US2105518A - Selectivity control system - Google Patents

Description

' Jam. 18, 1938. G. l.. BEERS SELECTIVITY CONTROL SYSTEM Filed July 31, 1955 NNN.

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Q Tb z/vE/v'rofr GeorgeL.Beer6 ATTORNEY Patented `ian. 18, 1938 NTE STATES PATENT GFFICE SELECTIVITY CONTROL SYSTEM of Delaware Application July 31, 1935, Serial No. 34,004

3 Claims.

. The present invention relates to means for controlling the selectivity of radio receiving systems. More particularly, it relates to a selectivity control system particularly adapted for superhetero- 5 A; dyne receivers.

In accordance with the invention, a combination of inductive and capacitive coupling may be employed between two tuned signal circuits in a receiver, in conjunction with suitable switch- 101 ing means, whereby the capacitive coupling may be made to aid the inductive coupling thereby to adjust the selectivity and/or the fidelity characterstic of the receiver. In addition, the switchmg arrangement may include control of the audio -1 frequency tone characteristic and of the selectivity of certain other high frequency signal circuits.

It is also a further object of the invention to provide an improved selectivity-fidelity control system which may be applied to the usual coupling circuits in an intermediate or high frequency amplifier and which may be controlled by simple switching means.

With a control system of this character, a re- 2"5; ceiving system may be operated with a high degree of selectivity for the reception of programs from distant broadcasting stationsin adjacent frequency channels to local stations without appreciable interference, while for local stations broadcasting high fidelity programs,'the fidelity of the receiver may be increased by decreasing the selectivity. In the system of the present invention this change is effected by introducing capacity coupling into inductively coupled tuned S. circuits without appreciably changing the tuning thereof, the tuned circuits being preferably in the intermediate frequency amplifier.

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

In the drawing, the figure is a schematic circuit diagram of a portion of a s-uperheterodyne LSireceiver provided with a selectivity control system embodying the invention.

Referring to the drawing, the receiving system is provided with a radio frequency amplifier stage comprising a high frequency amplifier tube 5 suitably coupled to a signal source, such as an antenna l, through a coupling transformer 9. The secondary I I of the transformer is tuned by a variable capacitor I3 connected in shunt therewith through a series resistor I5 and a fixed byfpass capacitor I'I, the latter being of relatively (Cl. Z50-20) high capacity with respect to the tuning capacitor I3. The series capacitor has no appreciable effect upon the tuning of the circuit. The latter is designated at I9, and comprises the secondary I I, the capacitor I3 and the resistor I5.

The tuned circuit I9 is grounded at its low potential terminal 20 between the capacitor I3 and the resistor I5, as indicated at 2I, and the cathode 23 of the amplifier device 5 is also grounded through a self-bias resistor 25 and a 10 shunt by-pass capacitor 21 as indicated at 29. The control grid 3| is connected with the high potential terminal 33 of the tuned circuit I9 between the capacitor I3 and the winding II.

The circuit I9 therefore provides a tuned input 15 circuit for the high frequency amplifier 5 and includes in series therewith adjacent to the low potential terminal a resistor I5 for broadening the tuning of the circuit under certain conditons hereinafter described. The by-pass capacitor Ii permits biasing potential to be introduced into the circuit for the control grid 3I through a supply lead 35 which includes therein a filter resistor 3l. The control grid may, therefore, be independently biased while the tuned circuit is 25 grounded as indicated.

The high frequency amplifier 5 is preferably of the variable-mu screen grid pentode type as indicated, and is coupled through a suitable radio frequency coupling transformer 39 with the fol- 30 lowing tube 4I in the signal channel which, in the present example, is the rst detector. This is also of the screen grid pentode type, comprising a cathode 43, a control grid 45, a screen grid lil, a suppressor grid 49 and an output anode 5I. 35

The detector is also provided with a tuned input circuit 5S comprising the secondary winding 53 of the interstage coupling transformer 39, a shunt variable tuning capacitor 55 and a series resistor 5l corresponding to resistor I 5 in the pre- 40 ceding high frequency circuit for broadening the tuning of the circuit. The low potential terminal 6I of the tuned circuit 59 is grounded as indicated at 63 and the high potential terminal indicated at 65 is connected with the control 45 grid 45.

The cathode 43 is also connected to ground indicated at S'I through a cathode lead 69 in which is located a series resistor 'I I provided with a high frequency by-pass capacitor 'I3 and the 50 secondary 15 of a high frequency coupling transformer l?. 'Ihe primary '.19 of the coupling transformer is tuned by a shunt variable capacitor 8I and connected to the output terminals 83 of a suitable high frequency oscillator 85, the fre- 55 quency of the oscillator being controlled by the variable capacitor 8| or any suitable means to produce with the incoming signal a beat or intermediate frequency.

It will be seen that by this circuit arrangement, the oscillator signals are introduced into the cathode lead 59 through the coupling transformer il and are thus supplied between grid and cathode of the detector 4| along with the high frequency signals from the tuned circuit 53. The control of the resistor 5l will hereinafter be described.

The detector il is coupled to the'frst intermediate frequency amplier tube 87 through two inductively coupled tuned circuits 89 and Elprovided with means for supplementing the inductive coupling by capacitive coupling without changing the tuning adjustmenthof either of the circuits.

The tuned 'circuit 89 includes, as an inductance element, the primary winding 93 of an intermediate frequency amplifier coupling transformer 95 and is connected with the output anode 5| of the detector as a tuned output circuit therefor. The anode issupplied with anode operating potential from a supply lead 9i) through a suitable filter comprising a series resistor and a bypass capacitor |03 connected with the low potential terminal lil'of the primary winding 93.

The capacity tuning element of the circuit 89 is provided by an adjustable shunt tuning capacitor lul connected with the high potential side of the circuit Sl and of the primary winding 93 and with the low potential terminal |05 of the winding S3 through a lead |39 in which is located the circuit 89. rihe capacitor |91, therefore, provides the controlling element in the tuned circuit 89. c

The `switch means is provided with an additional contact Iii?) to which the arm may al- 'ternatively be connected to complete the tuned circuit between the winding 93 and the capacitor il', through a lead l2 a series resistor |23 and a second series fixed capacitor |25, of the same relatively large capacity as the capacitor to the same low potential terminal |05 of the primary winding 93.

The second series fixed capacitor |25 is also included in the tuned circuit 9| which comprises the secondary winding |21 of the intermediate frequency coupling transformer 95 and a shunt adjustable tuning capacitor |29, the capacitor 29 being therefore connected in shunt relation to the secondary winding |21 through the series capacitor |25.

Intermediate frequency signals developed across the tuned circuit 9| are applied to the control grid i3! of the intermediate frequency amplifier device '8l from a high potential terminal |33 through a suitable coupling capacitor The amplifier stage 8'! is provided with ground return connections |39 and is provided with a suitable tuned output coupling transformer i3? for further stages (not shown). The

ycoupled tuned circuitQI, therefore, isjconnected .to ground from the terminal |05 through the by-pass capacitor m3 whereby intermediate frequency signals are applied across the input elec-- supply lead |43 through a suitable filter |45 and grid leak resistor M1. The resistor |41 is of relatively high impedance with respect to the impedance of the tuned circuit 9 i, whereby the latter is not appreciably loaded, thereby since it is connected substantially in shunt to the tuned circuit through the coupling capacitor |35 and the capacitor element his of the lter |45.

With the coupling arrangement shown between the rst detector and the intermediate frequency amplifier, the coupling is controlled by the switch i-HS-HQ which may be termed the selectivity control switch. When the arm is connected with the contact i3, the tuning is sharp and the selectivity of the coupling means is maximum. Under this condition, the primary tuned circuit SQ, which includes the tuning capacitor lill, is completed through the relatively large fixed capacitor The seco-ndary tuned circuit 9| which includes thetuning capacitor |29 is completed through the fixed capacitor |25 which has the same capacity as the capacitor The coupling between the two tuned coupling circuits 89 and 9| with the selective control switch in the position as shown for sharp tuning and maximum selectivity, is obtained through the mutualiinductance between two transformer windings 83 and l2? which is adjusted to give a desired high degree of selectivity.

When the switch is thrown to the broad position, the arm iii making contact at H9, the primary tuned circuit is completed from the contact I9 through the resistor Q23 and the capacitor |25. Under this condition the capacitorA |25 is common to both tuned circuits and its capacity is so chosen that the coupling between the two tuned circuits, because this capacitive reactance is common to both tuned circuits, is of the correct value to give a frequency response characteristic of the desired band width.

Furthermore, by this circuit arrangement, the tuning of the coupled circuits is not changed as the selectivity is changed. The capacitive coupling provided by the capacitor l predominates to give a wide band width of frequency response of the receiving system and of the intermediate frequency amplifier. The hand width is changed merely by changing the coupling whereby one type predominates, without changing the tuning of the coupled circuits.

The series resistor |23 is provided to prevent the transformer characteristic from becoming double peaked when the selectivity control switch is thrown to the broad position and, in the present example, vmay have a resistance value of substantially 4Q ohms.

The tuning'capacitors |2 and l'l may have a capacity of 400 micro-microfarads, while the series capacitors 25 and il' may have a capacity of 6890 micro-microfarads for an intermediate frequency of 370 k. c.

The selectivity control switch is arranged to be Operated simultaneously with selector switch means for short-circuiting the resistors l5 and 5l in the high frequency tuned input circuit I9 and 5a when the selectivity control switch is in the position as shown for sharp response or maX- imum selectivity. rihe switch means for each f the resistors i5 and El comprises switch arms and |53 connected respectively through leads |55 and |521, with the ungrounded terminals |59 and |6| of the resistors l5 and 5l respectively. Each switch is provided with a grounding contact |63 connected to ground as indicated at |55,

.As shown in the drawing, the switches are closed to the grounding contacts IS3 thereby short-circuiting the resistors I5 and 5l to ground and rendering the tned high frequency circuits I9 and 59 more selective and providing a sharp selectivity characteristic corresponding to' the sharp selectivity characteristic provided by the circuits 85 and SI` when the selectivity control switch is in the position shown.

The switches are preferably interconnected, as indicated by the dotted connection I El, for si'- multaneous control through a common control knob indicated at |69. It will be seen that with this arrangement when the selectivity control switch is moved to close the arm EI! with the broad selectivity control contact l I9, the resistors I5 and 51 are, automatically at the same time, inserted in circuit with the tuned circuits I9 and 59 thereby broadening the tuning to permit an overall high delity of response in the receiving system.

It is preferable also to control the response characteristic of the audio frequency system of the receiver, which is represented by audio frequency input leads i'II connected with an audio frequency coupling transformer I'l' and, in turn, coupled through capacitors H5 on its secondary side with a suitable audio frequency amplifier represented by the rectangle Ill. The amplifier may in turn supply output signals to any suitable sound producing device such as a loudspeaker I'IQ.

To control the tone of the audio frequency signals, for example, by reducing the high frequency response when the receiver is adjusted for a sharp response or a high degree of selectivity, a capacitor IBI may be shunted across the audio frequency signal circuit I'II through selector switch means comprising a switch arm I83 and a switch contact |85, the switch means being mechanically connected with the other switches to be operated simultaneously therewith.

As shown, the switch I83-I35 is closed to connect the capacitor I8I across the audio frequency circuit III thereby to reduce the high frequency response with the control system adjusted for maximum selectivity and sharp tuning. This is desirable in order to minimize high frequency noises which are received when distant stations may be tuned in. The audio frequency response range may be increased when the receiver is adjusted for high fidelity, the switch I83--I85 being opened as the selectivity control switch arm Ill is moved to the contact IIS.

In order to prevent noise from the loudspeaker device I 'I9 during the switching operation, the audio frequency system may be controlled as by short circuiting the secondary I8? of the transformer I'l3 through a ground connection ISS and an elongated switch contact I 9| over which a grounded contact arm |93 moves while the control switches are in intermediate positions between ccntacts. The short circuit is removed at either end of the travel of the switch arm by connection with ungrounded contacts Ii5 corresponding in position to the switch contacts in the other switch elements. The tone control and audio frequency short-circuiting switch means are preferably connected through the means I6'I for simultaneous control with the selectivity control switch.

With this switching arrangement it is preferable that the audio frequency coupling transformer be isolated from the audio frequency ampliiier by the coupling capacitors H5. In this manner the switching contact arm E93 and one side of the transformer secondary may be interconnected through ground.

From the foregoing description, it will be seen that the improved selectivity control systemfis provided by simple switching means in connection with a tuned interstage coupling transformer. In the present example, the primary and secondary low potential terminals are connected together and with two series capacitors of equal value, one of which is in the secondary tuned circuit and the other of which is in the primary tuned circuit, with switching means for selectivity connecting one tuned circuit through either of said capacitors.

The switching means further includes contacts for controlling the selectivity in certain of the high frequency tuned circuits and the audio frequency tone characteristic of the system.

I claim as my invention:

1. In a superheterodyne radio receiving system comprising a, radio frequency amplier, a first detector, an intermediate frequency amplifier and an audio frequency amplier circuit, the combination of means connected with the radio frequency amplifier and rst detector for broadening the frequency response characteristic thereof, a coupling transformer for said intermediate frequency amplier comprising a primary winding and a secondary winding inductively coupled to provide a relatively high degree of selectivity, a shunt tuning capacitor for each of said windings, a capacitor of relatively high capacity value with respect to the secondary tuning capacitor in series therewith, a selector switch, a capacitor having a capacity value equal to that of the secondary series capacitor connected in series with the primary winding and the tuning capacitor therefor through one contact of said switch, said switch having a second contact providing a series connection between said primary winding and the tuning capacitor therefor through said secondary series capacitor whereby said circuits are capacitively and inductively coupled for decreasing the selectivity, tone control means for said audio frequency amplifier circuit, and means providing a connection between said tone control means, said selector switch and the first named selectivity control means for simultaneous operation thereof.

2. In an intermediate frequency amplifier, the combination with an inter-stage transformer having a primary winding and a secondary winding, of a tuning capacitor connected with a terminal of the primary winding, a second tuning capacitor connected with a terminal of the secondary winding, a conductive connection between the remaining terminals of the primary and secondary windings, a series coupling capacitor connected between said conductive connection and each of said tuning capacitors to provide primary and secondary circuits, selective switching means for connecting the primary tuning capacitor with said conductive connection through the secondary series coupling capacitor to the exclusion of the series coupling capacitor for the primary circuit, and means in said last named connection for inserting resistance in the primary circuit, thereby to prevent the response characteristic of the transformer from becoming double-peaked.

3. In an intermediate frequency amplifier, the cor'nbination with an inter-stage transformer having a primary winding and a secondary winding, of a tuning capacitor connected with a terminal of the primary winding, a second tuning capacitor connected with a terminal of the secondary winding, a conductive connection between the remaining terminals of the primary and secondary windings, a series coupling capacitor connected between said conductive connection and each of said tuning capacitors to pro- Vide primary and secondary circuits, selective switching means for connecting the primary tuning capacitor with said conductive connection 10 through the secondary series coupling capacitor

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