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Патент USA US3098997

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July 23, 1963
T. w. HOLDEN
3,098,937
VARIABLE BAND PASS AUDIO FILTER
Filed Jan. 12, 1962
2 Sheets~Sheet l
a //f
INVENTOR.
74,0 7114.5 hf/Va /a’a2z
w
Zia/{6.40M
A TTORNE YS
Jul)’ 23, 1963
T. w. HOLDEN
3,098,987
VARIABLE BAND PASS AUDIO FILTER
Filed Jan. 12, 1962
2 Sheets-Sheet 2
83 71
Wm!”
m
70
INVENTOR
7%»245 11/66/1672
w; $1M
é;
‘
A TTORNE YS
United States Patent O?ice
Patented July 23, 1963
l
2
M39832”
which is threaded into the mounting plate '32 and has its
lower end 490, FIGURE 1, abutting the upper surface of
VARIABLE BAND PASS AUDIQ FILTER
Thomas W. Holden, Sill W. 16th Place,
(Ihicago Heights, Ml.
Filed Ian. 12, 1%2, Ser. No. 165,881
7 Claims. (Cl. 333-71)
This invention relates to an audio frequency band pass
?lter adapted for use in radio-telegraph receivers and the
like.
It is an object of the present invention to provide a band
pass audio ?lter which will almost ‘completely reject un
desired frequency components while allowing the desired
the adjustment plate 67.
Similar adjustment plates indicated at 43 and 44 are
shown in FIGURE 2 as lying adjacent the base 11 and
being secured thereto by means of screws such as 46 and
4-7 shown in ‘FIGURE 1 and 49 shown in FIGURE 2.
Adjustment screws are indicated at 50 and ‘51 in FIGURE
2 which are threaded into the base ‘11 and have their
upper ends abutting the lower surfaces of the respective
adjustment plates 43 and 44- to de?ect the same upwardly
away from the base 11.
Mounted on the adjustment plates 43 and 44 are a pair
frequency components to pass.
of rectangular core members '55 and v56 which are en
It is a further object of this invention to provide such 15 circled by respective helical coils indicated at 57 and 58.
a ?lter having means for changing the band width thereof.
Each of the cores 5‘5 and 56 has a non-magnetic coating
A further object is to provide a selectively operative
at its upper end as indicated at '60’ and 61 which prevents
means for injecting the input frequency components into
actual contact between the magnetic material of the reeds
the output circuit.
25 and 26 and the respective core members 55 ‘and 56.
Still another object of the invention is to provide a band
The adjustment plate 37 carries a magnetic core mem—
pass ?lter wherein the amplitude of the output voltage
ber 65 of rectangular con?guration having a helical wind
may be limited to a predetermined value.
ing 66 encircling it. The core 65 has its broad cross sec
Other objects, features and advantages of the present
tional dimension extending transversely to the broad cross
invention will be apparent from the following detailed de
sectional dimensions of the cores ‘5‘5 and 56 so that the
scription taken in connection with the accompanying 25 end face of the core ‘65' overlies both of the reeds .25 and
drawings, in which:
26 as best seen in FIGURE 2. The end face has a non—
FIGURE v1 is a somewhat diagrammatic side eleva
magnetic coating 67 for preventing contact of the reeds
tional view of a structure in accordance with the present
‘25 and 26 with the end face ‘of the core member 65.
invention with certain parts broken away and in section;
For adjusting the band of the ?lter, a permanent
FIGURE 2 is a vertical sectional view taken generally 30 magnet 76 is mounted in direct proximity to the magnetic
along the line II—II of FIGURE 1, and with portions of
circuits linking the reeds 25 and 26. The magnet may be
the ?lter mechanism shown in vertical section for clarity;
mounted on a shaft 71 of magnetic or non-magnetic
FIGURE 3 is a vertical sectional ‘view taken generally
material and circular cross section carried by means of
along the line III—-III in FIGURE 1 and looking in the
a support bracket 72 ‘of magnetic or non-magnetic material.
direction of the arrows;
35 The bracket 72 is shown as being secured to the base 11
FIGURE 4 is a somewhat diagrammatic electric circuit
by means of a pair of screws 73 and 74, and a pair of
diagram for the embodiment of FIGURES 1-3; and
collar members 76 and 77 are shown as clamped to the
FIGURE 5 illustrates an alternative permanent magnet
shaft 71 by means of screws 78 and 79‘ at either side of
construction for use in ‘the embodiment of FIGURES
the support bracket 72. By loosening the screws 78 and
14.
40 79, the shaft 71 may be adjusted axially to position the
As shown on the drawings:
magnet 70 in a precise desired position. The screws 78
As seen in FIGURES 1 and 2, a band pass audio ?lter
and 79 are then tightened to prevent axial shifting of the
in accordance with the present invention may comprise a
magnet 70 while permitting rotation of the shaft 71 in its
vertical panel or plate 10 of magnetic or non-magnetic
bearing within support post 72. A knob is indicated at 80
material secured ‘to a base plate '11 by means of brackets 45 at the exterior side of the panel 10 for rotating the magnet
12 and 13. The base plate may be of suitable magnetic
70 about the axis of shaft 71 to adjust the strength of the
material such as soft steel.
magnetic field produced thereby which impinges upon
As best seen in FIGURE 3, a pair of fastening members
the magnetic circuits of the ?lter.
15 and 16 of non-magnetic material extend through aper
FIGURE 5 illustrates a modi?ed permanent magnet
tures in the base plate 11 and through a pair of plates 50 70" which may be secured to the shaft 71 and positioned
of magnetic material indicated at 2.0’ ‘and 21 which form
so that its magnetic ?eld will adjustably in?uence the
parts of the magnetic circuit of the device. A perma
magnetic circuits of the ?lter. The magnet 70' may be
nent magnet 22 is con?ned between the base '11 and in
\of circular cross section and magnetized along a diametric
termediate plate 20, and a second permanent magnet 23
plane so that rotation thereof will vary the strength and
is illustrated as being con?ned between the intermediate 55 polarity of the magnetic ?eld impinging on the magnetic
plate 24} and the upper plate 21. The permanent magnets
circuits associated with the reeds 25 and 26. The means
22, and 23 may be magnetized in the vertical direction as
indicated at '83 in FIGURE 2 and 83’ in FIGURE 5
viewed in FIGURE 3 and may have polarities as indi
securing the magnets 70 or 7 0’ to the shaft 71 may
cated by the letters N ‘and S.
comprise a suitable glue or low melting point solder.
As best seen in FIGURES 1 ‘and ‘2, the intermediate 60
Referring to FIGURE 1, the cylindrical hole ‘87 in
plate 20 has a pair of vibrating ‘reeds 25 and 26 of mag
panel 10 may receive a suitable phone jack such as
netic material clamped thereto by means of a bar 27 and
indicated at 88 in FIGURE 4. The hole 89 in panel I10
screw means 28 and 29.
may receive a single pole double throw switch such as
The upper plate 21 has a further plate of magnetic ma
indicated at 90‘ in FIGURE 4, and a hole 91 in panel 10
terial 32 secured thereto by means of the nuts $3 and 34 65 may receive a potentiometer such as indicated at 92 in
threaded onto the ends of the bolts I15 and 16. The plate
FIGURE 4.
Adjustment 0]‘ the Filter
32 extends from the upper plate 21 in the same direction
as the reeds .25 and 26 and has an adjustment plate 37 of
If the ?lter is to operate in the 1000 cycle region, for
magnetic material secured thereto ‘by means of a pair of
example, the ?lter may be adjusted as follows. With the
screws 38 and 39. The plate 37 is sufficiently ?exible so 70 rotatable magnet 70‘ or 70’ removed, the length of reed 25
as to be de?ected by means of an adjustment screw 40
is adjusted until it resonates at approximately 925 cycles
'4
per second. Reed 26 is then adjusted to resonate at 1000
through the mounting plate 32 for the mounting screws
cycles per second by suitably adjusting its effective length,
38 and 39 so that by loosening the screws 38 and 39
again through the use of screws 28 and 29.
the adjustment plate 37 may be shifted in a horizontal
plane together with the core 65 and coil 66 to the left
Next, adjust screws 40, 50 and 51, FIGURE 2, to
move coils 57, 58 and 66 ‘and their cores 55, 56 1and 65
until the space between the reeds 25 and 26 and the non
magnetic coatings 60, 61 and 67 is, about .005 inch. Coils
and right as viewed in FIGURE 4, or as viewed in FIG
URE 2.
Also, for reasons of economy, the movable magnet
57 and 58 are as nearly identical as it is possible to make
70 or 70’ may be omitted, but in this case the band width
them.
Will be ?xed as determined by the reed adjustments and
Referring to FIGURE 4, the total effective resistance
provided by potentiometer 92 and ?xed resistor 94 is
under some circumstances.
equal to the resistance of ?xed resistor 95.
With the adjustments properly made, the output at 750
cycles per second and at 1250 cycles. per second will be
—40 decibels taking the output at 10010 cycles per second 15
other parameters.
Capacitor 98 may also be omitted
The permanent magnet v23; may be replaced by a
block of low retentivity magnetic material such as soft
steel.
as a refernece (zero decibels).
The permanent magnet 70 or 70" is now installed and
Summary of Operation
With the ?lter properly adjusted as just described, an
input signal is supplied to the input element 99 having a
positioned as indicated for the magnet ‘70 in FIGURES 1
and 2 so that the ?eld of the permanent magnet impinges
on the ?lter magnetic circuits in the region of the reeds
25 and 26. When the magnet is rotated so that its ?eld
relatively wide range of audio frequency components.
If, for example, the ?lter is to pass frequencies in the
reinforces the magnetic flux in the ?lter magnetic circuits
in the region of the reeds 25 and 26, the reeds will vibrate
have an effective length to resonate at 925 cycles per sec
0nd and reed 26 will have an effective length to resonate
at 1000 cycles per second. With the present ?lter con
region of 1000 cycles per second and deliver the same
to an output system associated with jack 38, reed 25 will
over a wider band of frequencies and the output of the
?lter will have a wider band width. For example, the 25 struction adjusted as above described, the output at jack
output frequency components may cover a range of 300
cycles per second. On the other hand, if the magnet 70‘
or 70’ is rotated by knob 80 until its ?eld cancels part
of the magnetic ?ux in the ?lter magnetic circuits in the
region of the reeds, the band width will be sharper ‘and 30
>88 for frequency components of 750 cycles per second
may effectively be limited to a range of 90 cycles per
second.
Referring to FIGURE 4, coils 57 and 58 are connected
of frequencies which are effectively delivered to the out
so that undesired frequency components cancel. In other
words, if the reeds 25 and 26 are held against movement,
and an input signal supplied to the coil 66, the net output
‘at the jack 88 will be zero if potentiometer 92 is set to
supply a maximum value \of resistance in series with
resistor 94 so that the sum of resistance introduced by
potentiometer 92 and resistor 94 is equal to the resistance
of resistor 95.
When it is desired to inject all of the input frequencies
into the output circuit, potentiometer 92 is adjusted to
and below and at 1250 cycles per second and above will
be at least 40 decibels below the output level at 1000
cycles per second.
By rotation of the permanent magnet 70, the range
{put jack 88 may be adjusted. For example, with the
permanent magnet producing a magnetic ?eld which
reinforces the ?ux in the region of the reeds 25 and 26,
the normal band pass range may be widened to 300
cycles per second, while if the permanent magnet 70 pro
rvides a magnetic ?eld cancelling part of the ?ux linking
the output windings 57 and 58, the effective band width
will be sharpened to a range such as 90 cycles per second
(centering about 1000 cycles per second).
When it is desired to inject the input frequencies from
source 99 into the output circuit including jack 88,
potentiometer 92 is adjusted to reduce the resistance of
components 92 and 94 as compared to the resistance of
desired amount of injection is produced. Capacitor 98 45 resistor 95.
tunes the circuit to approximately 950 cycles per second
The non-magnetic coatings 60, 61 and 67 prevent
in order to improve the ?ltering action. When the ?lter
sticking of the reeds to the coil ‘cores and limit the
is not to be used, switch 90, FIGURE 4, may be switched
head output so that headphones connected to the
to its upper position to connect an input element 99 such
output jack 88 will not excessive an excess amplitude
'as the secondary of a transformer directly to the originm 50 which would ‘cause discomfort to the ears.
load v100 which may comprise the primary of a trans
If the ?ltering action is not desired, switch 90‘, FIGURE
former, for example.
4 is switched to the upper position, to complete the normal
When the switch 90 is in its lower position, the jack ‘818
coupling between elements 99 and 100 of a radio
may be utilized to connect the output of the ?lter to
telegraph receiver or the like.
headphones or to the input of a suitable ampli?er or the
The ability to change band width readily as by means
like.
of the knob 80, FIGURE 2, to inject the input fre
The non-magnetic coatings 60‘, 61 and 67 on the cores
quencies into the output circuit by adjustment of poten
keep the reeds 25 and 26 from sticking to the cores 55,
tiometer 92, FIGURE 4, and to limit the output ampli~
introduce a reduced value of resistance such that the
56 and 65.
Also, the coatings will limit the output.
tude by means of the non-magnetic coatings 60, 61 and
When headphones are used with the ?lter, it is desirable 60 67 represent very desirable features when the filter is
to limit the output amplitude in order to prevent the sound
used with the radio-telegraph receiver, for example.
from causing discomfort to the ears.
[It will be apparent that many modi?cations and varia
The reeds 25 md 26 may be made from a semi-soft
tions may be effected without departing from the scope
steel such as a material used for audio transformer
of the novel concepts of this invention. By way of
laminations.
65 example, coil 66 may be used as the output coil and coils
The movable permanent magnet 70 or 70’ may be
57 and 58 maybe used as the input coils.
mounted in a number of di?erent ways, for example
I claim as my invention:
a suitable mounting may permit sliding or rolling of a
permanent magnet into and out of the operative position
such as ‘generally indicated in FIGURE 1. For reasons 70
1. An audio frequency band pass ?lter comprising
(a) a base of magnetic material,
of economy, resistors 94 and 95 and potentiometer 92
may be omitted. When this'is done, the input siganl may
be injected into the output circuit by moving coil 66 to the
(b) a permanent magnet secured to said base and
having one pole adjacent said base and an opposite
pole remote from said base,
right or to the left a small amount. Such an adjustment
(0) a pair of reeds of magnetic material having ?xed
may be accommodated by providing enlarged openings 75
ends adjacent said opposite pole of said permanent
5
3,098,987
6
magnet and having free ends extending in spaced
relation to said base,
(d) said reeds being resonant at respective audio fre
reed means, an input circuit comprising input coil means
coupled to said reed means for driving said reed means in
accordance with an input signal to be ?ltered, said input
quencies which together de?ne a desired band pass
Width,
coil means driving said reed means in accordance with
Cl
desired input signal frequencies of said input signal
(6) output vcoil means having core means extending
from said base toward the free ends of the respective
reeds and forming parts of respective magnetic
circuits with said reeds and said permanent magnet,
(f) reed driving means coupled to said reeds for driv 10
within said band pass width to produce a modulation of
ing said reeds in accordance with an input signal to
the unidirectional magnetic flux threading said output
coil means, said input coil means being responsive to un
desired input signal frequencies of said input signal out
side said desired band pass width to produce magnetic
flux components of corresponding undesired frequencies
threading said output coil means and producing spurious
voltages therein, output terminals connected with said out
be ?ltered and comprising input coil means having
core means terminating in close relation to the free
ends of said reeds, and
put coil means for providing an output signal from said
(g) one of said coil means comprising a single coil
having a core of oblong cross section disposed trans
?lter, at least one of said input and output coil means com
prising an even number of coils coupled to respective ones
_of said reed means, means connecting said even number of
versely to said reeds and in magnetic coupling rela
tion to the free ends of said reeds.
2. An audio frequency band pass ?lter comprising a
coils in balanced opposing relation with respect to unde
sired input signal frequencies to balance out any spurious
plurality of reed means having vibrating portions of mag 20 voltages at said output terminals, the number of coils of
netic material and being resonant at respective audio
said one of said input ‘and output coil means being not
frequencies which together de?ne a desired band pass
greater than the number of vibrating portions of said reed
means, and the output coil means being conductively iso—
width, output coil means coupled to the vibrating por
lated from said input coil means to prevent current flow
tions of the respective reed means, means for producing a
unidirection magnetic ?ux threading said output coil
means under the control of the vibrating portions of said
reed means, an input circuit comprising input coil means
from said input circuit through said output coil means, and
means for mechanicallylimiting the amplitude of vibra
tion of said vibrating portions of said reed means com
coupled to said reed means for driving said reed means
prising non-magnetic material engageable with said reed
in accordance with an input signal to be ?ltered, said input
coil means driving said reed means in accordance with 30
means and spaced therefrom about .005 inch.
5. An audio frequency band pass ?lter comprising a
desired input signal frequencies of said input signal within
plurality of reed means having vibrating portions of
said band pass width to produce a modulation of the uni
magnetic material and being resonant at respective au
dio frequencies which together de?ne a desired band pass
25
directional magnetic flux threading said output coil means,
said input coil means being responsive to undesired in
put signal frequencies of said input signal outside said
desired band pass width to produce magnetic ?ux com
ponents of corresponding undesired frequencies threading
said output coil means and producing spurious voltages
therein, output terminals connected with said output coil
means for providing an output signal from said ?lter, at
Width, output coil means coupled to the vibrating portions
of the respective reed means, means for producing a uni
directional magnetic ?ux threading said output coil means
under the control of the vibrating portions of said reed
means, an input circuit comprising input coil means cou
pled to said reed means for driving said reed means in
accordance with an input signal to be ?ltered, said input
least one of said input and output coil means comprising
coil means driving said reed means in accordance with
an even number of coils coupled to respective ones of
said reed means, and means connecting said even number
said band pass width to produce a modulation of the uni
desired input signal frequencies of said input signal Within
of coils in balanced opposing relation with respect to
directional magnetic ?ux threading said output coil means,
undesired input signal frequencies to balance out any 45 said input coil means being responsive to undesired input
spurious voltages at said output terminals, the number
signal frequencies of said input signal outside said de
of coils of said one of said input and output coil means
sired band pass width to produce magnetic flux compo
being not greater than the number of vibrating portions
nents of corresponding undesired frequencies threading
of said reed means, and the output coil means being con
said output coil means and producing spurious voltages
ductively isolated from said input coil means to prevent 50 therein, output terminals connected with said output coil
current ?ow from said input circuit through said output
means for providing an output signal from said ?lter, at
coil means, and adjustable permanent magnet means
coupled to said vibrating portions of magnetic material
for supplying to said vibrating portions an adjustable
magnetic ?ux separate from and additional to said uni
directional magnetic ?uX from said producing means,
said adjustable permanent magnet means having man
ually operable means for selectively incrementally ad
justing the position of said permanent magnet means to
selectively and adjustably modify the band pass width of 60
said ?lter.
3. The ?lter of claim 2 wherein said adjustable perma
nent magnet means comprises a permanent magnet
mounted for rotation adjacent said reed means, said
least one of said input and output coil means compris
ing an even number of coils coupled to respective ones of
said reed means, and means connecting said even num
ber of coils in balanced opposing relation with respect to
undesired input signal frequencies to balance out any
spurious voltages at said output terminals, the number of
coils of said one of said input and output coil means
being not greater than the number of vibrating portions
of said reed means, and the output coil means being con
ductively isolated from said input coil means to prevent
current ?ow from said input circuit through said output
coil means, said output coil means having core means of
magnetic material terminating in spaced relation to said
manually operable means being operative to rotate said 65 vibrating portions of said reed means, and said core means
permanent magnet to adjust the spacing of the poles of
having non-magnetic coating material thereon spaced
said permanent magnet from said reed means.
from said vibrating portions approximately .005 inch
4. An audio frequency band pass ?lter comprising a
and engageable with said vibrating portions to mechani
plurality of reed means having vibrating portions of
cally limit the amplitude of vibration of said reed means.
magnetic material and being resonant at respective audio
6. An audio frequency band pass ?lter comprising a
frequencies which together de?ne a desired band pass
plurality of reed means having vibrating portions of mag
width, output coil means coupled to the vibrating por
netic material and being resonant at respective audio fre
tions of the respective reed means, means for producing
quencies which together de?ne a desired band pass width,
a unidirectional magnetic flux threading said output coil
output coil means coupled to the vibrating portions of the
means under the control of the vibrating portions of said 75 respective reed means, means for producing a unidirec
3,098,987
8
tional magnetic flux threading said output coil means
under the control of the vibrating portions of said reed
means in accordance with an input signal to be ?ltered,
said input circuit driving said reed means in accordance
means, an input circuit comprising input coil means
with desired input signal frequencies of said input signal
coupled to said reed means for driving said reed means in
Within said band pass Width to produce a modulation of
accordance with an input signal to be ?ltered, said input
the unidirectional magnetic ?ux threading said coil means,
said input circuit being responsive to undesired input
coil means driving said reed means in accordance with
desired input signal frequencies of said input signal Within
signal frequencies of said input signal outside said desired
band pass width to produce magnetic flux components
of corresponding undesired frequencies threading said
directional magnetic flux threading'said output coil means,
said input coil means being responsive to undesired input 10 coil means, output terminals connected with said coil
means for providing an output signal from said ?lter, said
signal frequencies of said input signal outside said desired
coil means comprising an even number of-coils coupled
band pass width to produce magnetic ?ux components of
said band pass width to produce a modulation of the uni
corresponding undesired frequencies threading said output
to respective ones of said reed means, and means connect
ing said even number'of coils in balanced opposing‘rela
coil means and producing spurious voltages therein, out
put terminals connected with said output coil means for 15 tion with respect to undesired input signal frequencies to
balance out said undesired input signal frequencies at
providing an output signal from said ?lter, at least one of
said input and output coil means comprising an even
number of coils coupled to respective ones of said reed
said output terminals, and-adjustable permanent magnet
means coupled -to said vibrating portions of magnetic
material for supplying to said vibrating portions an ad
means, and means connecting said even number of coils
justable magnetic flux separate from and additional to
in balanced opposing relation with respect to unde
said unidirectional magnetic ?ux from said producing
sired input signal frequencies to balance out any spurious
means, said adjustable permanent magnet means having
voltages at said output terminals, the number of coils of
manually operable means for selectively incrementally
said one of said input and output coil means being not
adjusting the position of said permanent magnet means
greater than the number of vibrating portions of said reed
means, and the output coil means being conductively 25 to selectively and adjustable modify the band pass width
of said ?lter.
isolated from said input coil means to prevent current
flow from said input circuit through said output coil
References Cited in the ?le of this patent
means, and one of said input and output coil means com
prising a single coil having a core of oblong cross section
UNITED STATES PATENTS
disposed transversely to said reed means and in magnetic 30
coupling relation to the vibrating portions of the reed
means.
7. An audio frequency band pass ?lter comprising a
plurality of reed means having vibrating portions of
magnetic material and being resonant at respective audio
frequencies Which together de?ne a desired band pass
width, coil means coupled to the vibrating portions of the
respective reed means, means for producing a unidirection
al magnetic ?ux threading said coil means under the con
trol of the vibrating portions of said reed means, an input 40
circuit coupled to said reed means for driving said reed
1,821,181
1,887,713
Gumm ______________ __ Sept. 1, 1931
Gumm _____________ __ Nov. 15, 1932
. 2,143,437
Fisher _______________ __ Jan. 10, 1939
2,148,578
Pullis _______________ __ lFeb. 28, 1939
2,160,876
2,164,541
Lakatos ______________ __ June 6, 11939
Och _________________ __ July 4, 1939
2,222,796
2,928,057
2,981,905
Devol _______________ __ Nov. 26, ‘1940
Targer _______________ __ Mar. 8, 1960
Mason _____________ __ Apr. 25, 19611
356,803
Great Britain _________ __ Sept. 14, 1931
FOREIGN PATENTS
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