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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to spectral analyzers and, more particularly, to
the calibration thereof.
2. Description of the Prior Art
The use of dectector arrays for spectral analysis is well-known in the
prior art. These prior art devices are often complex and/or not amenable
to self or automatic calibration and/or are subject to error. Furthermore,
this is particularly disadvantageous when the spectral analyzer uses a
data processor such as, for example, a multiprocessor or a general purpose
computer to process the spectrometric data derived therefrom and/or is
under computer control.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a spectral analyzer employing
a detector array which is capable of self-calibration in a simple and/or
automatic manner.
It is another object of this invention to provide a spectral analyzer of
the aforementioned kind in which the detector array is capable of being
illuminated by at least two portions of a radiation spectrum image.
Another object of this invention is to provide a spectral analyzer of the
aforementioned kind which is readily amenable to computer data processing
and/or control.
Still another object of this invention is to provide a spectral analyzer of
the aforementioned kind which is embodied as a spectrophotometer.
Still another object of this invention is to provide a spectral analyzer of
the aforementioned kind that is a dual beam type and is capable of
self-calibration with either or both of the two beams.
Still another object of this invention is to provide a spectral analyzer of
the aforementioned kind that is capable of attenuation compensation for
different radiation transmission conditions.
According to one aspect of the invention, there is provided spectral
analyzer apparatus that has a source of radiation which provides at least
one beam of radiation with a predetermined spectral bandwidth
characteristic. A spectral band image converter, which includes radiation
dispersing means that is incident to the beam and spectrum imaging means,
provides a spectral band image, i.e. a spectrum image, of the beam.
Selective irradiating means irradiates an array of detectors with at least
two portions of the spectral band image. One of the portions is associated
with a predetermined first spectral range. The other portion is associated
with a predetermined second spectral range. The first and second ranges
include mutually exclusive first and second spectral components,
respectively, as well as a common spectral component intermediate of the
first and second spectral components. The detectors of the array are
electrically scannable in a predetermined sequence. Calibrating means for
calibrating the detectors for the first and second ranges are provided
which includes first and second narrow band optical filters and means for
disposing these filters in the radiation beam. The first and second
filters are single and double narrow bandpass filter types, respectively.
The first filter has a spectral transmission characteristic corresponding
to one of the three aforementioned spectral components. The second filter
has a spectral transmission characteristic corresponding to the other two
spectral components.
The foregoing and other objects, features and advantages of the invention
will be apparent from the following more particular description of the
preferred embodiments of the invention as illustrated in the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a perspective and exploded view, partially broken away, of a
preferred embodiment of the spectrum analyzer apparatus of the present
invention;
FIG. 2 is an end view of certain fiber optic cables shown in FIG. 1;
FIG. 3 is a more detailed elevation view of the monochromator portion of
the apparatus of FIG. 1 as viewed from the direction of the arrow AA shown
therein;
FIG. 4 is a partial enlarged detail view, shown partially in cross-section,
of the selective irradiating means shown in FIG. 3;
FIG. 5 is a schematic plan view of a preferred photodetector array shown in
FIGS. 1 and 2;
FIG. 6 is an elevation view of the chopper wheel shown in FIG. 1 as viewed
from the direction of the arrow AA shown therein;
FIG. 7 is an idealized waveform timing diagram of an illustrative
illumination sequence cycle example used to describe the operation of the
embodiment of FIG. 1;
FIG. 8 is an idealized waveform diagram of the bandpass transmission
characteristics of the calibration filters of the chopper wheel shown in
FIGS. 1 and 6;
FIG. 9 is a simplified view shown in schematic form of another embodiment
of the present invention;
FIG. 10 is an elevation of the chopper wheel shown in FIG. 9, as viewed
from its direction of the arrow AA' shown therein;
FIG. 11 is an idealized waveform diagram of the bandpass transmission
characteristics of the calibration filters of the chopper wheel shown in
FIGS. 9 and 10; and
FIG. 12 is a view of a modification of the probe tip that can be used for
the embodiments of FIGS. 1 and/or 9.
In the FIGURES, like elements are designated with similar reference numbers
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, there is shown a simplified schematic of a
preferred embodiment of the spectral analyzer apparatus, also sometimes
referred to herein as a spectrum analyzer apparatus, of the present
invention. In FIG. 1, the apparatus is configured as a dual beam system.
As will become apparent from the description hereinafter, either or both
of the beams of the system can be used to calibrate the apparatus.
In general, the two radiation beams are provided by a source generally
indicated by the reference numeral 1. Each of the beams from source 1 is
transmitted along a common path A towards a spectral band image converter
that includes radiation dispersing means 2 which is incident to the
particular beam. Means 2 disperses the incident beam into a spectrum and
is preferably a reflection type plane diffraction grating. The dispersed
beam in turn is reflected by spectrum imaging means 2A, which is part of
the aforementioned spectral band image converter sometimes referred to
herein also as the spectrum image converter.
An array 3 of plural optical detectors is selectively irradiated with two
portions of a spectral band image that is provided by the spectrum image
converter via its imaging means 2A. In FIG. 1, the preferred means for
accomplishing the selective irradiating of the array 3 is omitted for sake
of clarity, but is described hereinafter in greater detail with reference
to FIGS. 3 and 4.
One of the two aforementioned spectral band image portions is associated
with a predetermined first spectral range and the other portion is
associated with a second spectral range. The first and second ranges
include mutually exclusive first and second spectral components,
respectively. The two ranges also include a common spectral component
intermediate of the first two components.
As contemplated by the present invention, calibrating means, which in the
embodiment of FIG. 1 include two narrow band optical filters 4 and 5, are
provided to calibrate the array 3 of detectors for the two aforementioned
ranges. The filters 4 and 5 are disposed in the radiation beams in a
predetermined manner by suitable means, which in the preferred embodiment
includes a rotatable chopper wheel assembly 6 having a chopper wheel 6A
that is driven by a motor, not shown for sake of clarity. Filter 4 has a
spectral transmission characteristic corresponding to one of the
aforementioned three spectral components associated with the two ranges.
Filter 5 has a spectral transmission characteristic corresponding to the
other two spectral components. Filters 4, 5 are utilized during the
calibration mode of the apparatus of FIG. 1.
The spectrum analyzer apparatus of FIG. 1, is furthermore preferably
embodied as a spectrophotometer. It includes, inter alia, a light source 7
such as an adjustable high intensity lamp. Lamp 7 is adapted to be
connected to an external adjustable power supply, not shown, and
preferably one that is controllable by a data processor. Source 7 provides
light with a broad band emission characteristic preferably in the visible
spectrum, i.e. white light. Lamps suitable for this purpose are
quartz-iodine, tungsten filament or Xenon arc lamp types, for example.
Furthermore, the apparatus of FIG. 1 may be modified to utilize
interchangeable different lamp types if desired. To form the radiation
from lamp 7 into two beams, the light from lamp 7 is divided into two
channels or optical paths 8, 9, hereinafter sometimes referred to as the
reference and test channels respectively.
More specifically, each of the optical paths 8, 9 includes inter alia an
optical path folding lower mirror 10, a condenser lens system 11, an
optional compensating filter 12 and an upper folding mirror 13. The two
respective lens systems 11 of paths 8, 9 focus the light from lamp 7 into
the input ends 14A, 15A, respectively, of a pair of flexible fiber optic
cables 14, 15, which are also part of the paths 8, 9, respectively.
Filters 12, if used, attenuate abnormal high intensities which may be
present at a specific wavelength in the light.
Disposed between the ends 14A, 15A and mirrors 13 is the aforementioned
chopper wheel 6A. In addition to carrying the two calibration filter
members 4, 5, the wheel 6A of assembly 6 also carries a transparent window
member or opening 16. Member 16 is used during the spectrum analysis mode
of the apparatus of FIG. 1. The three members 4, 5 and 16 are
symmetrically disposed, both angularly and radially, about the axis 17 of
rotation of wheel 6A. As the wheel 6A rotates, the ends 14A, 15A are thus
alternately illuminated through the members 4, 5 and 16 in rotational
sequence.
As also contemplated by the present invention, optimal compensation is
preferably provided so as to allow the apparatus to handle a wide range of
attenuation levels. To this end, in the embodiment of FIG. 1, two
indexable filter wheels 18, 19 are disposed in the paths 8, 9,
respectively. Each of the filter wheels 18 and 19 carries a series of
plural, e.g. six, neutral density filters F0, F1, . . . F6, and F0', F1',
. . . F6', respectively. For sake of simplicity, these filters are
sometimes referred to herein simply by the letter prefix F of these
reference characters. Each filter F of the same series is capable of
attenuating the intensity of the light passing therethrough by a
predetermined discrete different amount and uniformly over the entire
spectral bandwidth of the light. Preferably, filters F1 - F6 have
increasing attenuation characteristics, as do the filters F1' - F6'.
The wheels 18, 19 are indexed by respective suitable indexing mechanism.
The indexing mechanisms are only partially shown in FIG. 1 for sake of
clairty, and include the respective detent wheels 20 and 21 that are
affixed to the respective shafts 22, 23. The shafts 22 and 23, which are
affixed to filter wheels 18 and 19, are driven by the aforementioned
chopper wheel motor, not shown, through gear trains, not shown, and
appropriate slip clutches, not shown, or the like. Detent wheels 20, 21
coact with their respective detent arms 24, 25 which in turn are connected
to their respectively associated electrically operated solenoids 26, 27.
With an arm 24 or 25 located in a respective detent of its associated
wheel 20, 21, rotation of the particular shaft 22, 23 is prevented while
the aforementioned slip clutches still allow continuous rotation of the
chopper wheel 6A.
When a solenoid 26 or 27 receives a short actuating pulse, the respective
arm 24, 25 is lifted from the detent and the particular wheel 20, 21 is
indexed to the next detent position. At this next detent position the arm
24 or 25 engages the particular detent and as a result, rotation of the
particular wheel 20, 21 is again stopped or prevented. Each detent
position on the wheels 20 and 21 is associated with a mutually exclusive
one of the filters F of the wheels 18 and 19, respectively.
By way of example, an optical shaft position encoder and detector system is
provided for each detent wheel 20, 21. The particular encoder and detector
system provides an output signal indicative of the particular filter F
which is presently in the particular path 8, 9 with which the wheels 18,
19 are associated.
Fiber optic cable 15 together with another fiber optic cable 28 are
terminated in a bifurcated end 29 which forms an optical test probe P. In
the embodiment of FIG. 1, the test probe P is preferably adapted to be
immersed in a cell C containing a fluid or liquid sample S to be analyzed.
It is provided with a reflector 30, which faces the bifurcated end 29.
Reflector 30 is mounted a fixed distance D from end 29 by a detachable,
i.e. demountable, adaptor shown as a pair of elongated narrow brackets 31
or the like. Thus, a fixed or uniform volume of liquid is placed between
the reflector 30 and bifurcated end 29 of the cables 15, 28 each time the
probe P is immersed in a sample S. Reflector 30 has preferably a concave
first surface spherical reflector configuration. Alternatively, reflector
30 may have other configurations such as, for example, a second surface
concave spherical reflector configuration, a second surface convex-concave
spherical reflector configuration, a second surface plano-concave
spherical reflector configuration, or in less efficient systems first or
second surface planar reflector configurations. In addition, several
adapters with different focal length reflectors 30 can be used
interchangeably in the probe P, thus providing different optical path
lengths, if desired. The fiber optic cables 15, 28 and probe P are part of
the test channel. In the bifurcated end 29, the individual optical
elements of cables 15 and 28 are arranged in a circular cross-sectional
array, the elements of cables 15 and 28 being confined to mutually
exclusive semi-circular cross-sectional halves thereof.
It should be understood that other types of probes and/or cross-sectional
configurations for the end 29 can be interchangeably used for the
apparatus of FIG. 1 with the aid of appropriate adaptors. For example, the
reflective probe P' of FIG. 9 or transmissive probe p" of FIG. 12 may be
utilized and, for example, the elements of cables 15 and 28 may be
arranged randomly in the end 29, or the elements of one cable may be
arranged concentrically about a core formed of the elements of the other
cable.
Fiber optic cable 14, which is part of the reference channel, and fiber
optic cable 28 are also terminated in a bifurcated cable end 32. As shown
in FIG. 2, the individual optical elements of cables 14, 28 are terminated
at end 32 in a slit-like rectangular cross-sectional array having an
elongated axis 32A. Half the array includes exclusively the fiber optic
elements of cable 14 and the other half includes exclusively the elements
of cable 28.
Referring now to FIGS. 1 and 3, the monochromator portion 33 of the
spectrophotometer shown therein will next be described, the radiation beam
source 1 being its illumination section. The bifurcated end 32 of the
fiber optic cable pair 14, 28 is mounted in a T-shaped pluglike structure
34. A compatibly configured elongated ring-shaped receptacle structure 35
is affixed to a bracket 36, FIG. 3, that is mounted to the frame assembly,
not shown. When the pluglike structure 34 is inserted in the direction of
arrow Z into the receptacle 35, the end 32 is placed a fixed distance d
from and in close proximity to the elongated symmetrically adjustable slit
opening 37 formed by the knife edges of two partially shown movable plates
37A, 37B, in FIG. 3, and the center elongated axis 32A of end 32 is in
parallel symmetrical alignment with the elongated slit 37. Adjustment of
the slit opening 37 is effected by a suitable mechanism, not shown for
sake of clarity. The slit adjustment mechanism moves the plates 37A, 37B
in unison towards and away from each other, cf. bidirectional arrow X, via
the bidirectional rotatable adjusting knob 38 between fully open and fully
closed positions. The slit opening 37 is judiciously adjusted to optimize
the resolution and intensity of the light image projected on the
diffraction grating 2.
The light passing through slit 37 passes through a conical light baffle
formed on the inner surface of hollow member 39, FIG. 3, to reduce or
minimize stray light into the system. Member 39, which has a cylindrical
outer shape, is terminated with a concentric light stop member 40 having a
square aperture 40A. For sake of clairty, baffle 39 and member 40 are
omitted in FIG. 1 and are shown in cross section in FIG. 3. The light
exiting from aperture 40A strikes the diffraction grating 2 which is
blazed in the direction indicated by the arrow B. Aperture 40A confines
the emerging light beam so that only the effective grating area of grating
2 is substantially illuminated with the beam and thus further reduces the
stray light in the system.
Referring now to FIGS. 3 and 4, grating 2 is mounted ot a pivotable lever
assembly 41. More particularly, lever assembly 41 has a resilient
springplate member 41A interconnected to a rigid member 41B. Suitable
mounting means such as a clamp member 41C and screws 42 clamps the grating
2 directly to the rigid member 41B. An end portion 43 of resilient member
41A is configured as a yoke. The yoke portion 43 straddles the armature 44
of a solenoid 45 and is pivotable secured to the armature 44 by a pivot
pin 46. The yoke portion 43 and armature 44 are disposed in an opening 47,
cf. FIG. 4, provided in the rigid member 41B. Lever assembly 41 pivots
about a pivot shaft 48, FIG. 3, which is concentric with the pivot axis R,
FIG. 1. A return spring 49 is connected to the lever assembly 41 via a
link 50. The lever assembly 41 is positioned between two adjustable stops
51, 52. In the unretracted position of the solenoid armature 44, spring 49
maintains the lever assembly 41 in a first position as shown in FIG. 3
against stop 52. When the armature 44 is retracted, the lever assembly 41
is placed in a second position, not shown, against stop 51. Member 41A
dampens the high energy impact forces caused when the lever assembly 41
strikes the stop 51 as a result of the retraction of the armature 44 which
occurs when solenoid 45 is energized. The spring characteristics of spring
49 are such that the impact forces, which are caused when the lever
assembly 41 strikes the other stop 52 by the action of the return spring
49, are less severe and/or cushioned, i.e. dampened, by the spring 49 per
se.
In either of the two positions, the light beam dispersed by the grating 2
is passed through the opening of an adjustable iris diaphragm 53 and is
reflected back through the diaphragm opening by the imaging means 2A which
is preferably a concave spherical reflector 54. Means 2A forms an image of
the spectrum that is projected toward the array 3 of photodetectors.
Juxtaposed in front of the array 3 is an elongated cylindrical lens 55
which is substantially co-extensive with the array 3. For sake of clarity,
the diaphragm 53 is shown in schematic outline form in FIG. 3.
The opening of iris diaphragm 53 is adjusted to optimize the quality, i.e.
resolution, and intensity of the particular portion of the spectral band
image projected on the array 3 of photodetectors D0, D1 . . . DN. Array 3,
which is shown schematically in FIG. 5, is preferably a monolithic array
of semiconductor photoconductors. The photodetectors are adapted to be
periodically electrically scanned in synchronization with the rotation of
wheel 6A. Cylinder lens 55 reduces the respective height of the two
separated and alternate spectral band images derived from the reference
and test channels, respectively, via the cables 14 and 28 at end 32 and
focuses them onto a more centralized common portion of the detector array
3. Each of the two spectrum images, which are derived from the test and
reference channels, respectively, and projected towards array 3, have two
orthogonal planar dimensions referred to as its height and width
dimensions. The individual spectral components of the particular spectrum
image are aligned normal to and along its width dimension. The width and
height dimensions of the spectrum images are parallel to the length W and
height h, respectively, of the photodetector array 3, cf. FIG. 5. For the
configuration of cable end 32 shown in FIG. 2, the test channel derived
spectrum image and reference channel derived spectrum image are projected
spatially separated and in substantially parallel alignment with their
respective width dimensions. The width dimensions of the images are
greater than the length W of array 3 and the corresponding co-extensive
axial length of cylinder lens 55. Cylinder lens 55 thus reduces the height
of that portion of the particular spectrum image projected therethrough
and also focuses or concentrates the particular spectrum portion onto a
more centralized and common region, shown in FIG. 5 as a dashline 3A of
array 3.
Referring now to FIG. 6, symmetrically disposed on the periphery of the
chopper wheel 6A are six illumination sequencing timing magnets 4T, 4R,
5T, 5R, 16T and 16R. A sensing device such as a Hall cell 56 is mounted to
the apparatus frame, not shown, in close proximity to the trajectory of
the six magnets. Hall cell 56 provides an output signal, each time it
senses one of the six magnets 4T, 4R, etc. In addition, a reference
position magnet 57 is mounted inwardly on the chopper wheel 6A and is
sensed by a sensing device 58, which can also be a Hall cell, that is
mounted on the apparatus frame, not shown. The output signal of cells 56,
58 are utilizable as control and synchronization signals for the apparatus
of FIG. 1.
The apparatus of FIG. 1 is suitably mounted to a frame assembly which
includes the front panel 59A of a housing assembly 59. An integral cover
of the assembly 59 has two light-tight compartments 59B, 59C that house
portions 1 and 33, respectively, except for those external portions of the
fiber optic cables 14, 15, 28, and the calibrated control knob 38 for
adjusting slit 37 and the calibrated control knob, not shown for sake of
clarity, for adjusting the diaphragm 53. Compartment 59B also houses the
aforementioned motor, gear trains and slip clutches associated with the
chopper wheel 6A and filter wheels 18, 19. Housed in compartment 59B is
also the aforementioned optical shaft position encoder and photodetector
systems associated with the neutral density filter wheels 18, 19.
Compartment 59B can also house a blower system, not shown, if it is
required to keep the illumination section 1 below a desired temperature
level, as well as any detector circuitry that may be employed, if desired,
to monitor light fluctuations in the lamp 7. Compartment 59C also houses
appropriate scanning and detector circuits, not shown, that are associated
with array 3 and which are also preferably configured as integrated
circuits. Appropriate electrical terminals, not shown, are provided on the
panel 59A and/or compartments 59B, 59C for electrical power and signal
interconnection to the apparatus contained therein.
As is apparent to one skilled in the art, the apparatus of FIG. 1 is
particularly adapted to be operated in a fully automatic mode by a data
processor system, not shown, which not only analyzes the output signals
derived from the array 3, but which also in response to these output
signals can provide control signals for operating the solenoids 26, 27,
and/or 45 or is programmed to operate the solenoid 45 on a periodic basis.
In addition, the data processor in response to the signals derived from
sensor devices 56, 58 provides a synchronization signal for synchronizing
the scanning of the array 3 with the rotation of the wheel 6A.
Alternatively, the apparatus of FIG. 1 can be modified to operate in a
semi-automatic and/or manual mode, the output signals, array 3 being fed
to an appropriate utilization means such as, for example, a computer or
display device.
The operation of the apparatus will be described hereinafter with respect
to its preferred aforementioned spectrum analyzer and calibration modes
under corresponding appropriate headings. However, prior to the
description of these last two mentioned modes, the preferred
initialization operation of the apparatus of FIG. 1 is next described
under the headings System Turn-On and Attenuation Compensation.
System Turn-On
Prior to system turn-on, chopper wheel 6A is stationary, and each of the
neutral density filter wheels 18, 19 has a filter F aligned with the end
14A, 15A, with which the particular wheel 18, 19 is associated. In
addition, grating 2 is in the position shown in FIG. 3, being held against
the stop 52 by the action of spring 49.
When the system power is turned on, the lamp 7 is lit and its intensity
level set by the operator to a predetermined value. Also, the chopper
wheel motor, not shown, is energized and causes the wheel 6A to rotate
continuously at constant speed in the direction of the arrow CW shown in
FIG. 6. It is assumed that at this time the probe P is not immersed in a
sample S to be analyzed. During the initial start-up period, appropriate
adjustments are made by the operator, if required, to the slit 37 and
diaphragm 53.
Attenuation Compensation
Preferably, in the present invention, the spectrum analysis of the sample
is done by correlating the two sets of output data signals from the
photodetector array obtained when the test probe is in operative and in
non-operative relationship, respectively, with the sample all other things
being the same as hereinafter explained in the description of the spectrum
analysis mode. That is to say, the two sets of data are obtained using the
same lamp intensity level and filter F of wheel 19 in the test channel for
the aforesaid operative and non-operative relationships, and hereinafter
referred to sometimes as the loaded and unloaded test channel,
respectively. For the particular test probe P, the test channel of the
apparatus of FIG. 1 is loaded and unloaded when the probe P is immersed
and not immersed, respectively, in the liquid sample S.
Attentuation compensation of the present invention gives the spectrum
analyzer the ability to analyze a wide range of absorption levels so that
the light passed through the loaded and unloaded transmission channel does
not drive the photodetector array 3 below its threshold level or into
saturation.
After start up and during the initialization period, the test channel is
loaded with the sample to be analyzed. Each time the wheel 6A rotates and
its window 16 fully exposes the end 15A, the array 3 is synchronizing
scanned as is explained in greater detail hereinafter in the description
of the spectrum analysis mode. The attenuation compensation is begun by
detecting the output signal condition of the array 3 in response to the
light being transmitted through the test channel during one of the
revolutions of wheel 6A.
If an output signal condition, other than a saturation condition, is
detected, the resultant data output signal is correlated by the data
processor with the data of the attenuation characteristics of the filters
F of wheel 19 and the particular lamp intensity level utilized so as to
determine the optimal lamp intensity level and filter F of wheel 19
combination which allows the most efficient amount of light to be
transmitted to the array 3 for both the loaded and unloaded conditions of
the test channel. The data processor provides control signals that
positions the optimal filter F of wheel 19 in the test channel, adjusts
the lamp power supply to provide the optimal intensity level, and also
computes and provides control signals that positions a filter of wheel 18
on the reference channel that allows the most efficient amount of light to
be transmitted to the array 3 through the reference channel for the
aforesaid optimal lamp intensity level. The apparatus is then ready for
the spectrum analysis mode.
If a saturation condition is detected when the test channel is first
loaded, the data processor in response thereto provides control signals to
position successive filters F of wheel 19 with successively increasing
attenuation characteristics into the test channel. If one of these filters
causes a non-saturation output signal, the operation described in the
previous paragraph is then performed. If, on the other hand, no output
signal condition is detected when the test channel is first loaded, the
data processor in response to this condition, provides control signals to
position successively filters F of wheel 19 with successively decreasing
attenuation characteristics into the test channel. If one of these filters
cause a non-saturation output signal, the operation described in the
previous paragraph is then performed. In either of the lastmentioned
cases, if no filter is found which can produce a non-saturation output
signal, the data processor then provides control signals to the
aforementioned lamp power supply, not shown, that causes the lamp
intensity to be incremented in increasing or decreasing amounts, depending
on the case, and if a detectable output signal is obtained, the operation
described in the previous paragraph is then performed.
The aforedescribed attenuation compensation is performed for either one or
both positions of the grating 2, as required.
If desired, a manual operable override with an external calibrated control
knob can be provided on each shaft 22, 23, to allow the operator to
position any filter F of the wheels 18, 19 in their respective aligned
relationships with the ends 14A and 15A.
With optimum attenuation compensation achieved, the apparatus of FIG. 1 is
now ready for operation in its spectrum analyzer and calibration modes.
Spectrum Analyzer Mode
By way of explanation, the wheel 6A is illustrated in FIG. 6 in its zero
degree position relative to a fixed or stationary reference point. In the
zero degree position, the Hall cell 58 is aligned with its associated
magnet 57 and its resultant output signal indicates the beginning of one
revoltuion of wheel 6A about its rotational axis 59. Also shown in FIG. 6,
are the respective radii 0.degree., 30.degree., 60.degree., 120.degree.,
150.degree., 180.degree., 240.degree., 270.degree., and 300.degree. of
wheel 6A which coincide with the respective elements 16R, 16, 5T, 4R, 4,
16T, 5R, 5, and 4T, respectively. In the zero degree position, the
0.degree. radius and, consequently, the magnet 16R are aligned with the
Hall cell 56. As a result, cell 56 provides an output signal in response
to the magnet 16R. The ends 14A, 15A are shown in FIG. 6 in their
respective positions relative to the wheel 6A. More particularly, the
centers of the ends 14A, 15A are symmetrically aligned along the
horizontal diameter of the wheel 6A as viewed in FIG. 6. Moreover, the
centers of the ends 14A, 15A and the centers of the members 4, 5 and 16
are at equal radial distances from the axis 59, the | | |
