O
Umted States Patent [191
[11]
4,409,569
Potash
[45]
Oct. 11, 1983
[54] FILTER CIRCUITS HAVING
TRANSFORMERS AS FILTER ELEMENTS
[75]
Inventor:
-
Jerome Potash, Flemington, NJ.
,
-
[73] Asslgnee'
-
-
Industries’ Inc" phlnlpsburg’
OTHER PUBLICATIONS
Jordan et al, "Fundamentals of Radio”, Prentice-Hall,
New York, 1952; Title page and pp. 1l3—l l5.
Primary Examiner—-M. Nussbaum
Attorney. Agent, or Firm-Arthur L. Lessler
[57]
[2]] App!‘ NO': 3"’438
[22] Filed
Nov 2 1981
'
'
ABSTRACT
Filter circuits of the type having inductance and capaci
tmce elements, wherein the number of magnetic com
’
ponents is reduced by using two transformers to per
lnt. GL3
7/09,
7/075
[52] us. Cl. ............................. 335/177‘ 333/168
form Th8 fIlIlCliOIlS 0f four individual II'ldUCIOI'S. Each
transfmme' has One end 01" the Primary winding C0“
[581
"wed ‘0 one end of the sewndary winding’ and a Pi °r
Field of Search .............................. .. 333}167-l71
333/174 175 177480 24
’
[56]
’
'
References Cited
6/1938 United Kingdom
values is equal in magnitude and opposite in sign to a
corresponding equivalent circuit inductance value of
the other transformer.
FOREIGN PATENT DOCUMENTS
495455
T equivalent circuit in which one of the inductance
333/177
7 Claims, 19 Drawing Figures
US. Patent
0m. 11, 1983
Sheet 2 0M
Ac2BC3
w.‘
LC
IILG"LbLd.
D
FIG.5
c
T4c
.
4,409,569
US. Patent
0a. 11, 1983
Sheet 3 of4
a-35-5,3
Q635:lL
FIm
_QM
WUYEN
row
.Tm
QH45W_EN._OJ
|,| h
AmOEn:
w.0_|.
0.0K
4,409,569
1
4,409,569
2
?lter according to a ?rst embodiment of the present
FILTER CIRCUITS HAVING TRANSFORMERS AS
FILTER ELEMENTS
BACKGROUND OF THE INVENTION
invention;
FIG. 5 shows a ?lter according to said ?rst embodi
ment of the invention;
FIG. 6 illustrates an intermediate step (in conjunction
5
with FIGS. 2, 3(0), 3(b), and 4(b) in converting the ?lter
This invention relates to electric ?lter circuits and
processes for manufacturing the same, and more partic
ularly to ?lters which include a number of inductors.
In the design of ?lters having desired frequency re
sponse characteristics, it is frequently necessary or de
sirable to include a number of individual inductors.
of FIG. 1 to a ?lter according to a second embodiment
of the present invention;
FIG. 7 shows a ?lter according to said second em
bodiment of the invention;
FIG. 8 shows a second type of prior art ?lter; having
more than four inductance elements;
FIG. 9 shows an intermediate step in converting the
Such inductors are relatively expensive magnetic com
ponents and occupy a relatively large volume.
Accordingly, an object of the present invention is to
provide ?lter circuits having desired frequency re
sponse characteristics similar to those of prior art ?lters
?lter of FIG. 8 to a ?lter according to a third embodi
ment of the present invention;
FIG. 10 shows a ?lter according to said third embodi
ment of the invention;
FIG. 11 shows a third type of prior art ?lter having
four inductance elements;
of the aforementioned type, at reduced cost and in a
smaller space.
SUMMARY
20
FIGS. 12, 13(0), 13(b), 14(0) and 14(b) show interme
diate steps in converting the ?lter of FIG. 11 to a ?lter
according to a fourth embodiment of the present inven
As herein described, there is provided an improved
?lter circuit comprising at least two transformers and
tion; and
FIG. 15 shows a ?lter according to said fourth em‘
having a frequency characteristic corresponding to that
of a given ?lter circuit including at least four individual 25 bodiment of the invention.
inductors, comprising a pair of transformers each hav
DETAILED DESCRIPTION
ing a T or pi network equivalent circuit wherein two
The
present
invention, as hereinafter described in
inductance values correspond to those of two of said
further detail, utilizes two transformers (or more than
individual inductors, the third inductance value of one
one group of two transformers) having parameters such
of said equivalent circuits being positive and the third
that they provide frequency response characteristics
inductance value of the other of said equivalent circuits
similar to those of four individual inductors, thus reduc
being negative and equal to the third inductance value
ing the number of individual magnetic components by
of the ?rst-mentioned of said equivalent circuits, said
transformers being interconnected so that said equiva
two, resulting in a saving of cost and of space occupied
lent circuit third inductance values are in series or in 35 by the ?lter.
One of the two transformers has its windings coupled
in series aiding relationship [as shown for example in
parallel with each other, whereby the third inductance
values do not affect the frequency characteristic of said
?lter circuit.
Also herein described is a process for manufacturing
an improved ?lter circuit comprising at least two trans
formers and having a frequency characteristic corre
sponding to that of a given ?lter circuit including at
least four individual inductors, comprising the steps of:
representing said given circuit schematically; adding to
the representation of said ?lter circuit a pair of positive
and negative interim inductors having equal absolute
values of inductance, the inductors of said pair being
coupled in series or in parallel with each other so that
FIG. 4(b)], so that the mutual inductance m between the
primary and secondary windings has a negative sign;
the other transformer having its windings coupled in
series opposing relationship [as shown for example in
FIG. 4(0)], 80 that the mutual inductance therebetween
45
has a positive sign. In this arrangement, the equivalent
circuit of one transformer has two positive inductance
values and one negative inductance value (see FIG.
3(b), for example), while the equivalent circuit of the
other transformer has three positive inductance values
(see FIG. 3(a), for example).
By properly setting the design parameters of each
the frequency characteristic of said ?lter circuit is not 50 transformer, and the manner in which the transformers
altered thereby, and so that each inductor of said pair
are interconnected, the negative equivalent circuit in
cooperates with two adjacent individual inductors to
ductance value of one transformer may be "cancelled
form a T or pi network, wherein in each such network
out” by one of the inductance values of the equivalent
the absolute value of the inductance of the correspond
circuit of the other transformer, thus resulting in an
ing interim inductor is less than the square root of the
equivalent circuit comprising four inductors. The trans
product of the inductances of the individual inductors
formers can be so designed that their combined equiva
therein; and fabricating said improved ?lter circuit by
lent circuit, comprising four inductors, has the same
inductance values as those of the four individual induc
providing a transformer to perform the function of each
tors of the prior art ?lter circuit.
of said networks, each such transformer having an
A ?rst embodiment of the invention based on these
equivalent circuit corresponding to that of the respec
principles is shown in FIG. 5, which circuit has a fre
tive network.
quency characteristic similar to that of the prior art
IN THE DRAWING
circuit shown in FIG. 1.
In converting the circuit of FIG. 1 to that of FIG. 5,
FIG. 1 shows a ?rst type of ?lter circuit according to
the prior art, having four individual inductance ele 65 two parallel inductors are added to the schematic repre
sentation of FIG. 1, said inductors having inductance
ments or inductors;
FIGS. 2, 3(a), 3(1)), 4(0) and 4(b) illustrate intermedi
ate steps in converting the prior art ?lter of FIG. 1 to a
values +L and —L respectively. The positioning of the
added positive and negative inductance values is se
3
4,409,569
4
lected, as shown in FIG. 2, so that each of said added
inductance values forms a delta or pi network with two
Le :
adjacent individual inductors of the original prior art
(38)
circuit. As seen in FIG. 2, the inductance value +L
forms a ?rst pi network with individual inductors L1
and L2; while the inductance value —L forms a second
pi network with individual inductors L3 and L4. These
pi networks are shown in FIGS. 3(a) and 3(b) respec
(3c)
tively.
The pi network shown in FIG. 3(a) is then replaced
by the transformer shown in FIG. 4(a), said transformer
and the resulting circuit is as shown in FIG. 7.
A somewhat more complex prior art circuit is shown
in FIG. 8, which conduit includes seven individual
having primary and secondary windings La and Lb
connected in series opposing relationship to provide a
positive sign for the mutual inductance mab therebe
inductors. Using techniques previously described with
reference to FIGS. 1 to 5, the filter circuit shown in
FIG. 10 can be provided, with only four magnetic com
tween.
The pi network shown in FIG. 3(b) is replaced by the
transformer shown in FIG. 4(b), said transformer hav
ing primary and secondary windings L‘- and L4 con
nected in series aiding relationship to provide a negative
sign for the mutual inductance mcd therebetween.
The positive and negative inductors L may be given
ponents, and having a similar frequency characteristic
to that of FIG. 8.
As shown in FIG. 9, which is similar in principle to
20
FIG. 2, schematic representations of a pair of positive
and negative inductance values +L and —L are pro
vided, in such a way that the elements L5, L6 and +L
any desired inductance value, so long as each said in
ductance value is less than the square root of the prod
form one pi network, while the elements —L, L7 and 2
L3 form an adjacent pi network. The representation of
uct of the other two inductance values in the corre 25 the inductor L3 is changed to two parallel inductance
sponding pi network. That is,
values of 2L5 each (combined inductance equal to L3),
in order to facilitate formation of third (2L3, L9, +L’)
and fourth (—L’, L10, L“) pi networks including a
second pair of positive and negative inductance values
|L| < qLlLz and |Ll < ‘L3L4
30 + L’ and — L’.
Each of the pi networks is then replaced by a trans
The equations which interrelate the inductance pa
former having a corresponding equivalent circuit.
rameters of each of the transformers shown in FIGS.
The equations which interrelate the parameters of the
4(a) and 4(b) with the inductance values of the corre
left hand pair of transformers shown in FIG. 10 with the
sponding or “equivalent” pi networks shown in FIGS.
35 equivalent pi networks shown in FIG. 9 are as follows:
3(a) and 3(b) respectively are as follows:
_ L1(Lz + L)
L“ ‘ L] + L: + L
(la)
L8: Ls(Ls + L)
4
(1)
Lb :
(4b)
L5 + La + L
L _ L(LZ + [4)
" _ L1 + L2 + L
LIL
(lb)
(Ic)
"'“b = L. + L; + L
my, =
(46)
45
L _
Ida-4 — L)
(2a)
‘ _ L3 + L4 - L
L
_ L4(L3 — 1-)
d _ L3 + L4 - L
L3L4
(2b)
50
(2c)
mg:
When the transformers shown in FIGS. 4(a) and 4(b)
are substituted for their equivalent pi networks as
(56)
The equations which interrelate the parameters of the
right hand pair of transformers shown in FIG. 10 with
shown in FIG. 2, the resulting ?lter is as shown in FIG.
5. Since each of the transformers occupies little more
space, and entails little more cost than a single one of
the equivalent pi networks shown in FIG. 9 have the
same form as equations 4a through 50, with correspond
the inductors L1 through L4, the filter of FIG. 5 pro
While all of the aforementioned transformer filter
circuits have been arrived at by use of intermediate pi
networks, it is also possible to utilize intermediate wye
vides a considerable improvement in terms of cost and
space, over that of FIG. 1.
ing changes in similar variables.
Instead of using the transformer con?guration shown
or T networks to arrive at equivalent transformer ?lter
in FIG. 4(a), the transformer con?guration shown in
circuits, as illustrated in FIGS. 11 to 15, by adding series
FIG. 6 may alternatively be employed. In such event 65 combinations of positive and'negative inductance val
the equations which interrelate the parameters of the
ues to prior art filter circuits.
transformer of FIG. 6 with the equivalent pi network of
As shown in FIG. 12, the schematic representation in
FIG. 3(a) are as follows:
FIG. 11 is modi?ed (without altering its frequency
5
1
4,409,569
response characteristics) by adding a series combination
of inductance values +L and —L, ‘at a position in the
circuit such that the added inductance element +L
6
rectly connected to one end of one winding of the other
transformer.
3. The ?lter circuit according to claim 1, wherein the
other end of said one winding of said one transformer is
forms a T network with the individual inductors Liz
directly connected to one end of the other winding of
and L13; while the added inductance element —L forms
said other transformer.
a T network with the indivdual inductors L14 and L15.
4. The ?lter circuit according to claim 1, 2, or 3.
These T networks are shown in FIGS. 13(a) and 13(b)
wherein the magnitude of said third inductance value of
respectively, while the corresponding transformers are
each of said equivalent circuits is less than the square
shown in FIGS. 14(0) and 14(b) respectively.
The equations which interrelate the parameters of 0 root of the product of said two inductance values
thereof.
each transformer with the inductance values of the
5. An improved ?lter circuit comprising at least two
corresponding equivalent circuit are as follows:
transformers and having a frequency characteristic cor
responding to that of a given ?lter circuit including at
15 least four individual inductors, comprising a pair of
transformers each having a T or pi network equivalent
circuit consisting of three inductance values, wherein
two inductance values correspond to those of two of
said individual inductors, the third inductance value of
20 one of said equivalent circuits being positive and the
third inductance value of the other of said equivalent
circuits being negative and equal to the third inductance
value of the ?rst-mentioned of said equivalent circuits,
said transformers being interconnected so that said
and the resulting transformer-based circuit is shown in
equivalent circuit third inductance values are in series
FIG. 15, said circuit having a frequency characteristic
or in parallel with each other, whereby the third induc
similar to that of the circuit of FIG. 11, but requiring
tance values do not affect the frequency characteristic
two less magnetic components.
of said ?lter circuit, each transformer having a primary
Since the only restriction on the inductance values
winding and a secondary winding, one end of the pri
added to the prior art ?lter circuit is that the magnitude
mary winding of each transformer being connected to
of each such value should be less than the square root of
one end of the secondary winding thereof so that said
the product of the other two inductors of the corre
windings are effectively connected in series with each
sponding pi or T network, a great deal of circuit design
other, the windings of one transformer being connected
?exibility is available, allowing the design of the trans 35 in series aiding relationship and the windings of the
formers employed in the ?lter circuits to be optimized.
other transformer being connected in series opposing
In ?lter circuits where high permeability magnetic
relationship, the primary and secondary windings of
cores for the inductors, e. g. ferrite or iron, are required,
one transformer being connected in series across one
the savings provided by the circuits shown in FIGS. 5,
winding of the other transformer.
7, l0 and 15, for example, can be quite substantial.
6. An improved ?lter circuit comprising at least two
transformers and having a frequency characteristic cor
I claim:
1. An improved ?lter circuit comprising at least two
transformers and having a frequency characteristic cor
responding to that of a given ?lter circuit including at
responding to that of a given ?lter circuit including at
least four individual inductors, comprising a pair of
transformers each having a T or pi network equivalent
least four individual inductors, comprising a pair of 45 circuit consisting of three inductance values, wherein
two inductance values correspond to those of two of
transformers each having a T or pi network equivalent
said individual inductors, the third inductance value of
circuit consisting of three inductance values, wherein
one of said equivalent circuits being positive and the
two inductance values correspond to those of two of
third inductance value of the other of said equivalent
said individual inductors, the third inductance value of
circuits being negative and equal to the third inductance
one of said equivalent circuits being positive and the
value of the ?rst-mentioned of said equivalent circuits,
third inductance value of the other end of said equiva
said transformers being interconnected so that said
lent circuits being negative and equal to the third induc
equivalent circuit third inductance values are in series
tance value of the ?rst-mentioned of said equivalent
or in parallel with each other, whereby the third induc
circuits, said transformers being interconnected so that
said equivalent circuit third inductance values are in 55 tance values do not affect the frequency characteristic
of said ?lter circuit, each transformer having a primary
series or in parallel with each other, whereby the third
winding and a secondary winding, one end of the pri
inductance values do not affect the frequency charac
mary winding of each transformer being connected to
terisitc of said ?lter circuit, each transformer having a
one end of the secondary winding thereof so that said
primary winding and a secondary winding, one end of
windings are effectively connected in series with each
the primary winding of each transformer being con
other, the windings of one transformer being connected
nected to one end of the secondary winding thereof so
in series aiding relationship and the windings of the
that said windings are effectively connected in series
other transformer being connected in series opposing
with each other, the windings of one transformer being
relationship, the series combination of the primary and
connected in series aiding relationship and the windings
of the other transformer being connected in series op 65 secondary windings of one transformer being con
nected in parallel with the series combination of the
posing relationship.
primary and secondary windings of the other trans
2. The ?lter circuit according to claim 1, wherein at
least one end of one winding of one transformer is di
former.
4,409,569
8
7. An improved ?lter circuit comprising at least two
transformers and having a frequency characteristic cor
responding to that of a given ?lter circuit including at
value of the ?rst-mentioned of said equivalent circuits,
said transformers being interconnected so that said
equivalent circuit third inductance values are in series
or in parallel with each other, whereby the third induc
tance values do not affect the frequency characteristic
of said ?lter circuit, the magnitude of said third induc
tance value of each of said equivalent circuits being less
than the square root of the product of said two induc
least four individual inductors, comprising a pair of
transformers each having a T or pi network equivalent
circuit consisting of three inductance values, wherein
two inductance values correspond to those of two of
said individual inductors, the third inductance value of
one of said equivalent circuits being positive and the
third inductance value of the other of said equivalent
circuits being negative and equal to the third inductance
O tance values thereof.
i
20
25
30
35
45
50
55
65
i
i
t
t