Note: Descriptions are shown in the official language in which they were submitted.
3~
FIEL~ OF THE INVENTION
The invention relates to a single-mode oscillator operating
in the microwave range using the magnetostatic waves that can be
propagated on the surface of a thin layer of magnetic material
with the control of an ext_rnal magnetic field, this oscillator
being tunable by varying the magnetic field intensityO
DESCRIPTION OF THE-PRIQR ART
Tunable oscillators under the control of a magnetic field
have already been designed including a delay line formed in a
thin magnetic layer. These oscillators use the propagation of
magneto-elastic waves resulting from the interaction of the
magnetic waves set up by a magnetic field in a magnetic layer
and the elastic waves generated by piezo-electric effect in
the delay line constituted between an input transducer to which
lS an electric voltage is supplied and an output transducer. As the
propagation of magneto-elastic waves is bound up with the displa-
cement of substances, the oscilla~ion frequency that can be
obtained by interconnecting the delay line in a feedback loop
via an amplifier is in the range from about 10 MHz to about
1 GHz and practically between 50 and 500 MHz. On the other hand,
the tuning range which can be obtained by varying the magnetic
field is very small, about 1 to 5 %.
In order to obtain oscillation frequencies in approximately
the 1 to 10 GHz range, it is necessary to use another type of
propagation that is possible in a magnetic material, i.e. that
of magnetostatic waves. These waves, which are due to the movement
of magnetization in the material, are of a purely magnetic nature.
Excitation and detection of those waves ~ achieved by means of
two short-circuited microstrip couplers made of conductive material
which receive or supply ul~r~-high frequency currents. As the
propagation of these waves is associated with spin displacements,
an oscillator operating at frequencies around one GHz can be
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obtained by interconnecting the two couplers via an amplifier.
It can be noted from simple reasoning in connection with the
conditions of oscillation that, for a given magnetic field
intensity, there e~ist several modes of oscillation that ~re
5j equidistant in fre~luency. In order to obtain a single-mode
oscillator, arrangements must be made to ensure that the pass~
band of the couplers covers only one mode. It is not desirable
to increase the interval between modes excessively as the more
this interval increases, the poorer is the quality factor of
the oscillator. It i5 conceivable in this case to reduce the
pass-band of the transducers. One known means is to use multi-
strip couplers for the input and output of the delay line. The
pass-band is far smaller than for a one strip coupler and it is
thus possible to select a single mode. Howeverl a problem
arises when the magnetic field is varied in order to adjust
the oscillation frequency : when the magnetic field varies, the
frequency of the modes shifts and the pass-band is also displaced,
but not at the same speed. In order for the selected mode to
remain within the pass-band, the latter being small, it is
necessary for the variation in the magnetic field to be very
small, which restricts the possibility of tuning to a hundred
or so MHz.
SUMMARY OF THE INVENTION
The invention proposes an oscillator comprising, on the
surface of the same magnetic layer, two magnetostatic wave delay
lines of differing lengths. A simple method of forming said two
lines is to place three transducers on the layer, the two lines
being constituted by the distance between one of the transducers
and each of ~he other two, that is to say, they have in common
either the inpub transducer or the output transducer. The two
lines are then looped up by means of a sinyle amplifier. The
oscillation modes of the two line-oscillator correspond to the
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frequencies for which the modes of the two lines coincide when
taken separately. By suitably selecting the lengths of the two
lines, it is possible to select the interval between modes at
will, which obviates the need to reduce the pass-band of the
transducers. The tuning range for the oscillation frequency
can then extend over several hundreds of MHz, and even l to 2 GHz.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the invention will become
apparent from the following description and the accompanying
figures, wherein :
- Fig. 1 is a perspective vlew of a known magnetostatic
surface waves delay line ;
- Fig. ~ is a schematic diagram of a ~ own~ oscillator
comprising a delay llne ;
- Fig. 3 is a diagram showing the oscillation modes of
the oscillator represented in fig. 2 ;
- Fig. 4 is the curve representing the transfer function
of the loop of the oscillator in fig. 2 ;
- Fig. 5 is a diagram showing the drawbacks of known
oscillatars using multistrip couplers ;
- Fig. 6 is a schematic diagram of an oscillator in accor-
dance with the present invention ;
- Fig. 7 is a diagram showing the oscillation modes of
the oscillator in fig. 6 ;
- Fig. 8 illustrates an embodiment of the invention ;
Figs 9a to 9 G are explanatory diagrams showing how the
parameters of the oscillator according to the invention are
to be selected.
DESCRIPTION OFiTHE PREFERRED EMBODIMENTS
Fig. l shows a known magnetostatic surface wave delay line.
It is constituted in a magnetic medium having a magnetization M
to which is applied a constant magnetic field H. From Maxwell's
~3~4
e~uations, it can be deduced that the interaction between the
applied field and the magnetization enables surface waves to be
propagated in the material. The device in fig. l comprises a
non-magnetic substrate l, for example gadolinium gallium garnet
(G.G.G.~ on which is deposited a thin layer 2 o~ a ferrimagnetic
garnet obtained, for example, by epitaxial growth in liquid phase
on the gadolinium gallium garnet. This ferrimagnetic garnet can
be a pure yttrium iron garnet (Y.I.G.) ; the magnetic losses of
this material are small and the surface waves generated can thus
~ lO be used in devices of a planar struc-ture such as delay lines,
; resonators, oscillators
These waves are coupled by means of transducers which are
metallic strips deposited on the layer 2. The delay line is
formed between an input strip 3 and an output strip 4~ One of
the extremities of each transducer is connected to a metallic
layer S located opposite the ferrirnagnetic layer 2 in relation
to the substrate l and which constitutes the earth of the device.
The metallic layer 5 extends, covering one of the lateral faces
of the substrate l and a portion 6 of the outer surface of the
ferrimagnetic layer 2, which permits connection with the strips
3 and 4. To the other extremity of strip 3 is applied an input
signal E whose fre~uency f is in the ultra-high frequency band
(around one Gigahertz). At the other extremity of the strip 4,
a signal S is recovered after propagation in the layer 2 between
the two strips. As the latter are short-circuited, the electrical
current is maximum there. It produces magnetic induction with a
frequency f which has a movement of precession around the external
magnetic field H. This precession is propagated between the two
strips with a propagation speed V which depends on the properties
of the ferrimagnetic material and on the magnetic field H. As a
function of the length L between the two strips, the recovered
current S is lagging in relation to the input current E by a
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delay ~ . The delay line represented in fig~ 1 can be used
in various devices, and more particularly in an oscillator.
F~ig. 2 is a schematic diagram of a kno~n oscillator. On
layer 2 are placed the strips 3 and 4 each having a first
extremity connected to earth. The second extremity of strip 4,
which supplies the current S, is connected to the input of an
amplifier 7 ~hich supplies the current E applied to the second
extremity of strip 3. A directionnal coupler 8 also enables a
fraction Eo of the current E to be recovered. The oscillation
conditions or such a device are that the loop gain is greater
than 1 and that the total phase-shift caused by the delay line,
the amplifier and the connecting leads is a whole multiple of
2~ . f being the oscillation frequency, ~ the delay of the
delay line, and ~e~ the phase-shift effected by the amplifier
lS and the leads, the latter condition is expressed as follows :
~e ~ 2~r f~ = n 2~r , n being an integer. ~he phase-shift ~
is equivalent to a delay time to = 2~r f ; the device of figO 2
- is capable of oscillating at all the frequencies of the form
f = t +~,com~rised in the pass-band of strips 3 and 4 and
such that the loop gain is greater than 1.
Figure 3 represents the frequency spectrum of the oscillator.
The interval between the frequencies of two adjacent modes is
equal to ~ f = t +~ ~ The proximity of the modes thus increases
the greater is the time delay 1~. Furthermore, the quality factor
of the oscillator is proportional to ~ ~
Fig. 4 is a diagram representing the loop transfer function
of the oscillator, that is to say the gain of loop G as a function
of frequency J. In order to obtain an oscillation, the loop gain
G must be greater than 1, and the amplifier gain must thereore
be greater than the insertion losses. The transfer function has
two cut-off frequencies, ~ 1 and J2~ The value o J~ is a
~unction of the width of the strips. It i5 known, in fact, that
~391L'~
a strip ~ith a width b enables waves with a wave r~u~ber compri-
sed between O and b to be coupled. On the other hand the
insertion losses increase with the frequency~ This is due to an
increase in wave delay with the frequency, which leads to the
increase in propagation losses. By adjusting the amplifier gain
in such a way that the loop gain is equal to 1 in the vicinity
of the frequency ~2,one obtain an oscillator whose loop gain
does not vary by more than 3dB over the entire oscillation range.
This range is centered around a frequency Jo and has a width of
B = ~2
Typical ~alues obtained with an oscillator comprising a
layer of YIG with a thickness of 10 ~m, aluminium strips with
a width of 50 ~m, a length of 3 mm and distant by L = 1 cm, ~he
magnetic field applied being H = ~00 Oe are B ~P 600 MHz,
~JO~- 2 GHz, to +~120 ns, ~f~ 9 MHz. Comparison between B
and ~ f shows that this oscillator is not single-mode. A known
means of making it single-mode is to increase the value of ~ f
by decreasing the length L, but this solution is rejected as it
would be necessary to increase ~ f very considerably and the
quality factor decreases when ~f increases. Another known means
is to reduce the pass-band by using multistrip couplers, that is
to say couplers made of several metallic strips in parallel.
With 6 strips having a width of 100 um, disposed in parallel,
one obtains a pass-band of 35 MHz. This method is suitable for
~5 an oscillator whose frequency is fixed or which is to be tuned
over a reduced range. The object of the invention is to obtain
a single-mode oscillator that can be tuned over a wide range,
using the tuning facility offered by variation in the magnetic
field. Now, as the delay ~Ghas a variation that is subtantially
proportional to that of H, the variation l~fi of the frequency
fi of a given mode is such that fi = ~+l . The pass-band
is also displaced. Its displacement can be calculated on the
~asis of the variation of the central frequency Jo such that
. The displacement of the pass-band is thus more
rapid than that of the oscillation modes. If the pass-band is
centered on a value fi for a given magnetic field, in such a
way as to select the single ~requency mode fi, for another
value of the magnetic field, the pass-band having shifted in
relation to the mode spectrum, the pass-band may co~er another
frequency, or two frequencies fj and fk, or, on the contrary,
no frequency may be present in the pass-band, if the latter is
very small. Fig. 5 illustrates a variation curve giving the
oscillation frequency (or frequencies) f as a function of the
magnetic field H. This curve is formed by sections of straight
lines separated by jumps which correspond to the interval ~ f.
Without jumps, the tuning range is very small : approximately
100 MHz with the values given above.
In a schematic diagram, Fig. 6 illustrates an oscillator
according to the invention which is single-mode and can be tuned
over a range of several hundreds of MHz. This oscillator compri-
ses two delay lines placed in parallel in the same oscillation
loop ~ith a single amplifier. Three strip couplers are placed
on the ferrimagnetic layer : an input strip 3 receiving a current
E as before, and two output strips 41 and 42 supplying currents
Sl and S2. One of the extremities of each strip is connected to
earth. In this way, two delay lines are obtained on the same
surface : a line with a length Ll between the strips 3 and 41,
giving a delay ~1 and a line with a length L2, greater than Ll
between the strips 3 and 42, giving a delay ^~~2~ ~1 The signals
Sl and S2 are added at the input of an amplifier 7. A sma~ll
fraction EO of the signal obtained at the output of the amplifier
is recovered at the output of the oscillator while the rest forms
the input signal ~ of the delay line. The signal Eo is extracted
by means of the directionnal coupler 8. As layer 2 is subjected
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to a magnetic field H from electromagnetic means 9, the delay
times -rl and -~2 can be varied when the intensity of the magne-
tic field is varied. An oscillator according to the invention
can also comprise two lines with lengths Ll and L2 having a
common output strip, the output of the amplifier being connected
to the respective input strips of the two lines. The arrangement
of the two delay lines enables numerous variants to be obtained :
on the same layer but not having common portions ; each provided
with their own input and output strips : on different layers,
possibly formed of different materials, whether juxtaposed or
otherwise, etcØ Whatever its form of embodiment, the operation
of an oscillator with two different delay lines can be explained
on the basis of the frequency spectra of fig. 7 corresponding
to a given va,lue for H. This Fig. is described from the top to
the bottom. First, it shows the frequency spectrum for an oscil--
lator formed rom a line with delay 1~1, whose total delay is
~1 + tol The modes are spaced at intervals of ~ f = ~
Fig. 7 then shows the frequency spectrum of an oscillator formed
from a line with delay ~2 whose total delay is ~2 + to2. The
intervals between the modes are ~ f2 = ~ 1 to . Fig. 7 further
shows the frequency spectrum of the oscillator of figO 6. It
will be noted that the oscillation modes are those for which
the frequencies coincide in the two first spectra. In the case
of the figure, ~2 ~-tt~ = 2/3 , there is an interval between
modes of ~ f = 3 ~fl = 2 Qf2. The use of two delay lines has
thus made it possible to increase the interval between modes
without any loss in quality factor. Finally, is represented
the pass-band of the two delay lines. In order for the oscillator
to be tunable, the variation of H must not eliminate the coinci-
dence in frequency between the two first spectra, that is to say
+ to = ~ ~~to now, ~ ~1 = fCl h
1 + ~ L ~
_ ~ _ _
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By suitably selecting the value of the relation 2 + to2-
it is possible to determine at will the inter~al between the
modes of the oscillator in such a way that said interval is
sufficiently large as a function of the pass-band. The latter
must be centred in relation to a given mode so as to enable
the oscillation frequency to be tuned by varying the magnetic
field H over a large range (in the same order of magnitude as
the interval between modes).
Fig. 8 illustrates a form of embodiment of the oscillator
according to the invention. It comprises a substrate of GGG, 1,
500 ~m thick on the surface of which a layer of pure YIG, 2,
with a thickness of 10 ~m is epita~ially grown in liquid phase.
The layer is parallel to a plane permitting easy orientation
of the GGG (1, 1, 1). In this way a layer of uniform magnetiza-
lS tion is obtained. Electro magneti~ means 9, such as a permanentmagnet, provides the magnetic field H, parallel to said plane
and to the magnetization M of the layer.
The transducers 3, 41, 42 are very narrow strips ~about
5G ~m wide~ made of any conductive material such as aluminium,
copper, gold in order to obtain a wide pass-band (about 500 MHz~.
These strips can be obtained b~ vacuum spraying a layer of con-
ductive material 5 ~m thick and by photo-etching. The strips are
3 ~m long. Their orientation on the layer 2 determines the angle
between the direction of propagation and the field H, said angle
2S being selected in such a way as to obtain maximum interactions.
The distances between the strips, Ll and L2, can be very precise
(to within a few ~m) owing to the techniques of masking and
photo-etching. The free surface area of the substrate 1 and
one of its transverse faces is covered with a layer S, of con~
ductive material which may be the same as for the strips connected
to the layer 6 formed on the surface of the layer 2 at the same
~ime as the strips. The amplifier 7 can be an integrated circuit
3~
contained in a housing and located on the layer of YIG 2. This
is, for example, a conventional amplifier constituted by field
effect transistors. The directionnal coupler 8 is also located
on the YIG layer. The strips 3, 41, 42, the amplifier 7 and
the coupler 8 are linked up by coaxial cables whose sheath
is connected to the earth layer 5.
As it has been seen above, the choice of lengths Ll and L2
depends on the desired performances of the oscillator. Approxi-
mative values for the different physical dimensions entering
into this choice can be given and result from the design of
an oscillator with Ll = 10 mm and L~ = 15 mm. When applying
a magnetic field H of 180 Oe, we measured an oscillation fre-
quency of fO = 2.1 GHz and delay times of tol ~1 = 77 ns
and to2 ~r~2 = 115 ns. The corresponding values of ~ fl and
~ f2 are ~ fl = 13 MHz and ~ f2 = 8.69 MHzo The interval between
the modes of the described oscillator is ~ f = 26 MHz. The
oscillation frequency is, furthermore, noted to be proportional
to the field H. These values for Ll and L2 are given only for
the purpose of verifying the operation of an oscillator compri-
sing two delay lines. However, they are not suitable for asingle-mode oscillator ; in fact, the interval ~ f is far smaller
than the pass-band, which is 500 M~lz with 50 ~m wide transducers.
The diagrams of Figs 9a, 9b and 9c explain the method recom-
mended for selecting the different parameters of the oscillator.
In the case of the example taken in the fig., the pass-band B is
500 MHz and the interval between modes is also 500 MHz. It is
desired to be able to tune the oscillation frequency in a range
of 500 ME]z between 2.5-GHz and 3 GHz. The three figures are
frequency diagrams for 3 values Hl, H2, H3 of the magnetic field
corresponding respectively to oscillation frequencies of 2.5,
2.75 and 3 GHz. As central frequency ~0 of the pass-band varies
more considerably as a function of H than the oscillation fre-
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quency fi of a given mode, the centering of the pass~band inrelation to the oscillation mode is selected in such a way
that the oscillation frequency for the field Hl (2.5 GH7) is
within the high frequencies of the band and, for the field H3
(3 GHz) in the low frequencies of the band, while, in the case
of H2, the oscillation frequency (2.75 GH~) is in the centre
o the band. In this way, by acting on the gain of the amplifier,
the value of ~0 equal to 2.275 GHz, 2.75 GHz and 3.225 GHz was
obtained for Hl, H2 and H3 respectively. In all cases, the value
selected for B enables only one mode to be selected. For
fi = 500 MHz, we thus have ~ ~ ~ = 950 MHz, giving the rela-
tionship ~ 01 = ~ ~ = 955 = 1.9~
The values of ~fl and ~f2 are such that ~f = S00 MHz = nl ~fl -
n2 ~ f2, wherein nl and n2 are incommensurable numbers. For
lS example, nl = 100 and ~ fl = 5 MH7. ; n2 = 111 and ~ f2 = 4.5 MHz
and we deduce therefrom tol + ~ = 200 ns and to2 +~2 = 222 ns-
The above relationship necessarily makes ~ 1 = 105 ns and
~2 = 117 ns ; the values tol = 95 ns and to2 = 105 ns can be
obtained owing to the delays provided by the amplifier and the
cables. In order to measure them, one can remove the delay lines
from the circuit and measure the intervals of oscillation frequen-
cy obtained for the two loops. The lengths Ll and L2 are adusted
so as to obtain the above delay times~Cl and ~ , the relation-
ship L being equal to 1,11. They are approximatively 15 mm.
In order to adjust the oscillation frequency between 2.5 GHz
and 3 OEIz, one must vary the magnetic field between approxima-
tely 210 oe and 260 oe. All these values are given only by
way of example and to show how to design a single-mode oscillator
according to the invention. The applications according to the
invention are for oscillators operating in the microwave range,
such as radar, measuring instruments, etc. The device proposed
is particularly advantageous by reason of its simplicity, its
small size owing to the technology used and its large tuning
capacity.
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