Note: Descriptions are shown in the official language in which they were submitted.
~2~73~
A MATCHING AN~ ISOLATING DEVICE
COMPRISING A FERRIT~ CIRCUhATOR
BACRGROUND OF THE INVENTION
The present invention relates to a matching and
iæolating device comprising a ferrite circulator for
coupling a circuit to be matched to a connecting point,
the circuit to be matched being comparable from an
impedance standpoint with a series xesonant circuit.
Broad-band matching devices for microwave
circuits are based on th~ use of resonators which, in the
event that the circuit to be matched is not comparable
with a parallal resonant circuit, make it possible to
provide an impedance corresponding to that of the circuit
to be matched in order to obtain a wide band of operating
frequencies. Furthermore, it is already known to make use
of a circulator between the micro~ave circuit with its
matching and access device under consideration with a view
to providing isolation or to improving the standing-wave
ratio (SW~). The connection between the circulator and the
microwave circuit to be ma~ched ls effected in the con-
ventional manner by means of a passage at a standardized
characteristic impedance (50 ohms, for example). This
entails the need for two ma~ching devices, namely one for
~he circulator and one for the circuit to be matched, and
consequently for a reduction of the passband.
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BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to over-
come these disadvantages which arise from the use of two
matching devices.
This object is attained by producing the
circuit-matching device by means of the ferrite circulator.
According to the invention ~here ~s provided a
matching and isolating device for coupling a circuit to be
matched to a connection point, the circuit to be~matched
being comparable from an impedance standpoint with a
series resonant circuit. The device comprises a matching
~ircuit of the parallel resonant circuit type which is
coupled to the circuit to be matched and a ferrite cir-
culator having a first access coupled to the connection
point and a second access coupled to the circuit to be
matched. In this device, the circulator comprises a
switching circuit with accesses and said accesses have
impedances which are comparable with those of parallel
resonant circuits so tha~ the circulator itself constitutes
the matching circuit.
In a related field, it should be noted that an
isolator is known in which provision is made for a circu-
lator. In this circulator, in order to ensure optimum
matching of one of the three inputs of the circulator pro-
per (corresponding to the switching function) with which a
121Z73iL
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dissipative load is coupled, the coupling element chosen
is a series oscillating circuit whose impedance added to
that of the dissipative load is as clos~ as possible to
the conjugate impedance of the circulator ; thls series
oscillating circuit replace~ the type ~4 impedance trans-
former which is usually employed for the matching operation
and normally forms part of the circulator. This arrange-
ment bears only a remote comparison with the invention
which does not involve replacement of a typs A/4 impedance
transformer (forming part of the cir~ulator) by a series
oscillating circuit (added to the circulator) but dis-
penses with the need for a ma~ching circuit of the
parallel resonant circuit type usually employed for
coupling a circuit to be matched and a circulator. The
circuit to be matched is thus coupled directly to the cir-
culator without using any element as a substitute for the
matching circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features of the invent~on will be more
apparent upon consideration of the followtng desaription
and accompanying drawings, wherein :
- Figs. 1 and 4 show representative impedances
of circuits ;
- Figs. 2 and 3 show circuit assemblies relating
respectively to the prior art and to the invention ;
- Figs. S and 6 are views showing a ferrite
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circulator employed ln the circuit assembly of Fig. 3.
In ~he different figures, corresponding
elements are designated by the same references.
DETAILEn DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description will be concerned with
a circuit to be matched and more especially a microwave
circuit such as an amplifier, fllter, mlxer, and so on.
There is shown diagrammatically in Fig. 1 the
impedance of a circuit to be matched in the most~general
. 10 case, that is, in the case in which said impedance is
comparable with that of a series resonant circuit Rl-L-Cl.
If the mid-band operating frequency F0 of the circuit to
be matched is different from its resonance frequency, an
impedance is added in series with the circuit to be matched
in order to make these two irequencies equal. In the case
of Fig~ 1, this impedance is obtained by means of an
inductance ~'1. It will be considered in the following
description and in the appended claims that thls additional
impedance forms part of the circuit to be matched and this
latter will be considered as a series resonant circuit
~ormed by the elements Rl-Ll-Cl with L1 = ~ + L'l ; and
the angular frequency ~0 corresponding to the mid-band
operating frequency is such that : Ll.Cl.~o = 1.
The impedance, at the angular frequency w, of the
circuit to ~e matched as shown in Fig. 1 is :
Zl = Rl ~ ~(Ll.~ ~ Cl ~) (1)
os
and its Q factor is :
Ll.~
Ql Rl (2,
When a circuit to be matched of the type con-
sidered in the foregoing has to be coupled tG a connection
point via an isolation circult, it is a known practice to
employ for this purpose a ferrite circulator in a circui~
arrangement which i5 shown diagrammatically in Fig. 2.
Flg. 2 shows a circuit 1 to be matched which is
coupled to a connection point A by means of a matching
circuit 10 followed by a circulator 20 to 23. The match-
ing circuit 10 is constructed in the conventional manner
by means of one or a number of parallel resonant circuits.
The elements of said circuit 10 are chosen so as to present
a purely ohmic input impedance having a given value Zc
~hroughout the operating frequency band when its ou~put is
connected to the input of the circuit to be matched.
Although consisting of a single Plement, the circulator
is represented by four blocks in order to distinguish
between its different functions : a block 20 de~ignated as
a switching circult and relating to the "circulator"
function proper, and three blocks 21 to 23 designated as
impedance ~ransform~rs and connected to each of the three
accesses of the switching circuit 20 in order to restore
the impedances of ~he three~circulator accesses to the
value Zc mentioned earlier. The "circulator" function
7~ 1
represented by the block 20 serves to isolate the circuit
1 to be ma~ched from another circuit to be matched and
connected downstream with respect to the point A in order
~o improve the standing-wave ratio at the point A. In the
same manner as the matching circuit 10, the lmpedance
transformers 21 to 23 have an impedance matching func~ion
but, in contrast to the circuit 10, do not call or the use
of resonators in order to perform this function by reason
of the fact that, as will be indicated hereinafter, a
circulator is provided at its accesses with impedances of
the same type as those of the parallel resonant circuit~.
The impedance transformers 21 to 23 consist of quarter-wave
transformers in the majority of instances. The impedance
transformer 21 is connected to the point A, the impedance
1~ transformer 22 is connected to the first end of a resistor
R, the second end of which is connected to ground. In
regard to the impedance transformer 23, this latter is
coupled to the circuit 1 to be matched via the matching
circuit 10.
Fig. 3 shows how the circuit arrangement shown
in Fig. 2 is modified in accordance with the invention.
The arrangement shown in Fig, 3 is distinguished from the
arrangement of Fig. 2 by the fact that the impedance
transformer 23 and the matching circuit 10 have been dis-
pen~ed with and replaced, according to requirements, either
by a direct connection or by a simple impedance transformer
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3 which is not provided with a parallel resonant element.
This alternativ~ is represented in the figure by the block
3 in dashed outline~ By eliminating the impedance trans-
former 23 and the matching circuit 10 and therefore lts
resonator, a circuit arrangement of this ~ype has a pass-
band of greater wid~h than the arrangement shown in Fig. 2.
Moreover, by reason of the reduction in number of elements,
the circuit of Fig. 3 is less subject to losses than the
circuit of Fig. 2~
The reason why the arrangement shown in Fig. 3
is possible will be sho~n in the following description.
In the case of a ferrite circulator, it is known
that the impedances of the accesses are comparable with
those of parallel resonant circuits as can be vexified by
means of measurements. Reference may be made in this
connection to the article entitled "Operation of the
ferrite junction circulator" which appeared in the January
1965 issue of IEEE transactions on microwave theory and
techniques, pages 15 to 27.
Fig. 4 illustrates a parallel resonant circuit
formed by a resistor R2 ln parallel with an inductance ~2
and a capacitor C2. This figure is the schematic repre-
sentation of the impedance of an access of a ferrite cir~
culator whose impedance at the angular frequency ~ is :
Z2 = 1 1 (3)
R2 ~ i (C2.~ ~ L2 ~-)
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The Q factor of the resonant cireuit according
to Fig. 4 is :
Q2 = R2.C2.~o (4)
where ~O is the resonant angular frequency of the
S circulator and ls such that L2.C20~2 = 1.
When a circulator is employed in a cir~uit
assembly as an isolation device for a microwave circuit,
its resonance ~requency is chosen for reasons of bandwidth
of operating frequencles so as to be equal to or at least
as near as possible to the mid-band operating frequency
and therefore the resonance frequency of the microwave
circuit. It is for this reason that, in the form
~l.Cl.~o = 1 relating to the circuit to be matched of
Fig. 1 and in the formula L2.C2.~o = 1, the resonant
angular frequency is represented by ~ in both instances.
In order to ensure that the parallel resonant
circuit in accordance with Fig. 4 can constitute a matching
circuit for the series resonant circuit in accordance with
Fig~ 1 or, in other words, in order to ensure that the
diagram of Fig. 2 can be replaced by ~he diagram of Fig. 3
with a coupling between the circuits 20 and 1 without an
lmpedance transformer, the impedance Z2 of the parallel
resonant circuit must be equal to the conjugate of the
impedance Zl. In other words, it must be ensured that
12 ~ j(C2.~ ~ ~ = Rl ~ j(Ll.~ ~ Cl ~)
~2~%73~
Cl and L2 being replaced in this eguation by their valuec
derived from the formulae
Ll.CL ~o = 1 and L2~C2.~o = 1
then it follows that by establishing equality between the
real parts and the imaginary parts
+ Ll.C2.~ ( 2 o)2 = 1 (5)
Ll.~ ~ _ ~2
(Rl.C2.~ ~ R2 ) ~ 2 ~ = ~ (6)
when ~ = ~0, it follows from relation (5) that :
Rl = R2
when ~ is different from ~0, it follows from relation (6
that .
Rl.c2.~ = R2
which produces, by multiplying by ~
Rl.c2-~o = R2
This may be written, taking into account formulae (2)
and (4),
Ql = Q2
When the values of Rl and of Ql at the resonant
angular frequency ~0 of A circuit to be match~d are known,
it is always possible to construct a circulator in which
the resonant angular frequency, the Q factor and ~he
resonance impedance are respectively e~ual or at least
close in value to l~o~ Ql and Rl. Thus it is possi~le to
~2~73~
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achleve optim~n matchlng in a broad band between the
circular and the microwave circuit.
It ~hould be noted that, if the resistance value
of the circulator resistor R2 is distinctly different from
S the value of the resistor Rl o.~ the circult to be matched,
it is possible to carry out matching by means of an
impedance transformer without any parallel resonant
element and mounted in the s~me manner as the imp~dance
transformer 3 of Fig~ 3. The assembly constituted by the
impedance transformer 3 associated with the circuit 1 to
be matched always has the same resonant angular frequency
as the circuit 1 to be matched and its resistance is
so determined as to be equal to R2, Moreover, its Q factor
at the angular frequency ~0 is no longer equal to Q1 but
is modified by ~he impedance transformer 3. ~he circulator
must therefore have been chosen ~o as to ensure that its
resonance quality factor Q2 ls as close as possible to the
Q factor of the assembly consisting of circuit 1 and
transformer 3.
One example of construction of a circulator
which is intended for an assembly in accordance with Fig. 3
with direct coupling without the transformer 3 between the
circuit 1 to be matched and the switching circuit 20 will
.hereinafter be described with reference to Figs. 5 and 6.
The circuit to be matched made use of a gallium-arsenide
field-effect transistor which operated within the fre~uency
3~. ~
range of 3.6 - 4.2 GHz and had the following character-
istics :
- resonance frequency : FO ~ 3.9 GHz
- ~uality factor : Ql = 3.15
- resonance impedance : Rl = 15 ohms.
The general aim of matching of this circuit was
to obtain a field-effect transistor having a minim~n noise
factor.
The completed circulator (as shown ln Figs. 5
and 6) has the following characteristics :
- resonance frequency : FO = 3.9 ~Hz
- quality factor : Q2 - 2.12
- resonance impedance : R2 = 15 ohms.
It may be conceded ~hat the circulator shown :in
Figs. 5 and 6 do~s not wholly satisfy the condition Q2 = Ql.
However, measuxements have demonstrated the fact that,
within the range of operating frequencies (3.6 - 4.2 GHz),
~h~ difference between the optimum impedance which makes
it possiblP to obtain the minimum noise factor and the
~0 impedance produced by the circulator remains of small value
and the noise factor thus obtained is practically at a
minimum.
Fig. 5 shows in dashed lines the periphery C of
the assembly constltuted by the casing of the circulator
and the casing of the circuit to be matched, sald casings
being placed in juxtaposed relation. There are shown in
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Fig. 5 only a component 4 locat~d at the center o~the
circulator, two resonator connection plugs 31, 32 and the
field-effect transistor 11 of the circuit 1 to be matchPd
and shown in Fig. 3.
The component 4 constitutes the lnternal
conducting strip in a structure of the three-plate wave~
guide type or in other words a structure formed by two
parallel ground planes and by a strip placed between the
two ground planes in parallel relation to these latter.
Said two ground planes are shown in Fig. 6 and designated
by the reference numerals 61 and 62. The component 4 i5
constituted by a metal plate of silver-plated brass
1.2 mm in thickness and comprising a resonator disk 40
having a diameter of 14 n~l and pierced by a centering hole
44. Around said disk are arranged three branches 41, 42,
43 relatively spaced at angulax intervals of 120, The
branches 41 and 42 are of distinctly greater length than
the branch 43 and are soldered to the internal conductor
of the coaxial connecting plugs 31, 32. Said plugs have a
characteristic impedance equal to 50 ohms and permit
connection to the point A and to the resistor R (shown in
Fig. 3). The outer conductor is not shown in Fig. 5 but is
connected to the circulator casing. ~he branch 43, which
is shown only by way of indication for connecting purposes,
is soldered to the end of the gate connection G of the
field-effect transistor 11 of the circuit to be matched.
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The length of the ga~e connectl~n of the ~ransis~or 11, or
in other words the length of the connection between the
transistor casing and the disk 40, determi~es the
inductance L'l mentioned earlier in the description of
Fig. 1. For the sake of simplification, and because it
would not be conducive to easier understanding of the
invention, the elements connected to the drain D of the
transistor 11 axe not shown in the drawings. In regard to
the source of the transistor 11, said source comprises two
output connections Sl and S2, which are both soldered to
the casing of the circuit to be matched~
A point worthy of note is the fact that, in the
embodiment described, the gate of the transis~or 11 is
biased by the circulator. In order to s~mplify the
drawings, the resistor R as shown in Fig~ 3 is connected
directly between the circulator and ground. In actual
practice, however, the first end of said resistor is
connected to the circulator (transformer 22 associated
with the branch 42) and the second end is connected to a
decoupling circuit. Said decoupling circuit behaves as a
short-circuit at the operating frequencies of the circuit
to be matched and as an infinite ~mpedance at the direct~
current bias voltage. Said bias voltage is applied to the
second end of the resistor R. ~he decoupling circuit can
be formed in the conventional manner by open quartex-wave
lines which bring back a short-circuit and/or by a
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capacitor,
In Fig. 3, consideration was glven to the
impedance transformers 21 and 22 and ~he switching circuit
20 which con~tituted the circulator. In Fig. 4, ~he
resonator disk 40 and the branches 41 and 42 correspond
re~pectlvely to the switching circuit 20 and to the
impedance transformers 21 and 22, Said lmpedance trans-
formers are of the quarter-wave type. In other words, the
length of the branches is substant1ally equal to one-
quarter of the wavelength, within the circulator, at themean frequency of 3.9 GHz. In more precise terms, the
blocks 20, 21 and 22 of Fig. 3 are to be identified with
the association of the elements 40, 41, 42 and of the
elements between which said blocks are inserted in the
circulator and which will be described with reference to
Fig~ 6.
Fig. 6 is a sectional view of the circulator,
taken along the axis X-X of Fig. 5. This view shows two
structures arranged symmetrically with respect to the strip
4 on each side of this latterO Starting from the strip,
said structures comprise :
- a ferrite disk 51, 52 having a diameter which is sub~
stantially equal to the diameter of the resona~or disk 40
(shown in Fig. 5) and a silica component 53, 54 which
covers practically the entire portion of the strip 4
which is located outside the disk 40 (shown in Fig. 5) ;
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the compo~ents 51 to 54 have a thickness of 2.1 mm ; the
ferrite disk 51, 52 is pierced with a central hole ;
- an aluminum plate 61, 62 constituting one of the afore-
mentioned ground planes and pro~ided in ~he face remot~
5 from the strip ~ with a recess having the shape of a
circular cup and located opposite to the ferrite disks
51, 52 ;
- within the cup-shaped recess : an air layer 71, 72, a
mild steel washer 81, 82 having a thickness of 0.5 mm,
and a permanent magnet 91, 92 having a thickness of
1.5 mm ; the magnetic circuit formed by the two magnets
91, 92 produces a field of 400 gauss ;
- a U-section member 609 the two opposite flanges of which
cover respectively the two magnets ~1, 92 in order to
close their field lines ; said U-section member is
fabricated from a mild steel strip having a thickness
of 1 mm and a width of 17 mm.
A cylindrical insula~ing rod 45 made of nylon
(registered trademark) passes through the holes pierced in
the strip 4 (hole 44) and through the ferrite disks 51, 52
in order to effect positioning of these components with
respect to each other.
~ very thin silver foil (not shown in Fig. 6)
is placed between the disk 51 and on the one hand the
silica component 53 and on the o~her hand the plate 61 in
order to ensure a good electrical contact between these
~lZ~7~
-16
element~. Another foil (not shown) ~nsures a good contact
between the disk 52 and on the one hand ~he silica component
54 and on the other hand the pla~e 62.
The circulator casing ls provided between the
plates 61 and 62 and the U-section member 60 with a
connector 31 which is fixed on the plates 61 and 62 by
means of screws (not shown in the figure).
Sald ferxite circulator has been fabricated as a
function of the characteristics to be obtained in order to
achieve matching of the circuit which has been considered
in the foregoing and is intended to be ma~ched. To this
end, preference was initially given to a circulator of a
current type having charactexistics which were fairly close
to those contemplated. ThPn~ after successive tests, the
circulator described with reference to Figs. 5 and 6 was
finally adopted.
It should be noted that the invention is no~
limited to the example hereinbefore described. From this
it follows in particular that, in the circuit arrangement
of Fig. 3, it would be poss~ble to dispense with the
impedance transformer 22, that is to say in practlce, to
eliminate the branch 42 of the strip 4 (shown in Fig. 5~.
In such a case, it would only be necessary to give the
resistor R the value of resistance existing at the corre-
sponding output of the resonator in the resonant state.
Another noteworthy point is that the cixculator
-17
can be employed as a matching element for a circuit to be
matched, not only at the input of the circuit as ln the
example described but also at the output of the circuit.
Thus the same circulator can serve as a matching element
for two circuits to be matched : the ou~put of a first
circuit to be matched is coupled to one of the accesses of
the circulator and the input of a second circuit to be
matched is coupl0d to another access of the circula~or.
