US2245340A - Tuned radio signal circuit - Google Patents

Description

June l0, 1941. R, LI HARVEY .2,345,349

TUNED RADIO SIGNAL CIRCUIT Filed May 5l, 1939 Patenieci l@ lfill Robert L. Harvey, Qalilyn, N. ll., assigner to adio Corporation of America, a corporation of melaware Application May 3l, w33, Scial No. 2i

il Claims. (Cl. Titi-37D The pre-sent invention relates to tuned radio sine-,ll variations in the strength of the applied signal circuits, and more particularly to signal signal or in response to variations in the applied circuits of the type having ferro-magneiic cr signal voltage.

inagne-tite core inductance tuning. It is also a further object of the present in- In conventional circuit design with inductance 5 c ltion to provide an improved tuned signal contuning of the type above referred to, the change veying circuit, the resonant frequency of which in permeability is made relatively small and usu be var ed in response to Variations in the ally is not measurable because of the low flux strer th of the applied signal or in response to density and relatively large Volume of the cores variations in the app-lied signal voltage when ordinarily provided. It has been found, however, lo such voltage lariations are relatively wide and that with magnetite core tuning inductances, the the applied signal is relatively strong. permeability may be varied appreciably as the lo providing a circuit embodying the invenflux density in the core is increased to a relatively tion, a tuning inductan'ce or inductive reactance .high value. of `the ferro-magnetic core type in which the core The flux density may be increased bythe presl5 material is of niagnetite is arranged to operate ence of strong signals as in an IF amplifier, to inwith a relatively high linx density for high frecrease the signal current flow through thc inquency signal circuits. With magnetite cores in ductance, or the core Volume may be decreased the tuning inductances, this has been found to to a relativelysmall percentage of anormal core f 'case the effective permeability and to cause volume. It has been found that as a result of a 2O an i crec.se in the inductan-ce value and a correlatively higher flux density than has heretofore espendin heen used for high frequency signal circuits, .it of the tun d circuits. is poesie-lesto shift or vary the resonant frequency lt is, therefore, a still further object of the of a tuned circuit embodying such an inductance, present inven ion to provide an improved tuning over a relatively Wide frequency 'range With a y25 i .l element for high frequency radio sigrelatively narrow range of variation in the nal circuits wherein the eifective permeability strength of the applied signal or signal voltage, of a ferromiagnetic core may be varied in the because of the change in permeability, and that, presence of strong signals and with a relatively for maximum change of flux density with relalow variation in signal strength or voltage. tively small voltage change, a lovv L/C ratio in 30 In. a tuned radio signal cir-cui't such as the cirthe tuned circuit is necessary. cuit of an intermediate frequency amplifier, it It is, therefore, an object of the present inhas been found that the permeability variation of vention to provide an improved tuned radio siga ferro-magnetic core tuning inductance may be nal circuit embodying a tuning inductance or increased by making the L/C ratio of the circuit inductive reactance device of the magnetite core 35 loi-v, that by making the inductance relatively type, wherein a higher than normal flux density and the associated shunt tuning capacity relfor radio signal. circuits is provided in the core atively high in value, thereby to provide an inin response to an applied signal vol-tage to cause crease in sig al current flow since tbe flux in the frequency response of the tuned circuit to be l the core depends upon the factor IZL. The flux, varied in response to variations in applied signal M theefore, is proportional to I2. It is, therefore, strength or voltage. an object of the present invention to provide a It is also an ooiect of the present invention to permeability tunable inductance for high freprovide improved radio signal circuit of the quent-y tuned radio circuits aranged to take adinagnetite core inductance tuned type which is vantage of the increased current available in a responsive to variations in signal strength to l5 circuit having a lovv L/C ratio to increase the efcause a corresponding variations in pernielabillect of frequency variation with variations in ity and frequency response, whereby ythe circuit permeability. is adapted for use in compensating circuits for It is also an obje-ct of the present invention to high frequency amplifiers, for example, as ernprovide an improved high frequency radio signal ployed in the and IF circuits of a superlo circuit of the ferro-magnetic tuned type comheterodyne receiver, and also in connection with ng a inductance having a magnetite AVC and AFC circuits for radio receiving app'acore and which, in the presence of strong signals, ratos and the like. responds to variations in signal strength or volt- The invention is particularly adapted for use age to provide a change in the permeability of in connection with the tuned signal conveying 5;3 the core the inductance value thereof,and circuits of radio and other high frequency apirnproved control circuit, such as an AVC circuit paratus, has for its furet-her object to provide for radio receiving apparatus and the like, ernan improved tuned signal conveying circuit, the ing such tuning inductance or inductances.

resonant frequency of which may be varied over a S, ice an increase in signal strength or voltage relatively Wide range in response to relatively J in a tuned circuit embodying the invention provides a decrease in resonance frequency, the circuit may be utilized to compensate for variations in capacity on the circuit caused by variations in controlling potentials on associated amplifier tubes such, for `example, as variation in capacity provided by an associated control grid in response to variations in AVC potential thereon to maintain the resonance frequency substantially constant.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing,

Figure l is a schematic circuit diagram of a tuned radio signal circuit embodying the invention;

Figure 2 is a graph showin-g curves representing operating characteristics of the circuit of Fig. 1 under diifering conditions of adjustment;

Figure 3 is a circuit diagram of an intermediate frequency amplifier and second detector for a superheterodyne radio receiving system also embodying the invention; and

Figure 4 is a graph showing curves illustrating an operating characteristic of the circuit of Fig. 3.

Referring to Fig. 1, a tuned radio signal circuit 5 is provided with a tuning inductance 6 and a shunt tuning capacitor 1 for said inductance. A signal input circuit 8 is coupled through a winding 9 with the indurctance 5. This represents any suitable signal conveying circuit adapted to be tuned to resonance at a predetermined frequency.

In accordance with the invention, the inductance 6 is of the ferro-magnetic core type having a core I!) of magnetite of such volume or crosssection that the flux density therein is relatively high.

The magnetic circuit of the core I@ is preferably closed through a suitable path provided by magnetic core material providing a relatively large volume or Ycross-sectional area. In. the present example, the ends of the core lil engage the inner surfaces of a ring Il of magnetic material having a large cross-sectional area Vwith respect to that of the core Ill.

By this arrangement, the core volume or crosssection under the winding is decreased. The signal intensity applied to the input circuit 3 and the voltage E in the tuned circuit 5 may be relatively high as in an intermediate frequency amplifier, with the result that the ux density in the core I El is increased to such a point that the permeability of the core is affected and, as hereinfbefore pointed out, the effect is to increase the permeability with increase in the iiuX density so that it is possible to shift the resonant frequency of the tuned circuit 5 over an appreciable frequency range by variation in the signal intensity or voltage E across the tuned circuit.

As shown in Fig. 2 by the curve l2, it has been found to be possible to shift the resonant frequency of the tuned circuit 5 from 450 kc., for

example; to 430 kc. with an RF voltage changeof Afrom zero to substantially ten volts. This is with a low L/C ratio in the tuned |circuit '5. With a higher L/C ratio, the response curve may be as indicated at I3 in Fig. 2, requiring a wider variation of voltage E across the tuned circuit to obtain the same shift of the resonance point.

' The value of the capacitor 1 or the value of C in the :circuit 5 was 400 mmf. for obtaining the curve I3, while for obtaining the curve i2, a capacity of 4000 mmf. was employed across the tuning inductance. rIhe lower L/C ratio provides a higher current through and a lower voltage across the tuning inductance. With a higher value of tuning capacity, it is obvious that for the same frequency or frequency range the value of the inductance B and the number of turns on the winding must be lower. However, it has been found that with a lower number of turns, the flux through the core is increased appreciably because of the fact .that the flux varies as the square of the icurrent and only as .the first power of the inductance. A

The ratio of the reactance of the inductance 6 to its high frequency resistance, or the Q of the coil, is reduced with increase in the strength of thesignal applied through the circuit toV the circuit 5, because the high frequency resistance increases at a higher rate than the inductance or the effective permeability, and this results in a decrease in the gain of any amplifier stage in which the circuit is connected, which is a desirable feature since it reduces the gain for strong signals and permits increased gain automatically for weak signals.

This is for the reason that, in the present system, the core of the inductance operates with high frequency currents far below the knee of the permeablity-magnetomotive force or saturation characteristic rcurve, but still relatively higher than has heretofore been used in such systems. This causes the permeability to rise with increased RF voltage at E. The relatively high ux density herein referre-d to is a flux density which is high for that ordinarily obtainalble in high frequency signal circuits and should not be confused with that used in power and audio frequency circuits. On a usual saturation curve, for example,v a point above the upper bend or knee is that which is ordinarily considered to represent a high fiux density for power and audio frequency circuits, while a point on or above the lower bend of the same curve l would indicate a high radio frequency ux density. The flexing of the curve is opposite for the two cases, and it will therefore be seen that the tangent to the curve more nearly approaches the vertical for the radio frequency circuits and, accordingly, Yin this region, as the RF voltage and current increases, the permeability increases, since the tangent to the curve becomes further more nearly vertical.

As will be seen from the icurves of Fig. 2, the voltage E required to change the resonant point or tuning o-f the circuit 5 through a relatively wide frequency range may be varied over a relatively wide range or a relatively narrow range, depending upon the L/C ratio 0f the tuning elements in the circuit. This permits considerable latitude in the design of the circuit.

In any case, however, the range of frequency variation and the Variation of the Q of the coil is relatively high only when the flux density is increased to a relatively high value for such high frequency lcircuits as hereinbefore noted, and this is accomplished not only by providing an inductance winding having a magnetite core of relatively low volume or cross-section whereby the flux density for high frequency circuits is high per unit volume of the core, but is also made possible at a lower input voltage E, by providing the tuned circuit with a low L/C ratio.

In any tuned circuit, it is unneccessary to depart from the desirable features of magnetite core tuning to obtain a shifting of the resonant point or a frequency variation in the tuned circuit, since to obtain this characteristic, it is merely necessary to decrease the length and diameter of the coil and core asseinhly in such a manner that the iiux density per unit of voluine of the core is increased, and for a maximum change in frequency with low voltage change, a 10W L/C ratio is used in the tuned circuit.

Referring new to 3, an intermediate ire-- quency amplier circuit is shown in which a tube iii is provided as the last intermediate frequency amplifier stage precening a second detector tube in the signal channel of the receiver, The detector is of the diode rectine type coinprisint,T a pair of diode electrodes il connected to the secondary it oi IE coupling transformer is which is connected in series with a coupling coil 2t and is tuned by a shunt adjustable capacitor i.

The output circuit of the iF ampliiier tube i5 is connected with the tuned primary winding also provided with a shunt tuning capacitor 2li so that the two tuned circuits rnay respond to the saine fred ncy, such as 450 kc., for example.

The secondary circuit liB--Il-Zl is connected at its. low potential end through a diode output resistor ii to the cathode Audio frequency signal output is derived` from the resistor 2d through an output leasj 2i', connected in the present example to an ain grid 2S in the same envelope with the diode detector. Audio irequency signals for further amplification and utilization are derived from the output anode il@ asu sociated with the grid is, through the output circuit 3S.

The Ei tuned input circui tube i5 is provided with a comprising a tuning inductance 35i and shunt tunii e capacitor 3,; therefor forming the tuned secondary cir-cui* of an L? transformer 3l ed primary of which is indicated at secondary is tuned by the shunt cap and also by a core Si? of magnetic m the ein in f1 cross-sect to normal. s; sunpiied through the channel, the voltage the circuit 35i- 36 causes the flux density the core to he relatively high for high fre uel :y circuits although far below the upper bend the usual saturation curve for inductanoe elements in audio frequency and low frequency power circuits, thereby to decrease the frequency of resor ce ci the input circuit by an amount lest su cient to overcome the eiect on the circuit c at or control grid of tire en pliiier connected thereto and y with a source oi AVC As is well bias provided in Ai strength in an AVC l such as inagnetite arranged -iy suitable manner and being of such area or volume that, in response ril notential known, -e response to systeni cans-s to be door i Vrequency response to he shifted toward a higher resonant frequency. This tendency is overcome by the present arrangement.

To take oi the variable permeability in response to variations in signal strength to the tuning' ci a resonant circuit having inductance ferro-magnetic core tuning` means, a tuned cir comprising a tuning inductance fit and a 'nt capacitor lil, is associated with the coupling wii in such a manner that, as indicated in the drawing, the latter Winding is As ereinleeiore described, the increase in perineao ity causes an increase in inductance of the windings tti and tunes the circuit to a lower frequency, to increase this effect, the magnetic circuit is preferably closed, as in the arrangement i, furthermore, the ratio of the inductance it to the capacity at that is, the C ratio, is rrwe low. This provides a relatively high current and low inductance in the winding for a given frequency.

With an intermediate frequency circuit tuned to kc., the circuit may be tuned to 486 kc., for example, with weak signal input. As the applied sional strength is increased, the 480 kc, circuit [is will shirt in resonant frequency to a lower frequency, approaching the intermediate frequency 45) irc. as a liinit, thus increasing the coupling to the secondary and causing the output voltage from the .,'ied circuiu i5 to increase at a higher rate than the signal.

A signal of the diode type indicated at having a output resistor 5l may be connected across the tuned circuit to provide controlling potentials for the AVC circuit (il, the connection being made indicated, through a suitable i'lter ccinpri a series resistor 52 and a shunt niter .capacitor This causes a better AVC characteristic than is normai y revidecL as indicate the curves o; 4, in which the curve is a response curve for a norrnal AVC connect-i n, W creas the curve "it represents the AVC control cn racteristic of the circuit of Fig. 3, clearly showing that the AVC characteristic is much flatter the prese. t system.

I claim as my invention:

l. In a tuned radio signal circuit, an inductive reactance responsive to the flow of relatively strong signal currents therethrough to vary the tuning oi said circuit from higher to a lov/er resonant frequency, reacts-ace comprising an inductive winding a core element thereior so proportioned that the and perlneuhility :i crease with an increase in signal potential across said circuit and resulting increase in signal current through said winding.

2. In a tuned radio signal circuit, an inductive reactance responsive to relatively strong signal current the r'ethrougli to vary the tuning of said circ from a higher to a lower resonant frequency with rease in signal potential across said circuit, inductive reactance comprising an inductance win and a ferro-magnetic core element therein su Vt to a relatively high flux de sity in response to si'f'nal current of a predernlined value through. said reactance and an increase in the permeability oi said core element in res ense to an increase in flux density effective to e said lewe ing of the resonant frequency cuit, said inductive reactance comprising an inductive windinghaving a ferro-magnetic core element providing a relatively high flux density in response to the iicw of signal currents in the winding and an increase in the permeability of said core element.

4. In a tuned radio signal circuit, an inductive reactance responsive to relatively strong signal currents therethrough to vary the tuning of said circuit from a higher to a lower resonant frequency with increase in signal potential across said circuit, said inductive reactance comprising an inductive winding having a. ferro-magnetic core element providing a relatively high flux density in response to the flow of signal currents in the winding and an increase in the permeability of said core element, and means providing a shunt tuning capacitor for said inductive windingY of a capacity value such that the L/C ratio of said circuit is relatively low.

5. In a tuned radio signal circuit, the combination of a tuning inductance and a shunt tuning capacitor therefor providing a circuit for the rlow of signal currents through said inductance, a ferro-magnetic core element for said inductance of relatively small volume providing a relatively high flux density therein in response to an increase in signal currenm through said inductance and an increase in the permeability,

whereby the frequency response of said circuit is decreased with increased signal strength, and the L/C ratio of said circuit being relatively low for maximum frequency change in the resonance of said circuit in response to a relatively small signal voltage change in said circuit.

6. In a radio signal circuit, means for varying the tuning of said circuit in response to variations in a signal voltage applied thereto, said means comprising an inductance, a tuning capacitor in shunt therewith, a magnetite core element for said inductance providing a relatively high flux density in the core in response to signal current iiow in said inductance, and means for applying radio signals to said circuit to cause the flux and the permeability of said core to increase with increased signal strength, thereby to increase the value of said inductance and to lower the resonance frequency of said circuit.

7. In a tuned radio signal conveying circuit, means responsive to variations in signal voltage across said circuit for varying the resonance frequency of said circuit, said means comprising an inductive tuning reactance in said circuit, a ferromagnetic core for said reactance providing an increase in flux density and permeability with an increase in said voltage, and means for applying signals of a predetermined strength to said circuit, said last-named means comprising an intermediate frequency amplifier circuit, a rectifier device connected with said first-named circuit, and a control circuit for deriving an automatic volume control potential therefrom.

8. In a tuned radio signal conveying circuit, means responsive to variations in signal voltage across said circuit for varying the resonance frequency of said circuit, said means comprising an inductive tuning reactance in said circuit, a ferro-magnetic core for said reactance providing an increase in flux density and permeability with an increase in said voltage, means for applying signals of a predetermined strength to said circuit, said last-named means comprising an intermediate frequency amplifier tuned circuit, a rectier device connected withY said first-named circuit, and a control circuit for deriving an automatic volume control potential therefrom, said intermediate frequency amplifier circuit being tuned to a predetermined intermediate frequency and said first-named circuit being tuned to a higher frequency by an amount substantially equal to a range of frequency variation of said first-named circuit corresponding to a variation of the signal voltage across said first-named circuit from a minimum to a maximum value.

9. In a radio signal conveying channel, the combination of means providing a tuned signal circuit, an amplifier tube having a. control grid connected with said tuned circuit, means for applying a biasing potential to said control grid to vary the gain of said tube and the capacity load provided thereby on said tuned circuit, and means in said circuit for compensating for variations in said capacity load to maintain the resonance frequency of said circuit substantially constant, comprising an inductive reactance Winding in said tuned circuit and a ferromagnetic core element therefor providing a relatively high flux density per unit volume in response to signals through said circuit thereby to increase the permeability of said core and to decrease the frequency response of said circuit With increased signal strength.

10. In a radio signal conveying circuit, the combination of a tuned signal circuit comprising a tuning inductance and a shunt capacitor therefor, an amplifier tube having a control grid connected with said circuit, means for applying automatic volume control potentials to said control grid in response to received signals, the capacity load provided by said control grid being thereby decreased with increased signal strength tending to increase the resonance frequency of said circuit, and means for compensating for said increase in resonance frequency comprising a magnetite core element for said inductance providing a relatively high flux density per unit volume in response to signals through said tuned circuit and a corresponding increase in the permeability and a decrease in the frequency response of said circuit substantially equal to the increase in frequency response tending to be effected by volume control of said grid,

11. In a radio receiver, an automatic volume control system, comprising in combination, a tuned signal circuit, a second tuned signal circuit coupled therewith, said second circuit being tuned to a higher frequency than said firstnamed circuit and comprising a tuning inductance having a magnetite core element of relatively srn'all volume within said inductance to provide a relatively high flux density per unit volume in response to signals applied to said second circuit and of a value such that the permeability of .said core element is increased thereby by an amount suicient to decrease the frequency response of said second circuit with increased signal strength to substantially the resonance frequency of the first-named circuit, a rectifier device connected with said secondnamed circuit, and means for deriving automatic volume control potentials therefrom for said receiver.

ROBERT L. HARVEY.

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