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BACKGROUND OF THE INVENTION
The present invention relates in general to an improved passive microwave
detection technique for early detection of cancerous tumors. More
particularly, the invention relates to a system adapted to provide
localized heating of subsurface tissue with the use of an active microwave
transmitter in combination with a passive radiometer for detecting a
temperature differential occasioned by the differential heating between
the tumor and adjacent tissue.
Studies have been conducted on the ability to measure temperature gradients
particularly deep within the body tissue in connection with clinical
medicine and research. For example, see the articles to A. H. Barrett, P.
C. Myers and N. L. Sadowsky, "Detection of Breast Cancer by Microwave
Radiometry." Radio Science 12, No. 6(S), 167, 1977; Ronald A. Porter and
Harry H. Miller, "Microwave Radiometric Detection and Location of Breast
Cancer." (Preprint.); J. Bigu del Blanco and C. Romero-Sierra, "MW
Radiometry: A New Technique to Investigate the Interaction of MW Radiation
with Living Systems." 27th ACEMB, Philadelphia, Pa., Oct. 6-10, 1974.
These temperature gradients occur, it is theorized, because of vascular
insufficiency associated with the thermal characteristics of tumors. It is
well known that a carcinoma or malignant tumor is normally hotter than the
surrounding tissue. It is also known that, from "black body" theory, any
perfectly absorbing body emits radiation at all frequencies in accordance
with Planck's radiation law. A recent article on the differential heating
characteristics is one by B. C. Giovanella, "Correlation of
Thermosensitivity of Cells to Their Malignant Potential." Conference on
Thermal Characteristics of Tumors: Applications in Detection and
Treatment; New York Academy of Sciences, Mar. 15, 1979.
The application of thermal therapy (i.e., localized heating) has been used
to reduce tumor size or to even destroy the tumor. It has been found that
tumor temperatures greater than 45.degree. C. can be maintained with the
normal tissue adjacent to the tumor at the same time remaining at or near
normal body temperature. It has been reported by several investigators
that cell tumor tissue will necrose at temperatures above 42.degree. C.
See the articles by David N. Leff, "Hyperthermia-Hottest News in Cancer
Therapy." Medical World News, May 14, 1979; Jozef Mendecki, Esther
Friedenthal and Charles Botstein, "Effects of Microwave-induced Local
Hyperthermia on Mammary Adenocarcinoma in C3H Mice." Cancer Research 36,
2113-2114, June 1976; James Schaeffer, "Treatment of Metastatic Osteogenic
Sarcoma in Mice with Whole Body Hyperthermia and/or Irradiation."
International Symposium on Cancer Therapy by Hyperthermia and Radiation,
Washington, D.C., 1975. Thermal therapy used in conjunction with other
conventional techniques involving drugs or radiation has proven to be
effective (i.e., anti-cancer drugs act more effectively at elevated
temperatures and, similarly, permit lower level X-ray treatment). The
combination of microwave detection with infra red detection is reported by
Barrett and Myer, supra.
In accordance with the present invention, there is provided a sensitive
microwave radiometer technique for sensing subsurface temperatures wherein
the technique is not invasive. It has been common in the past to employ a
conventional thermistor probe inserted in the area of the tumor, and
studies have been made with regard to the effect on the heating patterns
induced by microwave diathermy apparatus. See the Articles by Thomas C.
Cetas, "Temperature Measurements in Microwave Diathermy Fields: Principles
and Probes." International Symposium on Cancer Therapy by Hyperthermia and
Radiation, Washington, D.C., 1975; Len Yencharis, "Temperature Probe
Designed For Cancer Therapy." Electronic Engineering Times, 18, Jan. 9,
1978. The results of these studies indicate that the heating pattern is
altered considerably by the presence of the sensor.
The microwave radiometer of the present invention is in effect a very
sensitive radio receiver capable of measuring temperature differentials
down to 0.1.degree. C. or less. The receiver, when provided with a highly
directional antenna and technique of observation, provides a reading of
power picked up by the antenna. As mentioned previously, any perfectly
absorbing body emits radiation at all frequencies in accordance with
Planck's radiation law. The distribution of radiation is a function of
both the temperature and wavelength or frequency. As the temperature of
the body increases, the density of the radiation at all frequencies also
increases. From this viewpoint, infra red thermography or radiometry,
appears to be effective, however, the depth of penetration (depth of
effective emission) becomes a limiting factor. The highest value of
radiation density occurs in the optical region. Nevertheless, an
appreciable amount of radiation exists at the microwave segment of the
spectrum. In accordance with the present invention the power accepted in a
known bandwidth by an antenna having defined characteristics can be
accurately computed as a function of the temperature of the emitter.
As mentioned previously, a carcinoma or malignant tumor normally radiates
more heat than the surrounding tissue. See the article by R. N. Lawson and
M. S. Chughtai, "Breast Cancer and Body Temperature." Canadian Medical
Association, Vol. 88, Jan. 12, 1963. Early detection, namely detection
prior to invasion or metastases, requires the detection of tumors less
than five millimeters in diameter with an associated temperature deviation
of less than 0.2.degree. C. It has been found in accordance with the
techniques of this invention that such early detection is quite accurate,
and that tumors of relatively small size can be detected which heretofore
have not been capable of detection by such conventional techniques as
X-ray mammography.
Accordingly, one of the objects of the present invention is to provide an
improved technique for the diagnosis and treatment of cancer employing a
non-invasive microwave detection system.
Another object of the present invention is to provide in a single unit the
combination of both a microwave transmitter or source and a passive
detector or microwave radiometer.
A further object of the present invention is to provide a microwave system
employing a sensitive passive microwave radiometer particularly adapted
for sensing subsurface temperatures in combination with a solid state
microwave transmitter for providing localized heating of subsurface
tissue. With such a combined system, there is essentially a highlighting
of the tumor to enhance detection, thus taking advantage of the
differential heating characteristics of the tumor with respect to the
surrounding tissue.
Still another object of the present invention is to provide an improved
microwave system for the early detection of cancer and which is adapted
for use, not only for detection purposes but also for treatment purposes.
Still a further object of the present invention is to provide a microwave
system for cancer diagnosis which is totally battery operated to thus
eliminate possible problems associated with line transients and the like.
Another object of the present invention is to provide an improved microwave
system for the detection of cancerous tumors and which is non-invasive,
thus, not requiring the use of any temperature sensing probes. The present
invention employs a sensitive passive microwave radiometer particularly
designed to sense subsurface temperatures.
Still another object of the present invention is to provide an improved
microwave system for the detection of cancerous tumors and which is
capable of sensing at a temperature resolution down to at least
0.1.degree. C.
A further object of the present invention is to provide an improved
microwave system for the detection of cancerous tumors and which is
particularly adapted for the detection of relatively newly-formed tumors
of extremely small size.
Another object of the present invention is to provide, in a microwave
system, an improved, extremely sensitive passive radiometer capable of
measurements of temperature deviations even less than 0.1.degree. C.
SUMMARY OF THE INVENTION
To accomplish the foregoing and other objects of this invention, there is
provided in accordance with the present invention, a microwave system for
the diagnosis of cancerous tumors employing non-invasive microwave
techniques. This system may also be employed in the treatment of cancer.
The system is preferably totally battery operated, thus eliminating any
possible problems associated with line transients, pickup, etc. The system
comprises a sensitive passive microwave radiometer particularly adapted
for sensing subsurface temperatures, in combination with a solid state
transmitter that provides localized heating of the subsurface tissue. This
localized heating essentially enhances the tumor from a temperature
differential standpoint, taking advantage of the differential heating due
to vascular insufficiency associated with the thermal characteristics of
tumors. This technique highlights and enhances the early detection of
cancer tumors. The selection of both the radiometer and the transmitter
frequencies is based upon the following factors:
1. Emissivity, which increases with increasing frequency;
2. Spatial resolution; and
3. Microwave transmission characteristics.
In the embodiment disclosed herein, the frequency for the radiometer is
selected at 4.7 GHz sufficiently removed from the selected microwave
heating frequency of 1.6 GHz.
An applicator forms the means by which the system couples to the body. This
applicator employs a simple TE.sub.1-0 mode aperture that is placed in
direct contact with the radiating or emitting surface. The aperture is
formed by a single-ridged waveguide which is preferred because its use
lowers the frequency at which cutoff occurs. To further reduce the size of
the aperture, dielectric loading is employed. The waveguide dimensions for
operation at L-band and the dimensions of the ridged portion of the L-band
ridged wave guide are selected to allow propagation of the higher
frequency associated with the C-band radiometer. By having the radiometer
input contained within the single-ridged waveguide L-band transition, the
point of maximum field of the source of the heat is in close proximity
with the area of thermal detection. The cut-off characteristics of the
C-band waveguide are utilized in addition to other filtering that is
provided; the waveguide forming a high pass filter to isolate the high
power L-band source from the sensitive radiometer. A heater and
proportional thermostat are provided in the dual mode transition or
antenna (applicator) to maintain a constant temperature at or very near to
the temperature of the human body.
Another advantage of the system of this invention is that when the
applicator is uncoupled from the human body, the level of radiation is
very small and well within safety standards. This advantage is realized by
the large mismatch associated with the low impedance ridged waveguide when
left open-circuited. When so removed, there is a mismatch with the
atmosphere which has a low dielectric constant. The measured radiation
level one inch from the waveguide opening with the L-band source fully
operating is well within safety standards. For example, one power
measurement was less than 0.4 mW/sq. cm. The safety standard established
by the federal government is 10 mW/sq. cm.
BRIEF DESCRIPTION OF THE DRAWINGS
Numerous other objects, features and advantages of the invention should now
become apparent upon a reading of the following detailed description taken
in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic diagram of the microwave radiometer employed in the
system of this invention;
FIG. 2 is a schematic diagram of the L-band transmitter employed in the
system;
FIG. 3 shows the body transition element in the form of a hand-held
applicator;
FIG. 4 is a perspective view of the dual mode antenna and waveguide
construction;
FIG. 5 is a side view of the waveguide construction depicted in FIG. 4;
FIG. 6 is a graph of frequency response for the bandpass filter of FIG. 1;
and
FIG. 7 is a graph of transmitter output level associated with the
transmitter of FIG. 2.
DETAILED DESCRIPTION
The microwave system of this invention comprises an extremely sensitive
passive radiometer capable of measurements of temperature deviations of
less than 0.1.degree. C. The dual mode microwave system also employs a
solid state transmitter to provide localized heating of the cancer site.
In the particular arrangement described herein, the C-band radiometer
frequency is 4.7 GHz and the L-band transmitter frequency is 1.6 GHz. The
system also includes a dual mode antenna comprising a C-band aperture in
combination with an L-band applicator. The microwave transmitter causes an
elevation of the temperature of the tumor above that of the surrounding
normal tissue to thus enhance the detection by highlighting the tumor with
respect to the surrounding or background tissue. The heating of the cancer
site results in a differential heating of the tumor with respect to the
surrounding tissue. Also, because temperatures above about 42.degree. C.
are lethal to tumor cells, the system is also applicable for the treatment
of cancer.
As mentioned previously, the system is preferably totally battery operated,
allowing approximately 8 hours of continuous service prior to requiring a
recharging. Such a battery operated system is employed as it eliminates
possible problems associated with line transients, pickup, etc. A battery
charging circuit is included in the system with an overnight charging
cycle being designed to provide the batteries in a fully charged condition
for the next day's use. Three sealed, maintenance free, lead acid
batteries are connected in series, providing a maximum voltage of 36
volts.
FIG. 1 is a schematic diagram of the microwave radiometer of this
invention. FIG. 2 shows a schematic diagram of the transmitter employed in
this system. The radiometer and transmitter both couple to the dual mode
antenna with the radiometer receiving its signal from the C-band aperture
and the transmitter directing its signal to the L-band applicator.
Accordingly, a discussion of the dual mode antenna receives a discussion
hereinafter of the radiometer and transmitter schematic diagrams.
As previously mentioned, the frequency selected for localized heating is
1.6 GHz. For this frequency, a normal waveguide transition that would be
used would have dimensions of 5.100" (12.95 cm).times.2.550" (6.48 cm).
These dimensions correspond to a WR-510 guide. Thus, to reduce the
physical size of the applicator aperture a single ridged waveguide
construction is used. The use of a ridged waveguide lowers the cutoff
frequency allowing use at a lower operating frequency or, in the present
situation, allowing the use of a smaller aperture size. To further reduce
the overall size of the aperture, dielectric loading is employed. The
dielectric that is utilized is preferably aluminum oxide having a relative
dielectric constant, e.sub.r, of 9.8. With the utilization of both a
ridged waveguide and dielectric loading the aperture size is substantially
reduced with respect to the tumor thus providing greater resolution and
improved focusing.
FIGS. 4 and 5 show the dual mode antenna construction which comprises an
L-band applicator 10 and a C-band aperture 12. The applicator 10, as noted
in the drawing, is in the form of a single ridge waveguide. This waveguide
receives a signal from the probe 14 which couples in turn to the coax line
16. Similarly, there is provided a probe 18 associated with the C-band
aperture 12 coupling to an associated coax line 20.
The ridged waveguide dimensions as identified in FIG. 4, are as follows:
A.sub.1 =9.296 cm
A.sub.2 =4.648 cm
b.sub.1 =4.648 cm
b.sub.2 =2.154 cm
reduced due to dielectric loading, e.sub.r =9.8, to
A.sub.1 =3.66 cm
A.sub.2 =1.83 cm
b.sub.1 =1.83
b.sub.2 =0.85
The following calculated parameters apply, namely
.lambda.g guide wavelength=26.63 cm
.lambda.o free space wavelength=18.64 cm
.lambda.c cutoff wavelength=27.89 cm
Zo.infin. characteristic impedance at an infinite frequency=150 ohms
Zo characteristic impedance=214 ohms
For a calculation of these parameters see Samuel Hopfer, "The Design of
Ridged Waveguides." IRE Trans., Vol. MTT-3, No. 5; October 1955 and S. B.
Cohn, "Properties of Ridged Waveguide." Proc. IRE, Vol. 35, pp. 783-788;
August 1947.
The insertion loss may be obtained by measuring the total loss of two
identical transitions in series (i.e., mated at the waveguide opening).
Assuming the two transitions to be equal in loss, the single transition
loss is 0.2 dB maximum. The VSWR, when held against the human body was
approximately 1.5:1. Since the human body does not represent a fixed
termination but rather a variable match, a reflectometer is included at
the transmitter output to enable determination of the reflected and
incident power levels. Both of these measured levels may be easily
combined to provide a single output reading.
The dimensions of the ridged portion of the L-band ridged waveguide are
selected to allow propagation of the higher frequency associated with the
C-band radiometer. As indicated in FIG. 4, the C-band transition or
aperture has dimensions of a height of 0.92 centimeters and a width of
1.83 centimeters.
The plated surfaces of the dielectric-loaded C-band waveguide form and
coincide with the single ridge of the L-band waveguide as depicted in FIG.
4. The plating may be of nickel, copper or gold, for example. With regard
to the C-band aperture, the insertion loss is measured to be less than 0.3
dB. The VSWR, when held against the human body is less than 2:1.
By placing the radiometer input within the single ridged waveguide L-band
transition, the point of maximum field of the source of heat is in close
proximity with the point of thermal detection. The cutoff characteristics
of the C-band waveguide are used along with other filtering to form a
highpass filter for isolating the highpower L-band source from the
sensitive radiometer.
As indicated in FIGS. 4 and 5, and in particular in FIG. 5, there is
provided a heater 24 which is disposed between the applicator and the
aperture. This heater may be of conventional design and is in the form of
a thin sheet having associated therewith a proportional thermostat for
maintaining a constant temperature at or very near to that of the
temperature of the human body. The microwave assembly is then contained in
an insulated housing 28 having indexing lines 30 on the outer surface as
shown in FIG. 3. The indexing lines are located 90.degree. apart on the
perimeter of the housing to allow accurate positioning of the C-band
radiometer input. To allow accurate and repeatable positioning of the
antenna, an indexed silk screen and frame (not shown) may be provided. The
use of the tightly drawn silk screen allows flattening of the portion of
the body to be scanned. The mismatch and loss associated with this thin
silk screen is negligible.
The microwave system of this invention is also quite safe to use. One of
the characteristics of the system is that there is a large mismatch on the
order of 12:1 associated with the low impedance ridged waveguide when left
open circuited. (i.e., in the atmosphere removed from the human body with
its high dielectric constant to which the waveguide is matched). Utilizing
a Narda Model No. 8607 power meter placed within one inch from the
waveguide opening with the L-band power source fully on, the measured
level was less than 0.4 mW/cm.sup.2. The safety standard established by
the government is 10 mW/cm.sup.2 for electromagnetic radiation, regardless
of frequency. For example, microwave ovens are permitted to radiate at a
level of 5 mW/cm.sup.2 at a distance of 2" from the oven.
Referring now to FIG. 1, there is shown a schematic diagram of the
microwave radiometer showing the signal coupled from the receiver antenna
(C-band aperture) to the switch SW1. The microwave radiometer that is
depicted is of special design in accordance with the present invention but
is generally of the common load comparison, or Dicke, type. The radiometer
design substantially reduces the effects of short term gain fluctuations
in the radiometer. The receiver input is switched by means of switch SW1
at a constant rate between the antenna and a constant temperature
reference load. The switched, or modulated RF signal is therefore inserted
at a point prior to RF amplification and as close to the antenna as
possible; in turn, it is then amplified and coherently detected. The final
output is proportional to the temperature difference between the antenna
and the reference load.
In FIG. 1 a second switch SW2, referred to as a calibration switch, is also
employed. With this switch, the reference load as defined by the noise
diode 36 and the fixed attenuator 38, is compared with a base load 40
rather than the signal from the antenna. If the base load is equal in
temperature with the reference load, the DC output of the radiometer is
thus nulled to zero.
In the case where long integration times are involved, long term gain
variations in the receiver are considered. The long term, or slow, gain
variations can degrade the minimum detectable temperature sensitivity,
.DELTA.T, in accordance with the following expression:
##EQU1##
where .DELTA.G=receiver gain change G=nominal receiver gain
T.sub.1 =temperature of reference load .degree.K.
T.sub.2 =temperature of base load or antenna, .degree.K. (function of
calibration switch position)
If the temperature T.sub.1 and T.sub.2 are maintained approximately the
same, the effect of long term receiver gain variations becomes negligible.
Therefore, it is advantageous to maintain the temperatures of both the
base load 40 and the reference load 42 approximately equal to the
temperature of the antenna.
The radiometer described herein employs at least one low noise RF amplifier
in conjunction with a simple single-ended square law detector rather than
the more complex superheterodyne which employs a local oscillator and IF
amplifier. The square law detector of this arrangement minimizes the
potential drift and noise associated with the superheterodyne approach.
The components that comprise the radiometer are discussed in detail
hereinafter.
Associated with FIG. 1 is table I set out herein which lists the individual
components shown in FIG. 1 along with their identifying part number and
brief description of their purpose or function.
TABLE I
__________________________________________________________________________
ITEM PART NO.
PURPOSE OR FUNCTION
__________________________________________________________________________
RECEIVER ANTENNA MA-56825
COAX-TO-WAVEGUIDE TRANSITION -
INTEGRATED WITH TRANSMITTER
ANTENNA
SWITCH-1 MA-56829
SPDT COAXIAL MECHANICAL SWITCH -
GREATER THAN 60 dB ISOLATION;
LESS THAN 0.1 dB LOSS
ISOLATOR-1 MA-56831
STRIPLINE FERRITE ISOLATOR WITH
INTEGRATED STRIPLINE-TO-
WAVEGUIDE TRANSITION
ISOLATOR-2 MA-56834
WAVEGUIDE FERRITE ISOLATOR
WITH INTEGRATED TRANSITION
TO COAX
SWITCH-3 MA-56832
WAVEGUIDE FERRITE ISOLATOR
SWITCH - DICKE SWITCH
REFERENCE LOAD 42
MA-56836
REFERENCE LOAD - COAXIAL
TERMINATION WITH INTEGRATED
HEATER AND PROPORTIONAL CONTROL
BASE LOAD 40 MA-56836
BASE LOAD - COAXIAL TERMINATION
WITH INTEGRATED HEATER AND
PROPORTIONAL CONTROL
FIRST RF AMPLIFIER
AMPLICA RF AMPLIFIER (FET) HAVING 2.2 dB
MODEL NOISE FIGURE AND 35 dB GAIN
3131CS1
ISOLATOR-3 MA-56837
COAXIAL FERRITE ISOLATOR -
20 dB MINIMUM ISOLATOR WITH
LESS THAN 0.3 dB LOSS
FILTER 44 MA-56838
STRIPLINE BANDPASS FILTER -
500 MHz BANDWIDTH
SECOND RF AMPLIFIER
AMPLICA RF AMPLIFIER (FET) HAVING 2.6 dB
MODEL NOISE FIGURE AND 33 dB GAIN
3441CS
SQUARE LAW DETECTOR
MA-56841
FIRST RF DETECTION HAVING
AND VIDEO AMPLIFIER 20 dB VIDEO GAIN
LOCK-IN AMPLIFIER 50
PRINCETON
PROVIDES IMPROVED SIGNAL-TO-
APPLIED NOISE RATIO THROUGH FREQUENCY
RESEARCH
LOCK AND NARROW BANDWIDTH -
MODEL PROVIDES SYNCHRONOUS
5101 DETECTION
SWITCH-2 MA-56829
SPDT COAXIAL MECHANICAL SWITCH
PROVIDING GREATER THAN 60 dB
ISOLATION AND LESS THAN
0.1 dB LOSS
NOISE DIODE 36 MSC NOISE SOURCE - 30 dB
MODEL EXCESS NOISE
MC5048
FERRITE SWITCH DRIVER/56
MA-56839
PROVIDES 100 Hz SQUARE WAVE
REFERENCE TO LOCK-IN AMPLIFIER
ALSO PROVIDES LATCHING FERRITE
SWITCH DRIVE.
__________________________________________________________________________
The minimum detectable temperature sensitivity, .DELTA.T is expressed as
follows:
##EQU2##
In the case of the Dicke switch employing square wave modulation, the value
of k is 2.0.
F=noise figure (first amplifier stage), which in our case is 2.2 dB (1.66
ratio).
L=input losses, expressed as a power ratio. The total loss is 2.0 dB (1.58
ratio).
The effective noise figure, FL, is therefore 2.2+2, or 4.2, which
represents a power ratio of 2.63.
T.sub.1, is the ambient radiometer temperature (microwave portion), namely,
290.degree. K.
T.sub.2, the source temperature (i.e., temperature seen by antenna), namely
310.degree. K.
B, the receiver bandwidth (i.e., the 3 dB bandwidth of the bandpass filter
following the first RF amplifier); namely, 500 MHz.
.tau., the radiometer output time constant in seconds.
Utilizing a three-second time constant, there is a minimum detectable
temperature sensitivity of:
##EQU3##
Increasing the time constant, T, to 10 seconds results in a .DELTA.T of
0.02.degree. K. Similarly, reducing the time constant to one second
results in a .DELTA.T of 0.07.degree. K.
The signal level at the input to the square law detector 46 of FIG. 1 is
determined as follows:
Noise Temp., NT=(FL-1)T.sub.o, .degree.K.
F=noise figure (first amplifier stage), which in our case is 2.2 dB (1.66
ratio).
L=input losses, expressed as a power ratio.
T.sub.o =ambient temperature of the radiometer, .degree.K.
The losses at 4.7 GHz, prior to the amplifier, are as follows:
______________________________________
Antenna or Applicator
0.3 dB
Cable 0.7
Calibration Switch SW1
0.1
Isolator/Waveguide Adapter
0.3
Dicke Switch SW3 0.3
Ferrite Isolator 0.2
Waveguide-to-Coax Adapter
0.1
2.0 dB (1.58 ratio)
______________________________________
The effective noise figure, FL, is therefore 2.2+2, or 4.2 dB, which
represents a power ratio of 2.63.
.thrfore.NT=(2.63-1)290=473.degree. K.
To calculate the noise power at the input to the radiometer, we have
P.sub.N =kTB watts
k,=Boltzmann's constant=1.38.times.10.sup.-23 .mu.joules/.degree.K.
T=473.degree. K. (calculated above)
B=bandwidth of radiometer, Hz; namely, 500 MHz (equivalent to the 3 dB
bandwidth of the bandpass filter)
P.sub.N =1.38.times.10.sup.-23 .times.473.times.500.times.10.sup.6
3.26.times.10.sup.-12 watts
Converting dB, we have
##EQU4##
The combined amplifier gain less the loss of the bandpass filter is 64 dB,
resulting in an input level to the square wave detector of (-84.9+64) or
-20.9 dbm which is well within the square law region.
With regard to the microwave radiometer schematic of FIG. 1, at its input
there is shown the connection which is preferably by way of a coax cable
from the receiver antenna (applicator aperture) to one input of switch
SW1. This may be termed a calibration switch which is a solenoid-operated,
mechanical, single pole/double-throw switch used to disconnect the antenna
and in its place connect the base load 40 by way of a second switch SW2.
The switch SW1 has an isolation, or switching ratio, of greater than 60 dB
with a corresponding insertion loss of less than 0.1 dB. The switch SW2 is
used in the calibration circuit to disconnect the base load and to insert
in its place the calibrated noise source as represented by the fixed
attenuator 38 and the noise diode 36 referred to hereinafter.
As indicated in FIG. 1, there are three ferrite isolators used in the
receiver path. These are identified as isolators ISOL-1, ISOL-2 and
ISOL-3. The first isolator, is located between the calibration switch SW1
and the Dicke switch SW3. This isolator is used to terminate the output of
the reference load when the Dicke switch is in the low loss state. In this
state, the reference or base load is circulated in the direction of the
antenna which, in this case, functions as a ferrite isolator. The isolator
ISOL-1 employs a coaxial-to-waveguide transition. The insertion loss of
this isolator and the transition is less than 0.2 dB, with a corresponding
isolation of greater than 23 dB.
The second isolator ISOL-2 in FIG. 1, is disposed between the switch SW3
and the first stage RF amplifier to maintain a constant load match to this
amplifier. Any reflections from the RF amplifier would therefore be
terminated in the isolator. Again, this isolator, which is a waveguide
isolator with a coax-to-waveguide transition, has an insertion loss of
less than 0.2 dB with an isolation of greater than 23 dB.
There is also provided in FIG. 1 a third isolator ISOL-3 which is located
between the output of the first RF amplifier and the bandpass filter 44.
The purpose of this particular isolator is to present a constant load
match to the output stage of the first RF amplifier, and also to present a
matched input to the bandpass filter 44.
A switchable ferrite circulator, designated switch SW3 in FIG. 1, forms the
load comparison, or Dicke switch, function. A ferrite device is preferred
over a semi-conductor approach primarily in view of the lower insertion
loss, typically less than 0.3 dB, and elimination of noise generated by
the semi-conductor junction over and above the measured insertion loss.
Briefly, the device SW3 is a switchable ferrite junction circulator
utilizing the remnant, or latching, characteristics of the ferrite
material. The principle of latching action is as follows: Using the
intrinsic properties of a hysterisis loop of a ferrite toroid, a
transverse magnetic field is used across a portion of the ferrite exposed
to an RF signal. The biasing field is actually the residual inductance of
the ferrite toroid; therefore, the device needs no holding power and can
be reversed in polarity using merely enough energy to overcome the natural
coercive force of the toroid.
For the system of this invention, the latching circulator has been
constructed in waveguide having a single ferrite element contained within
the microwave circuit. The insertion loss is less than 0.3 dB, having
isolation in excess of 20 dB.
The first-stage RF amplifier may be a four stage FET device constructed in
microstrip with integrated biasing circuitry. The noise figure of the
first amplifier (Amplica Model No. 3131CSI) is 2.2 dB with a gain of 35
dB. The second RF amplifier (Amplica Model No. 3441CS) has a noise figure
of 2.6 dB, with an associated gain of 33 dB. In both instances, the noise
figure includes the input ferrite isolator as depicted in FIG. 1. With the
input and output VSWR at less than 1.5:1, the gain compression at signal
levels of between -55 dbm to -10 dbm is less than 0.1 dB.
In FIG. 1 the filter 44 is a bandpass filter and the bandwidth of the
microwave radiometer is basically determined by the bandpass
characteristics of this filter. The filter is disposed after the first
stage of RF amplification to minimize the impact of the insertion loss of
the filter on the overall system performance. The filter characteristics
are chosen to minimize possible interference due to nearby microwave
communications or radar bands. FIG. 6 shows the filter characteristics.
The filter is preferably an 8-section bandpass filter constructed in
stripline. The pass band loss is less than 3 dB and the bandwidth is
approximately 500 MHz.
As indicated in FIG. 1, there are basically two loads provided, a base load
40 and a reference load 42. The load design is coaxial, employing a
stainless steel RF connector to provide thermal isolation betwen the load
and the remainder of the system. The coaxial termination is contained
within an insulated housing and utilizes an integrated heater and
proportional control to maintain constant temperature. The absolute
temperature of both the base and the reference loads is monitored and
displayed on a digital temperature indicator (not shown).
The calibration circuit comprises a precision, solid state, noise source
having an excess noise ratio, ENR of 33 dB. This allows noise to be
injected into the receiver front end via the high isolation mechanical
calibration switch. The output level of the noise source is reduced
through the use of a precision calibrated pad (43.3 dB). This calibration
circuit is shown in FIG. 1 as including a fixed attenuator 38 and the
noise diode 36.
The temperature sensitivity of the noise diode is less than 0.01
dB/.degree.C.
The apparent output noise temperature, T.sub.NO, at the SPDT switch is
##EQU5##
where T.sub.1 =temperature, ambient, of the source; namely,
273.13.degree.+22.25.degree. or 295.38.degree. K.
T.sub.2 =temperature of component in lossy path; namely, 295.38.degree. C.
.epsilon.=emissivity or, in this case, excess noise ratio (ENR) of the
noise source (33 dB corresponds to a ratio of 1995)
L=attenuation expressed as a power ratio (43.3 dB corresponds to a ratio of
21,380);
therefore,
##EQU6##
thus providing a 12.76.degree. differential with respect to the base load
of 310.degree. K.
The lock-in amplifier 50 shown in FIG. 1 is one made by Princeton Applied
Research, Model No. 5101. This amplifier enables the accurate measurement
of signals contaminated by broadband noise, power line pickup, frequency
drift or other sources of interference. It accomplishes this by means of
an extremely narrow band detector which has the center of its pass band
locked to the frequency of the signal to be measured. Because of the
frequency lock and narrow bandwidth, large improvements in signal-to-noise
ratio are achieved. This allows the signal of interest to be accurately
measured, even in situations where it is completely masked by noise. In
addition, the lock-in amplifier 50 provides the synchronous function
associated with the Dicke switch; i.e., the unit supplies the 100 Hz
reference clock frequency to drive the ferrite switch driver.
The system is provided, of course, with a power supply comprising three
12-volt, 50 amp. maintenance free, lead-acid batteries in series, fused at
10 amps per battery. The outputs from the battery assembly include 12, 24,
and 36 volts. These voltages are appropriately applied to the receiver,
lock-in amplifier and transmitter. There may also be provided a voltage
converter and regulator. Status indicators may be employed for indicating
operating voltages. The main operating switch may have three positions
including an on position, an off position and a "charged" position. In the
charged mode, a meter is used to monitor the charge current to the
batteries which is limited to approximately 6 amps. With a 3-9 amp-hour
discharge rate (a normal 8 hour operate mode), the recharge cycle is
approximately 10-12 hours (overnight).
The microwave transmitter embodied in the system of this invention is shown
in FIG. 2. This is an L-band transmitter operating at a frequency of 1.6
GHz. The transmitter includes a 1.6 GHz, 30 W, solid state source 60 which
couples to an RF power amplifier, filter, and microwave reflectometer.
There are two series connected filters 66 and 68 which are low-pass
filters connected in series for providing 120 dB of attenuation at the
third harmonic. The third harmonic of the 1.6 GHz source is 4.8 GHz, which
is within the radiometer passband. It is intended that the microwave
transmitter operates simultaneously with the microwave radiometer to
provide localized heating of subsurface tissue, while simultaneously
monitoring the temperature with the radiometer described previously. The
reflectometer employed in the transmitter of FIG. 2 allows determination
of both the reflected and incident power levels. The detector 70 measures
the incident level while the detector 72 measures the reflected level.
FIG. 2 also shows the output terminal 74 which is the RF output coupling
to the applicator.
The output power level from the transmitter of FIG. 2 is adjustable from 0
to 25 watts (measured at the input to the L-band antenna) and, therefore,
includes all microwave circuit and coaxial cable losses. FIG. 7
illustrates the approximate power input plotted as a function of "output
level" control setting. This measurement is made into a matched load and,
therefore, to be more accurate is reduced according to the load mismatch.
A 2.1 load VSWR, for example, correspnds to a 10% power reflection. For an
"output level" setting of 70 which corresponds to 10 watts (per FIG. 7),
therefore, there are actually 9 watts of incident power with 10% or 1 watt
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