US2127816A - Multichannel radio receiving system - Google Patents

Description

Aug. 23, 1958. P. F. G. HOLST ET AL 2,127,815

MULTICHANNEL RADIO -RECEIVING SYSTEM Filed Aug. 29, 1936 o MAW m 30K G GK E@ L WM PM N .Jill Q1. rmmwm o@ SS \Nm\ m Pw m. l KNQ I.

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UNITED Patented Aug. 23, 1938 TES PATENT OFFFIQE Collingswood, N. J., assignors to Radio Corporatonlof America, a corporation of Delaware i Application August 29,

l1 Claims.

The present invention relates to radio receiving systems having two or more receiving channels which may provide a combined output signal as shown in our copending application Serial No. 98,443, led August 29, 1936, for I-Iigh fidelity radio receivers, and assigned to the same assignee as this application.

A receiver to which the present invention may be applied may comprise a pair of signal receiving 110 and amplifying channels or circuits, one of which is broadly responsive to received signals and the other of which is sharply responsive thereto, said channels being further controllable to render the broad channel effective only in response to signals of a predetermined high amplitude whereas the sharp channel is utilized for weaker signals.

In this manner, the fidelity or overall response of the receiving system improves with increased carrier wave strength, giving a relatively high degree of fidelity for local radio programs or in response to relatively strong modulated carrier waves. 'Ihe signal output from a plurality of amplifying channels is preferably applied to a common audio frequency amplifier and loud speaker device for the reason that a single amplifying channel results in desirable cost reduction and for the further reason that the sound from both channels preferably is derived from a common loudspeaker output device without switching connections.

It is therefore an object of the present inven tion to provide an improved audio frequency or second detector and audio frequency amplifier system for a multiple channel radio signal receiving system wherein the signals from either channel may be combined in a common audio frequency amplifier and applied to a single loud speaker device.

It is a further object of the present invention 4:0 to provide an audio frequency circuit for combining the signal output from a plurality of signal channels in a radio receiving system wherein either or both signal amplifying channels may r function to amplify signals, and the output therefrom, separately or in combination, may be applied to a common audio frequency amplifier and loud speaker or other output device, without the intermediary of switching means.

50 It is also an object of the present invention, to

1936, serial No. 98,444

connection with a single audio frequency amplier channel therefor.

The invention will be better understood upon reference to the accompanyingdrawing` wherein a schematic circuit diagram of a radio receiving system embodying the invention `is shown in Fig. 1, and connections for the circuit of Fig. 1 with a high gain amplifier are shown in a similar circuit diagram comprising Fig. y2. The scope of the inventionwill be pointed out in the appended claims.

Referring to the drawing, the figure is a schematic circuit diagram of a superheterodyne radio receiving system embodying the invention.. This comprises two signal channels operating at radio or intermediate frequency, or both as shown, with one receiving and amplifying chan nel sharply tuned and the other broadly tuned as indicated. In -a superheterodyne receiver, each channel may comprise a high or radio frequency amplifying and tuning unit '5, a first detector 6 and an intermediate frequency amplifier l, connected to the same or separate signal'sources as indicated at 8 and 9 and tunable to the same or differing frequencies.

In the present example, the superheterodyne system indicated, includes also an oscillator I0 in association with the first detector of each channel and the signal channelsfurthermore, preferably are provided with suitable automatic volume control means II and I2 for controlling the signal output in accordance with the strength of a received carrier wave, inany suitable well known and understood manner. In the present system, the automatic volume control means may be effective to provide substantially constant signal amplitude with carrier wave strength variations at the output of the intermediate frequency amplifiers, whereby the second detectors are provided with substantially constant signal strength.

Each of the separate signal amplifying channels is terminated in a second detector, the sharp channel detector being indicated at I4 and the broad channel detector being indicated at I5` Both detectors may be of the same type, such as diode rectifier devices, having cathodes It and anodes II, and are supplied with intermediate frequency signal energy through suitable coupling transformers I8 preferably of the iron core fixed-tuned type.

The primary winding I9 of each transformer is connected with the output circuit 'ZG of the preceding intermediate frequency amplifier, while the secondary 2 I, together with a shunt fixed tuning capacitor 22 provides a tuned input circuit for the rectifier in which circuit the high potential terminal 23 is connected with the anode. The low potential terminal 24 of the input circuit for the detector I4 is connected to the negative terminal 25 of a diode output resistor 26, the positive terminal 2l of which is connected to the cathode I6 and to ground as indicated at 28. Signals received through the diode input circuit from the preceding receiving system in the sharp channel are applied to the diode anode II and are rectif-led in the detector I4 and then appear as a direct current potential across the resistor 25 between the positive terminal 2l and the negative terminal 25, the terminal 25 being negative with respect to ground or chassis 2B. The audio frequency envelope or modulation component of the signal also appears across the output resistor 26 between ground and the terminal 25. A suitable intermediate frequency by-pass capacitor 36 is provided across the resistor 26 to eliminate therefrom any intermediate frequency currents which may be present.

In a similar manner, the broad channel detector I5 is provided with a diode output resistor 3| having the positive terminal thereof 32 con-- nected to ground or chassis 33 and to the cathode I6, while the negative terminal indicated at 34, is connected with the loW potential terminal 35 of the tuned diode input circuit.

Signals received through the broad channel and amplified therein, are applied to the detector I5 and appear as a direct current potential across the resistor 3| to establish on the terminal 34 a negative potential with respect to ground, and variable in accordance with the signal strength, while the modulation or audio frequency component also appears across the resistor BI between ground and the terminal 34.

The diode detector arrangement, therefore, comprises two diode rectifier devices, one in each channel, each having a cathode connected to chassis or ground and having a diode output resistor connected between ground and the low potential terminal of the tuned input circuit for the diode rectifier.

The terminals 25 and 34 are negative terminals and may be considered as the output terminals for the two rectier devices and for the two signal channels from which may be conveyed the audio frequency or modulation component of the received signals.

Each of these terminals is coupled to a common audio frequency amplifier comprising an input amplifier stage having an amplifier device 3B comprising a cathode 39 connected to ground 40, a control grid 4I connected to an input lead 42, and an output anode connected with an output circuit 46 leading to the remainder of the audio frequency amplifying system represented by the rectangle enclosure 4l. This is connected to an output loud speaker device indicated at 48.

The audio frequency output coupling to the circuit 46 includes an output coupling reactor 0.9 and iilter resistor 50 provided with a by-pass capacitor 5I for supplying to the anode 45 the operating potential as indicated from any suitable source.

The input lead 42 for the single audio frequency amplifier and the output terminals 25 and 35 are interconnected through a resistor network com prising a pair of resistors 53 and 54 connected in series between the output terminal of one diode rectifier device and the output terminal of the other diode rectifier device through output leads indicated at and 56. The lead 42 is connected to an intermediate tap 51 between the two series connected resistors.

Since the cathode 39 of the first audio frequency amplifier 38 is connected to ground and therefore to the positive terminals of both diode rectifier output circuits, the grid 4I is subjected to and receives a portion of both the biasing and modulation potentials from both signal channels, and one or the other of the signal channels may predominate in supplying signals depending upon the operation of their respective automatic volume control means I I and I2, without the intermediary of any switching means. If both channels are operating it will be seen that the output of each diode is impressed upon the rst amplifier grid 4I.

In order however to keep the inter-channel coupling through the resistor output network as low as possible the resistance of the resistor 53 should be relatively large or high with respect to the resistance of the diode output resistor 26 and similarly the resistance of the resistor section 54 should be relatively high with respect to the resistance of the diode output resistor 3I. Assuming the resistances of the diode output circuits to be R1 and the resistance of each section of the resistor elements 53 and 54 to be R2 then the ratio B2 Ri should be maintained as large as practically possible in any receiving circuit. The limit of this ratio is generally determined by the resistance of the diode output circuit which may be 100,000 ohms, for example, and the resistors 53 and 54 may then have a resistance preferably of 2 megohms, for example.

The circuit has the further advantage that either channel may be cut off by simple switching means comprising a two point switch having a switch arm and two contacts 6I and 62. The switching arrangement is effected by connecting the arm 60 to ground or chassis as indicated at 63, and by connecting each contact 6I and 62 to the respective negative output terminals for the diode circuits and in this case with the output leads 56 and 55 respectively at the input terminals of the resistors 53 and 54.

By connecting the arm 60 to the terminal 6I, for example, it will be seen that the diode output resistor 3I is short-circuited and grounded while the input end of the resistor 53 remains connected to the terminal 25. If the resistor sections 53 and 54 are equal as is the usual case where aqual output is desired from both receiving channels, the grid connection 42 is then effectively tapped down halfway on the whole output resistor comprising both sections 53 and 54 which is then effectively connected across the output resistor 26. In this manner the audio frequency channel receives only the signals from the sharp channel.

In a similar manner, by connecting the arm 60 to the contact 62 the sharp channel may be cut off and the output from the broad channel may then be taken through the lead 42 at a voltage tap provided by the terminal 51. The portion of the total output voltage available is determined, as before, by the relation between the resistances of the two sections 53 and 54 which are preferably equal, although not necessarily so.

For example if it is desired to receive a signal voltage from the sharp channel in a greater proportion with respect to that from :thebroad channel, the tap 51 may be connected electrically more adjacent to the output terminal 25 by reducing Athe value of the resistor 53. This may be done in case that the broad channel is responsive to relatively strong signals and the same output voltage is desired for both the -broad and sharp channel without adjusting the automatic volume control means for this purpose.

The operation of the system described is as follows: With the switch B,6|-62 open, signals received through the sharp channel and through the broad channel are rectified in the separate detectors |4 and l5 and both the audio frequency and direct current components thereof appear between ground and theA leads..55 and 55 connected with the output terminals 25 and 34 respectively. -Since the cathode `39 is connected to ,ground 40 and the grid 4| of therst audio frequency amplifier is connected to the `tap 57|, the audio frequency and signal voltages appearing in the leads 55 and 56 with respect to ground are applied between the grid 4| and the `cathod 39.

This results in an `increasing signal bias for the grid 4| as the signal strength increases and the tube 38 may then operate without overloading. The combined signals are applied to the output circuit 46 and appear at the loud speaker 48. If both signal channels are tuned to the same carrier wave and the automatic volume control l2 is effective to cut off the signals from the broad channel until a predetermined higher signal amplitude is obtained, then the sharp channel alone or to a greater degree supplies the signals to the loud speaker 43. However as the broad channel becomeseffective to supply signals the .same are applied through the coupling network to the grid 4| without switching change and without intercoupling between the two channels, by reason of the relation of the impedances or resistance of the output resistors 26 and 3| to the coupling resistors 53 and 54. Any suitable arrangement may be provided for controlling the two signal channels and any suitable automatic volume control meansmay be provided for operating the two channels in any suitable relation to eachother, since the mixing of the signals and the provision of biasing potential for the audio frequency amplifier is automatically obtained without switching connections. It should be understood that the switch 6B is provided merely for cutting ofi` one or the other of the channels as may be desired, and does not interfere with normal operation when open.

With this arrangement, for example, the two channels may be turned to separate signals, such as a broadcast program on the broad channel, for example, and with a sharp channel tuned to police or S. O. S. signals which will be applied automatically to the loud speaker 48 when received, without further attention from the listener. Also, the switch 6U may be operated to listen selectively to either channel.

This provides a desirable arrangement in police or aircraft receivers, for example, where the two channels are tuned to differing transmitting stations and both may be heard or either one may be selected by the simple switching means in the audio frequency mixing circuit. In this case, a noise suppresserin each channel is desirable, to cut ofi either of the channels when not receiving a carrier wave.

In certain applications it may be desirable to connect the receiving channels with a high gain audio frequency amplifier tube, as shown in Figure 2, to which attention is now directed and in which the same reference numerals are used for like parts as in Figure 1.

Referring to Figure 2, a. high gain audio frequency amplifier tube of the `pentode type, is provided with a control grid 66 connected with a movable contact 61 of a volume control potentiometer 68. The grid circuit is completed through the potentiometer 68 and a ground or chassis connection 69, and biasing potential is derived from a self-bias resistor 10, as shown. The output anode of the amplifier, indicated at 1|, is connected to a suitable output circuit 12 through a coupling capacitor 13, in conjunction with a coupling resistor 14 in the anode circuit. Positive B potential for the anode is supplied to the coupling resistor 14 through a filter resistor vl5, as indicated.

The input grid circuit, comprising the resistor element, is coupled to the output lead 42 from the output terminal 51, between the output resistors 53 and 451|, through a coupling capacitor 81 which serves to transfer the audio frequency signal to the amplifier grid circuit while isolating the two circuits against .direct current potential.

With this arrangement, the amplifier tube 65 may be self-biased lor biased in any suitable manner yindependently of the signal potential, and the potentiometer E'l-GB serves to providev volume control means directly for the audio frequency circuit, following the signal combining network in connection with the two detectors.

By this means, the signal volume may be adjusted to `any desired level for both channels jointly. With the signal channels controlled by suitable automatic volume control and noise suppression means, the volume control device 61--68, following the channel mixing network, may provide the only volume control means for the receiving system.

While the invention has been described in con.- nection Witha superheterodynereceiving system, it is `apparent that it may be applied to any receiving system having two signal receiving channels suitably controlledby automatic volume control and/or noise suppression means, with the channels terminating in suitable diode rectifier devices and followed `by a single audio frequency amplifier channel.

We claim -as our invention:

1. In a multiple signal .channel radio receiving system, the combination of a pair of diode signal rectifier devices each having a cathode, a rectified signal output terminal and an output impedance connected vbetween said cathode 4and said terminal, a pair of direct current conducting output impedance elements connected in series between said terminals, an audio frequency amplifier device having a cathode and having a control grid conductively connected between said impedance elements, and means interconnecting the `cathodes of said rectifier devices and the cathode of said amplifier device, whereby signals applied to said rectifier devices are rectified and jointly applied with the direct current component thereof to said control grid.

2. In a multiple signal channel radioxreceiving system, the combination of a pair of vdiode sig-,nal rectifier devices each having a cathode and `a rectified signal output terminal, a pair of output resistance'elements connected in series between said terminals, an audio frequency ampliiier'ldevice having a .cathode `and having a control grid connected between said resistance elements, and means interconnecting the cathodes of said rectifier devices and the cathode of said amplifier device, whereby signals applied to said rectifier devices are jointly applied to said control grid therefrom, said resistance elements having a relatively high resistance each with respect to the rectifier output circuits, whereby said signal channels are effectively decoupled while being interconnected with said control grid.

3. In a radio receiving system having a pair of signal receiving and amplifying channels, the combination of a pair of diode rectifier devices, one coupled to each of said channels to receive and to demodulate the signal output therefrom, automatic volume control means for each of said channels effective to control the amplitude of signals applied to said diode rectifier devices, each of said diode rectifier devices comprising a cathode connected to ground and having a diode output resistor connected at its positive terminal to said cathode and ground, a pair of signal and biasing potential output leads, one from a negative terminal of each of said output resistors, a pair of series connected resistor elements each having a relatively high resistance with respect to each of said diode output resistors connected between said output leads, and a common audio frequency amplifier for said channels having an input amplifier tube provided with a cathode connected to ground and a control grid connected to an intermediate tap point between said series connected resistor elements whereby the signal output and bias potential from said signal channels are applied jointly to said amplifier device,

4. In a radio receiving system having a pair of signal receiving and amplifying channels, the combination of a pair of diode rectifier devices one coupled to each of said channels to receive and to demodulate the signal output therefrom, automatic volume control means for each of said channels effective to control the amplitude of signals applied to said diode rectifier devices, each of said diode rectifier devices comprising a cathode connected to ground and having a diode output resistor connected at its positive terminal to said cathode and ground, a pair of signal and biasing potential output leads one from a negative terminal of each of said output resistors, a pair of series connected resistor elements each having a relatively high resistance with respect to each of said diode output resistors connected between said output leads, a common audio frequency amplifier for said channels having an input amplifier tube provided with a cathode connected to ground and a control grid connected to an intermediate tap point between said series connected resistor elements whereby the signal output and bias potential from said signal channels are applied jointly to said amplifier device, and means for selectively grounding either of said output leads thereby to render ineffective either of said signal amplifying channels selectively.

5. In a radio receiving system, the combination with means providing two signal amplifying channels therein, of means for combining the signal output from said channels comprising an audio frequency amplifier, a diode rectifier device connected with the signal output end of each of said channels, a diode rectifier output resistor in circuit with each of said diode rectifier de vices, a resistor network connecting said diode output resistors substantially in parallel relation to each other, and a circuit connecting said amplier with said network whereby said amplifier may receive biasing potential from each of said channels and signal potentials jointly substantially in parallel.

6. In a radio receiving system, the combination with means providing two signal amplifying channels therein, of means for combining the signal output from said channels, comprising an audio frequency amplifier, a diode rectifier device connected with the signal output end of each of said channels, a diode rectifier output resistor in circuit with each of said diode rectifier devices, a resistor network connecting said diode output resistors substantially in parallel relation to each other, and a circuit connecting said amplifier with said network whereby said amplifier may receive biasing potential from each of said channels and signal potentials jointly, substantially in parallel, said resistor network providing a relatively high resistance between said channels whereby they are substantially decoupled while being maintained in parallel connected relation to each other in connection with the amplifier.

7. In a radio receiving system, the combination with means providing two signal amplifying channels therein, of means for combining the signal output from said channels, comprising an audio frequency amplifier, a diode rectifier device connected with the signal output end oi each of said channels, a diode rectifier output resistor in circuit with each of said diode rectifier devices, a resistor network connecting said diode output resistors substantially in parallel relation to each other, a circuit connecting said amplifier with said network whereby said amplifier may receive signal potentials jointly from said rectifiers, Substantially in parallel, and means in connection with said resistor network providing a single variable volume control element interposed in circuit between said amplifier and a potential supply point on said network.

8. In a radio receiving system, the combination with means providing two signal amplifying channels therein, of means for combining the signal output from said channels, comprising an audio frequency amplifier having an amplifier tube, a diode rectifier device connected with the signal output end of each of said channels, a diode rectifier output resistor in circuit with each of said diode rectifier devices, a resistor network connecting said diode output resistors in parallel relation to each other through the negative terminals thereof, a circuit connecting said amplifier tube with a potential supply terminal of said network, to receive signal potentials through said network from each of said channels jointly, substantially in parallel, said resistor network providing a relatively high resistance between said channels whereby they are substantially decoupled while being maintained in parallel connected relation to each other in connection with the amplifier tube, said amplifier tube being of the high mu type, impedance coupling means including a coupling capacitor between said tube and said resistor network, and means for applying a biasing potential to said amplifier tube.

9. In a radio signal receiving system having a pair of signal amplifying channels, the combination of a diode rectifier device in each channel having a cathode and a rectifier output resistor connected to ground with the cathode, a pair of resistors connected in series between the diode output resistors, switch means for selectively connecting either of the outer terminals of said series connected resistors to ground, and an audio frequency amplifier having a cathode connected to ground and a control grid connected with a point along said series connected resistors to receive rectified signals jointly from said amplifying channels.

10. In a radio signal receiving system having a pair of signal amplifying channels, the combination of a diode rectifier device in each channel having a cathode and a rectifier output resistor connected to ground with the cathode, a pair of resistors connected in series between the anode ends of the diode output resistors, switch means for selectively connecting either of the outer terminals of said series connected resistors to ground, and an audio frequency amplifier having a cathode connected to ground and a control grid conductively connected with a point along said series connected resistors to receive rectified signals and biasing potential jointly from said arnplifying channels.

11. In a radio signal receiving system having a pair of signal amplifying channels, the combination of a diode rectifier device in each channel having a cathode and a rectifier output resistor connected to ground with the cathode, a pair of resistors connected in series between the anode ends of the diode output resistors, switch means for selectively connecting either of the outer terminals of said series connected resistors to ground, and an audio frequency amplier having a cathode connected to ground and a control grid connected with a point along said series connected resistors to receive rectified signals jointly from said amplifying channels, said last named connection including a Volume control potentiometer device for jointly controlling the signal output from both amplifying channels.

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