Amplitude and phase modulation in fin-lines by electrical tuning

A fin-line waveguide device for modulating signals passing therethrough. The device comprises a fin-line waveguide including two channel members longitudinally extending in the direction of signal propagation through the device. The channel members are physically and electrically separated by a ferrite dielectric slab which is oriented in a longitudinally extending plane parallel to the electric field lines of the signal passing through the device. A magnetic field is applied to the ferrite slab to magnetically bias the slab to ferromagnetic resonance. A voltage is applied across the ferrite slab to alter the ferromagnetic resonance characteristics of the slab. The phase and amplitude of the signal passing through the waveguide are modulated in response to the applied voltage.

BACKGROUND OF THE INVENTION 
The present Invention relates, in general, to a novel device for modulating 
the amplitude and phase of millimeter wavelength signals and, more 
particularly, to a novel device for modulating the amplitude and phase of 
millimeter wavelength signals in a fin-line waveguide configuration. 
Recently there has been a dramatic interest in the development of 
millimeter wavelength components and, in particular, in non-reciprocal 
components. Fabrication cost, propagation losses, and device efficiency 
are all important criteria to be considered at millimeter wavelengths. 
Presently the fin-line waveguide configuration is believed to offer the 
best opportunity in meeting all of the above criteria. 
The fin-line waveguide configuration is a relatively new type of waveguide 
transmission line which, in addition to the above mentioned criteria, 
offers distinct advantages in power handling capacity and bandwidth as 
compared to conventional waveguides. In its most basic form, the fin-line 
comprises a rectangular waveguide loaded with a slab of dielectric 
material positioned across the center of the waveguide in the plane of the 
electric field. The dielectric slab has the effect of adding capacitance 
to the dominant mode of resonance while only slightly changing the 
capacitance associated with adjacent modes. Thus the dielectric slab 
effectively increases the separation between the first two modes of 
propogation thereby providing a wider useful bandwidth than conventional 
waveguides. Additionally, because the dielectric slab has an inherently 
high breakdown strength relative to air, the slab adds a material having 
high breakdown strength to the waveguide in a region where breakdown is 
more likely to occur. As a result, the power handling capability is 
increased in the fin-line waveguide as compared to conventional 
waveguides. A thorough description of fin-line waveguides appears in P. J. 
Meier, "Integrated Fin-Line Millimeter Components," IEEE Transactions on 
Microwave Theory and Techniques, Vol. MTT-22, No. 12, December 1974, pages 
1209-1216. The contents of this article are incorporated herein by 
reference. 
The present Invention provides a novel means for modulating the phase and 
amplitude of millimeter wavelength signals passing through a fin-line 
waveguide. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present Invention is to provide a novel 
apparatus for modulating millimeter wavelength signals. 
Another object is to provide a novel low cost device for modulating 
millimeter wavelength signals. 
These and other objectives and advantages are provided by a novel fin-line 
waveguide device for modulating an electromagnetic signal passing 
therethrough which comprises a waveguide including first and second 
channel members longitudinally extending in the direction of signal 
propagation through the device. A ferrite dielectric slab is disposed in 
the waveguide between the first and second channel members to 
longitudinally bisect the waveguide and to electrically separate the first 
and second channel members. The ferrite slab is located in a 
longitudinally extending plane parallel to the electric field lines of the 
signal passing through the waveguide. A means produces a magnetic field in 
the ferrite slab to magnetically bias the ferrite slab to or near 
ferromagnetic resonance. A voltage is applied across the ferrite slab to 
shift the ferromagnetic resonance characteristics of the ferrite slab. 
Thus, the magnetic permeability of the ferrite slab is altered upon the 
application of the voltage across the slab. Since the phase and amplitude 
of the electromagnetic signal propagating through the fin-line waveguide 
depends on the permeability and dielectric constant of the ferrite slab, 
the device thus modulates the phase angle of the signal passing 
therethrough in response to the applied voltage.

DETAIL DESCRIPTION OF THE PREFERED EMBODIMENTS 
Referring now to the drawing, a fin-line modulator device 10 is illustrated 
in cross-section. The modulator 10 includes a rectangular waveguide 12 
formed from two channel members 14 and 16. The channel members 14 and 16 
are formed from standard non-magnetic waveguide materials, such as copper, 
silver, brass, etc. The channel member 14 includes two edge surfaces 18 
and 20 while the channel member 16 includes two edge surfaces 22 and 24. 
The channel members 14 and 16 are oriented such that their respective edge 
surfaces 18, 20 and 22, 24 oppose each other. 
Positioned between the respective edges of the channel members 14 and 16 is 
a slab or panel of ferrite dielectric material 26, having a thickness "c", 
the characteristics of which will be described in detail below. The 
ferrite slab 26 includes a first side surface 28 and a second side surface 
30. The first side surface 28 is coated with metallic layers 32 and 34 
spaced apart by a distance "d" while the second side surface is coated 
with metallic layers 36 and 38 spaced apart by the distance d. The 
metallic layers 34 through 38 are thus positioned so as to expose an area 
having a width d on each of the side surfaces 28 and 30. The metallic 
layers 34 through 38 are comprised of the same non-magnetic material as 
that of the channel members 14 and 16 and are formed on the ferrite slab 
26 by any standard means, such as by vapor deposition or evaporation. The 
metallic layers should have a thickness greater than the skin depth 
penetrated by the electromagnetic wave to be carried by the fin-line 
waveguide. A typical layer thickness would be approximately 5 .mu.m or 
greater for frequencies in the range of 1 to 100 GHz. 
The modulator device 10 is assembled such that the edge surfaces 18, 20, 
22, and 24 of the the channel members 14 and 16 are in firm mechanical and 
electrical contact with the respective metallic layers 32, 34, 36, and 38. 
Thus, the channel member 14 is in electrical contact with the side surface 
28 while the channel member 16 is in electrical contact with the side 
surface 30 of the ferrite slab 26. The assembled waveguide 12 has an 
overall internal width "a" and an internal height "b". The ferrite slab 26 
thus spans the height b of the waveguide 12 and, therefore, is positioned 
in parallel with the direction of the electric field developed within the 
waveguide 12 by the transmitted wave. 
The fin-line waveguide wavelength .lambda.g is given by: 
EQU .lambda.g=.lambda..sub.o [K.sub.e -(.lambda..sub.o /.lambda..sub.c).sup.2 
].sup.-1/2 (1) 
where K.sub.e is the effective dielectric constant of the ferrite slab 
which includes the permeability as well as the actual dielectric constant 
of the ferrite slab. .lambda..sub.o is the free space wavelength and 
.lambda..sub.c is the cut-off wavelength which can be calculated for given 
dimensions a, b, and d as described in the above-mention article by P. J. 
MEIER, incorporated herein by reference. For example, for a=0.71 cm, 
b=0.35 cm, and d=0.035 cm the cut-off wavelength .lambda..sub.c will be 
2.148 cm. If the effective dielectric constant K.sub.e is assumed to be 
approximately 3 for a typical ferrite material, the waveguide wavelength 
.lambda.g from Equation (1) for a free space wavelength .lambda..sub.o of 
0.86 cm (35 GHz) is 0.51 cm. Typically, the thickness of the ferrite slab 
c can be 250 .mu.m or less for the above stated values of a, b, and d. As 
a rule, the dimensions a, b, c, and d will be smaller for higher 
frequencies of operation. Additional fin-line design equations and 
dimension graphs are presented in P. H. Vartanian et al., "Propagation in 
Dielectric Slab Loaded Rectangular Waveguide", IRE Transactions on 
Microwave Theory and Techniques, Vol. MTT-6, 1958, pages 215-222, and in 
A. M. K. Saad et al., "Electrical Performance of Finlines of Various 
Configurations", IEE Journal of Microwave, Optics, and Acoustics, Great 
Britain, Vol. 1, No. 2, January 1977, pages 81-88. The contents of these 
articles are incorporated herein by reference. 
Although not illustrated, it should be understood that the fin-line 
modulator 10 has a length "L". The length L should be equal in length to 
at least one wavelength of the transmitted wave. A length of 2 cm would be 
sufficient for waves in the range of 30 to 50 GHz. 
The ferrite material used to form the ferrite panel 26 can be any material 
which exhibits a magnetoelectric effect at any temperature. The 
magnetoelectric effect manifests itself in ferrite materials which have no 
center of inversion symmetry or in ferrites with very low symmetry. The 
magnetoelectric effect is usually observed at low temperatures on the 
order of 4.2.degree. Kelvin. However, in flux grown single crystal lithium 
ferrite (Li.sub.2 Fe.sub.3 O.sub.4) the magnetoelectric effect is observed 
as high as 150.degree. Kelvin. In the case of lithium ferrite, an 
appropriate ferrite slab can be obtained from a bulk of single crystal 
lithium ferrite material flux grown from lead rich melt. In such a bulk, 
the skin region closest to the growth flux exhibits the highest level of 
magnetoelectric effect. Therefore, the ferrite slab 26 should be prepared 
from a layer of this skin material. Skin layer lithium ferrite sheets may 
be readily obtained from ferrite manufacturers such as Allied Corporation. 
Other ferrite materials exhibiting the magnetoelectric effect can also be 
used, as should be apparent to those of skill in the art. 
Provision must be made to maintain the ferrite slab 26 at an appropriate 
temperature consistant with the magnetic characteristics of the particular 
ferrite material used. In the case of lithium ferrite, the slab 26 must be 
cooled to a temperature of approximately 150.degree. Kelvin. This can be 
readily accomplished by enclosing the entire fin-line modulator 10 in a 
suitable non-magnetic vessel (not illustrated) filled with liquid nitrogen 
or liquid argon, or by equivalent well-known cooling means. Other ferrite 
materials will require similar cooling temperature considerations, as 
should be apparent to those of skill in the art. 
In general, the equivalent dielectric constant K.sub.e of a ferrite slab in 
a fin-line structure is not a strong function of the waveguide dimensions. 
The effective dielectric constant is, however, a function of the material 
magnetic parameters of the actual dielectric ferrite material used. Thus 
by changing the material parameters of the dielectric material, the 
effective dielectric constant K.sub.e may be changed. From equation (1), a 
change in the dielectric constant K.sub.e results in a change in the 
waveguide wavelength .lambda.g of the fin-line which has the effect of 
imparting a change in phase of the signal transmitted by the fin-line 
waveguide. 
In the fin-line modulator device 10 of the present Invention, modulation of 
signals traveling through the device 10 is accomplished in a manner 
similar to that described above with respect to equation (1) by altering 
or modulating the magnetic parameters of the ferrite dielectric slab 26. 
The magnetic parameters of the ferrite are altered or modulated by 
applying a variable voltage across the ferrite slab in the presence of a 
fixed DC magnetic field. The fixed magnetic field acts to magnetically 
bias the ferrite slab to or near ferromagnetic resonance for the 
particular ferrite material utilized. Applying a voltage across the 
ferrite slab results in a shift in the absorption or dispersion curve for 
the ferrite material which is linearly proportional to the amplitude of 
the applied voltage. The ferromagnetic resonance characteristics of a 
particular ferrite material, in particular, thin surface skin lithium 
ferrite, in the presence of both magnetic and electric fields are 
discussed in G. T. Rado et al., "Linear Electric Field Shift of a 
Ferromagnetic Resonance: Lithium Ferrite", Physical Review Letters, Vol. 
41, NO. 18, 30 October 1978, pages 1253-1255. The contents of this article 
are incorporated herein by reference. 
Referring again to the drawing, the magnetizing force vector H of the fixed 
DC magnetic field may be applied to the fin-line modulator device 10 
parallel to the plane of the ferrite slab 26 as indicated by the vector 
H.sub.p or by applying the vector perpendicular to the plane of the slab 
26, as indicated by the vector H.sub.T. The magnetic field may be supplied 
by appropriately orienting the device 10 within the air gap of a permanent 
horseshoe-type magnet (not illustrated) or by positioning the device 10 
within an electromagnet solenoid (not illustrated) or within a Helmholtz 
coil (not illustrated). 
The ferromagnetic resonant frequency .omega. for ferrite materials is 
related to the magnitude of the parallel field vector H.sub.p by: 
EQU .omega.=.gamma.[H.sub.p (H.sub.p +4.pi.M)].sup.1/2 (2) 
where .gamma.=ge/2mc, g is the gyromagnetic splitting factor, e is the 
electron charge, m is the electron mass, c is the velocity of light, and M 
is the saturation magnetization for the particular ferrite material used. 
Similarly, the ferromagnetic resonant frequency .omega. is related to the 
magnitude of the perpendicular field vector H.sub.T by: 
EQU .omega.=.gamma.[H.sub.T -4.pi.M] (3) 
For lithium ferrite, .gamma. is equal to 2.8 MHz/Gauss while the quantity 
4.pi.M equals 4000 Gauss. 
A voltage source 50 produces a voltage V across two conductors 52 and 54. 
The conductor 52 is electrically coupled to the first channel member 14 
while the conductor 54 is electrically coupled to the second channel 
member 16. As previously described, the first channel member 14 is 
electrically coupled to the first side surface 28 of the ferrite slab 26 
through the metallic layers 32 and 34 while the second channel member 16 
is electrically coupled to the second side surface 30 of the ferrite slab 
26 through the metallic layers 36 and 38. Thus, due to the dielectric 
characteristics of the ferrite material, essentially the entire voltage V 
appears across the ferrite slab 26. 
The frequency of the voltage V may range from DC up to 100 MHz or higher, 
the upper limit being controlled by the relaxation time constant of the 
ferrite material. The maximum amplitude of the voltage V is limited by the 
dielectric breakdown strength of the ferrite material. 
The fin-line modulator device 10 may be used to simultaneously amplitude 
and phase modulate a signal passing through the device. When used for this 
purpose, the bias magnetic field strength (H.sub.p or H.sub.T) should be 
set according to Equations (2) or (3) to produce resonance in the ferrite 
slab 26 at the frequency of the signal to be transmitted by the device. 
For example, for lithium ferrite material a parallel field H.sub.P of 3000 
Gauss will produce resonance at 12.8 GHz. The amplitude and phase of the 
transmitted signal can be varied or modulated by varying the amplitude of 
the applied voltage V. The rate of change in the amplitude and phase of 
the transmitted signal is proportional to the frequency of the applied 
voltage V. 
In many circumstances phase modulation with no change in amplitude is 
desired. Phase modulation with little or no amplitude variation can be 
achieved with the device 10 by magnetically biasing the ferrite slab 26 to 
a ferromagnetic resonant frequency, as given by Equations (2) or (3), 
which is at least 10 times the absorption bandwidth (FMR linewidth) of the 
particular ferrite material used away from the frequency of the 
transmitted signal. The resonant frequency may be either above or below 
the signal frequency. For example, lithium ferrite material has an 
absorption bandwidth of 2 Gauss or approximately 6 MHz. Thus, the resonant 
frequency should equal the signal frequency plus or minus 0.06 GHz. The 
phase of the transmitted signal will be altered in proportion to the 
amplitude of the applied voltage V. For example, a lithium ferrite slab 
125 .mu.m thick in a bias field of 3000 Gauss will linearly shift the 
phase of a 12.74 GHz transmitted signal at the rate of 1 degree per 100 
volts applied voltage. 
It should be noted that the ratio of the phase shift to the applied voltage 
is inversely proportional to the thickness c of the ferrite slab 26. Thus 
reducing the thickness c will increase the change in phase for a given 
voltage change. The minimum useable thickness of the ferrite slab 26 is 
controlled by the maximum applied voltage and the dielectric breakdown 
strength of the ferrite material used. 
The fin-line modulator device 10 is a non-reciprocal device. Thus, in 
general, transmitted signals traveling in one direction through the device 
will be affected differently from signals traveling in the opposite 
direction through the device. The device 10 may be used to form an 
isolator by redirecting the parallel bias field vector H.sub.p to run in 
parallel with the direction of signal transmission through the device 
(longitudinal direction). Thus, in the drawing, the vector H.sub.pp is 
directed into the plane of the drawing to run in parallel with the length 
L of the device 10. The resonant frequency produced by this vector can be 
determined from Equation (2). In this configuration, signals traveling in 
one direction through the device 10 can be phase modulated or phase and 
amplitude modulated by means of the applied voltage V while signal flow in 
the opposite direction will be blocked or substantially reduced. 
Obviously, numerous (additional) modifications and variations of the 
present Invention are possible in light of the above teachings. It is 
therefore to be understood that within the scope of the appended claims, 
the Invention may be practiced otherwise than as specifically described 
herein.