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Патент USA US3056091

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Sept. 25, 1962
3,056,080
P. A. CRANDELL
PARAMAGNETIC WAVEMETER
Filed Aug. 17. 1959
2 Sheets-Sheet 1
FIG. I
7 l5
D.C.
POWER
SUPPLY
44
46
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AUDIO
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NULL
NUCLEAR RES.
DE-IECTQR
FREQ. GEN.
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64
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FIG_ 2
INVENTOR.
PAUL A. CRANDELL
BY/f’
-
ATTORNEYS
L
Sept- 25, 1962
P. A. CRANDELL
3,056,080
' PARAMAGNETIC WAVEMETER
Filed Aug. 17, 1959
I05
I02
. .
I00
2 Sheets-Sheet 2
92
94
96
98
B [3
RP.
L'P
- FILTER
FILTER
AUDIO
99
FILTER
FIG. 3
84
FREQ. /88
COUNTER
|O5
I04
LP
AUDIO
FILTER
102
'
%>_
FIL'T'ER
82
FIG.4
FREQ.
COUNTER
INVEN TOR.
PAUL A. CRANDELL, .
)%&J 4 WM
4 vf
ATTORNEYS
'
United States Patent O??ce
3,0500%
Patented Sept. 25, 1962
1
2
It is known that if a sample of material, containing
3,056,080
protons which are not “shrouded” by an electron cloud
such as water or mineral oil for example is subjected to a
PARAMAGNETIC WAVEMETER
Paul A. Crandell, Bedford, Mass, assignor to National
Company, Inc, Maiden, Mass, a corporation of
steady state magnetic ?eld and to an oscillating magnetic
?eld, these two ?elds mutually perpendicular, at a par
ticular frequency of the oscillating ?eld a resonance will
occur, this resonance being the result of the absorption of
energy by the processing protons. A similar resonance will
My invention relates to a novel method and apparatus
occur, but at a much higher frequency because of absorp
for measuring frequencies in the high radio frequency 10 tion of energy by the electrons. If the same steady~state
(hereinafter “RF”) and microwave regions; in particular
magnetic ?eld is applied in both cases, the ratio ‘of the two
it relates to a method and apparatus which makes use of
frequencies, that of the nuclear and the electron resonance,
the electron and nuclear resonance effects respectively, of
can be very accurately calculated. This frequency ratio
appropriate source materials in the determination of the de
is a constant and since it is ratio of gyromagnetic ratios,
sired frequency.
15 it is referred to herein as the “gyromagnetic constant.”
In many measurements frequency is the independent
As used herein and in the claims the term “nuclear res
variable against which other parameters are plotted. If
onance” and “proton resonance” will be used interchange
the results of such measurements, e.g. gain band width
ably, it being understood that both of these terms have the
products, circuits Q’s, resonance, etc., are to be meaning
same meaning.
ful, a reliable measurement of frequency is required. 'Sim 20
In accordance with my invention, I provide a sample of
ilarly, in the calibration of other high frequency measuring
‘a material having free electrons therein to which I apply
instruments, a dependable frequency standard is necessary
the frequency which I desired to measure. This sample is
whose frequency is known rather exactly.
located in a variable static magnetic ?eld, this static ?eld
‘In the low frequency regions, i.e. up to about 30 mega—
being normal to the direction of the magnetic ?eld of the
cycles, frequency can be measured with great accuracy
unknown signal. By Varying the strength of the static
‘by employing frequency counters which measure the out
?eld, I can adjust the system to an electron resonance
put of the unknown frequency source against an oscillator
at the applied frequency.
controlled ‘by a crystal in a temperature controlled environ
I also provide, in the same static magnetic ?eld a sample
Massachusetts
Filed Aug. 17, 1959, Ser- No. 834,171
4 Claims. (Cl. 324—-.5)
ment as a standard. With this technique, measurements
which have an accuracy of 1 part in 106 can be carried
containing protons by means of which a nuclear resonance
may be obtained. With the static magnetic ?eld set at
the same level at which the electron resonance was ob
out consistently and reliably.
At the higher RF and microwave frequencies, however,
tained, another electromagnetic signal is applied to the
frequency counters are useful only when ‘a heterodyning
process is employed for the crystal controlled oscillator.
When this technique is used, the frequency of the crystal
controlled counter is multiplied in several steps through
lumped and distributed parameter circuits. Many stages
of multiplication are necessary to arrive at an X- or K
‘band, frequency (approximately 10 kilomegacycles and
sample containing protons, in a manner such that its mag—
netic component is at right angle to the static ?eld. The
35
‘frequency of this latter signal is variable. By adjusting
this variable frequency a resonance will ‘be observed at a
frequency which is below 30 mos. and which can be
measured accurately by means of conventional techniques.
The unknown frequency can now be determined with the
30 kilomegacycles respectively) from the low frequency 40 same accuracy with which the known frequency can be
of the crystal-controlled oscillator. The ?nal power which
is available is generally only a few microwaitts. Such a
measurement technique is further limited by the instability
of temperature-dependent crystals, as well as by the aging
measured since it is a function of ‘the known frequency
, and of the gyromagnetic constant previously de?ned. Ac
cordingly, accurate frequency measurements at electron
resonance frequencies may be carried out with this
of the many multiplier stages which must be employed. 4.5 method.
Nevertheless, the accuracy obtainable with this technique
is generally accepted today as a secondardy standard in
microwave spectroscopy.
Among other available frequency measuring instru
It is, ‘accordingly, a primary object of the present in
vention to overcome the disadvantages which are inherent
in the high frequency measurement methods heretofore
employed and to provide measuring and calibrating meth
ments in the higher RF and microwave frequencies which 50 ods which are simple, accurate and reliable.
are in use today, are cavity wavemeters and the like.
It is another object of this invention to provide appara
These devices utilize a cavity whose dimensions are con
tus for instrumenting the foregoing methods which is rela
trollable to make it resonant at a single frequency. By
tively simple in construction and which is insensitive to
comparision to the results obtainable with the technique
changing ambient conditions.
discussed above, the measurements which can be carried 55
Other objects of the invention will in part be obvious
out with cavity wavemeters are very inaccurate. For ex
and will in part appear hereinafter.
ample, a good, commercially available cavity wavemeter
The invention accordingly comprises the several steps
will measure to an accuracy of 5 mos. in 10,000 mcs., ie
and the relation of one or more of such steps with respect
to an accuracy of .05%. Further limitations of cavity
to each of the others, and the apparatus embodying fea
wavemeters arise as a result of their sensitivity to ambient 60 tures of construction, combination of elements and ar
temperature and humidity variations during a given
rangement of parts which are adapted to effect such steps,
measurement. Additionally, cavity wavemeters ‘are us
all as exempli?ed in the following detailed disclosure,
able over a limited frequency range only. It is generally
and the scope of the invention will be indicated in the
necessary to use a number of cavity wavemeters in order
claims.
to get wide coverage of a wide spectrum of frequencies. 65 For a fuller understanding of the nature and objects of
None of the presently available high frequency measur
ing techniques are free from ‘at least some of the forego
ing disadvantages. My invention, relates to a method and
to apparatus for instrumenting the method utilizing in
the invention, reference should be had to the following
detailed description taken in connection with the accom
panying drawings, in which:
FIG. 1 is a diagram partially in block form and par
herently stable nuclear and electron resonances to provide 70 tially pictorial, showing one embodiment of an improved
frequency measurements in the higher RF and micro
Wavemeter incorporating my invention;
wave regions.
FIG. 2 is a side elevation of a magnet structure which
3,056,080
5*
low so that it may be determined accurately by the use of
a crystal-controlled oscillator and counter as described
ment of a wavemeter made according to my invention;
and
4
quency at which nuclear resonance occurs is su?‘iciently
may be used in connection with the apparatus of FIG. 1;
FIG. 3 is a block and line diagram of another embodi
above. Thus, by measuring the frequency at which a
_
nuclear resonance occurs with a crystal-controlled oscil
FIG. 4 is a block and line diagram of an automatically
operated wavemeter of the type illustrated in FIG. 3.
1. Theoretical Considerations
It has long been known that there are precessional
frequencies peculiar to unpaired, or free, electrons and
?eld itself may be determined to an accuracy of one part
in 106 or 107. If an electron resonance sample is placed
in this same ?eld, the frequency at which an electron
10 resonance occurs, although much higher than the nuclear
to some nuclei which constitute proton sources i.e. in
resonance may be determined to a corresponding accuracy.
which a proton in the nucleus exhibits an observable
II. Construction and Operation
With reference now to the drawings, FIG. 1 illustrates
one instrumentation of the method which forms the sub
ject matter of my invention. With the apparatus shown,
the measurement of an unknown high frequency can be
carried out or, alternatively, a secondary frequency meas
uring device can be calibrated.
An electromagnetic having a C-shaped core whose
20
pole pieces 10 and 12 are illustrated, sets up a steady
state magnetic ?eld in the gap between the pole pieces in
the vertical direction of the drawing. In a preferred em
magnetic moment. If the frequency of an applied elec
tromagnetic signal is matched to these precessional fre
quencies under proper conditions a resonance-absorption
condition may be set up. The precessional frequencies
are proportional to the magnetic ?eld strength In which
the nuclei, or the unpaired electrons are placed. Thus,
the nuclear resonance frequency, f,,, is de?ned by the
equation:
where
'yn=a constant which depends upon the mass of the pro
ton and is 4.424763 kc./gauss, and
.
lator-counter, for a given steady-state magnetic ?eld, the
_
Ho=the intensity of the steady state magnet1c ?eld 1n
gauss.
bodiment of the invention, the ?eld intensity is approxi
mately 3000 gausses. The core of the magnet carries at
least one excitation coil 14 which is energized from a
variable D.C. power supply 15. Also, at least one of the
pole pieces carries a Helmholtz coil 16 which is energized
by an audio oscillator 18. The Helmholtz coil is adapted
by the equation
30 to superimpose a low intensity, magnetic audio frequency
fez'l’eH?
?eld on the steady-state magnetic ?eld. The frequency
of the audio oscillator is preferably between 60 and
where ve=a constant which depends on the mass of the
1000 cycles.
electron and is 2.802490 megacycles/gauss.
A hollow RF probe 20, which may consist of an induc
An examination of these two equations will show that
tive tank coil, is positioned in the gap between the pole
the frequency of the energy which must be supplied to a
Similarly, the electron resonance frequency, fe, is de?ned
pieces and contains a nuclear sample, e.g. water or min
nuclear system or to .an electron system in order ‘to ob
eral oil, which constitutes a source of protons. The
serve a resonance-absorption condition is a function of
probe 20 is energized from a source of electromagnetic
the steady-state magnetic ?eld in which the nucleus or
energy whose frequency is variable over the appropriate
the free electrons are located. In practically achievable
magnetic ?elds of the order of a few thousand gausses, 40 range, and which is accurately controlled and measured.
In practice, a calibrated oscillator capable of providing
nuclear absorption occurs between 10 to 20 megacycles
an output signal in the frequency range from 140 kc. to
while electron absorption occurs between
to 13.0
16 megacycles, and whose frequency may be read from a
kilomegacycles. In order to excite such transitions 1n a
dial setting to an accuracy of ‘1 part of 105 has been found
magnetic resonance process, it is necessary to supply an
satisfactory for the source 22. An oscillator of this type
oscillating magnetic ?eld, i.e. electro-magnetic radiation,
may be coupled to the probe 20 by its tank circuit coil,
in which the magnetic vector oscillates in a plane that is
perpendicular to the steady-state or static magnetic ?eld.
for example.
tion of the steady-state magnetic ?eld, causes all the par- '
The output of the probe 20‘ is connected to a demodu
lator 23, illustratively shown as a diode recti?er which is
connected to an ampli?er 24. The output of the am
pli?er 24 is applied to a null detector 26 which will be
ticles which were precessing about the static magnetic ?eld
direction and which had their spins aligned by the static
magnetic ?eld to undergo a change in their precessional
angle. Ultimately, these particles flip over so that the1r
spins are aligned in a different direction. The energy re
positioning of the two probes exposes them as nearly as
The oscillating magnetic ?eld must also be circularly
polarized. The application of such an oscillating circu
larly polarized magnetic ?eld at right angles to the direc
quired to carry out this process is the energy which is
absorbed from the applied oscillating magnetic ?eld. If
this oscillating ?eld has a frequency which is different
from the precessional frequency, or if it is circularly
polarized in the wrong sense, it will produce only pertur
bations of the moving particles, and substantial amounts
of energy will not be absorbed.
If the equations above are used to solve for H0,
,E='YeH0:Zg
fn 'YnHO 'Yn
Substituting the values previously given for we and 15,,
fe_2.802490 megacyeles/gauss; or, lrilocycles/gauss
if 4.25763
fe:658.2277 in, where the number 658.2277 is termed
the gyromagnetic constant.
It will be seen from the foregoing relationship that, if
either one of the two resonant frequencies is known, the
other resonant frequency can be calculated. The fre
hereinafter described.
A second probe 28 consists of a tuneable waveguide
cavity which supports the TE01 mode of propagation and
is placed as closely as possible to the probe 20. Close
possible to the same magnetic ?eld intensity and reduces
the chance of errors caused by a non-uniform magnetic
?eld in the gap between the magnetic pole pieces 10 and
12. In a preferred embodiment, the sample contained
within the probe 28 consists of a paramagnetic salt, such
as for example diphenyl-picryl-hydrazyl
( csHs) 2N—NCeH2(NO2 ) a
This sample is placed near the right hand end of the
cavity 28 at a point of maximum steady-state magnetic
?eld H0. The cavity 28 is preferably frequency-tuned
by means of an adjustable plunger (not illustrated) to
which input and output loops are attached. Basically,
70 therfore, this is a transmission cavity which absorbs en
ergy from an oscillator connected to the input loop only
when it is tuned to resonate at the applied frequency. It
will be understood, however, that the cavity, in addition
to its own tuned resonance, displays an additional reso
nance effect due to the energy absorption on the part of
3,056,080
5
.
the paramagnetic salt when the critical electron resonance
in turn, abut a neutral Zone 60 of the yoke. A pair of
steel shunt rings 62 and 64- are slidably disposed on the
legs of the yoke and are adapted to expose a variable
is to be measured and which may have a frequency be
amount of the pole pieces. Normally, flux leakage occurs
tween 100 mcs. and 10,000‘ mcs., is supplied by a signal 5 between the pole piece 50 and the leg 56 and between
source 30 and this signal is coupled to the probe 28 by
the pole piece 52 and the leg 58». By varying the hori
frequency f., is applied to the cavity.
An unkown high frequency signal whose frequency
the waveguide 34. The output signal from the probe 28
is applied to a demodulator 40, and the demodulated
zontal position of the steel shunt rings, it is possible to
control the amount of flux leakage and thereby con~
signal is ampli?ed by ampli?er 42, and after passing
trol the effective steady-state magnetic ?eld in the gap.
through a low pass audio ?lter 44 is applied to the indica 10 Thus, by moving the shunt rings 62 and 64, toward each
tor 46.
To measure the frequency of the signal source 3%), the
transmission cavity 28 is tuned approximately to reso
nance so that a substantial radio frequency signal is
other, the flux leakage is increased and the ?eld strength
is decreased. The opposite effect results when the shunt
nings are moved apart. The movement of the shunt coils
is preferably synchronous and can be effected with great
being passed by the cavity. A sharp absorption of this 15 precision by means of suitable gearing so that both rings
signal will then indicate a resonance condition.
move to increase or decrease the ?ux leakage simultane
The steady-state magnetic field applied between the
pole pieces 10 and 1.2 is modulated by the audio oscil
lator ‘18, and accordingly the signal passed to the detector
ously. Under certain conditions, the apparatus illus
trated in FIG. 1 is subject to magnetic ?eld distortion,
signal at the fundamental frequency of the audio signal
same steady-state magnetic ?eld, they may distort the
even though the permanent magnet which is illustrated
40 will include a component modulated at this frequency. 20 in FIG. 2 is employed. When the tunable cavity probe
When detected this modulated component will provide a
28 and the inductive coil probe 20 are positioned in the
generator. Filter 4M is a low pass audio ?lter which passes
?eld and react on each other. In this case, the two
this fundamental component, but not higher harmonics
samples are not exposed to ?elds of like intensity, nor
to the detector 46. When the magnetic ?eld in which 25 to a ?eld which is homogeneous throughout the volume
probe 28 is located is exactly adjusted for an electron
occupied by the sample. Resultant errors may be due
resonance, the fundamental component of audio modula
to any one of these causes or to a combination thereof.
tion will disappear and the signal passed by the cavity 28
Thus, they may be due to the physical size of the two
will be modulated only by second and higher harmonics
samples, the ?eld distortion due to the presence of one
(depending uponthe shape of the resonance curve) of 30 sample with the other, or due to the lack of homogeneity
the audio frequency modulating signal. However, be
of the magnetic ?eld itself throughout the volume which
cause of ?lter 44, the detector 46 is responsive only to
is taken up by both samples.
the fundamental of the modulating frequency. Hence an
The apparatus which is illustrated in FIG. 3 overcomes
electron resonance is indicated by a sharp null on the indi
these disadvantages by combining both the electron
cator 46.
Accordingly after cavity 28 is tuned, the current
through winding 14 is varied until an electron resonance
occurs in the crystals in the probe 28, as indicated by a
null on indicator ‘46.
Having achieved an electron resonance, the steady-state
magnetic ?eld is held constant and the frequency of the
nuclear resonance frequency source 22, is adjusted until
35 resonance and nuclear resonance samples in a single sub
stance which is contained in one sample holder. This is
done by dissolving a crystalline form of the paramagnetic
salt (diphenyl-picryl-hydrazyl) in water or mineral oil.
Alternatively, proxylamine disul?nate ions,
can be used in an aqueous solution while, by some dy
a similar null is obtained on the null indicator 26, this
namic chemical process, the free radical (ON(SO‘3)2) is
null indicator being similar to that described in connec
kept active as a source of protons. Such solutions exhibit
tion with the electron resonance circuit described above,
respective nuclear and electron resonance phenomena.
in that it incorporates both a low pass audio ?lter and 45
Even if there is an interaction between the nuclei and
an indicator.
the electrons, it is still possible to determine the two
Theoretically, the energy absorption due to nuclear
resonance effects and, by a suitable initial calibration, to
resonance of the nuclear sample contained in the probe
determine the new proportionality constant.
20, should occur at a single frequency fn of the nuclear
In this context, Overhauser has discovered that if a
50
resonance frequency source. Practically however, since
single substance is used to provide both sources, Le. a
the sample is not a true point sample, the magnetic ?eld
source of electrons and a source of protons, the inter
may vary over the volume which the sample occupies.
action which occurs can enhance the intensity of the
When this condition holds true, the response will be
proton resonance and shift the resonant frequency of
spread over a narrow band of frequencies. This spread
55 the electrons (“Polarization of Nuclei in Metals,” A. W.
is called the line width of the response and is extremely
narrow when the steady-state magnetic ?eld is uniform.
Although the ‘accuracy of the nuclear reference fre
quency source 22 may be only one part in 105, an accuracy
of one part in 106 or 107 can be obtained if more than
one reading is taken. Once the nuclear resonance fre
quency is known, the frequency of the source 30 can be
determined to this same accuracy by multiplying the nu
Overhauser, page 411, Physical Review, vol. 29, No. 2,
October 1953). It will be clear that this effect may be
used to advantage in the invention herein. Korringa
carried the work of Overhauser one step further and
showed that the same phenomenon can be observed in
paramagnetic salts in which the nuclei of the paramag
netic ions have spin (“Orientation of Nuclei by Satura-V
tion of Paramagnetic Resonance,” Physical Review
clear resonance frequency by the gyromagnetic constant.
1953).
Certain disadvantages are inherent in the electromag 65
The apparatus illustrated in FIG. 3 employs a coaxial
net which is illustrated in the apparatus of FIG. 1. A
probe 72 which allows the simultaneous passage of low
more uniform steady-state magnetic ?eld is obtainable by
RF frequencies for determination of the nuclear resonance
using the permanent magnet illustrated in FIG. 2. With
and of microwave frequencies in the TEM mode for deter
this arrangement, a uniform steady-state magnetic ?eld
mination of the electron resonance. The volume of the
is provided whose ?eld intensity may be varied between 70 sample holder 72 is approximately 1 cubic centimeter,
approximately 350 gausses and 3500 gausses. A pair
depending to some extent on the material used and on the
of pole pieces 50 and 52 de?ne a ?ux gap 54 between
band width of the observed resonance.
A permanent
them in which the sample probes are positioned. The
magnet includes a pair of magnetic pole pieces 74 and
pole pieces consist of magnetic material and directly abut
76. These pole pieces de?ne the gap in which the co
the legs 56 and 58‘ of the C-shaped steel yoke. The legs, 75 axial cavity sample holder 72 is disposed at right angles‘
3,056,080
7
to the direction of the steady state magnetic ?eld. For
the sake of clarity, a single moveable steel shunt ring 78
similar to the rings 62 and 64 of FIG. 2 is illustrated
in connection with one of the pole pieces, although it
will be understood that the pole piece 74 also carries such
a ring. One or both of the pole pieces may carry a
Helmholtz coil 80 which is energized from an audio oscil
lator 82. This coil is so positioned that it superimposes
a weak magnetic ?eld at audio frequency on the high
8
As shown in FIG. 4, I provide a phase sensitive dc
modulator 106 to which the audio frequency signal sup
plied to indicator 105 is also supplied. The audio modu
lating signal supplied by generator 82 is also supplied to
demodulator 106 as a reference signal. The output sig
nal from the demodulator, a direct voltage whose polarity
depends on the phase of the audio frequency signal passed
by ?lter 104 with respect to the audio modulating signal
is connected as an input signal through a switch 108 to a
intensity steady-state magnetic ?eld for modulation pur 10 servo power ampli?er 110. The output signal from am-.
pli?er 110 drives servo motor 112, whose shaft is mechan
poses, as previously described. A stable low frequency
ically connected, through an appropriate gear box 114 to
oscillator 84 for generating the nuclear resonance signal
position the steel shunt rings 78 and to control the fre
is coupled to one arm of a ?rst coaxial T-section 86 and is
quency of nuclear resonance oscillator 84.
adapted to apply energy thereto at a low RF frequency
Before operating the wavemeter shown in FIG. 4 auto
which is variable between approximately 140 kc. and 16 15
matically, switch 108 is opened (thus opening the servo
mos. The output of the oscillator 84 is further coupled to
loop) and the wavemeter is adjusted as previously de
a frequency counter 88 to measure the nuclear resonance
scribed to measure the frequency of source 90. When
frequency with an accuracy of at least one part in 105
correctly adjusted, no signal will be indicated by the in
and preferably one part in 106. The signal having energy
dicator 105 and hence demodulator 106 will not be pro
at an unknown microwave frequency which may vary be
ducing an output signal. The switch 108 may now be
tween 100 mcs. and 10,000 mcs. is applied to the opposite
arm of the coaxial T-section 86 from a microwave source
closed without affecting system operation.
90. The third arm of the T-section 86 is coupled to the in
put of the sample holder 72 whose output, in turn, is con
If, after switch 108 is closed, the frequency of source
90 changes, the previous setting of the shunt rings will
nected to one arm of a second coaxial T-section 92. The 25 not provide the proper magnetic ?eld for an electron res
onance and an audio frequency signal will be produced at
second arm of the T-section 92 is coupled to a band pass
?lter 94. The pass band of the ?lter 94 extends approxi
indicator 105, the phase of this signal indicating the di
mately between 140 kc. and 16 mcs. to cover the fre
rection of error.
This error signal will drive motor 112
through demodulator 106 and power ampli?er 110 to re
quency range of the stable low frequency oscillator 84.
The output of the band pass ?lter 94 is coupled to a de 30 position the rings 78 to the correct position for an elec
tron resonance, at the same time changing the frequency
modulator 96 and is subsequently applied to a. low pass
of the nuclear resonance oscillator 84 to maintain a nu
audio ?lter 98 and indicator 99‘. If desired, the signal
clear resonance. The nuclear resonance is linearly re
may be ampli?ed after demodulation and before appli
lated to the electron resonance and hence the setting of
cation to the ?lter 98. The third arm of the coaxial T
section 92 is coupled to a microwave band pass ?lter 100 35 oscillator 84 may be varied directly with changes in fre
which has a pass band that includes the frequency range
of the microwave source 90. The output of the band pass
quency of source 90.
If for any reason a nuclear res
onance is not obtained, a reading other than a null indi
cation will be observed on indicator 99. The indicator
?lter 100 is coupled to a demodulator 102 which in turn
thus serves as a check for proper system operation.
is connected to a low pass audio ?lter 104 and indicator
105. The signal may again be ampli?ed, if necessary 40 If it is desired to control additional apparatus in ac
cordance with the frequency of the source 90, it is of
course possible to do so by mechanically coupling to the
output shaft of the servo 112.
Although the instrumentation of the method which
ilar to that shown in FIG. 1. In order to determine the
unknown frequency at which energy is applied by the 45 forms the subject matter of my invention herein is only
somewhat simpler than the equipment discussed above
source 90, the steady-state magnetic ?eld is varied in in
which is commonly in use today, the accuracy and re
tensity by moving the shunt rings until an electron res
liability obtainable with my method and apparatus is far
onance absorption effect is indicated by the null detector
superior to that heretofore available. As previously
105. The band pass ?lter 100 passes only signals having
the proper microwave frequency to the demodulator 102. 50 pointed out, prior methods generally depend on the mul
tiplication of the frequency of a stabilized low frequency
As in the case of the aparatus of FIG. 1, the microwave
standard by zero beating or IF heterodyning the ampli?ed
energy is modulated at a low audio frequency by the low
intensity audio frequency variations in the magnetic ?eld
harmonics. vBecause of the many stages of multiplica
prior to its application to the ?lter 104.
‘Except for the activity within the sample holder 72,
the operation of the apparatus illustrated in 'FIG. 3 is sim
generated by the Helmholtz coil 80.
With the shunt coils ?xed at the position where electron
tion which are necessary to arrive at X-
or K~band
frequencies the available output power is very small. Fur
ther limitations of this method arise from the instability
resonance was obtained, the ‘frequency of the output
signal of the oscillator 84 is varied until an absorption
of temperature-dependent crystals, aging tubes, and the
effect is indicated by a zero signal on indicator 99. This
critical tuning of the many multiplier stages. In con
absorption effect shows that the nuclear resonance fre
trast, the methods and apparatus which I have disclosed
quency in has been reached. This frequency is deter 60 are not subject to these in?uences and hence provide a
mined by the frequency counter 88 and can be converted
far more reliable and precise measurement which is suit
into the unknown frequency of the source 90 by multiply
able for modern high-precision requirements.
ing it by the gyrornagnetic constant. In a preferred em
The preferred embodiments of the method and appa
bodiment, a dual scale on the frequency counter simul
ratus herein disclosed are intended to be illustrative only.
taneously provides both frequency measurements.
'Since, for any given unknown input frequency, the
Numerous modi?cations, departures and equivalents will
desired condition is to obtain electron resonance and
within the true spirit and scope of this invention.
It will thus be seen that the objects set forth above,
now occur to those skilled in the art, all of which fall
nuclear resonance simultaneously, the operation of the
entire system may be made completely automatic. This
among those made apparent from the preceding descrip
is possible because of the fact that the frequencies at 70 tion, are efficiently attained and, since certain changes
which these resonances occur, are linearly related by vir~
may be made in carrying out the above method and in
tue of the gyromagnetic constant. Apparatus for carry
the construction set forth without departing from the
ing out this operation automatically is illustrated in FIG.
4, applicable reference numerals having been carried for
ward from FIG. 3.
scope of the invention, it is intended that all matter con
75 tained in the above description or shown in the accom
3,056,080
panying drawings shall be interpreted as illustrative and
not in a limiting sense.
It is also to ‘be understood that the following claims
10
trons, means for providing a high intensity steady mag
netic ?eld, means positioning said sample holder in said
magnetic ?eld in a direction such that its axis is perpen
dicular to said ?eld direction, means for varying the in
tensity of said steady magnetic ?eld, means for modulat
ing said magnetic ?eld at a low audio frequency, a vari
are intended to cover all of the generic and speci?c fea
tures of the invention herein described, and all statements
of the scope of the invention which, as a matter of lan
guage, might be said to fall therebetween.
able, low radio frequency oscillator having its output
Having described my invention, what |I claim as new
coupled to second arm of the input T-section, a frequency
and desire to secure by Letters ‘Patent is:
counter coupled to the output of said low radio frequency
11. A wavemeter for measuring an unknown frequency 10 oscillator, means for coupling an input signal of unknown
comprising, a cavity adapted to hold a sample, said sample
frequency to the third arm of said input T-section, a ?rst
including a material having nuclear protons capable of
band pass ?lter coupled to a second arm of the output T
exhibiting resonance effects and a material having therein
section, said ?rst band pass ?lter admitting frequencies in
free electrons, means for applying a ?rst magnetic ?eld
the low radio frequency region only, a second band pass
at said unknown frequency to said sample in a ?rst direc 15 ?lter coupled to the third arm of said output T-section, said
tion, means for applying a steady-state magnetic ?eld to
second band pass ?lter admitting frequencies in the micro
said sample in a second direction normal to said ?rst di
wave region only, a demodulator coupled to the output
rection, means for varying the intensity of said steady
of each of said ?lters, and means indicating the presence
state magnetic ?eld to bring about electron resonance
of said audio modulating signal connected to the output
of said sample in cooperation with the ?eld applied at 20 of said demodulators.
said unknown frequency, means for modulating said mag
‘3. The apparatus de?ned by claim 2 which includes
netic ?eld at an audio frequency, means for applying a
a servo means responsive to the output signal from the
second magnetic ?eld at a known frequency to said sample
demodulator associated with said second band pass ?lter
in said ?rst direction, means for varying said known
for varying the intensity of said steady magnetic ?eld to
frequency to bring about nuclear resonance of said sam 25 maintain the electrons in said sample in resonance with
ple in cooperation with said steady state magnetic ?eld
at an intensity set for electron resonance, means for de
said unknown frequency.
4. The apparatus of claim 3 which includes means
tecting the respective resonance effects. said detecting
coupled to said servo means for varying the frequency
means comprising sensing means responsive to said audio
of said low radio frequency oscillator as the intensity of
frequency, said nuclear resonance frequency being related 30 said steady magnetic ?eld is varied.
to said electron resonance frequency by a predetermined
constant and said unknown frequency being equal to the
References Cited in the ?le of this patent
product of said known frequency and said predetermined
UNITED STATES PATENTS
constant.
2. A microwave frequency wavemeter comprising a 35
2,837,712
Mackey ______________ __ June 3, 1958
1,180,455
France ______________ __ Dec. 29, 1958
coaxial cavity sample holder, said sample holder permit
FOREIGN PATENTS
ting the sirnultaneous passage of low radio frequency en
ergy and of microwave energy, a pair of T-sections each
having one arm coupled to the input and output respec
OTHER REFERENCES
tively of said sample holder, a sample disposed in said 40
sample holder, said sample comprising a paramagnetic
salt in solution in a liquid, said liquid containing nuclear
protons capable of exhibiting nuclear resonance effects,
said paramagnetic salt constituting a source of free elec
Feher: Physical Review, vol. 103, No. 2, July 1956,
pp. 500 and S01.
\Montchane et al.: Academic des Sciences, Comptes
rendus, vol. 246, No. 12, March 1958, pp. 1833 to 1835,
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