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Патент USA US2405999

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Aug. 20, 1.946..
- é. ECOLLAR EI'AL
2,405,
I CRYSTAL FILTER CIRCUIT
Filed Mar-ch 14., 1945'
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INVENTOR
BY
GLENETH F‘. CO LLAR
RICHARD G. YOUNG
M9, 4.1a,
ATTORNEY
2,405,999
Patented Aug. 20, 1946
UNITED; STATES yATENT OFFICE
2,405,999
CRYSTAL FILTER CIRCUIT
Gleneth F. Collar and Richard 0. Young,
Seattle, Wash.
Application March 14, 1945, Serial No. 582,783
3 Claims. (01. 25o-20)
(Granted under the act of March 3, 1883, as
amended April 30, 1928; 370 0. G._757)
1
2
The invention described herein may be manu
factured and used by or for the Governmentfor
signal, a wave trap‘for absorbing noise energy,
a simple ?lter tuned to a particular frequency
2b shows the curve then obtained by the use of
is difficult, if not impossible, to effect.
a band spreading network ‘in combination with
and a second crystaladapted to pass the desired
frequency in association with a band spreading
governmental purposes, without the payment to
network for controlling the Q of said second crys
us of any royalty thereon.
tal and thereby the band-pass width thereof.
The present invention relates generally to elec
For a better understanding of the present in
tromagnetic-wave ?lter circuits, and more par
vention, as well as further objects and features
ticularly to ?lter circuits, incorporating piezo
thereof, reference is made to the ensuing de
electric crystals, adapted to suppress strong in
scription to be read in connection with the an
terfering signals and. reduce noise level when
10 nexed drawing. The scope of the invention will
installed in conjunction with a radio receiver.
be pointed out in the accompanying claims. ‘
It is well known that interference due to at
In the drawing: Figure 1 schematically illus
mospheric conditions or man made static is
trates a preferred embodiment of a crystal ?lter
caused by waves of highly damped form which,
arrangement in accordance with the invention;
therefore, are not restricted to any de?nite or
particular frequency. The elimination of such 15 and Figure 2a shows the band-pass curve secured
with a high Q crystal ?lter circuit, while Figure
disturbances in a receiver by the utilization of
Such
said high Q crystal circuit.
means, while eliminating a certain de?nite fre
Referring now to the drawing, and more par
quency from the incoming signal energy, are not 20
ticularly to Figure 1, there is shown a crystal '
capable of effectively handling the band of fre
filter arrangement completely enclosed in a metal
quencies which is present incident to the damped
lic shield box In having three compartments.
nature of the interfering energy. Moreover,
The receiving antenna II is connected to one
side of the primary winding l2 of a radio fre
quency transformer, the other side thereof be
standard ?lter means are unable to suppress a
closely adjacent, stray or disturbing frequency,
especially where the value of interfering fre
wing grounded. Inductively coupled to the pri
quency is such that it lies within the band width
mary winding l2 of the transformer is a second
ary winding l3 which is balanced to ground
through a pair of condensers l4 and I5. Also in
of the’ resonance curve of the receiver selector
circuit.
Where a high Q, narrow-pass, crystal ?lter cir
cuit is employed as a solution to the above-de
scribed difliculty a new problem arises. By rea
son of the steep selectivity characteristic of these
circuits, there results a cutting off of a portion
of the side bands of the desired signal as well as
discrimination against unwanted and interfering
signals. Thus, while interference is thereby re
ductively coupled to the primary winding I2 is
a wave trap [6 consisting of an inductor H and
a variable condenser ! 8. Electrostatically shield
ing primary winding l2 and inductor i‘! from sec
ondary winding is is a Faraday screen l9 serving
to prevent stray capacitative coupling between
these elements and thereby tending to eliminate
high frequency disturbances, said screen however
having no eifect on the inductive coupling there
duced, the intelligibility of the desired signal is
impaired.
Accordingly, it is the principal object of this in
vention to provide a crystal ?lter arrangement
to be used in combination with a radio receiver
having a resonant input circuit tuned to a de
between. ‘
'
sired signal whereby a strong interfering signal
is suppressed, noise level is reduced, and the de
sired incoming signal is passed without cutting
off a portion of the side bands thereof.
It is another object of this invention to provide
a crystal ?lter arrangement of the above-men
tioned type wherein the width of the band ac
commodating the side bands of the desired sig
Shunted across secondary winding is is a ?rst
Also provided is a second
quartz crystal 2! connected in series with a vari
able condenser 22, the series combination being
shunted across the secondary winding E3. The
juncture of crystal 2i and condenser 22 is con
quartz crystal '29.
nected to one side of a band spreading network
consisting of an inductor'23 in parallel with a
nal may be adjustably shaped.
Brie?y stated, the objects of the invention are
variable capacitor 25, the other side of said net
work being grounded. The antenna terminal of
the receiver is connected to an adjustable tap 29
on inductor 23, so that the input impedance of
the receiver, which is principally resistance, is
attained by the use of a circuit entailing a ?rst
connected across a portion of inductor 23.
crystal arranged to reject a strong interfering 55
The dimensions of crystal 2| are such that it is
2,405,999
3
resonant to the desired incoming frequency. That
portion of the circuit comprising secondary wind
ing l3, variable condensers l4 and I5, crystal 2|
and variable condenser 22 behaves as a bridge
arrangement. Variable condenser 22 serves as a
phasing condenser which has a maximum ca
pacity somewhat higher than the capacity of the
cryst 1 2! in its holder. In operation, secondary
win ng I3 is tuned by condensers l4 and I5 so
that resonance with the desired incoming signal
is o tained. When phasing condenser 22 is set to
balance the crystal-holder capacity, the resonance
curve of the crystal circuit is substantially sym
metrical; the crystal 2! acts as a series resonant
circuit of very high Q and thus introduces a low 15
impedance path to signals of the desired fre
quency. Without phasing condenser 22, the ca
pacity of the crystal-holder (with the crystal 2!
acting as the dielectric) would by-pass signals of
undesired frequency to the output circuit.
Crystal 20 is dimensioned so that it is resonant
to a strong interfering frequency which may be
closely adjacent to the desired frequency, and it
4
pended claims, to cover all such changes and
modi?cations as fall within the true spirit and
scope of the invention.
What is claimed is:
1. In combination with an antenna and a radio
receiver having an input circuit, a ?lter compris
ing a ?rst crystal circuit, resonant to an undesired
signal frequency, interconnected between the an
tenna and receiver in a manner whereby said in
terfering signal is absorbed, a second crystal cir
cuit resonant to a desired incoming frequency and
interconnected between the antenna and receiver
in a manner presenting a low impedance path
solely to the desired signal, and an inductance
capacitance parallel resonant network across the
output of said second crystal circuit, and means
connecting the input circuit of said receiver across
a portion of said resonant network whereby the
selectivity of said second crystal circuit is ad
justed to pass to the receiver the desired signal
carrier and side bands only.
2. In combination with an antenna and a radio
receiver having an input circuit, a ?lter compris
functions to cancel out or absorb the undesired
ing an inductor, capacitative means for balancing
signal.
,
25 said inductor to ground and tuning same to a
Wave trap i5 is tuned so that it is resonant to
' desired incoming frequency, means for applying
a frequency about 10 or more kilocycles .removed
energy from the antenna to said inductor, a first
from the desired signal, the wave trap serving to
crystal resonant to an undesired signal frequency
absorb noise energy and interfering signals.
shunted across said inductor, a second crystal
Because of the high Q of the crystal ?lter cir
‘resonant to the desired incoming frequency, a
cuit, its selectivity characteristic is extremely
capacitor in series with said second crystal, the
sharp as shown in Figure 2a wherein curve 25
series combination being shunted across said in
is obtained by plotting the current against fre
ductor, said capacitor having a value such as to
quency in the series-resonant circuit presented by
neutralize the off resonance capacitance of said
crystal 2!. Dotted line 26 represents the point
second crystal, an inductance-capacitance par
of resonance and the carrier frequency of the
allel resonant network connected between the
desired signal while dashed lines 2‘! and 28 indi
juncture of said series combination and ground,
cate the side band limits of the incoming signal.
and means connecting the input circuit of said
It will be seen that the side bands of the incom
receiver across a portion of said resonant network
ing signal are largely cut off, hence a circuit of 40 whereby the effective Q of said second crystal is
this characteristic would be unsuitable for re
lowered and there is provided a path passing only
ception.
the incoming carrier and the side bands thereof.
The band spreading network when tuned to
3. In combination with an antenna and a radio
parallel resonance presents an extremely high
receiver having an input circuit, a ?lter compris
impedance path in series with the crystal circuit
ing a radio frequency transformer having its pri
and the receiver with a consequent reduction in
mary winding connected to the antenna, capaci
Q of the ?lter circuit and a broadening of the
tative means for balancing the secondary winding
selectivity curve. The connection to tap 29 in
of said transformer to ground and tuning same
troduces in a varying degree the input impedance
to the desired incoming frequency, a ?rst induct
of the receiver into the band spreading network.
ance—capacitance parallel network inductively
This impedance is principally resistive. Any re
coupled to said transformer and resonant to'a
active component of this impedance combines
frequency adjacent to the desired incoming fre
with the reactances of the band spreading net
quency, a Faraday screen shielding said second
work. It may be desirable to detune the band
ary winding from said primary winding and said
spreading network slightly from resonance. Con
?rst inductance-capacitance network, a ?rst
denser 25 and adjustable tap 29 are adjusted so
crystal resonant to an undesired signal frequency
that the selectivity curve is spread to a width just
shunted across said secondary winding, a second
sufficient to accommodate the desired incoming
crystal resonant to the desired incoming fre
signal, as shown roughly in Figure 2b. In this
quency, a capacitor in series With said’ second
manner the desired incoming signal is received 60 crystal, the series combination being shunted
without distortion while all other signals and
across said secondary winding, said capacitor hav
noise are greatly attenuated.
ing a value such as to neutralize the off-resonance
In practice, windings I2 and I3 and inductors
capacitance of said second crystal, and a second
i‘! and 23 may be multi-tapped and a number of
inductance-capacitance parallel resonant network
different plug-in crystals 20 and 2! may be pro
connected between the juncture of said series
vided, thereby permitting the crystal ?lter circuit
combination and ground, and means connecting
to be used effectively at any one of a plurality of
the input circuit of said receiver across a portion
desired incoming frequencies.
of said resonant network whereby the effective Q
While there has been described what is at
of said second crystal is lowered and there is pro
present considered a preferred embodiment of 70 vided a path passing only the incoming carrier
the invention, it will be obvious to those skilled in
and the side bands thereof.
the art that various changes and modi?cations
may be made therein without departing from the
GLENETI-I F. COLLAR.
invention, and it is, therefore, aimed in the ap
RICHARD‘ C. YOUNG.
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