delay line [ 93 , 94 , 95 ]. The most notable advantage to other TDS architectures is that it measures the resonance frequency directly from a single interrogation but without the need to calculate the arithmetic-intensive FFT or other frequency estimation algorithms.…”

“…It is still used today in defense applications for real-time frequency identification of unknown signals over a very wide bandwidth but typically in a fully digital implementation [ 99 ]. However, especially the analogue approach, combined with digital compensation of non-idealities, is very promising for industrial sensor technology, such as wireless SAW readers, as it provides high measurement update rates at comparably low hardware costs [ 95 ]. Although the IFM concept has been used in military receivers since the 1950s, it was the work of Engen and Hoer in the 1970s [ 100 , 101 ] that made the six-port well-known for other civil applications.…”

“…Since the concept can only measure a phase difference and not an absolute phase, this calculation becomes ambiguous when the effective length of the delay line is longer than the wavelength of the highest frequency to be measured [ 98 ]. However, it has been shown that this is not a practical limitation for SAW readers as only a very small unambiguous measurement range is generally required for resonant sensors [ 95 ]. The above formulas are all based on idealized conditions such as perfectly matched components with no imbalances and completely linear power detectors that cannot be achieved in reality due to manufacturing tolerances, temperature influences and aging.…”

“…When a SAW resonator at 2.4 GHz, with a maximum frequency deviation of 1 MHz, is to be determined with an accuracy of 1% of the measurand, an RF frequency accuracy of approximately four parts per million (ppm) is required. Differential measurement can reduce this requirement [ 95 ], but it still remains a challenge. Possible forms of implementation for delay lines, e.g., as SMA cable [ 118 ], PCB-based [ 119 ] or as SAW delay line [ 120 ] all have a temperature sensitivity, typically in the two-digit ppm range.…”

“…IFM is a very new architecture for which the first detailed investigations were published in [ 93 , 94 , 95 ]. Measurement times of 3 µs for the interrogation of a single resonator at 2.4 GHz SAW with a quality of approximately 2300 could be demonstrated and the precision in a differential measurement was approximately 1% [ 95 ]. However, an oscilloscope with a limited resolution was used to acquire the baseband signals and the measurement time was also significantly shorter than with readers in the 433 MHz ISM band.…”