IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 25, NO.

9, SEPTEMBER 2015 621

Liquid Sensing Using Active Feedback

Assisted Planar Microwave Resonator
Mohammad H. Zarifi, Member, IEEE,
Samira Farsinezhad, Karthik Shankar, and Mojgan Daneshmand, Senior Member, IEEE

Abstract—A novel electromagnetic sensor operating at mi- Among different types of microwave sensors, planar mi-
crowave frequencies with quality factor of 22,000 at 1.4 GHz for crostrip resonators demonstrate additional advantages of
real-time sensing of fluid properties is presented. The core of the simplicity, ease of implementation and an inherent planarity
sensor has a planar microstrip resonator, which is enhanced using
(for analyte sampling) in their structure, which make them
an active feedback loop. The resonance frequency and quality
factor of the sensor show clear differentiation between analytes more attractive as sensor platforms [7]. They have been used
composed of common solvents. To evaluate the sensor for water for sensing gas [8], [9], viscosity [10], and several other
based concentration detection, we have demonstrated that KOH physical variables. One of the highlights of such sensors is
dilutions as low as 0.1 mM are detectable. The proposed sensor their amenability to microfluidic channels that provides the
has advantages of inexpensiveness and high resolution as well as opportunity of manipulation of liquids in a microscale structure
capability for miniaturization and CMOS compatibility. while sensing. Having microfluidic channels also provides a
Index Terms—Active device, liquid sensing, microwave passive constant flow of the target liquid and allows precise automated
resonator, S-parameters and high resolution sensing. fluid delivery with reduced reagent consumption in an enclosed
and potentially low-cost system [11].
Although planar microstrip resonators have demonstrated
I. INTRODUCTION great potential in sensing of liquid materials, they suffer from
low resolution and quality factor. Quality factor in any type

M ICROWAVE resonator sensors have demonstrated

major advantage of contact-less sensing for dif-
ferent applications ranging from biomedical diagnostics and
of resonant sensor, plays a critical role since it determines the
sensitivity, resolution and minimum detectable signal [12]. A
microwave device with high quality factor can lead to a sensing
microfluidics to sensors for the food industry and for envi- platform with higher sensitivity and resolution with respect to
ronmental monitoring [1]–[4]. The microwave sensors bring its conventional counterparts.
the advantages of cost, simplicity, design flexibility and speed Different techniques have been employed for increasing the
associated with label free detection and real time sensing. quality factor of the planar resonators [13], [14]. Among these
In particular, microwave sensing-based assays benefit from techniques, having an active feedback loop around the resonator
minimal sample preparation, fast and precise determination demonstrates promising results for increasing the quality factor.
of the changes in dielectric properties due to the target in- This technique has already been applied to communication ap-
teracting with the resonator, and the elimination of assay plications [14]. However, the knowledge regarding its behavior
complexity, binding changes and costs associated with con- for sensing application is very limited and more specifically, no
jugated markers [5]. These advantages of microwave sensors results have been reported for liquid sensing.
are particularly important for pH and pOH sensing wherein In this work, we report a state of the art sensing platform
electrochemical, ion-sensitive field-effect transistor (ISFET) based on a microwave resonator, which is reinforced by an ac-
and colorimetric/fluorimetric sensors have very short operating tive feedback loop to significantly enhance the quality factor. In
lifetimes in particle-rich and corrosive environments due to comparison with previous reported work [15], this device has
fouling, clogging and dye-leaching issues [6], [7]. Microwave a very high quality factor and a very high resolution since it
sensors also have advantage of CMOS compatibility and minia- employs an active feedback loop comparing to a pure passive
turization which make them more attractive for lab-on-a-chip structure reported in [15]. The proposed device demonstrates
applications [6]. high sensitivity not only to different types of liquids (such as
methanol, ethanol, isopropanol, and water), also to small con-
Manuscript received February 06, 2015; revised April 13, 2015; accepted centration variation. It has been shown that the concentration
May 20, 2015. Date of publication July 14, 2015; date of current version
September 01, 2015. of water based solutions can be sensed. In the proposed device,
M. Hossein Zarifi, S. Farsinezhad, and M. Daneshmand are with the De- complexity and cost of fabrication are kept low while the per-
partment of Electrical and Computer Engineering, University of Alberta, Ed- formance is improved tremendously.
monton, AB, T6G 2R3 Canada (zarifidi@ualberta.ca; samira.farsi@ualberta.ca;
kshankar@ualberta.ca; daneshmand@ualberta.ca).
K. Shankar is with the Department of Electrical and Computer Engineering, II. SENSOR ARCHITECTURE AND THEORY OF OPERATION
University of Alberta, Edmonton, AB, T6G 2R3 Canada and also with the Na-
tional Institute for Nanotechnology, National Research Council, Edmonton, AB, The sensor architecture consists of a planar ring resonator
Canada.
Color versions of one or more of the figures in this paper are available online
microwave microstrip resonator. The length of the microstrip
at http://ieeexplore.ieee.org. line determines the resonance frequency of this resonator. This
Digital Object Identifier 10.1109/LMWC.2015.2451354 resonator is a half-wavelength resonator and the length of the

1531-1309 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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622 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 25, NO. 9, SEPTEMBER 2015

microstrip line associated with the resonance frequency can be

determined using the following equation:

(1)

where and are the total length of the resonator and the
effective permittivity of the materials in the sensor ambient
respectively, is the velocity of light and is the resonance
frequency. This conventional structure has a moderate quality
factor of 220(in measurement) and 240 (in simulation). To
increase the quality factor of the resonator, a feedback loop
with an active device (BJT-Transistor) is added to the passive
resonator. This regenerative feedback loop creates 180 degree
phase shift on its output and another 180 degree phase shift is
introduced by the passive resonator, therefore a constructive
(positive) feedback is created around the passive resonator
which compensates the power loss and increases the quality Fig. 1. Schematic and Implemented low cost, high precision liquid sensor,
factor. The loss (positive resistance) of the resonator can be E-field simulation is presented for demonstrating the hot spots of the resonator.
completely compensated by the negative resistance provided
by the regenerative feedback if the gain of the amplifier is
driven from (2) [15]

(2)

where is the gain of the transistor, and are the quality

factors for the coupling between the resonator and the feed-
back loop and is the quality factor of the passive resonator.
Having active feedback loop design with significantly enhanced
quality factor is expected to enable high resolution sensing.
Fig. 1 shows the resonator schematic with a microfluidic tube
on its surface as well as physical realization of the sensor. The
designed microwave resonator is implemented on a low dielec-
tric-loss substrate 5880 from Rogers Corporation. Both sides
of the substrate were initially covered by 35 m copper layers
with a conductivity of 5.8 10 S m ; the dielectric constant
and the loss tangent of the substrate are and 0.0003
respectively. NE680, a low noise, high gain and low cost tran-
sistor with a typical cut-off frequency of 10 GHz at 10 mA bias
current from California Eastern Laboratories (CEL) is used as Fig. 2. S21 parameter of the designed sensor for the different liquids (a) active
an active amplifier in the feedback loop. DC bias couplers to mode (b) passive mode.
provide bias for the transistor are high frequency high quality
inductors (18 nH). A microfluidic tube with inner diameter of ison to the passive resonator response wherein the active loop
0.4 mm is fixed on the surface of the sensor with a strong scotch is off [Fig. 2(b)].
tape. The flow path of the fluid chosen in Fig. 1 is designed to The difference in permittivity and in the loss creates
coincide with areas of high field intensity in order to maximize differences between the S-parameters of these liquid samples.
interaction with the microwaves. The difference in electromagnetic properties of the material in
the tube is transferred to the frequency variation.
III. MEASUREMENT RESULTS AND DISCUSSION The high Q factor of the proposed sensor also enables high-
To experimentally verify the proposed sensor, two sets of resolution measurement such as concentration tests. Therefore,
measurements are presented; i.e., different liquid sensing, and in addition to being able to differentiate between solvents, the
concentration detection. For the first experiment, the tube is reported sensor is also used for concentration measurements of
filled by five different liquids, namely methanol soluble materials in solvents for which we use KOH in water as
, ethanol , isopropanol a prototypical example.
, and deionized (DI) water Fig. 3(a) shows the S21 parameter for the bare resonator,
and the S21 profile of the sensor is measured using a vector net- while different concentrations of the liquid flowed in the tube.
work analyzer (VNA-E8362) from Agilent. The results show a Resonance frequency and quality factor study of the sensor for
quality factor of 22 K for the bare sensor. Fig. 2(a) presents the KOH concentration of 0.125 to 100 mM diluted in water is re-
results of S-parameter measurement for different liquids inside ported in Fig. 3(b). It is shown that increasing the concentra-
the tube. A very clear and distinct difference is observed be- tion of the analyte reduces the resonance frequency and enables
tween different liquids for the active feedback case in compar- the detection of various concentrations. In addition, the quality

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ZARIFI et al.: LIQUID SENSING USING ACTIVE FEEDBACK ASSISTED PLANAR MICROWAVE RESONATOR 623

crostrip microwave resonator at 1.4 GHz with the initial quality

factor of 200. This resonator is assisted by an active feedback
loop with an increased measured quality factor of 22000 for bare
resonator with a liquid tube on its surface. Based on this tech-
nique, a clear and distinguishable difference between different
liquid samples and different concentrations of KOH in water
solution has been observed. This technique for liquid sensing
enables very small concentration measurements of different ma-
terials while preserving the simplicity in fabrication and inex-
pensiveness. To our knowledge, this is the first time that RF res-
onator for liquid sensing for such high resolution has been re-
ported.

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