Патент USA US2405999код для вставки
Aug. 20, 1.946.. - é. ECOLLAR EI'AL 2,405, I CRYSTAL FILTER CIRCUIT Filed Mar-ch 14., 1945' ‘FIG. 1 LIME1, 1w ‘ RECEIVER % FZutmDU INCREASING FREQUENCY—>- ' 2 -.rzumauv IiNzt 5 m a 8. S. , J1: F6 M Q u2 RI}i!: -m C I . G E a 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.