A WIRELESS ULTRAVIOLET SENSOR FOR HARSH
ENVIRONMENTS
By
RACHEL DUDUKOVICH
Submitted in partial fulfillment of the requirements
for the degree of Master of Science
Department of Electrical Engineering and Computer Science
CASE WESTERN RESERVE UNIVERSITY
January, 2015
CASE WESTERN RESERVE UNIVERSITY
SCHOOL OF GRADUATE STUDIES
We hereby approve the thesis/dissertation of
Rachel Dudukovich
candidate for the degree of Master of Science
Committee Chair
Christos Papachristou
Committee Member
Daniel Saab
Committee Member
Frank Merat
Date of Defense
November 3, 2014
2
Contents
Introduction ......................................................................................................................... 8 Motivation .................................................................................................................................................. 8 Problem Statement .................................................................................................................................. 9 Outline ......................................................................................................................................................... 9 Chapter 1 - Background..................................................................................................... 12 Current combustion control methods ............................................................................................. 12 SiC for Harsh Environments ............................................................................................................. 13 UV Sensing Capabilities of SiC ...................................................................................................... 14 Related Work ......................................................................................................................................... 15 Dual-SiC Photodiode Devices for Simultaneous Two-Band Detection .................... 15 Chapter 2 - Approach ........................................................................................................ 18 Use of Frequency Modulation to Convey Sensor Signal ........................................................ 18 Oscillator Circuits ................................................................................................................................ 19 Relaxation Oscillator ........................................................................................................................... 19 Differential Oscillator ......................................................................................................................... 21 Colpitts Oscillator ................................................................................................................................ 22 Clapp Oscillator .................................................................................................................................... 23 Chapter 3 - Simulation....................................................................................................... 24 Clapp Oscillator .................................................................................................................................... 24 Chapter 4 - Device Characterization ................................................................................. 27 Chapter 5 - Implementation ............................................................................................... 29 Silicon Prototype Design ................................................................................................................... 29 Sensor Selection and Characterization .......................................................................................... 30 Development of Silicon Carbide Based Oscillators .................................................................. 32 SiC Relaxation Oscillator Design ................................................................................................... 34 Sensitivity Characterization .............................................................................................................. 38 Circuit Power Consumption .............................................................................................................. 40 Telemetry Design ................................................................................................................................. 40 Summary ................................................................................................................................................. 44 Chapter 6 - Conclusion ...................................................................................................... 45 3
Bibliography ...................................................................................................................... 46 4
List of Figures
Figure 1. System View of UV Sensor Components .......................................................... 18
Figure 2. Relaxation Oscillation ........................................................................................ 20
Figure 3. Differential Oscillator ........................................................................................ 21
Figure 4. Colpitts Oscillator .............................................................................................. 22
Figure 5. Clapp Oscillator ................................................................................................. 23
Figure 6. Simulated Clapp Oscillator Circuit .................................................................... 25
Figure 7. Clapp Oscillator Simulation Output ................................................................... 25
Figure 8. Clapp Oscillator Fourier Transform ................................................................... 26
Figure 9. SiC Photodiode I-V Characteristics Under Illumination ................................... 27
Figure 10. Cadmium Sulfide Photoresistor I-V Characteristics Under Illumination ........ 28
Figure 11. Silicon relaxation oscillator and photodiode sensor ........................................ 30
Figure 12. Difference between photodiode capacitance under illumination and at dark. . 31
Figure 13. Current versus voltage of diode under UV illumination. ................................. 31
Figure 14. Cadmium-sulfide photocell I-V characteristics under illumination ................. 32
Figure 15. Clapp oscillator based UV sensor. ................................................................... 33
Figure 16. SiC differential oscillator. ................................................................................ 33
Figure 17. Final SiC relaxation oscillator based UV sensor. ............................................ 35
Figure 18. Relaxation oscillator circuit with Cree SiC MESFETs. ................................... 36
Figure 19. Initial ramp-up of relaxation oscillator. ........................................................... 37
Figure 20. The change in frequency under UVA, UVC, and darkness. ............................ 37
Figure 21. Relaxation oscillator output frequencies as a function of UV photon flux.. .... 38
Figure 22. Clapp oscillator output frequencies as a function of UV photon flux. ............ 39
5
Figure 23. The SiC relaxation oscillator frequency change under illumination................ 39
Figure 24. Receiver circuit design. .................................................................................... 41
Figure 25. The major components of the AD650 frequency to voltage converter . ......... 43
6
A Wireless Ultraviolet Sensor For Harsh Environments
Abstract
By
RACHEL DUDUKOVICH
This thesis discusses the development of an ultraviolet sensor for use in harsh
environments, in particular high temperature environments. The sensor is constructed of
silicon carbide components for this reason. The sensor creates a frequency modulated
signal that is based on the change in light spectrum. The signal is transmitted wirelessly
to a receiver circuit. Background information related to the sensor design is presented, as
well as design considerations that were explored during the sensor development. The
process of building the sensor is discussed, as well the resulting signal output
characteristics of the sensor when it is exposed to ultraviolet radiation.
7
Introduction
The subject of this thesis is a SiC-based wireless sensor that is designed to
monitor the combustion efficiency in jet turbines and combustion engines. In this type of
environment, the temperature may exceed 600 ˚C, and can be very corrosive, as well as
subject to noise. For these reasons, silicon carbide based components were selected for
use in the sensor circuit. Silicon carbide is a wide energy bandgap semiconductor and it
use in this type of environment has been well documented [1]. The sensor can be used to
determine the efficiency of combustion based on the ultraviolet emission intensity. The
circuit is comprised of a relaxation oscillator with silicon carbide metal–semiconductor
field effect transistors (MESFET) as the active components and a silicon carbide
photodiode as the sensing element. The photodiode is embedded within the oscillator
circuit and acts as a resistive element. As the light spectrum changes, the resistance of the
photodiode changes, thereby changing the frequency of oscillation. The sensor frequency
modulated RF signal is then transmitted using a dipole antenna. The signal is received
and converted to a DC voltage using a frequency to voltage converter so that it can easily
be measured with a standard data acquisition unit.
Motivation
The efficiency of combustion systems can be improved by implementing a
feedback based control loop, rather than open loop control [2]. In order to achieve closed
loop control of combustion processes, sensors capable of monitoring the exhaust gases,
UV and visible light emissions from the flame and other pertinent parameters must be
developed. The use of a feedback controller would allow for more efficient fuel
consumption and therefore reduce pollution and operation costs.
8
UV emissions can be used in combustion control systems to monitor fuel
efficiency. Chemiluminescence signals given off during chemical reactions taking place
during combustion could be used to estimate the fuel-air ratio of the combustion process
and could be used to control the injection of fresh gases to a desired set point [2].
Development of a sensor which takes advantage of the relationship between UV
emissions and the quality of gases being burned could be used to improve combustion
control methods.
Problem Statement
The goal of this thesis is to develop a wireless SiC-based UV sensor for use in
harsh environments, such as a combustion engine. This sensor could be mounted near the
combustion source, so it should be designed to be rugged and small in size. It is desired
to keep the number of circuit components to a minimum to allow for a simple design and
small footprint. The use for this sensor would be to allow for onboard monitoring of
combustion efficiency.
Outline
Chapter 1 - Background
•
Current combustion control methods - This section discusses the benefits
of closed loop control and the role sensors play in this.
•
Suitability of SiC for harsh environments – The properties of silicon
carbide as they pertain to high temperature and pressure electronics are
explained.
•
UV sensing capabilities of SiC – The UV sensing properties of silicon
carbide are discussed.
9
•
Related Work – Several other projects regarding silicon carbide
photosensors and silicon carbide oscillators are included as background
material.
Chapter 2 - Approach
•
Use of frequency modulation to convey sensor signal – The rationale of
using frequency modulation to transmit the sensor signal is explained.
•
Oscillator Circuits – The theory of different types of oscillator circuits is
presented.
•
Selection of UV sensitive components – The appropriateness of different
types of photosensors as it pertains to the circuit being developed is
explored.
Chapter 3 - Simulation
•
Comparison of different oscillator types – A brief discussion of
simulations of oscillator circuits is presented.
Chapter 4 - Device Characterization
This section provides data on the I-V characteristics of the different
photodetectors considered for use in the sensor circuit.
Chapter 5 - Implementation
•
Silicon Prototype Design – The development of a silicon based prototype
is explained.
•
Sensor Selection and Characterization – This section discusses the
rationale behind the photodetector selected for use in the circuit.
•
Development of Silicon Carbide Based Oscillators – Work that was
completed to develop an operational silicon carbide based oscillator is
presented.
•
SiC Relaxation Oscillator Design – The final design of the silicon carbide
based relaxation oscillator is discussed.
•
Sensitivity Characterization – The sensor’s response under UV light is
demonstrated.
10
•
Circuit Power Consumption – The power consumption of the circuit is
discussed, as well as possible methods for supplying power.
•
Telemetry Design – The receiver circuit design is presented.
Chapter 6 – Conclusion
The topics discussed in this paper and the results of the sensor
development are summarized.
References
11
Chapter 1 – Background
An overview of the use of optical sensors for combustion control methods is
given. The type of environment such a sensor would operate in is likely to exceed the
operating temperature of standard electronics. For this reason silicon carbide is an ideal
candidate for the circuit components of such a sensor. The properties are silicon carbide
related to operating temperature is discussed. Silicon carbide is also well known as a
photodetector and its properties as a UV radiation detector are detailed. Finally, related
work in high temperature silicon carbide oscillators is explored, as the RF generator is a
major component of the sensor circuit.
Current combustion control methods
Many combustion control processes still operate in an open loop method without
the feedback of relevant parameters to the injection system. Future advances to the
efficiency and performance of these systems could benefit greatly from a closed loop
system employing sensors to provide controllers with information to make operating
adjustments. Applications that would benefit from such feedback-based control include
automotive engines, biomass combustion systems and gas turbines. In gas turbines in
particular, a significant reduction of nitrogen oxide emissions has been achieved by
using a premixed mode of operation that allows the flame temperature to be reduced and
the level of nitrogen oxide be thereby also reduced. However, premixed combustion
requires a precise determination of the air-fuel ratio. Real-time measurements of the airfuel ratio would allow for improvements to this process [2].
Optical sensors may be used to monitor the light emitted by a flame. The light
12
spectrum from the flame is produced by chemical reactions occurring as the result of
combustion. This process called chemiluminescence produces electronically excited
radicals in the UV and visible range. Each radical produces a particular spectrum that is
related to its quantum properties and can be used as a means of identification [2]. In
addition, optical sensors have the advantage of not requiring direct contact with the
flame, can selectively sense a desired wavelength, are fast, and able to gather data even in
extremely hostile environments [3].
SiC for Harsh Environments
In most optical sensor applications, the sensor must be mounted within the line of
sight of the radiation source. If a sensor is to be mounted on-board in close enough
proximity to observe the combustion flame, the sensor must be designed to withstand
high temperatures. Most commercial and industrial sensors and electronics are rated to
operate up to a temperature of around 85 to 100 C. It is therefore necessary to utilize
components designed specifically with higher operating temperatures such as silicon
carbide based transistors and diodes. Silicon carbide is very frequently used in high
temperature applications due to several of its characteristics [1].
Silicon carbide has a wide bandgap, mechanical strength, high thermal
conductivity, high melting point and inertness to exposure in corrosive environments [4].
SiC does not melt, rather is sublimes at about 1800 ˚C. Silicon carbide is very scratch and
wear resistant and a Mohs hardness of 9. This is nearly as hard as diamond, which is
rated as a 10. Silicon carbide is also resistant to most acids. Due to all of these qualities
silicon carbide is often chosen for use in harsh environments. Applications for SiC
sensors include radio frequency devices, pressure sensors able to withstand high vibration
13
due to its hardness and strength, accelerometers in airplane engines and motors in harsh
environments, and optical sensors for control of light [1].
UV Sensing Capabilities of SiC
Silicon carbide has a wide bandgap (Eg=3.0 and 3.2 eV for 6H and 4H SiC), low
dark noise and low impact ionization ratio [1]. This makes it an excellent candidate for
photo detection. 3C-SiC has a bandgap of 2.3 eV which also it to be used to detect photon
in the 200-500nm range. 4H-SiC has an indirect bandgap of 3.23 eV which makes in
suitable to detect photons in the 220-380 nm range [4].
Electromagnetic radiation interacts with semiconductors through absorption
processes. Absorption is the relative decrease of the irradiance Ф, per unit length,
δΦ( x)
Φ
= αδx . The solution of this equation is given by Φ( x) = Φ 0 e −αx , where Φ 0 is the
incident irradiance, α is the absorption coefficient and x is a path length variable [5].
The photoelectric affect dominates for the ultraviolet spectrum (< 50 keV). Absorption by
the photoelectric effect results in a complete transfer of the electromagnetic radiant
energy from an incident photon to the interacting atom, which interjects a photoelectron.
The minimum photon energy required is approximately equal to the semiconductor
bandgap. Carriers already within the conduction band may absorb photons with energy
below the bandgap. However the probability of this is low. Photoelectric absorption
depends on the density of occupied states at the initial energy, the density of available
states at the final energy, and the transition probability. The absorption coefficient for
14
3
direct transition is
α (hv ) = A * (hv − E g ) 2
where
A* = q 2 (2mr )1.5 /(nch 2 me *) and
1 / mr = 1 / me * +1 / mh *. The electron, hole and reduced effective masses are given by
me *, mh *, mr respectively, n is the index of refraction, and E g is the bandgap [5].
Related Work
The related work presented in this section discusses the development of silicon
carbide photodiodes for UV detection and the use of silicon carbide transistors in high
frequency oscillators. This related work shows that silicon carbide MESFETs can be used
to create high frequency oscillators that are capable of operating at temperatures
exceeding 200 C. The circuit designs discussed below were instrumental in achieving
oscillation in the relaxation oscillator developed in this thesis. The designs below were
explored to gain knowledge about the biasing and start up conditions of a SiC based
oscillator.
Dual-SiC Photodiode Devices for Simultaneous Two-Band Detection
Silicon carbide is frequently used for UV detectors due to its large bandgap and
its ability to withstand harsh environments such as gas turbines [6]. Research conducted
by the GE Global Research group in Niskayuna, New York attempted to improve on
standard SiC photodiodes by making a single device with separate sensing components
which respond to multiple wavelength bands concurrently. 6H p-type substrates were
purchased from Cree Research. These substrates were approximately 350 µm thick,
having a resistance of approximately 1 Ω/cm. Epitaxy was then added with a 1 µm, ptype layer , a second p-type layer, 5 µm thick, and an n-type layer, 0.35 µm thick. In
15
order to create separate sensing components, reactive ion etching was first performed to
form trenches 1 µm deep between adjacent device components. As the p-n junction was
located approximately 0.25 µm below the surface, the trenches reduce unwanted
electrical cross talk between the filtered and unfiltered parts of the chip. Light-blocking
strips made from opaque layers of metal were then added to reduce absorption of UV in
the areas between adjacent fingers. Optical filters were then deposited directly over the
diodes and selectively etched to filter incident light to at least one of the device
components. The materials used for the filters were and, deposited in thin layers, for a
total thickness of approximately 3 µm [6].
An interdigitated geometry was employed to reduce sensitivity to chip
misalignment when placed in an optical system. Square chip sizes of 1, 1.5, and 2 mm on
a side were fabricated in order to determine the effect of area on overall performance [6].
The spectral responsivity curves of both diodes were measured using a 150 W Xe
lamp. A monochrometer was used to obtain individual response measurements with a
wavelength bandwidth less than 2 nm in the range between 200 and 400 nm. The
unfiltered diode had a peak responsivity of 290 nm and the filtered diode reached a peak
response at 320 nm that rapidly decreases at shorter wavelengths.
1-GHz, 200 C, SiC MESFET Clapp Oscillator
This paper discusses the design of an oscillator for use in high temperature environments.
The oscillator is intended for use as an RF signal generator in a frequency modulated
sensor circuit. The circuit uses Cree SiC MESFETs, ceramic chip capacitors, gold bond
wires and spiral inductor on an alumina substrate. The design is based on a Clapp
16
oscillator, which was selected since the frequency of oscillation is easily modulated based
on sensor capacitance. The circuit was tested at 30 C to 200 C [7].
High Temperature Performance of a SiC MESFET Based Oscillator
A differential oscillator was developed using Cree SiC MESFETs for pressure
sensors for use in high temperature environments. The oscillator consists of two crosscoupled SiC MESFETS, two LC tank circuits made of spiral inductors and chip
capacitors on an alumina substrate. The oscillator operates in the UHF band. The
substrate was placed on top of a ceramic heater and heated from 30 to 475 C while RF
measurements were taken. The biasing conditions were changed as needed to allow the
oscillator to function reliably up to 475 C [8].
17
Chapter 2 – Approach
The UV sensor design concept consists of an oscillator circuit and a UV photo detector.
The photo detector's resistance or capacitance will change when exposed to UV light
versus ambient light. This change in resistance or capacitance will cause a change in the
frequency of the oscillator. A dipole antenna is used to transmit the frequency signal to a
wireless receiver. The use of frequency modulation, oscillator design and photo detector
selection are discussed below.
Figure 1. System View of UV Sensor Components
Use of Frequency Modulation to Convey Sensor Signal
Frequency modulation was determined to be the best way to convey that the
sensor has detected UV light. The sensor changes the output frequency based on exposure
to darkness, ambient light, UVA and UVC spectrums. Frequency modulation is more
resilient to noise since most noise is amplitude based. Frequency modulation is also more
resistant to signal strength variations [9]. Both of these advantages allow for a simpler
circuit design. An oscillator circuit with relatively few components can be used for the
basis of the sensor circuit design. The effects of UV radiation on the sensing component
18
can then change the frequency of the oscillator. A simple dipole antenna can be used to
transmit the signal.
Oscillator Circuits
There are many types of oscillator circuits. For the purpose of creating a sensor,
the oscillator frequency would need to change with respect to changes in either a resistive
of capacitive component. The reason for this is that most photosensitive elements either
display a change in their resistance or capacitance. Several types of oscillators were
considered.
Relaxation Oscillator
The relaxation oscillator is a rather simple circuit consisting of two transistors,
three resistors, a capacitor and voltage source, as shown in Figure 2. Capacitor C1 and
resistor R2 control the frequency of oscillation. The basic principle of the relaxation
oscillator is the charging and discharging of C1. It is designed such that a current charges
the capacitor until a positive voltage threshold is obtained and the direction of the current
is reversed. Next, the negative threshold voltage is reached and the current changes
direction again. The process is repeated over and over again to produce the oscillations
[10]. In relaxation oscillators, the transistors act like on-off switches. Two regions of
operation determine the timing of the oscillation period. The first occurs when the
devices are operating in the active region or negative conductance region. The waveform
19
displays an exponential growth, rather than a slower growing sinusoid. This response is
referred to as the ‘regenerative-switching’ mode. The second stage of operation is called
the recovery mode. In this case, both transistors are in the normal operating region. A
small voltage or current excitation is plied to the base of J1 and is amplified by J1 with a
phase change. This is coupled directly to the base of J 2 where another amplification and
phase change occurs which leads to positive feedback at the base of J 1. Regenerative
build-up occurs if the loop gain is greater than one until one of the transistor leaves its
active region and enters saturation or cutoff. Regenerative switching is generally very
fast, and the voltages of the coupling capacitors do not change significantly. When one of
the transistors is cutoff, the biasing of the circuit attempts to return to the state of the
circuit without capacitors. This time regenerative switching will occur in the opposite
direction due to the fact that the saturated transistors re-enter the active region and the
loop gain is again greater than unity. This causes another and restarts the entire the cycle.
This cycling will produce a steady-state oscillation [11].
R1
R3
732
2.7k
V1
4.5V
J2
C1
68p
J1
R2
69.1k
0
Figure 2. Relaxation Oscillation
20
Differential Oscillator
The tail-current biased differential LC oscillator consists of a differential pair,
which commutates a tail current and creates an effective negative resistance across a
resonator. The differential inductors provide a higher Q while taking up less area than
single-ended inductors. Oscillation occurs when energy in the inductor is balanced by the
energy in the capacitor. The frequency of oscillation is determined by the resonator [12].
The amplitude of oscillation is proportional to the tail current. However if the current
source is increased until the transistor is driven into the triode region, the oscillation
amplitude is clipped at 2 Vdd. At this point, the current source is supplying as much
current as the oscillator can support. When this happens the current source can be thought
of as a short to ground. This region of operation is called supply limited. The linear
region of operation is called current limited [12].
L2
L4
60uH
C2
60uH
C4
3p
V2
4Vdc
3p
C1
C3
3p
J2
3p
J3
L1
L5
10uH
10uH
R1
10
V3
7.5Vdc
0
Figure 3. Differential Oscillator
21
Colpitts Oscillator
The Colpitts oscillator is a fairly simple and robust oscillator. It frequency of
oscillation is determined by two capacitors and one inductor. A voltage divider made by
the two capacitors provides the feedback that is necessary for oscillation. In a Colpitts
oscillator, the transistor conducts for a very short duration, which causes an impulse of
negative resistance current. The amplitude builds up in a Colpitts oscillator as follows:
When M1is off, the current source charges C1until the gate-source voltage of M1exceeds
VT . M1 then conducts and the resulting current reaches C1and C2. C2 recharges and the
transistor is shut off. This results in a train of narrow current pulses that are T seconds
apart, where T is the period of oscillation [12].
VCC_WAVE
L1
M1
VCC
C1
M2
VCC_BAR
C2
Figure 4. Colpitts Oscillator
22
Clapp Oscillator
Vd
L2
4 Vdc
0.5uH
L1
CT
0.5uH
15p
LT
0.5uH
J1
15p
C2
3p
Vg
6 Vdc
C3
C1
J2N3819
R1
50
3p
0
Figure 5. Clapp Oscillator
The Clapp oscillator is a variable frequency oscillator that is tuned with a series
LC tank rather than with a parallel tank as in the Colpitts or Hartley oscillator. The Clapp
oscillator is related to the Colpitts oscillator but it has an additional capacitor placed in
series with the inductor. A typical Clapp oscillator consists of a single inductor and three
capacitors. Two of the capacitors (C1 and C2) form a voltage divider that determines the
feedback voltage applied to the transistor input. A Clapp oscillator has some advantages
over the Colpitts oscillator when used as a variable frequency oscillator. In the Colpitts
circuit, the voltage divider contains the variable capacitor. This also causes the feedback
voltage to be variable, making it difficult to achieve oscillation over a portion of the
desired frequency range. This problem can be avoided by using fixed capacitors in the
voltage divider and a variable capacitor (CT) in series with the inductor for the LC tank
[12].
23
Chapter 3 – Simulation
Simulations were performed to help to determine a suitable oscillator design and
determine component values. Orcad PSpice was used to develop the simulations [13].
The circuit simulations were used to develop prototype silicon oscillators prior to
developing a silicon carbide based implementation. This was done so that cheaper,
readily available components such as silicon JFETs could be used until a promising
design was found that would then later be developed with more expensive, longer lead
time silicon carbide MESFETs. The JFETs used in the simulations were J2N4393. The
circuits selected for simulation were a Clapp oscillator and a relaxation oscillator.
Clapp Oscillator
A Clapp oscillator simulation was developed and produced. The oscillator
frequency is around 60 kHz. Possible options for placement of the photodiode were in
series with L2 and C1, or in series with R1. However, this circuit was deemed unsuitable
since the resistance of R1 required a high value of 1 Mohm or greater to produce
oscillations, further more changes in this resistance did not significantly impact the
frequency of oscillation. The diode’s capacitance does not change enough under UV
illumination to produce changes in oscillation. Figure 6 shows the Clapp oscillator circuit
that was simulated. A voltage source with a short pulse in the initial start up was used to
help set up the oscillations in circuit.
24
V1
L1
TD = .01u
TF = .01u
10uHPW = 1
PER = 2
V1 = 0
TR = .01u
V2 = 5
-1.066e-30V
1u
L2
0V
1uH
V
J1
0V
J2N4393
R1
10M
0
Figure 6. Simulated Clapp Oscillator Circuit
Figure 7. Clapp Oscillator Simulation Output
The waveform in Figure 7 has a time scale of 2 us per division on the x axis and 1 V per
division on the y axis. Figure 8 shows the Fourier transform of the oscillator frequency
output. The x axis based on 20 kHz per division and the y axis is 0.5 V per division.
25
Figure 8. Clapp Oscillator Fourier Transform
While several LC type oscillators were simulated successfully, the RC based
relaxation oscillator could not be made to produce oscillations in PSPICE. This is thought
to be due to the fact that the necessary start up conditions for oscillation cannot be
achieved in PSPICE. Several attempts were made to start up the oscillations with an
initial pulse of the power supply, as well as modifying the initial conditions of the
capacitor. It is sometimes suggested the circuit noise helps to start the oscillations in an
actual circuit, which is lacking in the SPICE simulation. It should be noted that the actual
circuit was built and oscillates with the parameters used in the simulation, simply
oscillation through the simulation process itself could not be achieved.
26
Chapter 4 - Device Characterization
Photodiodes and photoresistors where characterized under darkness, UVA and
UVC illumination. The differences in the I-V characteristics are shown below. It can be
seen that the SiC photodetector is more sensitive to the changes in the UV spectrum and
also can distinguish between ambient and white light better than the photoresistor. For
this reason, as well as the fact that the SiC photodetector is much more tolerant of high
temperature environments, it was determined that the photodetector was the better choice
for use in the UV detector circuit.
Figure 9. SiC Photodiode I-V Characteristics Under Illumination
27
Figure 10. Cadmium Sulfide Photoresistor I-V Characteristics Under Illumination
28
Chapter 5 – Implementation
This section discusses the general design of the UV sensor as well as preliminary
work such as the development of a silicon based prototype design and circuits built as a
study of SiC oscillators. The final oscillator design is discussed, as well as the wireless
transmitter and receiver circuits. It was determined that the relaxation oscillator was the
most suitable as a basis for the sensor since it had few components, was a simple design
and the frequency is dependent on a resistive component.
Silicon Prototype Design
A prototype of the relaxation oscillator was first constructed with silicon JFETs.
JFETS were selected due to their similarity to the MESFETs that would eventually be
used in the SiC design. This prototype provided approximate values for the input voltage,
resistors and capacitor suitable for the SiC relaxation oscillator design. The prototype was
also used to experiment with where to embed the photosensor within the circuit. SiC
photodiodes, cadmium sulfide photo-resistors, and silicon photocells were tested for UV
sensitivity in the relaxation oscillator circuit. It was found that the largest frequency
differential could be obtained by placing the SiC photodiode between the gate of the
JFET in the first stage of the circuit and ground. This allowed the time constant of the
oscillator to change appreciably but not so much as to cause the oscillations to become
unstable. Once the prototype was constructed in silicon, several attempts to transform the
design to SiC were made. It was found that the biasing conditions necessary to produce
oscillations in the SiC design were quite different from those of the silicon relaxation
29
oscillator. Figure 11 shows the schematic for the original silicon prototype.
R1
732
V1
4Vdc
68p
R2
2.7k
C1
Figure 11. Silicon relaxation oscillator and photodiode sensor
Sensor Selection and Characterization
Several options for the ultra-violet sensing component were explored. The first
choice was a silicon carbide photodiode. The diode’s change in capacitance while in
darkness and under UV illumination was compared. The difference was less than a
picofarad as can be seen in Figure 12. The diode showed much greater sensitivity when
used as a resistive component as shown in the I-V characteristics when in darkness and
when illuminated by a MAPP (methylacetylene-propadiene propane)torch. MAPP gas
produces approximately the same amount of UV radiation as the combustion of jet fuel,
so it was selected as a suitable approximation.
30
Figure 12. Difference between the UV photodiode capacitance under illumination and at
dark.
Figure 13. Current versus voltage of the same diode under the UV illumination at dark
and a MAPP torch at 5 cm: the difference is quite large. The UV illumination had 4eV
photon energy.
A cadmium sulfide photocell was also characterized and tested in the oscillator
circuits. However, as it was more sensitive to ambient light and not made of silicon
carbide it was deemed unsuitable for UV sensing in high temperature environments.
31
Figure 14. Cadmium-sulfide photocell I-V characteristics under illumination and at dark.
Development of Silicon Carbide Based Oscillators
The proper operating conditions were found by constructing a Clapp oscillator [7]
and a differential oscillator [8] using a SiC MESFETs from Cree, Inc. The Clapp
oscillator can operate with gate voltages approximately between 5 and -8 volts and drain
voltages between 1.2 and 5 volts. Its frequency varies between approximately 6 and 50
MHz depending on biasing conditions. The differential oscillator operated at a drain
voltage of approximately 1.5 to 6 volts and a gate voltage of approximately -4.5 to -10
volts. Its frequency varies from approximately 2 to 10 MHz, and also is dependent on the
applied voltages. The Clapp oscillator design in shown in Figure 15 and the differential
oscillator in shown in Figure 16.
32
Figure 15. Clapp oscillator based UV sensor.
L2
L4
60uH
C2
60uH
C4
3p
V2
4Vdc
3p
C1
C3
3p
J2
3p
J3
J2N3819
L1
J2N3819
L5
10uH
10uH
R1
10
V3
7.5Vdc
0
Figure 16. SiC differential oscillator.
These designs produced very stable oscillations when the gate and drain voltages
were adjusted to a suitable level. Experimenting with the differential oscillator confirmed
that a second power supply is necessary to apply a negative voltage to the MESFET gate
in order to produce oscillations and to control the drain current level. Both of these
circuits were eventually abandoned however, in favor of the relaxation oscillator due to
33
the fact that the frequency of oscillation is controlled by the tank capacitor CT in both
designs. The SiC photodiode junction capacitance does not change sufficiently to produce
a significant change in frequency. Also, as the designs are based on an LC tank, parasitics
tended to dominate the oscillations which produced poor sensitivity to changes in the
tank capacitance. It was determined that the RC based relaxation oscillator would be
more sensitive to UV emissions due to the photodiode’s larger change in resistance rather
than capacitance when under illumination.
SiC Relaxation Oscillator Design
With the knowledge gained from the Clapp and differential oscillators, the Si
based relaxation oscillator was redesigned for SiC and is shown below. A negative
voltage was applied to the gate of both transistors. The two stages of the circuit were
coupled using 830 nF blocking capacitors at the gate of each MESFET. It was also
necessary to considerably reduce the resistors R1 and R2. Oscillations occur with a gate
voltage of -6 to -8 V and a drain voltage of 6 to 10 V. When suitable biasing conditions
have been met, the gate voltage can be used to pinch off the MESFETs and keep the drain
current at a lower level, reducing heat output from the components.
34
V1
7.5Vdc
R1
R2
15
28
C1
C3
68p
830n
C2
SiC Photodiode
15k
SiC MESFET
SiC MESFET
830n
R3
R4
51
V2
8Vdc
Figure 17. Final SiC relaxation oscillator based UV sensor and antenna for wireless
transmission.
When the proper biasing resistor values were found and stable oscillations were
achieved, a printed circuit board was designed. All of the traces where made 0.04” inches
wide to easily account for a current of up to 0.5 A. All of the surface mount resistors
where replaced with two resistors of the same value in parallel to account for this as well.
The printed circuit board was manufactured by ExpressPCB according to the drawing
provided [14]. The completed circuit board is shown in Figure 18. The board was first
tested without the photodiode to ensure that the oscillator was functioning properly.
35
Figure 18. Relaxation oscillator circuit with Cree SiC MESFETs.
The sensor was tested by exposing it to ambient light, darkness, UVA, and UVC.
The device is contained in a metal package to block electromagnetic interference, with a
small opening to allow UV illumination of the sensing elements. The light source was
positioned 3 cm from the photodiodes. The sensor was exposed to each type of
illumination for a period of approximately 2 minutes before transitioning to the next type
of light in order to capture the rise and fall times associated with the changing light
spectrum. It was found that during the first fifteen minutes of operation the oscillator’s
frequency must ramp up and stabilize before reliable sensing can occur as shown in
Figure 18. The change in frequency under exposure to ambient light and UVC are shown
in Figure 19. The sensor was first exposed to ambient light for 20 seconds, then UVC at a
36
distance of 5 cm for 20 seconds. The light sources are then switched back successively in
20 second intervals. It can be seen that exposure to UVC causes the frequency to increase
by 20 to 30 kHz during each interval.
Relaxation Oscillator Turning On
23.35
Frequency (MHz)
23.30
23.25
23.20
23.15
23.10
23.05
23.00
22.95
22.90
22.85
1
46 91 136 181 226 271 316 361 406 451 496 541 586 631 676 721 766
Timed (s)
Figure 19. Initial ramp-up of relaxation oscillator.
Relaxation Oscillator Sensing UV Illumination
22.88 22.87 UVA UVC MHz 22.86 22.85 22.84 22.83 Darkness Darkness 22.81 1 29 57 85 113 141 169 197 225 253 281 309 337 365 393 421 449 477 505 533 561 589 617 645 673 701 729 22.82 Time (s)
Figure 20. The change in frequency under UVA, UVC, and darkness.
37
Sensitivity Characterization
The sensor was tested by illuminating it with ambient light, darkness, UVA,
and UVC. The sensor is packaged in a metal box with a cutout large enough to expose the
sensing elements in order to block electromagnetic interference. The light source was
positioned 1 inch from the photodiodes. The sensor was exposed to each spectrum of
light for a period of approximately 2 minutes before transitioning to the next type of light
in order to capture the rise and fall times associated with changing light frequency.
Figures 21 and 22 show the relaxation and Clapp oscillator output frequencies as a
function of UV photon flux. The UV photon flux is shown in log scale in both cases. The
relaxation oscillator’s frequency shift has a linear dependence on the photon flux.
62.85
F req u en cy (kH z).
62.80
62.75
E p h = 4 eV
62.70
62.65
62.60
62.55
62.50
62.45
62.40
1E +15
1E +16
1E +17
U V P h o to n F lu x (1/cm 2.s.)
1E +18
Figure 21. Relaxation oscillator output frequencies as a function of UV photon flux. The
UV photon flux is shown in log scale. The frequency shift has linear dependence on the
photon flux.
38
8.06
Frequency (MHz) .
8.05
Eph =4 eV
8.04
8.03
8.02
8.01
8.00
1E+14
1E+15
1E+16
1E+17
1E+18
UV Photon Flux (1/cm2.s.)
Figure 22. Clapp oscillator output frequencies as a function of UV photon flux.
Figure 23 shows the UV detector’s operation in conjunction with the wireless
transmitter and receiver. A UVC source placed 5 cm away from the sensor produces a 20
kHz increase in the oscillator’s frequency. This corresponds to a 125 mV increase in the
final DC output signal from the receiver.
Figure 23. The SiC relaxation oscillator frequency changes by 20 kHz when illuminated
by UVC. This produces a 125 mV change in the wireless receiver’s output voltage.
39
Circuit Power Consumption
The current draw of the circuit is typically around 0.150 A, while operating
around 6 to 8 volts. This level of power consumption could likely be reduced by
optimizing the circuit. If the power consumption could be reduced, this would open
several possibilities for powering the circuit. It is highly desirable to utilize properties of
the sensor installation environment for potential energy harvesting. However, most
energy harvesting technologies are much lower power and would be impractical to meet
the power requirements of the present circuit design. Optimization of the power
consumption of the circuit would have to be completed to make use of such energy
harvesting capabilities. This has been left as future work and has not been implemented
in the scope of this project. Of the several types of energy harvesting capabilities
available, thermoelectric technologies on average produce about 40 µW/cm3, vibration
technologies produce about 116 µW/cm3 and solar cells can generate about 15 mW/cm3
[14]. While heat is highly available in the combustion environment, thermoelectric
technologies are not very efficient and produce considerably less than the UV sensor
circuit requires, even if an optimization was completed to minimize the power
consumption. Solar cells have the highest efficiency and may be a promising method if
light from the combustion flame could be used to produce power.
Telemetry Design
The wireless transmission system was designed to minimize the size of the
footprint of the primary sensor/transmitter component. As such, all that is required to
40
transmit the FM signal is a small dipole antenna. The receiver consists of a dipole
antenna, variable gain amplifier, frequency divider, and frequency-to-voltage converter.
The receiver’s input is initially a 40 mV, 22 MHz FM signal. It is then amplified by a
National Semiconductor LM386 variable gain amplifier. This increases the amplitude to
approximately 5 V. The signal is demodulated by the Analog Devices’ AD650. The
AD650 is a voltage-to-frequency converter, but it can be wired to function as a
frequency-to-voltage converter. A schematic is shown below.
+5V
+5V
250u
250u
0.05 u
0.1u
0.05 u
0.1u
LM386
LM386
10u
10u
Vin
10k
10k
Rint
-15V
Cint 11.11k
0.1u
3u
Cos
Vout
88u
SN54LS294
AD650
20k
D1N914
0.1u
250k
+5V
2k
R4
1k
500
+15V
560p
Fin
+5V
Figure 24. Receiver circuit design.
The maximum input frequency the AD650 is capable of processing is 1 MHz, so a
41
SN74LS294N frequency divider from Texas Instruments is used to divide the input
frequency by a factor of 32. The SN74LS29N is programmed by specifying the desired
frequency divisor value on the four programming inputs, pins 2, 1, 15, and 14
respectively. To divide the input frequency by 32, or 25, the input 0101 (binary 5) is
chosen. Consequently, pins 2 and 15 are tied low and 1 and 14 are high. This will
produce and output frequency of 22 MHz/32 or 687.5 kHz, which is well within the range
of the AD650’s input range [15].
The signal can then be converted by the AD650. Analog Devices provides an
article detailing the use of the AD650 voltage-to-frequency converter as a frequency-tovoltage converter [16]. The major components of the AD650 are shown in Figure 25. It
consists of a comparator, a one-shot with a switch, a constant current source, and a lossy
integrator. An input signal at the comparator triggers the one-shot. The one-shot actuates
a single-pole-double-throw switch which determines when the current source is applies
to the summing junction or the output of the lossy integrator. When the one-shot is on,
there is current applied to the input of the integrator and its output increases. At the end
of the one-shot period, the current enters the output of the integrator. Since the output has
a low impedance, the current has no effect and the circuit is essentially turned off. During
this time the output falls due to the discharge of the Cint capacitor through the resistor
Rint. When the comparator is triggered constantly, the integration capacitor will maintain
to a steady value due to charging and discharging.
42
Figure 25. The major components of the AD650 frequency to voltage converter as
explained in the application note “Using the AD650 Voltage-to-Frequency Converter As
a Frequency-to-Voltage Converter” [13].
The values of the feedback resistor Rint, the integration capacitor Cint, and oneshot timing capacitor Cosmust be determined as follows. The input frequency fin has a
constant amplitude α and period tos. The average output voltage is given by the equation:
Voutavg = tos× Rint×α×fin. Thus, a suitable value for Rintcan be found by selecting the
desired output voltage for a given signal with frequency fin and amplitude α. Cos
determined by the relationship: Cos=
!!" !!.!µ!
!.!!"
!
. This equation is simply rearranged from
the equation for the one-shot timing period t !" as given in the AD650 datasheet [15].Cint
!"#$%&'#%( !"#$%&#" !"#$
is given by the equation: Cint=
!×!!"#
where N is the number of time
constants needed to settle and the mechanical response time is that of the switch actuated
by the one-shot shown in Figure 25.
43
Summary
The initial silicon based design had to be modified significantly in order to
function with SiC transistors. The input voltages and the amount of current drawn
increased, as well. Comparable output voltages were achieved, however the SiC
MESFET circuits operated with frequencies several orders of magnitude greater than that
of the Si JFETS. The use of the relaxation oscillator design provided superior sensing
performance over the Clapp and differential oscillators. This is due to the fact the period
of the oscillation is controlled by an RC time constant in the relaxation oscillator, rather
than a LC tank as in the Clapp and differential oscillators. The SiC photodiode exhibits a
must greater sensitivity to UV illumination when used as a resistive element rather than a
capacitive element.
The output signal of the oscillator was strong enough to be sent via a simple
dipole antenna to a receiver circuit that converts the change in frequency to a change in
voltage. There is a 125 mV difference in the output voltage of the receiver when the
sensor is illuminated by UV as compared to when it is under ambient light. This is a more
than sufficient change to be detected by most data acquisition modules and could easily
be integrated into a computer based system for data logging or control.
44
Chapter 6 – Conclusion
In closing, this thesis discusses the design and implementation of a silicon carbide
UV sensor for use in high temperature environments. This type of sensor has applications
in the aero and space industries, or in any industry where the combustion of fuels may
give off NO and other pollutants. The sensor prototypes a particularly useful design in
that it possesses an on board wireless transmission system and a remote receiver for ease
of integration into a monitoring system.
Future work that would be beneficial to this design would be to perform an
optimization of the power consumption of the circuit. This would potentially allow the
sensor to make use of various energy harvesting technologies. The trade-offs between
different types of energy harvesting methods could be explored and there practicality for
use in high temperature environments could also be studied.
45
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46