Abundant research reported in the literature has indicated that broadband dielectric spectroscopy (BDS), i.e., the measurement of material permittivity versus frequency, can serve a broad range of applications, including, but not limited to, biomedical, food, automotive, and agricultural industries. Adopting this technique in real-life application scenarios is directly dependent on the miniaturization of bulky measurement setups, currently in use for these (prototype) sensing systems. At the same time, a highly sensitive and precise permittivity readout is essential to distinguish between different materials or track variations in the material state composition. This work focuses on developing ultra-compact sensing elements, readout electronics, and measurement techniques to determine the localized complex permittivity with high accuracy, sensitivity, and spatial resolution at microwave operation frequencies. Firstly, various sensing elements and high-resolution measurement setups are discussed for their compatibility with CMOS integration. Application scenarios are directed towards the characterization of low-loss materials, which often present much higher impedance than the currently 50-Ω oriented measurement setups. An I/Q-mixer-based interferometric technique is introduced to re-normalize the readout system reference impedance and improve the measurement sensitivity at high-impedance loads. Experimental results underline the potential of this technique. However, its compatibility with CMOS technology to enable small-factor systems is challenging at the intended frequencies of operation. Therefore, a double-balanced, RF-driven Wheatstone bridge with programmable branch impedance implemented in CMOS technology is proposed and analyzed for the high-resolution measurement of high-impedance loads (chapter 2). Next, a high-sensitivity, ultra-compact BDS sensor system is introduced for localized permittivity sensing. As a sensing element, it utilizes a metal patch that performs the actual sensing by presenting permittivity-dependent admittance. This patch is best implemented on the top metallization layer of a CMOS technology such that it can directly interface with the material-under-test (MUT). High measurement sensitivity is achieved by embedding the patch in a double-balanced, RF-driven Wheatstone bridge followed by a frequency down-converting mixer. By driving the bridge with a square wave, permittivity information can be acquired at the fundamental and subsequent harmonics. This concept allows increasing the measurement speed and, at the same time, provides an extended measurement frequency range (chapter 3). The measurement of the complex permittivity of materials is enabled by developing a dedicated calibration procedure for the patch-based BDS sensor. Measurement results of known liquids show good agreement with theoretical values in the literature, and the relative permittivity resolution in these measurements is better than 0.3 over a 0.1–10 GHz range. The proposed sensor implementation features a measurement speed of 1 ms and occupies an active area of only 0.15×0.3 mm^2, enabling the realization of very compact sensor arrays that can facilitate (real-time) 2-D dielectric imaging of permittivity contrast (chapter 4). Such a real-time BDS sensor array has been implemented as a 5x5 array, illustrating the scalability of the proposed patch-based BDS concept. This matrix has been demonstrated for its functionality by resolving spatial permittivity variations in the sub-mm range (chapter 5). Last, the findings and conclusions of this dissertation, and recommendations for future work, are discussed (chapter 6). ... Read more

In this article, we present an analytical formulation based on an equivalent circuit model to support the challenging task of designing and analyzing single-ended patch sensing elements to be integrated in planar technologies. The proposed approach further allows for differentiating the permittivity values of the individual layers when sensing over dense and stratified mediums. The equivalent model of the sensing pixel is derived resorting to equivalence theorem and transmission-line theory. The relative impact of the material under test and the metal thickness of the sensing element is accurately included in the evaluation of the endpoint load of the radial transmission line, equivalent to the patch radius. This approach of representing the single-ended sensing element isolates the capacitance contributions associated with the patch radius, patch thickness, and the medium under test. The computationally fast tool is further utilized in absolute permittivity measurements using a 0.14- μm CMOS 2-D permittivity imaging matrix prototype operating from 100 MHz to 2.9 GHz, reporting excellent agreement with theoretical values. ... Read more

We present a compact, scalable, and broadband architecture for the implementation of complex microwave permittivity sensors in complementary metal-oxide semiconductor (CMOS) technology. The proposed architecture consists of a patch sensor embedded in a programmable balanced readout bridge and performs third and fifth harmonic downconversion for fast multi-frequency readout. Circuits designed can act as the basic building block for a wide span of biomedical applications, ranging from wearables to permittivity imaging. Experimental results of manufactured prototypes demonstrate measurement noise reduction through bridge balancing, Debye model parameter estimation of independent material with a 1.6% error using full frequency dataset, and 5.3% in high energy efficiency mode, as well as image construction based on material permittivity differences. ... Read more

A compact sensing pixel for the determination of the localized complex permittivity at microwave frequencies is proposed. Implemented in the 40-nm CMOS, the architecture comprises a square patch, interfaced to the material-under-test sample, that provides permittivity-dependent admittance. The patch admittance is read out by embedding the patch in a double-balanced, RF-driven Wheatstone bridge. The bridge is cascaded by a linear, low-intermediate frequency switching downconversion mixer, and is driven by a square wave that allows simultaneous characterization of multiple harmonics, thus increasing measurement speed and extending the frequency range of operation. In order to allow complex permittivity measurement, a calibration procedure has been developed for the sensor. Measurement results of liquids show good agreement with theoretical values, and the measured relative permittivity resolution is better than 0.3 over a 0.1-10-GHz range. The proposed implementation features a measurement speed of 1 ms and occupies an active area of 0.15x0.3 mm², allowing for future compact arrays of multiple sensors that facilitate 2-D dielectric imaging based on permittivity contrast. ... Read more

A 0.14-μ m CMOS 2-D permittivity imaging matrix prototype operating at microwave frequencies is presented. It comprises 25 permittivity-sensing pixels, each consisting of a sensing patch connected to a dedicated RF bridge. A trans-conduct-ance stage converts the imbalance voltage to a current signal, subsequently down-converted to an intermediate frequency and sampled. The implemented sensor matrix shows precise permittivity measurements over a range of 0.1-10 GHz, and successfully demonstrates permittivity contrast with a resolution of 0.1 - 2.3 from 0.1 to 10 GHz when the matrix is interfaced with various dielectrics. Owing to the matrix implementation a sub-mm, permittivity discontinuity is easily resolved by the presented sensor device. ... Read more

This paper presents a 0.15×0.3 mm² complex permittivity sensor integrated in a 40-nm CMOS node. A single-ended patch, employed as a near-field sensing element, is integrated with a double-balanced, fully-differential tunable impedance bridge that is driven by a square RF pulse. The multi-harmonic, interme-diate-frequency down-conversion architecture achieves a compact form factor and fast multi-frequency readout. Measurement results show good agreement with theoretical values and the measured relative permittivity variation remains below 0.3 over a 0.1-10 GHz range at a 1-ms measurement time. The energy efficiency resulting from the fast measurement time and the record-small active area allows integration in battery-operated wearables. ... Read more

In this paper, we apply various area reduction techniques on an inductor–capacitor (LC)-tank oscillator in order to make its size comparable to that of ring oscillators (ROs), while still retaining its salient features of excellent phase noise and low sensitivity to supply variations. The resulting oscillator employs a proposed ultracompact split transformer topology that provides a 1:2 passive voltage gain and is less susceptible to common-mode electromagnetic interference than are regular high-quality-factor LC tanks, thus making it desirable in systemon-a-chip environments. The oscillator, together with a proposed dc-coupled buffer, is incorporated within an all-digital phaselocked loop (ADPLL) intended for wireline, digital clocking, and less stringent wireless systems. The ADPLL architecture introduces a look-ahead time-to-digital converter that exploits a deterministic phase prediction to reduce power consumption and phase detection complexity. The ADPLL is realized in 40-nm CMOS and has the smallest reported area of 0.0625 mm2 among LC-tank oscillators while providing fractional-N operation, wide tuning range of 45% (from 9.4 to 14.8 GHz), very low voltage supply sensitivity of 80 MHz/V, and integrated figure-of-merit jitter (FoMjitter) better than −230 dB. A separate identical ADPLL was implemented using an RO instead, for completeness and systematic comparisons. ... Read more

This paper presents a complex permittivity sensor, integrated in 40-nm CMOS, for microwave dielectric spectroscopy. It utilizes a single-ended patch as a near-field sensing element, embedded in a double-balanced, fully-differential impedance bridge. A low-IF, multi-harmonic down-conversion scheme is employed to extend the characterization frequency range and increase the measurement speed. The implemented architecture is compact, accurate and fast, thus suitable for the realization of future real-time, microwave-based, 2-D dielectric imagers. Measurements on liquids show an rms error of <;1% over a frequency range of 0.1 - 12 GHz. ... Read more