Noise characterization of analog to digital converters for amplitude and phase noise measurements.

Improvements on electronic technology in recent years have allowed the application of digital techniques in phase noise metrology, where low noise and high accuracy are required, yielding flexibility in system implementation and setup. This results in measurement systems with extended capabilities, additional functionalities, and ease of use. In most digital schemes, the Analog to Digital Converters (ADCs) set the ultimate performance of the system; therefore the proper selection of this component is a critical issue. Currently, the information available in the literature describes in depth the ADC features only at frequency offsets far from the carrier. However, the performance close to the carrier is a more important concern. As a consequence, the ADC noise is, in general, analyzed on the implemented phase measurement setup. We propose a noise model for ADCs and a method to estimate its parameters. The method retrieves the phase modulation and amplitude modulation noise by sampling around zero and maximum amplitude, a test sine-wave synchronous with the ADC clock. The model allows discriminating the ADC noise sources and obtaining the phase noise and amplitude noise power spectral densities from 10 Hz to one half of the sampling frequency. This approach reduces the data processing, allowing an efficient ADC evaluation in terms of hardware complexity and computational cost.

Note: A dual-chip stroboscopic pulsed RADAR for probing passive sensors.

Stroboscopy provides an energy and computationally efficient means of sampling radiofrequency and microwave signals assumed to be reproducible under external excitation. While well known for impulse mode RADAR receivers, we here investigate its use for interrogating surface acoustic wave (SAW) transducers acting as passive cooperative targets. Amongst the originality of the implementation is the need to keep phase coherence between successive pulse generations which last up to tens of the radiofrequency periods to optimally transfer energy to the transducer. A two-chip receiver architecture is demonstrated, with a trigger signal compatible either with single-period avalanche transistor pulse excitation or frequency-agile direct digital synthesizer source.

Probing a dielectric resonator acting as passive sensor through a wireless microwave link.

Dielectric resonators, generally used for frequency filtering in oscillator loops, can be used as passive cooperative targets for wireless sensor applications. In the present work, we demonstrate such an approach by probing their spectral characteristics using a microwave RADAR system. The unique spectral response and energy storage capability of resonators provide unique responses allowing to separate the sensor response from clutter. Although the dielectric resonator is not designed for high temperature sensitivity, the accurate determination of the resonance frequency allows for a remote estimate of the temperature with Kelvin resolution.

Local oscillator phase noise limitation on the resolution of acoustic delay line wireless passive sensor measurement.

The role of the phase noise of a local oscillator driving a pulsed-mode RADAR used for probing surface acoustic wave sensors is investigated. The echo delay, representative of the acoustic velocity, and hence the physical quantity probed by the sensor, is finely measured as a phase. Considering that the intrinsic oscillator phase fluctuation defines the phase noise measurement resolution, we experimentally and theoretically assess the relation between phase noise, measurement range, and measurand resolution.

Fast contactless vibrating structure characterization using real time field programmable gate array-based digital signal processing: demonstrations with a passive wireless acoustic delay line probe and vision.

Vibrating mechanical structure characterization is demonstrated using contactless techniques best suited for mobile and rotating equipments. Fast measurement rates are achieved using Field Programmable Gate Array (FPGA) devices as real-time digital signal processors. Two kinds of algorithms are implemented on FPGA and experimentally validated in the case of the vibrating tuning fork. A first application concerns in-plane displacement detection by vision with sampling rates above 10 kHz, thus reaching frequency ranges above the audio range. A second demonstration concerns pulsed-RADAR cooperative target phase detection and is applied to radiofrequency acoustic transducers used as passive wireless strain gauges. In this case, the 250 ksamples/s refresh rate achieved is only limited by the acoustic sensor design but not by the detection bandwidth. These realizations illustrate the efficiency, interest, and potentialities of FPGA-based real-time digital signal processing for the contactless interrogation of passive embedded probes with high refresh rates.

Remote vibration measurement: a wireless passive surface acoustic wave resonator fast probing strategy.

Surface acoustic wave (SAW) resonators can advantageously operate as passive sensors which can be interrogated through a wireless link. Amongst the practical applications of such devices, structural health monitoring through stress measurement and more generally vibration characteristics of mechanical structures benefit from the ability to bury such sensors within the considered structure (wireless and battery-less). However, measurement bandwidth becomes a significant challenge when measuring wideband vibration characteristics of mechanical structures. A fast SAW resonator measurement scheme is demonstrated here. The measurement bandwidth is limited by the physical settling time of the resonator (Q/π periods), requiring only two probe pulses through a monostatic RADAR-like electronic setup to identify the sensor resonance frequency and hence stress on a resonator acting as a strain gauge. A measurement update rate of 4800 Hz using a high quality factor SAW resonator operating in the 434 MHz Industrial, Scientific and Medical band is experimentally demonstrated.

Note: A frequency modulated wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement.

We demonstrate the wireless conversion of frequency modulation to amplitude modulation by radio frequency resonators as means of accurately determining the resonance frequency of passive acoustoelectronic sensors. The emitted frequency modulated radio frequency pulses are generated by a pulsed radar for probing a surface acoustic wave based sensor. The sharp sign transition of the amplitude modulated received signal provides a signal on which a feedback loop is locked to monitor the resonance signal. The strategy is demonstrated using a full software implementation on a generic hardware, resulting in 2 Hz resolution at 1 s integration time limited by the proportional feedback loop.

A wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement.

Monitoring physical quantities using acoustic wave devices can be advantageously achieved using the wave characteristic dependence to various parametric perturbations (temperature, stress, and pressure). Surface acoustic wave (SAW) resonators are particularly well suited to such applications as their resonance frequency is directly influenced by these perturbations, modifying both the phase velocity and resonance conditions. Moreover, the intrinsic radio frequency (rf) nature of these devices makes them ideal for wireless applications, mainly exploiting antennas reciprocity and piezoelectric reversibility. In this paper, we present a wireless SAW sensor interrogation unit operating in the 434 MHz centered ISM band--selected as a tradeoff between antenna dimensions and electromagnetic wave penetration in dielectric media--based on the principles of a frequency sweep network analyzer. We particularly focus on the compliance with the ISM standard which reveals complicated by the need for switching from emission to reception modes similarly to radar operation. In this matter, we propose a fully digital rf synthesis chain to develop various interrogation strategies to overcome the corresponding difficulties and comply with the above-mentioned standard. We finally assess the reader interrogation range, accuracy, and dynamics.

Vibration amplitude of a tip-loaded quartz tuning fork during shear force microscopy scanning.

This Note reports on experimental results obtained with a recently published vision method for in-plane vibration measurement [Sandoz et al., Rev. Sci. Instrum. 78, 023706 (2007)]. The latter is applied to a tip-loaded quartz tuning fork frequently used in scanning probe microscopy for shear-force monitoring of the tip-sample distance. The vibration amplitude of the tip-loaded prong is compared to that of the free one and the damping induced by tip-surface interactions is measured. The tuning-fork behavior is characterized during approaches from free space to surface contact. Tip-surface contact is clearly identified by a drastic reduction in the prong vibration amplitude. However, no differences were observed between hydrophilic and hydrophobic surfaces. Experiments reported here show that the vibration amplitude of the quartz tuning fork in free space is a good estimate of the vibration amplitude of the tip interacting with the sample surface during shear force sample-tip feedback. The experimental setup for measuring the amplitude is easily integrated in an inverted microscope setup on which the shear force microscope is installed for simultaneous scanning probe and optical microscopy analysis of the sample.