U.S. patent application number 09/780754 was filed with the patent office on 2001-09-06 for variable microwave cold/warm noise source.
Invention is credited to Dunleavy, Lawrence P., Lardizabal, Steven M., Roeder, Robert S., Smith, Matthew C..
Application Number | 20010019572 09/780754 |
Document ID | / |
Family ID | 21863983 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019572 |
Kind Code |
A1 |
Roeder, Robert S. ; et
al. |
September 6, 2001 |
Variable microwave cold/warm noise source
Abstract
A radiometer calibrating system utilizes an adjustable noise
source for calibrating a radiometer. The noise source includes a
transistor configured as a noise equivalent circuit having a gate
port, drain port and source port. A source inductance providing
series feedback for the noise source has one end coupled to the
source port of the noise equivalent circuit and another end
connected to the ground. A bias circuit controls the amount of DC
bias applied to the noise equivalent circuit. In order to match the
impedances in the noise source, an output impedance matching
network is connected to the drain port and an input impedance
matching network is connected to the gate port of the noise
equivalent circuit. The output and input impedance networks have an
output port and input port, respectively. Included in the noise
source is a port switch that terminates a matched load to the
output port when a cold thermal radiation temperature is generated
at the input port, and alternatively, the port switch terminates
the matched load to the input port when the warm thermal radiation
temperature is generated at the output port.
Inventors: |
Roeder, Robert S.; (Dunedin,
FL) ; Smith, Matthew C.; (Largo, FL) ;
Dunleavy, Lawrence P.; (Land of Lakes, FL) ;
Lardizabal, Steven M.; (Andover, MA) |
Correspondence
Address: |
Harold E. Meier, Esq.
Baker Boots LLP
Suite 600
2001 Ross Avenue
Dallas
TX
75201
US
|
Family ID: |
21863983 |
Appl. No.: |
09/780754 |
Filed: |
February 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09780754 |
Feb 8, 2001 |
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09319520 |
Aug 23, 1999 |
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6217210 |
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09319520 |
Aug 23, 1999 |
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PCT/US97/20969 |
Nov 28, 1997 |
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60032262 |
Dec 3, 1996 |
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Current U.S.
Class: |
374/1 ; 374/2;
374/E11.003; 374/E15.001 |
Current CPC
Class: |
G01K 11/006 20130101;
G01K 15/00 20130101 |
Class at
Publication: |
374/1 ;
374/2 |
International
Class: |
G01K 015/00 |
Claims
What is claimed is:
1. An adjustable noise source, for calibration of a radiometer,
comprising: a noise equivalent circuit having a first port, a
second port and a third port; a source inductance providing series
feedback and having one terminal coupled to the third port of the
noise equivalent circuit and another terminal connected to the
ground; an output impedance matching network terminated at the
second port of the noise equivalent circuit, said output impedance
matching network includes an output port; an input impedance
matching network connected to the first port of the noise
equivalent circuit, said input impedance matching network includes
an input port; a bias circuit for applying the DC bias to the
output impedance matching network and the input impedance matching
network; a matched load having a first terminal connected to the
ground; and a port switch having a first position for connecting a
second terminal of the matched load to the input port and for
connecting a calibration line to the output port, and a second
position for connecting the second terminal of the matched load to
the output port and for connecting the calibration line to the
input port, where a warm thermal radiation temperature outputs from
the output port when the matched load terminates the input port and
a cold thermal radiation temperature outputs from the input port
when the matched load terminates the output port.
2. The noise source in accordance with claim 1 further including a
microprocessor connected to the bias circuit, said microprocessor
generating control signals to adjust the DC bias to establish the
cold thermal radiation temperature and the warm thermal radiation
temperature.
3. The noise source in accordance with claim 2 further including a
stabilizing compensation circuit in thermal contact with the noise
equivalent circuit and connected to the microprocessor to provide
temperature related control of the DC bias.
4. The noise source in accordance with claim 3 wherein the
stabilizing compensation circuit further includes a precision
thermistor in the thermal contact with the noise equivalent
circuit.
5. The noise source in accordance with claim 1 wherein the noise
equivalent circuit includes a microwave active FET having DC bias
and temperature dependent small signal parameters.
6. The noise source in accordance with claim 5 wherein the noise
equivalent circuit selectively operates at frequencies from less
than 2 GHz to greater than 90 GHz.
7. The noise source in accordance with claim 1 wherein the cold
thermal radiation temperature and the warm thermal radiation
temperature encompass a range of less than 100.degree. K to greater
than 2600.degree. K.
8. A radiometer comprising: a radiometer receiver; a switch
connected to said receiver for selecting between an antenna mode
and a calibration mode, the antenna mode enabling a brightness
temperature received from an antenna to be applied to the
radiometer receiver; a driver coupled to said switch for
controlling the operation of the switch; an adjustable noise source
connected to the radiometer receiver through said switch when in
the calibration mode, said noise source comprising: a noise
equivalent circuit having a first port, a second port, and a third
port; a source inductance providing series feedback having one
terminal coupled to the third port of the noise equivalent circuit
and another terminate connected to the ground; an output impedance
matching network terminated at the second port of the noise
equivalent circuit, said output impedance matching network includes
an output port; an input impedance matching network connected to
the first port of the noise equivalent circuit, said input
impedance matching network includes an input port; a bias circuit
for applying a DC bias to the output impedance matching network and
the input impedance matching network; a matched load having a first
terminal connected to ground; and a port switch having a first
position for connecting a second terminal of the matched load tq
the input port and for connecting a calibration line to the output
port, and having a second position for connecting the second
terminal of the matched load to the output port and for connecting
the calibration line to the input port, where a warm thermal
radiation temperature outputs from the output port when the matched
load terminates the input port and a cold thermal radiation
temperature outputs from the input port when the matched load
terminates the output port; and a microprocessor coupled to the
radiometer receiver, said noise source and said driver for
correcting the output of said radiometer-receiver.
9. The radiometer in accordance with claim 8 wherein the
microprocessor generates a port switch command controlling the port
switch of the noise source to alternate between the cold thermal
temperature and the warm thermal temperature.
10. The radiometer in accordance with claim 8 wherein the
microprocessor includes a memory for storing calibration curve
data, said microprocessor responds to calibration curve signals
from the noise source and the stored calibration curve data to
correct voltage outputs from the radiometer receiver.
11. The radiometer in accordance with claim 8 wherein said noise
source further includes a thermistor in thermal contact with the
noise source, said thermistor generating a signal to the
microprocessor corresponding to the physical temperature of the
noise source.
12. The radiometer in accordance with claim 11 wherein said
microprocessor further includes a memory storing a plurality of
reference curves for correcting the voltage output from the
radiometer receiver.
13. A method for calibrating a radiometer receiver using an
adjustable noise source, comprising the steps of: transmitting a
bias command from a microprocessor the noise source; generating at
outputs of the noise source a cold thermal radiation temperature
output from an input port or a warm thermal radiation temperature
output from an output port; coupling the cold thermal temperature
or warm thermal temperature to the radiometer receiver; and
alternating the position of a port switch in accordance with a
command transmitted from the microprocessor to the noise source to
alternate the coupling between the cold thermal radiation
temperature and the warm thermal radiation temperature to the
radiometer receiver.
14. The method in accordance with claim 13 further including the
step of stabilizing the voltage output from the radiometer
receiver.
15. A solid state noise source providing a warm thermal radiation
temperature and a cold thermal radiation temperature for
calibration of a radiometer, comprising: a FET microwave active
noise equivalent circuit having a gate port, a drain-port and a
source port; a feedback network coupled to the source port of the
FET and to ground for providing a series feedback; an output
impedance matching network terminated to the drain port of the FET;
an input matching network connected to the gate port of the FET; a
matched load having a first end connected to ground; a biasing
network coupled to the output matching network and the input
matching network; and a port switch having a first position for
connecting a second terminal of the matched load to the input port
and for connecting a calibration line to the output port, and
having a second position for connecting a second terminal of the
matched load to the output port and for connecting the calibration
line to the input port, where a warm thermal radiation temperature
outputs from the output port when the matched load terminates the
input port and a cold thermal radiation temperature outputs from
the input port when the matched load terminates the output
port.
16. The solid state noise source in accordance with claim 15
wherein the biasing network further includes a microprocessor for
adjusting the DC bias to control the magnitude of the warm thermal
radiation temperature and the cold thermal radiation
temperature.
17. The solid state noise source in accordance with claim 16
further comprising a stabilizing circuit coupled to the
microprocessor compensating and controlling the DC bias.
18. The solid state noise source in accordance with claim 17
wherein the stabilizing circuit includes a thermistor in thermal
contact with the FET.
19. The solid-state noise source in accordance with claim 15
wherein the FET selectively operates from less than 2 GHz to
greater than 90 GHz.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to calibration of a radiometer
with reference temperatures from a noise source and, more
particularly, to calibration of a radiometer with reference
temperatures from an electronically adjustable noise source
providing hot thermal radiation temperature from an output port and
cold thermal radiation temperature from an input port.
BACKGROUND OF THE INVENTION
[0002] Radiometers are used to measure thermal radiation or
brightness temperatures emitted from a segment of a remote object.
The segment is commonly referred to as a scene and may be a portion
of the earth's surface. Like most sophisticated instrumentation,
radiometers require periodic calibration to insure accurate
measurements. In practice, at least two known calibration
temperatures that abound the brightness temperatures of the scene
are used to calibrate a radiometer receiver. The lowest and highest
calibration temperatures are referred to as cold and hot thermal
radiation temperatures, respectively.
[0003] Radiometers are generally ground-based, airborne or
satellite-based systems that measure brightness temperatures in the
mostly cold range of 10.degree. K-300.degree. K. There are also
specialized radiometer applications where an instrument is needed
to measure hot brightness temperatures from forest fires and
burning dumps. For these applications the radiometer must measure
brightness temperatures in the range of 300.degree. K to greater
than 1000.degree. K. The ground-based systems may utilize closed
cycle refrigeration such as a sterling cycle cooler with liquid
nitrogen or is liquid helium to generate cold thermal radiation
temperatures "Tc". The closed cycle refrigeration systems are not
considered practical for the satellite-based systems.
[0004] Referring to FIGS. 1-3, there are illustrated three
traditional satellite-based systems for measuring the brightness
temperature "Ta" emitted from a portion of the earth's surface and
received by an antenna 36. The brightness temperature "Ta" is then
transmitted through an antenna feed 32 on an antenna-earth scene
line 12 to a radiometer receiver 16 of the radiometer 150.
Currently, satellite-based systems use calibration techniques that
are either externally-based (FIGS. 1 and 2) or internally-based
(FIG. 3).
[0005] Referring to FIG. 1, there is illustrated an
externally-based calibration technique known as the sky horn
approach. The sky horn approach utilizes a radiometer 150 which
includes a first RF switch 10 connected to either the antenna-earth
scene line 12 or a calibration line 14 to the radiometer receiver
16. In the calibration line 14 a second RF switch 18 alternately
switches between a sky horn 20 and in internal warm load 22. The
sky horn 20 outputs the cold space thermal radiation temperature
"Tc," approximately 2.7.degree. K, and the internal warm load "Tw,"
approximately 300.degree. K. A precision thermistor 24 in thermal
contact with the warm load 22 outputs an electrical hot thermal
radiation temperature "Td" that is equivalent to the hot thermal
radiation temperature "Tw." The electrical hot thermal radiation
temperature "Td" is utilized in the calibration of the radiometer
receiver 16.
[0006] The sky horn approach is a complex and expensive way to
calibrate the radiometer receiver 16. The main problem is that the
antenna-earth scene line 12 and calibration line 14 are separate
lines, thereby requiring precise knowledge of the RF losses,
mismatch losses and physical temperatures of each line to
accurately calibrate the radiometer receiver 16. Also, the use of
the sky horn 20 adds to the complexity of the calibration, because
of possible interference of the sky horn pattern by a spacecraft or
contamination caused by the earth or sun.
[0007] Referring to FIG. 2, there is illustrated another
externally-based calibration technique for satellite-based systems
using an antenna scanner 26. The antenna scanner 26 is a mechanical
mechanism employed during a calibration mode to alternately couple
a reflector plate 28 or an absorption target 30 to respectively
feed a cold thermal radiation temperature "Tc" or a warm thermal
radiation temperature "Tw" to the antenna feed 32. The antenna feed
32 is connected to the radiometer receiver 16. During an antenna
mode when the brightness temperature "Ta" is measured the antenna
scanner 26 connects the antenna-earth scene line 12 to the
radiometer receiver 16. The antenna scanner 26 does have an
advantage over the sky horn approach in that only one RF path is
utilized. However, the antenna scanner 26 is complex, bulky and
adds significant size and weight to the radiometer 150.
[0008] Referring to FIG. 3, there is illustrated an
internally-based calibration technique that may be used in a
satellite-based system. The internal approach is very similar to
the sky horn approach discussed previously and illustrated in FIG.
1. However, the internal technique may utilize a thermoelectric
cooler 34 to generate a cold thermal radiation temperature "Tc" of
approximately 270.degree. K, instead of the sky horn 20 used in the
sky horn approach. However, the warm and cold thermal radiation
temperatures "Tc" and "Tw" used in the internal is approach may
only be 30.degree. K apart. The 30.degree. K difference between the
cold and warm thermal radiation temperatures "Tc" and "Tw" does not
cover the full range of each brightness temperatures which are
approximately 100.degree. K to 300.degree. K, (exclusive of burning
materials) therefore, measurement accuracy of the radiometer
receiver 16 will likely degrade below the cold thermal radiation
temperature "Tc."
[0009] Accordingly, there is a need for an adjustable calibration
noise source to provide cold to hot thermal radiation temperatures
from a waveguide or coaxial port. There is also a need to provide a
noise source manufactured using microwave integrated circuit (MIC)
and/or monolithic microwave integrated circuit (MMIC) technologies.
These and other needs are satisfied by the adjustable calibration
noise source of the present invention.
SUMMARY OF THE INVENTION
[0010] The present invention is a radiometer calibration system
utilizing an electronically adjustable noise source and a method
for calibrating a radiometer. The noise source includes a
transistor configured as a noise equivalent circuit having a gate
port, drain port and source port. A source inductance providing
series feedback for the noise source has one end coupled to the
source port of the noise equivalent circuit and another end
connected to ground. A bias circuit controls the amount of DC bias
applied to the noise equivalent model. In order to match the
impedances in the noise source, an output impedance matching
network is connected to the drain port and an input impedance
matching network is connected to the gate port of the noise
equivalent model. The output and input impedance networks have an
output port and input port, respectively. The noise source
terminates a matched load to the output port while an adjustable
cold thermal radiation temperature is generated at the input port.
Alteratively, a port switch may be used to terminate a matched load
to the input port while an adjustable hot thermal radiation
temperature is generated at the output port.
[0011] According to the present invention there is provided an
adjustable noise source for calibrating ground-based, airborne, or
satellite-based radiometers.
[0012] Also in accordance with the present invention there is
provided a noise source that functions in the millimeter and
microwave spectrum.
[0013] Further in accordance with the present invention there is
provided a noise source implemented as an integrated circuit.
[0014] Further in accordance with the present invention there is
provided a calibration system having a noise source for measuring
the radiometer receiver transfer function or receiver
linearity.
[0015] Further in accordance with the present invention there is
provided a calibration system having a noise source with a
built-in-test capability providing noise figure measurements.
[0016] In accordance with the present invention there is also
provided a radiometer having adjustable calibration time intervals
to maximize the measurement of earth scenes.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A more complete understanding of the invention may be had by
reference to the following Detailed Description when taken in
conjunction with the accompanying Drawings wherein:
[0018] FIG. 1 is a schematic representation of PRIOR ART
illustrating a sky horn approach for calibrating a satellite-based
radiometer;
[0019] FIG. 2 is a schematic representation of PRIOR ART
illustrating a calibration technique using an antenna scanner;
[0020] FIG. 3 is a schematic representation of PRIOR ART where an
internally-based calibration technique uses a thermoelectric
cooler;
[0021] FIG. 4 is a schematic representation of the present
invention illustrating a satellite-based radiometer calibration
system incorporating an adjustable noise source;
[0022] FIGS. 5A-5D are illustrations of calibration curves for use
with the radiometer calibration system of FIG. 4;
[0023] FIGS. 6A, 6B and 6C are illustrations of calibration and
port switch commands respectively transmitted by a microprocessor
to a drive and the adjustable noise source illustrated in FIG.
7;
[0024] FIG. 7 is a schematic of the adjustable noise source;
[0025] FIG. 8 is a schematic of the adjustable noise source
(without a port switch, port driver and load) implemented as a
microwave integrated circuit;
[0026] FIG. 9 is a graph indicating noise temperature performances
for three types of FETS, each biased for a minimum-noise figure at
18 GHz;
[0027] FIG. 10 is a graph comparing input noise temperatures output
from an InP HEMT (FET) having various source inductances;
[0028] FIG. 11 is a graph of measured and simulated data
illustrative of cold and hot thermal radiation temperatures output
from the InP HEMT illustrated in FIG. 11;
[0029] FIG. 12 is a graph of cold thermal radiation temperatures
measured at the InP HEMT operating at 18 GHz;
[0030] FIG. 13 is a graph of hot thermal radiation temperatures
measured at the FET operating at 18 GHz;
[0031] FIG. 14 is a graph illustrating a measured variation of cold
noise source (port 1) reflecting coefficient magnitude at 18
GHz;
[0032] FIG. 15 is a graph illustrating a measured variation of warm
noise source (port 2) reflecting magnitude at 18 GHz; and
[0033] FIG. 16 is a graph illustrating a noise figure measurement
of the radiometer receiver.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Referring to FIG. 4, wherein like numerals represent like
parts throughout the several views, there is disclosed an
adjustable noise source 100 for calibration of a radiometer in
accordance with the present invention.
[0035] Although the noise source 100 will be described incorporated
with a radiometer calibration system 150, those skilled in the art
will appreciate such application is only one of many for utilizing
the noise source of the present invention. Accordingly, the
described noise source 100 should not be construed in a limiting
manner.
[0036] A noise source using a FET such as illustrated and described
in U.S. Provision application Ser. No. 60/032,290 has warm and cold
thermal radiation temperatures output at the gate port of the FET.
Robert Roeder and Matthew Smith, two of the inventors of the
present invention, are joint inventors of the noise source
illustrated and described in the U.S. Provisional application Ser.
No. 60/032,290, which is hereby incorporated into this
specification.
[0037] Major contributing errors associated with calibrating
satellite-based radiometers arise from the following factors: (1)
cold calibration brightness temperature; (2) warm calibration
brightness temperature; (3) radiometer receiver transfer function;
(4) ground retrieval algorithm; and (5) antenna brightness
temperature. Each of the major contributing errors must be
separately addressed and combined in establishing an overall
accuracy scheme for the radiometer calibration system 150. The
errors associated with the cold and warm calibration brightness
temperatures and the radiometer receiver transfer function are
addressed by the noise source 100. A detailed description of the
noise source 100 will be discussed after describing the Interaction
of the noise source with the radiometer calibration system 150.
[0038] Referring to FIG. 4, there is illustrated a block diagram of
the satellite-based radiometer calibration system 150 incorporating
the noise source 100. The brightness temperature "Ta" emitted from
a segment of the earth's surface is received by the antenna
reflector 36 and transmitted to the antenna feed 32. The antenna
feed 32 outputs the brightness temperatures "Ta" on the
antenna-earth scene line 12. The antenna-earth scene line 12 is
connected to a selector switch 62 for switching either the
antenna-earth scene line 12 or a calibration line 64 to an input
terminal 66 of the radiometer receiver 16. The calibration line 64
connects the noise source 100 to the radiometer receiver 16. The
selector switch 62 is preferably a low loss RF ferrite switch.
[0039] A driver 68 actuates and controls the selector switch 62
according to commands received from a microprocessor 70. Initially,
the microprocessor 70 receives a "test command" signal from an
external source (not shown) on line 80; the test command triggers
the calibration sequence.
[0040] Referring to FIGS. 4, 6A, 6B and 6C, the microprocessor 70
transmits ad command on line 72 to the driver 68 to actuate either
an antenna mode 82 or calibration mode 84 (FIG. 6A). In the antenna
mode 82 the selector switch 62 is actuated to connect the
antenna-earth scene line 12 to the input terminal 66 of the
radiometer receiver 16. In the calibration Mode 84 the selector
switch 62 is actuated to connect the calibration line 64 to the
input terminal 66 of the radiometer receiver 16. Selection of the
calibration mode at selected time intervals for short durations
maximizes measurements of the brightness temperatures "Ta".
[0041] The microprocessor 70 also transmits a port switch command
signal .86 (FIG. 6B) on line 74 to the noise source 100. The noise
source 100, in response to the port switch command signal 86,
alternately outputs a fixed cold thermal radiation temperature "Tc"
or a fixed warm thermal radiation temperature "Tw". The temperature
may be stepped from warm to cold as shown in FIG. 6C. The stepped
mode is used to measure the radiometer receiver transfer function.
Alternating between the warm and cold thermal radiation
temperatures "Tc", "Tw" occurs during the calibration mode 84. The
noise source 100 does not output the cold thermal radiation
temperature "Tc" or the warm thermal radiation temperature "Tw"
during the antenna mode 82.
[0042] Referring again to FIG. 4, the noise source 100 includes a
correction precision thermistor 76 in thermal contact with the
noise source and connected to the microprocessor 70 by a line 78.
The correction precision thermistor 76 provides compensation for
changes in the physical temperature "Td" of the noise source 100. A
thermal insulation blanket 71 may be provided to encompass the
noise source 100. The compensation, DC bias, and the correction
precision thermistor 76 will be discussed in greater detail
later.
[0043] Prior to suing the radiometer calibration system 150, the
noise source 100 is initially calibrated with a laboratory
radiometer (not shown). During the initial calibration of the
radiometer calibration system 150 there is generated a series of
reference calibration curves which are stored in the microprocessor
70. the calibration curves are accessed by the microprocessor 70
during the calibration mode 84 to adjust the uncorrected output
voltage from the radiometer receiver 16 on line 98 to output a
corrected output voltage on line 99.
[0044] Referring to FIGS. 5A, 5B, 5C and 5D, the calibration curves
include a precision thermistor calibration curve 88, a noise source
radiation temperature drift curve 90, a radiometer calibration
curve 92 and a corrected radiometer calibration curve 94. The
calibration curves illustrate the calibration procedure based on
using the two known calibration temperatures "Tc" and "Tw".
[0045] The precision thermistor curve 88 (FIG. 5A) illustrates the
change in the voltage "Vd" versus the physical temperature "Td" of
the noise source 100 sensed by the thermistor 76 and applied to the
microprocessor 70 along the signal line 78. "Vd" is a calibrated
thermistor output voltage corresponding to the known physical
temperature "Td."
[0046] The noise source radiation temperature drift curve 90 (FIG.
5B) on the line 64 and radiometer calibration curve 92 (FIG. 5C) on
the line 98 are combined into the corrected radiometer calibration
curve 94 (FIG. 5D). The corrected radiometer calibration curve 94
represents the amount of correction required of the uncorrected
output voltage generated by the radiometer receiver 16 on line 98
and input to the microprocessor 70. The radiometer calibration
curve 92 (FIG. 5C) illustrates the radiometer calibration
performance during the calibration mode 84. The uncertainty is due
to the variation in the physical temperature "Td" of the noise
source 100. The microprocessor 70 utilizing data represented by the
precision thermistor curve 88 adjusts the uncorrected voltage
output on line 98 to generate a corrected voltage on line 99. The
corrected voltage output represents the correct output by taking
into consideration the physical temperature "Td" of the noise
source 100. The shift in the calibration curves 88, 90, 92 and 94
have been exaggerated to illustrate the correction procedures of
the radiometer calibration system 150. Furthermore, data
represented by the calibration curves 88, 90, 92 and 94 is also
utilized to adjust the output signal of the radiometer receiver 16
when operating in the antenna mode 82.
[0047] Referring FIG. 7, there is illustrated a schematic of the
adjustable noise source 100. The noise source 100 includes a field
5 effect transistor (FET) configured as a noise equivalent model
114 and having a gate port 116, a drain port 118 and a source port
120. The noise equivalent model 114 is a microwave active circuit
designed to generate noise temperatures such as warm and cold
thermal radiation temperatures "TwIl" and "Tc" when DC bias is
applied.
[0048] The term "noise-temperature" is an expression for the noise
power spectral density at a specified f frequency and is derived
from Planck's blackbody formula. The average energy of an
oscillator at a temperature T is: 1 = hf exp ( hf / kT ) - 1 ( 1
)
[0049] where f is the frequency; his Planck's constant; and k is
the thermal conductivity. At high temperatures and low frequencies
<.epsilon.> approaches kT so the power in a bandwidth B will
be P=kTB (Nyquist's formula). A quantity .phi.=P/kB is taken as a
convenient unit of thermal noise power spectral density and is
referred to as "noise temperature." The noise source 100 includes a
source inductance 122 with one end coupled to the source port 120
of the noise equivalent model 114 and another end connected to
ground. The source inductance 122 provides series feedback for the
noise 100, where the source inductance 122 is typically in the
range of 20-700 pH.
[0050] A bias circuit 128 generates the DC bias that is applied to
the noise source 100, during the calibration mode 84. The bias
circuit 128 generates the voltage "Vgs" 140 (voltage across the
gate port 116 and the source port 120) and the voltage "Vds" 142
(voltage across the drain port 118 and the source port 120). The
microprocessor 70 adjusts the magnitude of the DC bias to change
the values of the cold and hot thermal radiation temperatures "Tc"
and "Tw". More particularly, the DC bias corresponds to the port
switch command signal on line 74 transmitted from the
microprocessor 70 (FIG. 4).
[0051] A stabilizing compensation circuit 130 in contact with the
noise equivalent model 114 and connected to the microprocessor 70
(FIG. 4) provides further control of the DC bias. The stabilizing
circuit 130 includes the precision thermistor 76 and measure the
physical temperature "Td" of the noise source 100. When the
stabilizing compensation circuit 130 is not used fluctuations in
the physical temperature "Td" of the noise source 100 may adversely
effect the performance of the noise source.
[0052] An output matching impedance network 124 includes an output
port 144 from which the warm thermal radiation temperature "Tw" is
outputted. The output matching impedance network 124 further
includes a plurality of output transmission lines and/or lumped
elements (FIG. 8) configured and sized to match the impedances of
the output port 144 and the drain port 118 of the noise equivalent
circuit 114. The output matching network 124 has one end connected
to the drain Port 118. The plurality of output transmission lines
and/or lumped elements may be manufactured on an Al.sub.2O.sub.3
substrate of approximately 0.015" thick for frequencies up to about
35 GHZ.
[0053] An input matching impedance network 112 includes an input
port 146 from which the cold thermal radiation temperature "Tc" is
outputted. The input matching network 112 further includes a
plurality of input transmission lines and/or lumped elements (FIG.
8) configured and sized to match the impedances of the input port
146 and the gate port 116 of the noise equivalent circuit 114. The
input matching impedance network 112 has one end connected to the
gate port 116 of the noise equivalent model 114. The plurality of
input transmission lines and/or lumped elements may be manufactured
on an Al.sub.2O.sub.3 substrate approximately 0.015" thick.
[0054] A port driver 151 actuates and controls a port switch 148
according to a port switch command received from the microprocessor
70. The port driver 151 preferably configured with low loss RF
ferrite switches. The port switch 148 has a plurality of contracts
connecting output port 144 and input port 146, to the calibration
line 64 (FIG. 4).
[0055] The port switch 148 selects either the cold thermal
radiation temperature "Tc" or the hot thermal radiation temperature
"Tw." The contacts of the port switch 148 are configured in a
predetermined manner such that a matched load 152 terminates the
input port 146 when the calibration line 64 connects to the output
port 144, or a matched load 153 terminates the output port 144 when
the calibration line 64 connects to the input port 146.
[0056] The hot thermal radiation temperature "Tw" exits the output
port 144 when the matched load 152 terminates the input port 146,
and a cold thermal radiation temperature "Tc" exits the input port
146 when the matched load 153 terminates the output port 144. The
two matched loads 152 and 153 have one end connected to the ground
and the other end connected to either the output port 144 or the
input port 146. The typical noise temperatures generated by the
noise source 100 have a range of less than 100.degree. K to greater
than 2600.degree. K.
[0057] Referring to FIG. 8, there is illustrated a schematic of an
adjustable noise source implemented as a microwave integrated
circuit. The microwave integrated circuit utilizes either microwave
integrated circuit (MIC) or monolithic microwave integrated circuit
(MMIC) technologies. The noise source 100 may be designed to
operate in the microwave and millimeter wave spectrum having an
operation frequency of less than 2 GHz to greater than 90 GHz.
[0058] Referring to FIG. 9, a series of graphs illustrate noise
temperature performances for three types of FETs, each biased for
minimum noise figure at 18 GHz. The FET types include a 0.25 .mu.m
GaAs MESFET, a 0.25 .mu.m GaAs PHEMT and a 0.15 .mu.m InP HEMT. The
FETs were enabled by noise circuit models and implemented in
HP-EESOF's Libra (TM). "Trev" represents noise power exiting the
input port of a two-port terminated in a reflection-less load held
at 0.degree. K.
[0059] "Trev", also referred to as reverse available noise, may be
used to predict a source temperature "Ts" (FIG. 7) which is either
the cold or warm thermal radiation temperature "Tc" and "Tw." The
source temperature "Ts" is indicative of the cold or warm thermal
radiation temperature "Tc" and "Tw" when the reverse available
noise "Trev" is added to an ambient temperature noise of the
opposite port termination transformed through the noise equivalent
model 114 using the appropriate forward or reverse power gain. The
source temperature "Ts" so calculated may be referred to as port 1
source temperature "Tout1" and port 2 source temperature
"Tout2".
[0060] Referring to FIG. 10, there is illustrated a graph comparing
input noise temperatures output from an InP HEMT (FET) having
various source inductances 122. The graph also includes
measurements for "Temin" the effective minimum noise temperature
defined as Tcmin=To(Fmin-1). The graph also indicates "Teq" the
equivalent noise temperature of the short circuit noise current in
the input port 146 having a resistance of 50 .OMEGA..
[0061] FIG. 11 is a graph illustrative of measured and simulated
data of cold "Tout1" and warm "Tout2" thermal radiation
temperatures "Tc" and "Tw" output from the InP HEMT. In the graph
the source temperature "Ts" is a function of the voltage "Vgs" 140
where "Vds" 142 equals one volt and source inductance 122 equals
0.24 nH.
[0062] FIG. 12 is a graph of cold thermal radiation temperatures
"Tc" measured from the InP HEMT operating at 18 GHz. A portion of
the measurements were made at the National Institute of Standards
and Technology (NIST) using an 18-26 GHz substitution radiometer,
referenced to a cryogenic waveguide noise standard. The remaining
measurements were taken by using the noise power measurement mode
of a 0.01-18 GHz HP8970B/HP8971B noise figure measurement system.
The remaining measurements were referenced to a HP346B solid-state
diode.
[0063] FIG. 13 is a graph of warm thermal radiation temperatures
measured at 18 GHz, from the output port with the input terminated
in a 50 OM load.
[0064] FIG. 14 is a graph illustrating a measured variation of cold
noise source (port 1) reflection coefficient magnitude. The
measured variation may necessitate the use of a circulator for some
applications, and is responsible for some of the differences
between the temperature data illustrated in FIG. 12. However, the
reflection coefficient does show minimal variation with bias in the
intended operation region.
[0065] FIG. 15 is a graph illustrating a measured variation of hot
noise source (port 2) reflection magnitude.
[0066] Referring to FIG. 16 there is a graph illustrating a noise
figure measurement of the radiometer receiver 16. Noise figure
measurement is the process of quantitatively determining the ratio
of the total noise power per unit bandwidth at the output of the
noise source 100 to the portion of the noise power due to the input
termination, at the standard temperature of 290.degree. K. The
noise figure (F) equation may be represented by the following
equation:
F=Tr To+1 (2)
[0067] where "Tr" is the receiver noise temperature and "To"
represents the temperature of the radiometer receiver 16. "To" is
measured using a receiver precision thermistor (not shown) mounted
on RF components in the radiometer receiver 16.
[0068] The following equations are derived by referring to FIG. 7
and are relevant in calculating the noise figure measurement
utilizing a linear radiometer receiver 16:
[0069] For the linear radiometer receiver 2 Vo - Vc Tin - Tc = Vw -
Vc Tw - Tc ( 3 )
Tin=Tc or Tw applied to the radiometer receiver (4)
[0070] For Tin=0 3 VR = Vc = Tc Vw - Vc Tw - Tc ; and ( 5 ) 4 Tr =
Vc Tw - Tc Vw - Vc - Tc ( 6 )
[0071] The noise figure is expressed by:
F=Tr/To+1 (where To.congruent.290.degree. (ambient)) (7)
[0072] where "Vc,", "Vr," and "Vw" are the radiometer output
voltages corresponding to "Tc," "Tr" and "Tw," respectively.
[0073] While the present invention has been described with
reference to the illustrated embodiment, it is not intended to
cover such alternatives, modifications and equivalents as may be
included in the spirit and scope of the invention as defined in the
following claims.
* * * * *