Circulator and components thereof

A circulator having integrally formed conductors (20, 21, and 22) which may be folded to form overlaying conductors of a circulator. The circulator includes a lens (44) for shaping a biasing magnetic field distribution to compensate for non-uniformity of magnetic field strength caused by irregularities of a magnetic circuit or the shape of a magnet (45) or ferrite (40, 41). The characteristics of ferrite discs (40, 41) are preferably correlated with the characteristics of a permanent magnet (45) so that variations of permeability of the ferrite (40, 41) are minimized over a specified temperature range.

TECHNICAL FIELD 
The present invention relates to a radio frequency circulator/isolator and 
components thereof. The circulator/isolator (called hereafter a 
"circulator" for brevity) is a non-reciprocal device often used for 
discriminating and/or diverting oppositely directed signals transmitted 
through a network. 
BACKGROUND OF THE INVENTION 
Circulators generally contain two basic parts: 
i/ a microwave circuit comprising an arrangement of conductors and ferrite 
blocks, and 
ii/ a magnetic circuit providing a magnetic biasing field applied to the 
ferrite blocks that act as a non-reciprocal media for propagating radio 
frequency signals throughout the device. 
An ideal three port circulator transmits power (as shown diagrammatically 
in FIG. 1) between any two ports in a forward direction only, i.e. from 
port 1 to port 2, from port 2 to port 3, from port 3 to port 1. In the 
reverse direction (from port 1 to port 3, from port 3 to port 2 and from 
port 2 to port 1) no power can be transmitted (i.e. port 3 is isolated 
from port 1, port 2 from port 3, and port 1 from port 2). 
A circulator can be converted to an isolator by connecting a matched load 
to one of the ports. For example, if port 3 is terminated with a matched 
load, a drive signal is applied to input port 1 and an antenna is 
connected to output port 2, then any power reflected from the antenna is 
directed to the terminated port 3 and dissipated in the load. 
A typical prior art strip line lumped element circulator is shown in FIGS. 
2 and 3. Conductors 4 connected to terminal ports are sandwiched between 
ferrite discs 5 and 6 which in turn are located in the gap between magnets 
7 and 8. Permanent magnets 7 and 8 are supposed to magnetise ferrite disks 
5 and 6 and provide a dc biasing magnetic field in the ferrite disks 5,6 
that is necessary for signal circulation between terminal ports. The 
direction of circulation is determined by the orientation of the applied 
dc magnetic field and may be reversed by reversing the polarity of the 
magnets 7,8. 
In such prior devices conductors 4a, 4b, and 4c form a multi-layered 
construction where individual strips are interwoven and their 
intersections are insulated during assembly. The conductor ends (9a, 9b, 
9c) are connected to terminal ports of the circulator and the other ends 
(10a, 10b, 10c) are attached to a common ground plane. 
The pattern of interwoven conductors 4 may be fabricated in two different 
ways. One approach is based on interweaving and joining separate insulated 
strip conductors. The other technique employs the technology of 
multi-layered metal and dielectric deposition on the surface of a ferrite 
disk. The former method is time consuming and the resulting conductor 
assemblies may have inconsistent topology. The latter procedure exploits 
thin film technology and is typically useful in fabrication of low power 
microwave integrated devices. Increasing power handling capacity may 
result in a substantial rise in manufacturing cost. Another problem 
encountered by both fabricating methods is the quality of the connections 
between conductor ends (10a, 10b, 10c) and the common ground plane, the 
inconsistent joints causing increased losses and degradation of overall 
circulator performance. 
Homogeneity of the biasing magnetic field inside the ferrite disks is 
normally desirable for optimum circulator performance. Non-uniformity of 
the biasing magnetic field associated with the shape of magnets and 
ferrite blocks may substantially degrade insertion losses and isolation 
between the circulator ports. The crucial problem of optimising 
distribution of the biasing magnetic field has been extensively explored 
and addressed in numerous publications and patents. 
In particular, to generate a uniform magnetic field inside ferrite disks it 
has been proposed to attach ferrite semi-spheres either side of the 
ferrite discs (see E. F. Schloemann. "Circulators for Microwave and 
Millimeter-Wave Integrated Circuits". Proceedings of IEEE, vol. 76, No.2, 
February 1988, pp 188-200). Semi-spherical ferrite segments surrounding 
the ferrite disks neutralise the demagnetising effect of the disk-shaped 
ferrites on distribution of the internal biasing magnetic field. They help 
to preserve uniformity of the internal magnetic field when the system is 
exposed to a uniform external magnetic field. However, such an arrangement 
is bulky and only employs the central part of the magnetic system due to 
tight requirements of homogeneity in the external magnetic field. Ferrite 
semi-spherical segments are also expensive to produce and, due to the very 
poor thermal conductivity of ferrite, they impede heat transfer from the 
ferrite disks. The latter problem may result in substantial degradation of 
circulator performance with increasing power and/or varying temperature. 
DE 2950632 discloses the use of frustoconical ferrites in a junction 
circulator. This is said to reduce noise and intermodulations by 
minimising the effect of irregularities in the biasing magnetic field 
nearby the edge of the ferrite. This, however, requires special 
fabrication techniques, thus increasing cost. This also increases the 
thickness of ferrite used, thus impeding heat transfer. 
Further, in prior art circulators the ferrite was considered simply as part 
of the microwave circuit not affecting the DC magnetic circuit. This often 
resulted in difficulties of thermal stabilisation and the need for complex 
temperature controlling devices. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a circulator and 
components thereof which overcome or at least minimise the disadvantages 
mentioned above, or which at least provides the public with a useful 
choice. 
According to a first aspect of the invention there is provided an integral 
conductor arrangement for a circulator comprising a plurality of overlying 
spaced apart crossing strips attached at one end to a base portion having 
an opening therein, and forming a first compartment adapted for receiving 
a ferrite block therein such that the ferrite block may be inserted into 
the first compartment with one face of the ferrite block located adjacent 
the strips while an opposite face of the ferrite block is exposed to allow 
direct contact with a circulator housing body. 
There is further provided a method of forming a conductor arrangement for a 
circulator comprising the steps of: 
i) forming an integral conductor arrangement consisting of a plurality of 
strips extending outwardly from a base portion having an opening therein; 
ii) folding the arrangement to define a first compartment to accommodate a 
ferrite block; and 
iii) folding the strips inwardly without a ferrite block being inserted 
into the first compartment to form an arrangement of spaced apart 
overlaying crossing strips.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 4 shows the topology of an integrally formed conductor arrangement 19 
for a circulator formed from a thin sheet or foil of copper, although any 
suitable electrically conductive material may be used. The pattern of 
conductor arrangement 19 may be obtained by any appropriate process such 
as etching, stamping, photolithography etc. Conductor arrangement 19 is 
seen to comprise strips 20,20', 21,21' and 22,22' (for concern referral to 
benefits collecting gas 20, 21 and 22) connected to a base portion 23, 
23', 23". As the strips 20, 21 and 22 are integrally formed with base 23 
it is ensured that ends 24, 25 and 26 of strips 20, 21 and 22 form a good 
electrical connection with one another and a common ground plane. A more 
preferred topology 69 is shown in FIG. 6 which incorporates stripes 70, 71 
and 72 to facilitate connection to trimming capacitors and tapered ends 
73, 74 and 75 to facilitate direct connection to strip line connectors. 
Once a conductor layout 19 is produced the strips 20, 21 and 22 must be 
folded to form the desired configuration of overlapping crossing strips. 
That part of the pattern to the right from the line AA' including strips 
22,22' pads 35,35' and stripe 26 is folded inwards 90.degree. along the 
line AA'. Then end 29 of strip 22 is further folded inwards 90.degree. and 
pads 35,35' are folded outward 90.degree. along the line BB'. The same 
manipulations are subsequently performed with the other strips. End 27 of 
strip 20, stripe 24 and adjacent pads 31, 31' are folded along the lines 
A"A' and B"B'. Finally, end 28 of strip 21, stripe 25 and adjacent pads 
33, 33' are folded along the lines A"A and B"B. 
This is shown diagrammatically in FIGS. 7 to 8 for the topology of FIG. 6. 
FIG. 7 shows the strips 76, 77, 78 folded 90.degree. inwardly and FIG. 8 
shows the strips 76, 77 and 78 after they have been folded inwardly 
through 180.degree.. 
Dielectric film spacers are inserted between overlapping conductors 20,21 
and 22 (or 76, 77 or 78) after each fold to avoid direct electrical 
contact between adjacent strips 20, 21, and 22. Ends 27, 28 and 29 of 
strips 20, 21 and 22 may be connected to respective terminal ports of a 
circulator in use. Ends 73, 74 and 75 of conductor arrangement 69 may be 
connected to ports of a circulator or directly to strip line connectors. 
It will be seen that the strips may be easily formed simultaneously from a 
single sheet by virtue of the integral conductor topology. The conductor 
pattern may be easily fabricated simply by folding sections to the 
position shown in FIG. 5. Chip capacitors 36, 37, 38 used for circulator 
impedance matching, may be fitted between conductors 20, 21 and 22 and 
ground conductor 23 between aperture pairs 30, 30'; 32, 32'; and 34, 34'. 
The integral formation of the conductor layout avoids losses and faults in 
multiple contact joints of strips to a common electrical ground 
experienced in prior art arrangements. The integral topology of the 
conductors locates the position of conductors and enables the topological 
symmetry of the structure to be consistently reproduced. 
FIG. 9 shows a cross-sectional view of the circulator incorporating the 
conductor arrangement 19 shown in FIG. 5. The region where strips 20, 21, 
and 22 intersect is sandwiched between ferrite discs 40 and 41. Silver 
plated aluminium or copper layers 42 and 43 are an integral part of the 
circulator housing body which act as ground planes and assist in effective 
heat transfer from ferrite discs 40 and 41. U shaped yoke 47 provides an 
easy path for the magnetic flux from permanent magnet 45 to ferrite disks 
40,41. Magnetic lens 44 is located adjacent to disc-shaped permanent 
magnet 45. A similar lens 46 is provided on the opposite side of the 
magnetic circuit. This means that the magnetic circuit effectively 
concentrates the magnetic field and enhances the uniformity of the 
internal magnetic field inside the ferrite disks 40,41. 
Referring now to FIGS. 10 and 11 the magnetic lens is shown in more detail. 
Lens 44 is seen to have a disc-shaped portion 49 and a frusto-conical 
portion 48. The top face of frusto-conical portion 48 is positioned 
adjacent to magnet 45 so that there is a cut-away section 50 providing an 
increasing air gap between lens 44 and the edge of magnet 45 in the radial 
direction of magnet 45. The cut-away section 50 compensates for 
non-uniformity of magnetic field distribution caused by irregularities of 
the magnetic circuit and/or shape of magnet 45. As the strength of the 
magnetic field varies across, the surface of magnet 45 due to the magnet 
"edge effect" or other discontinuities (stronger magnetic field at the 
edge of magnet 45) lens 44 serves to flatten the magnetic field profile 
and ensure a substantially homogeneous internal magnetic field inside 
ferrite disks 40 and 41. The more uniform the magnetic field the lower the 
magnetic insertion losses in the circulator. 
Referring now to FIG. 12 a modification is shown wherein the cut-away 
portion 51 of the lens is concave. FIG. 13 shows a variant in which the 
cut-away portion 52 is convex. The shape of the cut-away portion will 
depend upon the shape of the permanent magnet and the aspect ratio of 
magnet and ferrite disks to be compensated. The shape of frusto-conical 
lens 44 shown in FIG. 10 is preferred due to its ease of fabrication. The 
lens is preferably formed of a magnetically soft material, e.g. iron or 
magnetic steel. 
Referring now to FIG. 14 ends 27, 28 and 29 of conductors 20, 21 and 22 
(shown in FIGS. 4 and 5) are seen to be attached to connectors 60, 61 and 
62. Adjustable capacitors 63, 64 and 65 may be connected between ends 27, 
28, 29 and a ground plane for the purpose of impedance matching and tuning 
the circulator to different operating frequencies. When the operating 
frequency is fixed, chip capacitors 36, 37 and 38 may also be used for 
this purpose (as shown in FIG. 5) and adjustable capacitors 63, 64 and 65 
may be redundant. Conversely, chip capacitors 36,37 and 38 may be 
redundant at higher frequencies and only adjustable capacitors 63, 64 and 
65 may be sufficient for circulator operation. Tabs 33, 33', 31, 31', 35 
and 35' are tightly clamped between the halves of the housing body 66 so 
that they provide a reliable electrical connection between the ground 
plane of the housing 66 and the conductor arrangement. 
FIG. 15 shows a partially cut away plan view of a circulator incorporating 
a conductor arrangement as shown in FIG. 8. Ends 73, 74 and 75 are 
connected to respective connectors 83, 84 and 85. Strips 70, 71, 72 are 
connected to respective adjustable trimming capacitors 80, 81 and 82. 
Ferrite 86 is shown partially cut away. The circulator is mounted within 
housing body 87. 
Referring again to FIG. 9, it is important to note that ferrite disks 40 
and 41 form an essential part of the closed dc magnetic circuit and 
contribute to its reluctance. Thus, the ferrite internal dc magnetic field 
becomes a substantially nonlinear function of magnetisation of saturation 
4/.pi.M.sub.s --a fundamental magnetic characteristic of a ferrite. The 
ferrite disks 40 and 41 will preferably be formed of a material selected 
to have 4.pi.M.sub.s characteristics such that variation of effective RF 
permeability of the ferrite with temperature is minimised. To achieve 
this, the magnet, yoke, ferrite and lenses must all be considered as 
constituents of the closed magnetic loop where the particular combination 
of magnet and ferrite provides a mechanism of thermal feedback to the dc 
magnetic circuit in response to changes in temperature. 
The plots of FIGS. 16, 17a and 17b illustrate the concept of thermal 
stabilisation of the circulator in which the microwave ferrite acts as a 
part of the closed dc magnetic circuit. An ideal circulator would be 
temperature stable if the RF effective permeability .mu..sub.e of the 
ferrite remained constant across the specified temperature range. It 
implies that the external biasing magnetic field (H.sub.e) needs to change 
coherently with variations of the ferrite magnetisation of saturation 
(4.pi.M.sub.s). However, because of fundamental differences in physical 
properties of RF ferrites and permanent magnets, their typical temperature 
characteristics vary in different manners. Nevertheless pertinent 
combinations of ferrite and magnet allow thermal instability of the 
circulator to be substantially minimised. In the proposed circulator 
embodiment deviations of the central frequency with temperature have been 
reduced because this depends upon the difference between H.sub.e and 
4.pi.M.sub.s but not on each of these parameters separately. 
For example, a combination of the microwave ferrite Gd8E and an FB 
permanent magnet, both produced by TDK Corporation of Tokyo, Japan, are 
used in the circulator operating in the 80 MHz to 390 MHz frequency range 
in the above resonance mode (i.e. the operating frequency is below 
ferromagnetic resonance). The plot of FIG. 17a demonstrates that H.sub.e 
rises faster that 4.pi.M.sub.s when temperature is decreasing. It results 
in decreasing .mu..sub.e (FIG. 16) and subsequent shifting of the 
normalised central frequency towards higher values (.delta..omega.&gt;0) at 
temperatures below 20.degree. C. (FIG. 17b). However, deviation of the 
central frequency gives rise to insertion losses that in turn causes 
temperature rise and shifting the central frequency back towards the 
initial operating frequency. This mechanism provides a feedback for 
temperature auto-stabilisation at lower temperatures. 
At elevated temperatures the curve of 4.pi.M.sub.s intersects the H.sub.e 
curve again at a temperature of about 75.degree. C. owing to the essential 
non-linearity of the 4.pi.M.sub.s curve. Above the latter temperature 
4.pi.M.sub.s decreases faster than H.sub.e and, consequently, the central 
frequency of circulator may increase with temperature indefinitely. 
In the temperature range between the crossing points 20.degree. C. and 
75.degree. C.) the circulator is temperature stable i.e. thermal 
variations of the central frequency are confined between zero and the 
maximum deviation at 52.degree. C. 
Other combinations of permanent magnets and ferrite materials may be 
employed in the frame of this concept of thermal stabilisation. When using 
ferrites with more linear dependence of 4.pi.M.sub.s (such as Al or Ca 
doped garnets) the specific non-linearity in H.sub.e temperature 
dependence may be introduced by incorporating a thermocompensating 
material in the magnetic circuit. 
If required, to further stabilise the effective RF permeability of the 
ferrite with temperature change, a layer of thermocompensating material 
may be incorporated into the magnetic circuit between lenses 44, 46, and 
yoke 47. This material is preferably a Nickel-iron alloy, such as 
THERMOFLUX produced by VAC Gmbh, of Hanu, Germany. Preferably, however, 
the thermal performances of ferrite discs 40 and 41 and magnet 45 can be 
matched so that no such additional thermal compensation is required. 
Further, a magnetic shielding material may be provided about the circulator 
to decrease the strength of fringing magnetic fields emanating from the 
circulator. Such shielding may be achieved by securing a magnetic 
shielding material such as MAGNIFIER 75, produced by VDM Technologies of 
Parsippany, N.J., to the housing body of the circulator. The shielding 
material is preferably secured to at least the mounting side of the 
circulator and is to be positioned so that it does not affect the thermal 
compensation of the circulator. 
In use, variable air gap 53 (see FIG. 9) may be employed to adjust the 
central operating frequency by altering the intensity (not shape) of the 
biasing magnetic field. Matching and tuning may be effected by chip 
capacitors 36, 37 and 38 and/or adjustable capacitors 63, 64 and 65. 
Although the embodiment described above is based on one permanent magnet, a 
pair of permanent magnets or an electromagnet may be employed. When an 
electromagnet is used the magnetic field intensity may be varied to sweep 
the operating frequency or reverse the direction of the magnetic field to 
change the direction of circulation. It is also to be appreciated that the 
magnet may be placed at a different position in the magnetic circuit. For 
example, the magnet may replace the upright portion of the yoke so as to 
have lateral arms form the top and bottom of the magnet conveying magnetic 
flux to the lenses. 
Ferrite layers 40 and 41 (having very poor thermal conductivity) are 
preferably thin enough to enable heat dissipated in the ferrite to be 
efficiently transferred outside of the circulator. Thin polycrystal slabs 
or thick single crystal film ferrites may be used for this and 
incorporated with superconducting materials to further reduce insertion 
losses in the circulator. 
It will thus be seen that the present invention provides a conductor 
topology which is easily fabricated, provides good electrical connection 
between conductors, and enables improvements of assembly accuracy. It also 
provides good thermal and electrical connection between the ferrite and 
the ground plane. The use of magnetic lenses produces a substantially 
uniform internal magnetic field inside of the ferrite discs. Incorporating 
ferrite disks into a magnetic flux path enables thermal auto-stabilisation 
of circulator performance due to coherent thermal variations of the 
biasing magnetic field and magnetisation of saturation of the ferrite 
material. 
By selecting the thermal characteristics of the ferrite discs and the 
permanent magnet thermal stability of circulator may be achieved without 
additional temperature compensating components. 
Where in the foregoing description reference has been made to integers or 
components having known equivalents then such equivalents are herein 
incorporated as if individually set forth. 
Although this invention has been described by way of example it is to be 
appreciated that improvements and/or modifications may be made thereto 
without departing from the scope of the present invention as defined in 
the appended claims.