US2085196A - Selectivity control system - Google Patents

Description

`lllle 29, 1937. w, R KOCH 2,085,196

SELECTIVITY CONTROL SYSTEM Filed May 3l, 1935 Z c'a/vT/Pol. V0.4 T1965 /4/ L +B ,35 A

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Patented June 29, 1937 ATENT OFFICE SELECTEVITY CONTROL SYSTEM Delaware Application May 31, 1935, Serial No. 24,217

12 Claims.

The present invention relates to a selectivity control system for radio receiving apparatus and the like, and more particularly, relates to a system of that character for application to the tuned signal receiving circuits wherein a wide range of selectivity control is required.

It is a primary object of the present invention to provide an improved selectivity control system for radio receiving apparatus and the like, which may be applied to tuned circuits and interstage coupling means in the high frequency portion of a radio receiver to secure improved selectivity control thereof through a wide range of control of effective anode-cathode impedance of an electric discharge controi device included in shunt with said circuits and means.

In accordance with the invention, instead of utilizing the plate impedance of a triode or pentode tube to produce damping in a tuned circuit and thus secure selectivity control by controlling the grid bias, the principle of negative regeneration is used to secure improved results thereover. It has been found that, with the triode used as an impedance load for a tuned circuit, some distor- 2 tion may occur when the tube is biased near the cut-off point of the anode current, and the lowest shunt resistance which may effectively be placed across the tuned circuit with present tubes is not sufficiently low to provide proper control of the (.0 selectivity.

However, the importance of low effective impedance secured does not lie only in the selectivity change, but .also in the very effective volume control action of the shunt load.

Accordingly, it is a further object of the present invention to provide an improved selectivity control system providing a load across a tuned circuit, which system embodies a pentode type of tube to provide an effective load which may be made less than 1,060 ohms and may readily be varied by simple control means in the range of 1,000 to 10,000 ohms. It has been found that, by utilizing the pentode type of tube, the desired effective control may be obtained without introducing distortion. of the signal by operation of the selectivity control means.

The invention will, however, be better understood from the following description when considered in connection with the accompanying 00 drawing, and its scope wili be pointed out in the appended claims. In the drawing,

Figure i is a schematic circuit diagram of a radio receiving circuit having a selectivity control system embodying the DVSHOH;

Fig. 2 is a similar schematic diagram illustrating a modification of the system embodying the invention as applied to a radio receiving circuit of a different type; and

Fig. 3 is a curve diagram illustrating certain (Cl. Z50-20) operating characteristics of the control systems of Figs. l and 2.

Referring to Fig. l, an intermediate frequency amplifier is illustrated comprising a tuned intermediate frequency input transformer 5 connected between the grid 'l and the cathode 9 of an intermediate frequency amplifier tube ll of the pentode type. The type grid circuit is returned through a lead I3 to a common ground lead I5 which is also the common plate and grid return circuit. The receiver and the return circuit for the grid is completed through a suitable self -bias resistor Vi.

The first stage intermediate frequency amplifier tube Il is coupled to a second intermediate frequency amplifier tube I9 through a suitable tuned interstage coupling transformer 2| and it will be noted that the tube is the same as the tube I I being of the screen grid pentode type provided with biasing potential from a self-bias resistor Z3 connected with the common return circuit lead I5.

The intermediate frequency amplifier is provided with input terminal leads and output terrnina leads 2l, one of which is the positive anode potential supply lead derived from a suitable source of direct current represented by the voltage divider resistor 2S. It will be noted that the common return lead I5 is connected to an intermediate tap 3i on the divider resistor, so that, between the negative supply terminal indicated at 33 and the tap SI, a source of negative potentials is available with respect to the cathcdes of the receiving system. The screen grid connection is made through a suitable lead 35 to a voltage supply point 3i while anode potentials are provided through the lead indicated at 39. The voltage divider resistor represents any suitable source of operating potentials for the electric discharge device of the system and, since an amplifier of this character is known and well understood, further description is believed to be unnecessary.

For the control of selectivity in a system of the character shown, one or more of the tuned circuits may be loaded in accordance with the invention and, in the present example, the tuned output circuit of the first intermediate frequency amplifier is arranged to be controlled thereby to control the selectivity of the system. The output circuit comprises the primary winding fl! of the intermediate frequency transformer 2! and the shunt tuning condenser 43. The anode or high potential side of the circuit is connected through a lead i5 to a screen grid pentode electric discharge device or tube 41 with the plate 49 connected to the lead 5 and the cathode 5! thereof connected preferably through a self-biasing resistor 53 to the ground lead l5 whereby the device 4l forms a high frequency load across the tuned circuit. The anode circuit of the tube 4l' is, therefore,

completed through the same primary winding 4| as is that of the amplifier tube and variations in the anode current flow serves to variably load the tuned circuit.

In order to control the load current, the inner or control grid 55 of the selectivity control tube 151 is connected through a lead 51 and a lter resistor 59 with the negative or return lead l5 rthrough a source of negative biasing potential which may be varied and which is, therefore, included in the grid circuit in addition to the negative potential provided by the self-bias resistor 53.

The source of additional potential for controlling the tube may be derived selectively from either of two sources, through a selector switch 6| to which the resistor 59 is connected by a lead |53. 'I'he selector switch has one contact 65 leadingk to ay suitable source of automatic volume control voltage, as indicated in the drawing, and a second contact 51 connected through a lead` 69 with a variable contact 1| on the negative potential supply portion of the power supply or voltage divider resistor 29 between the terminals 3| and 33.

As shown in the drawing, the switch 6l is connected to the contact 61 to receive manually variable control potentials from the contacts 1| whereby the selectivity of the system may be varied by manual means. The automatic volume control voltage may be utilized for the same purpose by moving the switch to the contact 65. Screen grid potential is supplied to the device 41 to a supply lead 13 connected to a movable contact 15.

The selectivity control Voltage derived from the automatic volume control circuit or from the manually controllable selectivity control voltage at 1| must become more negative to make the transformer 2| broad in its tuning characteristic. By utilizing a pentode type tube at 41, the manual control may also be provided for varying the'contact at 15 to vary the screen grid voltage. By varying the control grid voltage or the screen grid voltage the mutual conductance of the selectivity control tube 41 is Varied.

Referring to Fig. 3, the variation of mutual conductance with variations in control grid voltage is indicated by the curve 11, while the loading effect or effective or operating plate impedance of the tube t1 is indicated by the curve 19. It will be seen that the mutual conductance is variable over a relatively wide range to provide a similar smooth wide control range of the effective plate impedance.

The control effect is further enhanced by connecting a coupling condenser 8| between the control grid 55 and the tuned circuit comprising the condenser @3 and the inductance lil, the coupling condenser being connected from a high potential point 83 on the circuit through a connecting lead 85. The reactance of the condenser 9| isrelatively low with respect to the resistance 59 in the grid circuit. In general, preferably, the resistance should be high and the condenser should have a reactance less than a few percent of the resistance used. Y

With this circuit arrangement, any voltage appearing across the tuned circuit 43-4| will be impressed upon the grid of the selectivity control tube through the coupling condenser 9 l. Because of the amplifying property of the selectivity control tube, a large voltage, 180 out ofY phase with the impressed voltage, will appear in the plate circuit of the tube. This will tend to oppose and reduce the original voltage, which in effect, is the same as adding resistance to the tuned circuit to broaden it. The higher the mutual conductance of the selective control tube the more this resistance damping will be. Therefore, by varying the mutual conductance of the tube, the broadening effect on the tuned circuit may be controlled and, with the circuits, as hereinbefore described, this may be caused to operate manually, or automatically with variations in signal strength.

A system in accordance with the invention, therefore, provides means for variably loading a tuned circuit over a relatively wide range of impedance by varying the mutual conductance of a pentode electric discharge device, the anodecathode space path of which is in the loading circuit, and permitting the tube further to amplify signals from the tuned circuit and to apply said signals in opposing relation to the original signal voltage, thereby to broaden the tuning of said circuit.

In this manner the tuned circuit of a radio receiver or a plurality of said circuits may be made automatically responsive in any desired degree to variations in signal strength, whereby the said circuit or circuits are more broadly tuned in response to increased average signal strength and the degree of response may simultaneously be placed under manual or automatic control such as the said variations in signal strength through automatic volume control means.

Furthermore, since, as indicated in Fig. 3, the response characteristic 11 has a long cut-ofi range, distortion is substantially eliminated while the control characteristic 19 is substantially smooth and gives in effect a straight line control.

While the invention has been described in connection with an intermediate frequency amplifier and is preferably so applied in the usual radio receiver, and while it has further been described as being responsive to increases in negative potential from a controlling circuit, it may also be applied to other receiving circuits and may be made responsive to a control voltage which is positive as applied to the control grid of the selectivity control tube. In other words, an increasing positive control voltage may be applied to the system to render the control grid of the selectivity control tube more positive to broaden the frequency response characteristic of the tuned receiving circuit and such a system is shown in Fig. 2 to which attention is now directed.

A radio frequency amplifier isY provided in conn nection with an antenna or other signal source S1 provided with the usual input coupling transformer 89 having a secondary 9i tuned by a variable condenser 93 connected therewith through a stopping condenser 95. The high potential side of the tuned circuit comprising the lead 91 is connected to the control grid 99 of a radio frequency amplier tube IUI of the pentode type through a coupling condenser |93 and grid leak resistor |95. The amplifier tube IGI is provided with a radio frequency output circuit comprising a tuned coupling transformer |01.

Across the tuned circuit is connected the anodehaving the cathode I I3 connected through a selfbias resistor I I5 with the opposite or ground side of the circuit through suitable grounds III and a lead H9. Anode potential is supplied through a lead l2i and the secondary QI to the anode III, the anode voltage being isolated from the grid S9 by the coupling condenser ID3.

In the present example, the control or inner grid I23 of the selectivity control tube I09 is coupled through a coupling condenser I25 of relatively low capacity to the high potential side 97 of the tuned circuit 9I-93--95. Grid biasing potential is supplied to the control grid I23 through a grid leak resistor I2'I which also provides a filtering and isolating means for the grid. The grid leak resistor is in turn connected through a lead |29 to a manual selectivity control device comprising a potentiometer I3 I, the variable contact IES of which is connected tc the lead IZB and the potentiometer in turn receives a xed direct current potential or voltage from supply terminals indicated at ISE through leads i39. With this arrangement, the device ISI is readily adapted to be placed at a remote point for remote control of selectivity by extending the leads I29 and |39.

It will be noted that the upper terminal IIlI of the volume control potentiometer I3I is positive while the lower terminal II3 is negative. The potential, therefore, applied to the control grid I23 from the contact 33, is caused to increase in opposition to the biasing potential derived from the self-bias resistor IIE, as the ccntact |33 is moved toward the positive terminal I 4I.

In other words, as the control contact |33 is moved toward the contact I4I, the control grid I 23 becomes less negative, or if the selectivity control voltage between the terminals I35 is increased the control grid I23 may be said to become more positive and, in either event, the arrangement is such that the tuning response characteristic of the tuned circuit, across which the space current path of the device IOS is connected and to which the control grid is coupled, becomes broader thereby decreasing the selectivity of the receiving circuit.

In this system the selectivity is greatest when the impedance across the tuned circuit is the highest. Therefore, the circuit becomes broad in its response as the control voltage increases in a positive direction. The amplifying property of the tube is utilized in the same manner as described in Fig. 1 to provide a further control effect, whereby the effective resistance across the tuned circuit may be reduced substantially below 1,000 ohms.

In order that the selectivity control tube |89 may be biased to cut-ofi with no signal, the terminal |43 of the control device ISI is connected to a negative supply lead |31 having a negative potential with respect to the cathode II 3 of approximately l5 volts for a device of the character shown.

With no signal the selectivity is highest since the loading of the tuned circuit is substantially zero.

In this circuit as in the circuit of Fig. l, signal voltages appearing across the tuned circuit BI-QS-SE will be impressed upon the control grid I23 of the selectivity control tube |09 and, because of the amplifying characteristic thereof, relatively high voltage, out of phase with the impressed signal voltage, will appear in the plate circuit of the tube and will be opposite the original voltage in effect to said resistance to the tuned circuit to broaden it. This circuit, therefore, differs from the circuit of Fig. 1 in that the selectivity is highest when the impedance across the primary 4I of the transformer 2l is lowest, whereas in the present circuit the selectivity is higher when the impedance across the secondary SI is highest.

I claim as my invention:

1. In a radio receiving circuit, the combination with a tuned high frequency transformer, of a pentode electric discharge device having its anode-cathode impedance connected effectively in shunt relation to a tuned signal circuit including said transformer, means for coupling the control grid of said device to the high potential side of said circuit to receive signal energy therefrom, and means for applying a control voltage to said control grid to adjust the mutual con-- ductance of said device and the selectivity of said circuit.

2. In a radio receiving system, the combination with a tuned high frequency signal circuit comprising an inductance and a shunt condenser and a coupling winding associated with said inductance, of a pentode electric discharge device having an anode connected directly with said tuned circuit and in series with the inductance thereof for conveying anode current thereto, and having a cathode connected with the opposite side of said circuit, and said device having a control grid associated with the cathode and a screen grid interposed between said control grid and the anode, means providing a series vrelatively high resistance in circuit between the grid and cathode, means for coupling said control grid to the high potential side of said tuned circuit comprising a condenser of relatively low reactance with respect to said resistance in series with the grid, and means for supplying a controlling potential through said resistance to the control grid.

3. In a radio receiving system, the combination with a tuned high frequency signal circuit comprising an inductance and a shunt condenser and a coupling winding associated with said inductance, of a pentodc electric discharge device having an anode connected directly with said tuned circuit and in series with the inductance thereof for conveying anode current thereto, and having a cathode connected with the opposite side of said circuit, and said device having a control grid associated with the cathode and a screen grid interposed between said control grid and the anode, means providing a series relatively high resistance in circuit between said grid and cathode, means for coupling said control grid to the high potential side of said tuned circuit comprising a condenser of relatively low reactance with respect to said resistance in series with the grid, and means for supplying a controlling potential through said resistance to the control grid comprising a source of potential, and variable control means therefor, for varying the control grid potential in a positive direction, thereby to broaden the tuning response characteristic of said tuned circuit.

4. In a radio frequency amplifier, a tuned high frequency circuit, an electric discharge amplifier device having a control grid connected therewith for receiving signals therefrom, an electric discharge device having an anode connected to the high potential side of said tuned circuit and a cathode connected with the low potential side thereof, the anode circuit of said device being completed through an inductance element in said tuned circuit, and said device further having a control grid coupled to the high potential side of said tuned circuit and connected conductively with a source of biasing potential, and means inserted in circuit therewith for varying the biasing potential applied to said control grid whereby the mutual conductance of said device is varied inresponse thereto, thereby to vary the loading applied to said tuned circuit in proportion to the strength of received signals and in proportion to the adjusted mutual conductance of said device.

5. In an intermediate frequency amplifier for a superheterodyne receiver, an intermediate frequency amplifier coupling transformer having a tuned primary winding, a pair of electric discharge amplifier devices each having a control grid, and an anode circuit completed through said primary winding, means for applying intermediate frequency signals to the control grid of one of said devices comprising a separate intermediate frequency input circuit, means for applying to the control grid of the other of said tubes a variable biasing potential, thereby to adjust the mutual conductance thereof, the range of variation being such that the Variation range is extended to a value whereby the tuned circuity is loaded with a resistance below `1,000 ohms in value, and means including a coupling condenser for coupling the last named control grid to said tuned primary winding.

6. In a radio receiving system, the combination with a tuned signal circuit comprising a winding of a coupling transformer, an electric discharge ampliiier device connected with said winding for the transmission of signals therethrough, a second electric discharge device having an anode circuit including said winding in common with said tuned circuit, a cathode, and a suppressor grid', a screen grid and a control grid in the order named between the anode and said cathode, means for coupling said control grid to the tuned circuit', said means including a condenser of relatively low capacity connected between the high potential side of said tuned circuit and said grid, and a grid leak resistor connected to the grid, the reactance of the condenser being less than a low percentage of the resistance of the grid leak resistor, means for applying a xed negative biasing potential to the control grid, and means for applying a control voltage to said grid which varies with variations in signal strength, the direction of variation being such that the mutual conductance of the last named device is varied through a range providing a relatively wide change in the effective plate impedance of said device whereby the selectivity of the tuned circuit is varied between wide limits.

7. In a radio receiving system, the combination with a tuned signal circuit comprising a winding of a coupling transformer, an electric discharge amplifier device connected with said winding for the transmission of signals therethrough, a second electric discharge device'having an anode circuit including said winding in common with said tuned circuit, a cathode, and a suppressor grid, a screen grid and a control grid in the order named between the anode and said cathode, means for coupling said control grid to the tuned circuit, means for applying potential to the*` control grid, and means for applying a control voltage to said grid which varies with variations in signal strength, the direction of variation being such that the mutual conductance of the last named device is varied through a range providing a relatively Wide change in the effective lplate impedance of said device whereby the selectivity of the tuned circuit-is varied between wide limits. Y

8. In a radio receiving system, a tunable signal circuit including a transformer winding, an electric discharge device of the pentode type having its plate-cathode impedance connected in shunt relation to said circuit, means for varying the control grid potential of said device, means providing a connection whereby the anode current of said device flows through said winding, and means providing a signal coupling connection for the control grid of said device with the high potential side of said circuit whereby the selectivity of said circuit may be varied through a wide control range by variation of said grid potential, and means providing variable potential for controlling said grid.

9. The combination with a radio signal amplifier including an electric discharge amplifier device having a tuned output circuit, of means for controlling the selectivity of said system including a second electric discharge amplifier device having an anode electrode connected with said tuned output circuit in parallel with said first named device and having a control grid coupled to said output circuit through capacity coupling means, a grid circuit for said grid having an impedance higher than the reactance value of said coupling means, and means for applying to said grid circuit a controlling potential for varying the anode-cathode impedance of said last named device, and said last named device being further provided with a screen grid between said anode and control grid.

10. In a radio signal amplifier, the combination with a tuned signal transmission circuit, of a pentode electricV discharge device having its anode-cathode impedance connected substantially in shunt across said circuit as loading means therefor, means providing a high impedance grid circuit for said device, and means providing capacity coupling between said grid circuit and said signal transmission circuit at the anode side thereof, said capacity coupling means providing an impedance lower in resistance value than the resistance of said grid circuit.

l1. In a Vradio signal amplier, the combination with a tuned signal transmission circuit, of a selectivity control tube of the screen-grid pentode type having the anode-cathode space path thereof connected substantially across said circuit, said device having a control grid coupled to the high potential side of said tuned circuit, a coupling condenser of relatively low impedance providing said coupling, means including a high impedance grid circuit for said control grid, and means for applying to said control grid through said gridk circuit a biasing potential responsive to variations in signal strength.

12. In a radio signal amplifier, the combination with a tuned signal transmission circuit of a selectivity control tube of the screen-gridY pentode type having the anode-cathode space path thereof connected substantially across said circuit, said device having a control grid coupled to the high potential side of said tuned circuit, a coupling condenser of relatively low impedance providing said coupling, means including a high impedance grid circuit for said control grid, and means for applying to said control grid through said grid circuit a biasing potential of a value to cause the anode-cathode impedance of said device to fall below 1,000 ohms.

WINFIEID R. KOCH.

Cil

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