Johnson Noise Thermometry for Harsh Environments
R. Kisner1, C. L. Britton1, 2, U. Jagadish1, J. B. Wilgen1, M. Roberts2, T. V. Blalock2, 3, D. Holcomb1,
M. Bobrek1, 2, M. N. Ericson1, 2
1
Oak Ridge National Laboratory, MS6006, Oak Ridge, TN 37831-6006,
[email protected]
2
Dept. of Electrical and Computer Engineering, The University of Tennessee, Knoxville TN
3
Deceased
thermocouples are known to exhibit large drifts in high
neutron fluxes [1]. The SP-100 temperature measurement
system had a 1% measurement uncertainty requirement at
1375 K, an 8 second response time limit, and a
requirement for 7 years of unattended operation. No
conventional thermometer could meet these goals.
However, Johnson noise thermometers had been
developed by the Oak Ridge National Laboratory
(ORNL), originally for fuel centerline measurement, and
were believed to be able to meet these stringent
requirements. Johnson noise is a first-principles
representation
of
temperature.
Fundamentally,
temperature is merely a convenient representation of the
mean kinetic energy of an atomic ensemble. Since
Johnson noise is a fundamental representation of
temperature rather than a response to temperature such as
electrical resistance or thermoelectric potential, Johnson
noise is immune from chemical and mechanical changes
in the material properties of the sensor. Johnson noise
thermometry has been applied to in-core temperature. An
overview of the application of Johnson noise thermometry
to space power reactors was made by Roberts, Blalock,
and Shepard in 1989 [2].
Abstract - Although Johnson noise has been proposed as a
thermometry method for several decades, it is only
recently that digital and analog electronics have made it
possible to economically fabricate measurement systems
based on Johnson noise. Johnson noise, which is a result
of fundamental physics, is caused by the random thermal
motions of electrons in all conductors. Its fundamental
nature allows us to construct temperature measurement
systems that do not require periodic calibration. Thus
long, unattended operating intervals are feasible. Several
unique implementations of Johnson noise thermometry
(JNT) are possible.
One permits temperature
measurement without contacting the measured surface —
inductive JNT. Another implementation measures the
Johnson noise of a resistance element in contact with the
measured surface — conductive JNT. The resistive
element in conductive JNT can be an RTD. Apparatus
have been recently fabricated demonstrating the
practicality of both JNT implementations.
A
demonstration of conductive JNT is planned at a nuclear
facility within two years. We will present new hardware
implementations that allow real-time calibration of the
signals that have the potential of allowing a fullyintegrated, physically small and robust system to be
achieved.
Signal Processing Electronics
The Nyquist equation describes the voltage produced by
the vibration of the electrons within a resistor at a given
temperature. For frequencies below a few gigahertz,
Equation 1 shows the relationship between the absolute
temperature of a resistor (T), its resistance (R), the
frequency band of measurement ∆f, and the measured
mean-square noise voltage.
TABLE OF CONTENTS
1. INTRODUCTION ....................................... 1
2. I-NERI JNT IMPLEMENTATION ............ 3
3. INDUCTIVE JOHNSON NOISE
THERMOMETRY ........................... 5
4. CONCLUSION ........................................... 7
REFERENCES............................................... 8
V 2 = 4k B TR∆f
(1)
where kB represents Boltzmann’s constant (1.38x10-23
joules/Kelvin).
A direct measurement of the Johnson noise for
temperature measurement has several challenges. First,
the amplifier gain needs to be both known and stable.
Second the amplifier passband and filtering effects of
connection cabling must be well known to within the
required measurement accuracy. Finally, the resistance of
the sensor must be independently and accurately
measured. To avoid these difficulties, early Johnson noise
thermometers performed a ratio of two noise voltage
measurements, one with a resistor at the measurement
1. INTRODUCTION
The SP-100 space reactor was designed to have a diverse
set of temperature measurements. The primary W/W-Re
0-7803-8155-6/04/$17.00© 2004 IEEE
IEEEAC paper #2170, Version 1, Updated October 9, 2003
1
temperature and the other at a known temperature,
switched onto a single amplifier channel. However,
changing the connection of the sensor to the high-gain
measurement circuit introduced noise and decreased
reliability [3].
Two additional signal-processing concepts were
investigated as part of the ORNL SP-100 thermometry
project (further work was done under Electric Power
Research Institute (EPRI) sponsorship). In the first of
these, the temperature measurement resistor is connected
in parallel to two separate high input impedance
amplifiers. The output of these amplifiers is partially
correlated since each consists of the sum of a correlated
noise voltage and uncorrelated amplifier noise voltage. If
two Johnson noise amplifier signals, connected to the
same resistance, are combined and time averaged, the
correlated part of the noise will persist, but the
uncorrelated amplifier noise will approach zero. Figure 1
illustrates the concept of cross-correlation; the measured
voltage from one amplifier channel is Fourier transformed
and correlated and with that from the other to form a cross
power spectral density (CPSD) effectively eliminating the
noise contribution from the amplifier electronics.
The approach that ORNL staff took to minimize these
difficulties in the SP-100 thermometry project was to
follow the work of Pepper and Brown [4] in which a
resistive sensor is connected in series with an inductor
and a capacitor forming a tuned circuit. In this approach
the ideal mean squared noise voltage is given by
V2 =
k BT
C
(2)
where C is the capacitance of the capacitor. The major
advantage of this technique is that, for lossless inductors
and capacitors (provided that the measurement bandwidth
is greater than the tuned circuit bandpass), the measured
voltage output is independent of the sensor resistance and
the inductance. No measurement of the sensor resistance
is needed. Another advantage of the technique is that for a
properly tuned circuit, most of the signal power lies in a
relatively small band near the resonant frequency. This
allows the amplifier bandwidth to be relatively small and
reduces its noise contribution to the measurement
uncertainty.
Johnson noise has a flat (white) spectral energy
distribution. The shape of the power spectral density
functions displayed in Figure 2 is a result of the combined
effects of filtering out both the low and high frequencies
from the noise and the frequency dependent gain of the
amplifier circuit. The low frequency filtering is applied to
eliminate the non-thermal noise generated by mechanical
vibrations. These microphonic signals are limited to less
than a few tens of kilohertz. The upper frequency
filtering is applied both to avoid aliasing higher
frequencies into the measurement band as well as to
minimize the impact of sensor-to-amplifier cable
capacitance induced restriction of high frequency
transmission.
A practical limitation with this approach, however, is loss
in the inductors. Real inductors have winding resistances
(typically frequency dependent) that dissipate energy. In
practice, this limits the overall accuracy obtainable with a
tuned-circuit implementation of Johnson noise
thermometry.
The CPSD function has units of volts squared per hertz
and expresses the voltage-squared content per unit
frequency of the measured voltage signal. Integrating the
CPSD function over frequency and averaging the result
over time provides the mean-square noise voltage for the
Nyquist equation as shown in Fig. 1.
Another implementation restriction for Johnson noise
thermometry in space reactors is the capacitive effect of
the cable connecting the sensing resistor to the first stage
amplifiers. If the cable has significant capacitance, it will
block the high frequency portion of the sensor noise
before it reaches the measurement system. This filtering
of the upper frequencies reduces the bandwidth of the
Johnson noise signal. Under the high temperature and
radiation environment of a space reactor, the cable
capacitance will change over time. One way of
compensating for the cable effect is to periodically
measure its input impedance and calculate its transfer
function. The preferred technique is to locate the firststage amplifier near the sensor.
The signal processing electronics for incorporating the
Johnson noise temperature measurement system into the
SP-100 reactor were not completed during the ORNL
project. A radiation and thermally hardened preamplifier
package had not yet been developed by the conclusion of
the project. Also a technique for verifying the pass band
and gain stability of the amplifier had yet to be
demonstrated.
Figure 1. Power Spectral Density Function (PSD) of
Each Amplifier Channel Containing Both Correlated and
Uncorrelated Noise and the CPSD Function From Both
Amplifiers Containing Only Correlated Noise.
Electromagnetic interference spikes and microphonics are
two of the biggest problems for a practical
implementation of Johnson noise thermometry. In many
2
The CPSD provides the mean-square noise voltage, and
the resistance is independently measured, so the
remaining variables that have to be known to obtain
temperature from the voltage measurements are the
amplifier gain as a function of frequency and the effective
measurement frequency band (∆f).
The technique
currently used to obtain the gain-bandwidth product is to
initially calibrate the measurement using a known
temperature and treating both properties thereafter as a
single constant.
situations, these effects can completely dominate the
noise measurement. This puts a premium on wellimplemented grounding, shielding, and filtering. A
complementary technique to reduce these effects is to use
both knowledge of the spectral energy content of Johnson
noise and digital signal processing to recognize and
eliminate
interferences.
Typically
narrowband
electromagnetic interference appears as spikes in the
long-term average CPSD that can be recognized and
removed with only a small reduction in measurement
bandwidth as illustrated in Fig. 2.
Figure 3. Johnson Noise Thermometry Measurement
Process Schematic.
Figure 2. CPSD With Narrowband EMI Spikes.
A layout of the system is shown in Fig. 4. High-gain,
wide-band Johnson noise preamplifiers with a precision
resistance bridge have been implemented in high-density,
discrete component electronics. A continuous amplifier
gain calibration scheme has also been implemented. The
digital signal processing logic has been implemented in
LabVIEW™ on a desktop computer and is in the process
of being implemented in a field programmable gate array
(FPGA) and dedicated digital signal processing (DSP)
chip format. The preamplifier head and resistance probe
have been packaged in a shielded aluminum enclosure
and a coaxial signal interconnection scheme has been
implemented. More electronic design details on the
implementation will be published as the project completes
over the next year.
2. I-NERI JNT IMPLEMENTATION
While the underlying concepts (under NASA
sponsorship) and a laboratory prototype (under EPRI
sponsorship) of both the cross correlation and digital
signal processing had previously been developed by
ORNL and the University of Tennessee, the high speed
digital signal processing required remained prohibitively
expensive in the early 1990s. Consequently, the crosscorrelation amplifier electronics were never implemented
in an integrated form, and the signal processing software
was never finalized.
The current International Nuclear Energy Research
Project, led by ORNL on the U.S. side and with the
Korean Atomic Energy Research Institute leading the
Korean JNT tasks, is focused on implementing a dual
mode resistance and Johnson noise thermometer in a
rugged, integrated, prototype form. The resistance
measurement serves the dual purpose of providing the
necessary impedance measurement for the Nyquist
equation as well as providing a prompt temperature
measurement. Since Johnson noise is a stochastic process,
some time is required to perform a measurement. The
temperature measurement in the dual mode thermometer
is therefore made as a simple resistance measurement
whose resistance to temperature conversion is quasi
continuously updated using Johnson noise. A schematic
illustrating the measurement process is shown as Figure 3.
Instrument
Low-Noise
Head
Wide
Bandwidth
Preamps
Differential
Line Drivers
Channel Calibration
Generator
Receiver
RTD AC
Noise
Channels
Shielded
Twisted
Pairs
µV AC
Signal
ADC
To
Computer
for DSP
R
ADC
Resistor in
Thermowell
Bridg
RTD DC
Resistance
Channel
V/F
Convert
PhaseLockedLoop
ADC
Low-Drift High
Impedance DC
Preamp
Figure 4. Principal Components And Conceptual
Configuration Of The Dual Mode Resistance And
Johnson Noise Thermometer Incorporating the CrossCorrelation Technique, Digital Signal Processing, and
Channel Gain Calibration.
3
that has a low-frequency cutoff frequency of
approximately 15 KHz.
The filtered signal is
discriminated to get a rail-to-rail equivalent frequency
signal which is then passed into a phase locked loop used
as a frequency-to-voltage converter. The resulting output
voltage is proportional to the dc equivalent of the
resistance measured at the transmitter front end. The
single-ended signals out of these three channels are then
sent to a computer based DSP using a 4 channel 20
M sample/second ADC board.
Electronics Implementation
Figure 5 presents a block diagram of the connectivity of
the new system. Each of the two noise channels connects
from the preamp/cal head-end box via two sets of coaxial
cables arranged in differential pairs. The preamplifier
board contains two identical differential-input JFET
preamplifiers/filters/gain sections each with a voltage gain
of approximately 80dB and a center frequency of
approximately 650kHz. Each of the two single-ended
signal paths is converted to differential with a highbandwidth differential amplifier capable of driving 300m
of coaxial cable. Each of these cables is terminated in 50
ohms at the receiver box with the shields tied together at
the head-end box and isolated. The conductors are also
isolated by using capacitive coupling instead of
transformer coupling to minimize pickup. The calibration
board contains a swept-frequency oscillator that supplies
the calibration signal over the noise bandwidth of the
amplification chain by sweeping continuously from
50kHz to 2.5MHz and back over an approximate 2 second
period and is coupled directly into the front-end
preamplifier. A 100µA current is supplied to the input
also and a separate differential amplifier measures the
voltage drop across the RTD. This voltage is amplified
and converted to a 20kHz-90kHz differential pulse stream
which is then sent to the receiver box which converts it
back into a voltage proportional to the value of the
resistor. The head-end box has an associated powersupply box which supplies all DC voltages necessary for
operation. The preamplifier and cal boards are mounted
as a sandwiched pair separated by a sheet steel shield.
Thermowell
and
RTD
Diff. Receiver Amp
2000
2
Receiver
Board and
Power Supply
cable
PCI 9812
4-channel
20MHz ADC
interface board
and DSP
Diff. Receiver Amp
2-Stage
Sallen-key
HPF
2000
2-Stage
Sallen-key
LPF
50
cable
JOHNSON NOISE CHANNEL B
cable
221
5v
Diff. Receiver Amp
Sallenkey
HPF
50
2-Stage
Sallenkey
LPF
Discrim
PLL-Based
Frequency
to Voltage
converter
Sallenkey
LPF
50
DC RESISTANCE CHANNEL
2000
5v
221
Figure 6. Block Diagram of the Receiver section.
Data
Test runs of the system using calibrated metal-film
resistors at room temperature in place of the RTD have
been performed and the data is shown below. The system
has been tested at room temperature with the chosen RTD
and works equally well. These tests represent an end-toend test using the entire system and the actual software
algorithm. In the coming months, collaborators at Ohio
State University will perform a full system calibration.
Figure 7 is a plot of the power spectral densities of each
of the two noise channels versus resistance. Also
included is the cross-power spectral density of the two
channels. Figure 8 is the voltage of the simultaneous DC
readout plotted against the same resistance.
Analog Data Out
DC
2
50
JOHNSON NOISE CHANNEL A
50
2
Ch. 2
2-Stage
Sallen-key
LPF
221
Ch. 1
Preamp and
Cal boards
(Head-end box)
2-Stage
Sallen-key
HPF
50
3
Power supplies
Preamp and cal
Relative Noise Spectral Density vs. Resistance
Figure 5. System Block Diagram
3500
Channel 2
Relative Noise Spectral Density (V2)
The receiver box of the JNT setup shown in Fig. 6
consists of two Johnson noise measurement channels and
one DC resistance measurement channel. The signals for
the three channels are received differentially by variable
gain 250MHz differential-to-single-ended amplifiers. The
Johnson noise channels consist of this amplifier followed
by a 4-pole Sallen-Key High pass filter with a lowfrequency cut-off of approximately 80 KHz. This is then
followed by a 4-pole Sallen-Key Low pass filter with a
high-frequency cut-off of approximately 2 MHz. The
entire channel has a variable gain of 1 to 10. The DC
resistance channel has a front-end differential receiver
amplifier similar to the Johnson noise channel. The signal
then passes through a 2-pole Sallen-Key high pass filter
3000
Channel 1
2500
2000
1500
Cross Power
1000
500
0
100
120
140
160
180
200
220
Source resistance (ohms)
240
260
280
Figure 7. Individual spectral density for each channel
and CPSD for both channels versus resistance.
4
temperature in conductive surfaces. In fact, the surface
resistivity can also be measured by an independent
calculation of the spectrum broadening (the quality factor
or Q). The relationship shown in (2) can be expanded for
a resonant antenna and simplifies in the limit to
DC Vout vs. Resistance
5
4.5
4
DC Vout (V)
3.5
3
V 2 = 4 k B TR
2.5
2
π f0
2 Q
=
K BT
C
(3)
1.5
where V is the measured voltage, KB is Boltzmann's
constant, T is absolute temperature, R is equivalent
resistance, f0 is the center frequency, Q is the resonance
quality factor, and C is the effective parallel capacitance.
1
0.5
0
100
150
200
Source resistance (ohms)
250
Figure 8. Voltage output versus resistance.
3. INDUCTIVE JOHNSON NOISE THERMOMETRY
L
R
C
Theory
A noise analysis technique based on an inductively
coupled measurement system has been attempted only
once previously for sheet steel by Varpula and Seppae
[5, 6].
The cited measurement method needed a
complicated preamplifier circuit and hardware signal
processing to decode the noise signal in contrast to what
can be done now with software processing. Also their
antenna’s noise contribution was too high for a low
resistance material such as aluminum. In our work, we
developed a dual-channel preamplifier of a simpler design
that couples with software processing algorithms to
perform real-time, cross-power spectral density (CPSD)
calculations. The integration of the CPSD data directly
yields the surface temperature. The CPSD rejects
uncorrelated noise contributions from each of the
preamplifiers making preamplification noise very low.
We also demonstrated the use of a cryogenically cooled
copper and a Litz-wire resonant antenna to reduce the
antenna’s noise contribution. The Litz-wire antenna is
necessary for low resistance materials such as aluminum.
Figure 9. Inductance, L, Capacitance, C, and Resistance,
R, of the antenna combine with the metal plate to form a
parallel resonant circuit.
The noise voltage becomes proportional to the
temperature and the capacitance.
Fortunately, the
capacitance of the circuit can be dominated by a fixed
parallel capacitor and the uncertainty from plate to
antenna movement minimized. This is the approach we
took in our apparatus.
The temperature measurement uncertainty is a function of
measurement statistics. Very broadly, with measurement
averaging times between 1 and 10 seconds and
bandwidths of 20 to 40 kHz, uncertainty of about one
percent of absolute temperature value is expected.
Experimental Apparatus
Our apparatus is contained in an aluminum cylinder,
which acts as a Faraday shield as well as a physical
support for the internal components. Externally mounted
to the cylinder are two preamplifier modules, for which
electrical shielding is critical. Inside the cylinder is a
cryogenic chamber made of polystyrene that holds the
copper coil antenna, an air supply cools the bottom of the
antenna chamber. Several layers of low-loss sheet
ceramic insulator act as a support and heat shield for the
antenna chamber. Below the sheet insulation is the metal
plate and heater, which sit in a bed of high temperature
insulation. A piston is located below the heated steel
plate to vary the distance between the antenna and the
steel plate being measured. The apparatus is shown
diagrammatically in Fig. 10. Two materials were used for
the heated metal plate: aluminum and steel.
The inductive Johnson noise thermometry principle
collects thermal noise magnetically radiated from a
surface by a special antenna located nearby. The Johnson
noise received in a resonant antenna circuit has the same
mathematical relationship as that of the resonant
conductive method. For the tuned circuit, the meansquared voltage across the capacitor becomes independent
of R and L although the voltage fluctuations themselves
(the actual time-domain behavior) depend on R and L.
The dissipation losses (which are the same as R) from the
material under measurement and the antenna are mutually
coupled as shown in Fig. 9 (i.e., the noise sources add).
To function, the dissipation of the material being
measured must dominate those of the resonant circuit.
Hence, the induction method can be used to measure the
5
in Fig. 11, are closely correlated as seen by the tight
scattering around a linear fit. The data are taken as a steel
plate cools from 500°C to 180°C.
Measurement
uncertainty of better than +/-2°C was shown.
This
uncertainty was better than one percent absolute. A
typical resonance curve is shown in Fig. 12.
Aluminum Cover
Cooling
Air
Supply
Preamplifiers
Cryo Chamber
Liquid Nitrogen
Antenna
Low Loss
Insulation
Type K
Thermocouple
Steel Plate
Insulation
Piston for Changing
Plate Position
Heater
Aluminum
Cylinder
Figure 10. Diagram of Experimental Apparatus Showing
Internal Components.
Results and Accomplishments
We discovered the FET preamplifiers’ extreme propensity
toward instability. The tendency to break into oscillations
under high-Q conditions plagued us throughout the
development. We further discovered that frequency
dependent loading by the gate of the FET transistors is
one of the chief limiting factors of the frequency at which
we can operate. Although we designed a very wide
bandwidth preamplifier, the anomalous loading of the
gates forced operation down to 1.5 MHz, which
coincidently was the same range as the Varpula and
Seppae instrument. We purchased and surveyed many
low noise high frequency field effect transistors in an
attempt to overcome the loading problem. This effect has
only been marginally studied by microcircuit researchers
and it is not modeled in SPICE. We believe that further
research into this effect would be very beneficial.
Figure 11. Tracking of Inductive Noise Temperature
with Litz Coil Antenna Compared with a Type K
Reference Thermocouple.
In our experiments we used both copper and Litz wire coil
with nine turns in a planar orientation. A typical copper
coil is shown in Figure 13. The coil resonates with a high
Q tuning capacitor placed directly across it, not shown.
Both coil and capacitor are submerged in a liquid nitrogen
bath. At 77K, the Q of the circuit more than doubles
compared with room temperature conditions. The Litz
wire produces another doubling in Q over the copper.
We determined that all calculations of temperature should
be done algorithmically in software. CPSD processing of
the two preamplifier signals worked to reduce the
effective noise contribution of the preamplifiers to a
fraction of their individual noise — an improvement of
better than 20dB was observed. The CPSD method is
especially useful when the preamplifiers noise varies.
Otherwise, a baseline must be established and a
calibration made. With the CPSD, no base noise
calibration is needed.
The CPSD is filtered using exponential averaging with a
10-second time constant. Simple integration of the
resonance curve gives a value directly proportional to the
measured temperature. Temperature as measured by type
K thermocouple is plotted against the area under CPSD as
derived from a 1.5 MHz resonant antenna consisting of
Litz wire cooled to 77K.
Temperatures of 77K
substantially reduced antenna noise. The results, shown
Figure 12. Typical Resonance Curve of the Measured
Noise Spectra with Cryogenically Cooled Copper Tubing
Coil.
6
hurdle remains to be overcome before the technology can
be adopted. A radiation and thermally hardened version of
the preamplifier electronics remains to be developed.
Both the radiation and temperature requirements for space
reactor implementation will push the state-of-the-art in
discrete electronics requiring significant engineering
effort. Further, to minimize the system mass, it would be
desirable to implement the electronics into a highly
integrated, high-radiation-tolerance package using hightemperature electronics (silicon on sapphire or silicon
carbide for example).
Figure 13. Copper Antenna Coil, shown without tuning
capacitor.
We have not only constructed a system for resistive
Johnson noise measurements, but also an apparatus for
demonstrating
Johnson-noise-based
temperature
measurements of heated metal plates. Our apparatus is
suitable for testing the performance of a variety of
components. We conducted tests with a mild steel plate,
which we heated to 500°C and allowed to cool back to
room temperature. The data show excellent linearity
compared with a type K thermocouple in direct contact
with the plate. Experiments were also preformed with an
aluminum plate.
Because aluminum has five times the electrical
conductivity of steel, the resonant noise signal will be
lower, which places more demands on the antenna design.
Antennas for aluminum applications must use
cryogenically cooled Litz wire coil. Loading curves
comparing aluminum and steel are shown in Fig. 14. In
addition, the bandwidth is reduced with aluminum.
We discovered that for temperature measurements of steel
plates, given an antenna diameter of seven inches, at a
working distance of 1.5 inches, 90 percent of the noise
emission originates from the plate. The equivalent
temperature of the antenna (and electronics) was
approximately 65K.
Significant improvements were demonstrated over the
previous work of Varpula and Seppae: (1) improved
preamplifier configuration, (2) direct real-time digital
signal processing that can be implemented with typical
computer components, and (3) cryogenic cooling of the
detection antenna.
This work demonstrates that the inductive Johnson noise
method works for steel and has great promise for
aluminum. Although the design is non-trivial, the
technology can be developed into a field instrument for
metal mills and other industrial applications.
Loaded Resonator Q-factor Measured as a Function of Plate Spacing at 1.5 MHz
350
300
L o a d e d Q -fa c to r
250
REFERENCES
200
150
Steel
100
304L Stainless
[1] Brixy, H., Hecker, R., Oehmen, J., Barbonus, P., and
Hans, R., “Development Work on Noise Thermometry
and Improvement of Conventional Thermocouples for
Applications in Nuclear Process Heat,” Specialists’
Meeting on Gas Cooled Reactor Core and High
Temperature Instrumentation, Windermere, England,
IWGGCR-5, IAEA-TC-389/6-7, International Atomic
Energy Agency, Vienna, June 14-17, 1982.
Aluminum Foil
50
No Plate
0
0
0.5
1
1.5
2
2.5
3
3.5
Plate-Coil Spacing (in)
Figure 14. Comparison of Antenna Loading from Steel,
Stainless Steel, and Aluminum.
[2] Roberts, M. J., Blalock, T. V., and Shepard, R. L.,
“Application of Johnson Noise Thermometry to Space
Nuclear Reactors,”Space Nuclear Power Systems, 10 &
11, The Proceedings of the 6th Symposium on Space
Nuclear Power Systems(1989), Krieger Publishing Co.,
Melbourne, Florida, January 1993, pp. 87-93.
4. CONCLUSION
Johnson noise thermometry is becoming progressively
more
possible
for
long-term,
high-reliability
implementation as signal processing technology
progresses and engineering expertise is applied. While
Johnson noise thermometry has been demonstrated by a
number of groups for nuclear reactor implementations
over the past thirty years, one significant engineering
[3] Garrison, J. B. and Lawson, A. W., “An Absolute
Noise Thermometer for High Temperatures and High
Pressures,” Review of Scientific Instruments, 20, 785-94
(1949).
7
[4] Pepper, M. G. and Brown, J. B., “Absolute HighTemperature Johnson Noise Thermometry,” J. Phys. E:
Sci. Instrum., 12, 31-34(January 1979).
[5] T. Varpula and H. Seppae, Inductive Noise
Thermometer: Practical Realization,” Rev. Sci. Instrum.
64 (6), June 1993, pp. 1593.
[6] H. Seppae and Varpula, “Inductive Noise
Thermometer: Theoretical Aspects,” J. Appl. Phys. 74 (2),
July 15, 1993, pp. 771.
8