US2083232A - Automatic selectivity control system - Google Patents

Description

Fig.1

June 8, 1937. I w. R. KOCH 2,083,232

AUTOMATIC SELECTI-VITY CONTROL SYSTEM Filed April 8, 1935 RECTIFIER 3 I 5 I IIII I i :4 25

85/1/7146/9/0 (OM E 7E INVENTOR W/A/F/ELD A. K0671 mm. ATTORNEY Patented June 8, 1937 UNITED STATES AUTOMATIC SELECTIVITY CONTROL SYSTEM Winfield R. Koch, Camden, N. J., assignor to Radio Corporation of America, a corporation of Delaware 7 Application April 8, 1935, Serial No. 15,168

11'Claims. (01. 250-) My present invention relates to automatic control circuits for regulating the selectivity of radio receivers, and more particularly to an improved type of control system for a radio broadcast re- 5 ceiver wherein the selectivity and tone of the receiver are automatically regulated.

One of the primary objects of the invention is to provide a receiver having dual control chan nels; one of the channels being sharp and feeding signals to a device which functions to regulate the radio frequency selectivity and the audio tonal characteristic of the receiver inresponse to received carrier amplitude variations, and the other channel being broad and feeding signals to an automatic radio frequency amplification control device.

Another important object of the invention may be said to reside in the provision of an improved selectivity control system for a radio receiver wherein the selectivity control network is fed with signals through a relatively sharp transmission channel whereby only by correct tuning will the full frequency range of the receiver be secured.

prove generally the efficiency and reliability of automatic selectivity control systems for receivers, and more especially to provide automatic selectivity and/or tone control systems which are 'not only of improved operating characteristics but are readily constructed and assembled in broadcast receivers.

The novel features which I believe to be characteristic of my invention are set forth in par ticularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connectionwith the drawing in which I have indicated a circuit organization whereby my invention may be carried into elfect.

In the drawing:- Fig. 1 shows a receiving systemernbodying the invention,

Fig. 2 shows the resonance characteristics of the dual control channels of the receiver. Referring now to the accompanying drawing, the latter shows in Fig. 1 a circuit diagram of a superheterodyne receiver embodying one form of the invention. ceiver are conventional in nature, and comprise a signal collector A, such as a grounded antenna circuit, followed by a tunable radio frequency amplifier of well known type using a pentode tube. The amplifier output electrode is coupled to the Still other objects of the invention are to im- 7 The general networks of the re tunable signal input circuit l of the following pentagrid converter network. The latter uses a tube of the 6A7 type, and the circuits associated therewith are fully described and claimed by J. C. Smith in application Serial No. 654,421, filed J anuary 31, 1933. It is sumcient forthe purposes of this disclosure to point out that the numeral 2 designates the tunable local oscillator circuit of the converter network. The rotors of the variable tuning condensers of circuits I and 2, as well as the input circuit I of the radio amplifier, are unicontrolled, as is well known to those skilled in the art.

The circuit 3 in the plate lead of the pentagrid converter tube is fixedly tuned to the operating. intermediate frequency, and the latter may be chosen from a frequency range of to 500 k. c. The tube 4, also a pentodetype tube, following, the circuit 3 is an intermediate frequency ampliher. The signal grid thereof is coupled to the high alternating potential side of resonant circuit 5 by a condenser 9. The low alternating potential sides of circuits 3 and 5 are connected together. The resonant circuit 1 is in the plate circuit of the following intermediate amplifier tube l, of the pentode type; the tuned input circuit 9 of tube 8 and the resonant output circuit of tube 4 have their low alternating potential-.sidesconnected together. The cathode returns for circuits 5 and 9 to tubes 4 and 8 respectively are through the usual common voltage supply bleeder resistor (not shown). v

The normal negative bias for amplifier 8 is se cured by connecting the resistor in between the grid side of coupling condenser l l and the ground.-

ed side of the grid bias resistor. The tuned circuits 3 and 5 are magnetically coupled. This is' equally true of circuits 1 andS. Each of these four circuits is tuned to the operating intermediate frequency. The normal signal transmission path of the receiver may be traced from the circuit 9, through coupling condenser l2, to the signalgrid electrode of the intermediate frequency pentode amplifier tube 53. The plate circuit'of-tube i3 includes the resonant circuit l4 tuned to the operating intermediate frequency. The second detector, or demodulator, is provided by'a diode whose electrodes are the cathode and diode anode l6 of the duplex r diode-triode tube H, which may be 'of the well known 55 type. The resonant circuit {5 acts as the demodulator input circuit, and is connected, in series with loadresistori l 3, between the cathode of tube l1 and anode It. The load resistor 18 is Icy-passed for intermediate frequency currents by condenser i9.

The circuit I5 is tuned to the operating intermediate frequency, and is magnetically coupled to circuit I l. The coupling M1 between circuits l4 and I5 is chosen so that a relatively broad resonance curve characteristic is obtained. The curve B in Fig. 2 denotes the type of characteristic desired for the network M, It. The control grid of tube 51 is connected to the anode side of resistor |8, while the plate of the tube I'Iis coupled to an audio amplifier 23.. The signal grid of the latter is connected to the plate of tube I! through a series connection of a condenser 2| and resistor 22, The condenser side of resistor 22 is connected to the grounded side of the grid bias resistor through a leak resistor, and the plate circuit of tube includes a coupling resistor 23. The plate circuit .of audio amplifier 20 includes a load resistor, and the latter may be coupled to a subsequent audio network which may comprise one, or more, audio amplifiers followed by a reproducer.

The signal carrier amplitude at the circuit I5 is maintained substantially uniform over a wide range of signal carrier variation at the collector A by employing an AVC arrangement. The latter comprises the anode l6 of tube l1, co-operating with the cathode of thelatter to provide a device of unidirectional conductivity. The anode |6' is grounded through resistor 24, while the variable resistor 25 is disposed in the cathode circuit of tube H. A voltage point of about volts is connected to the resistor 25. and the magnitude of the latter'is adjusted to a point such that the cathode of tube I1 is positive with respect to the anode l6 for the space current flow which exists when receiving signals below a predetermined amplitude.

In the absence of received signals the control grid of tube I1 is at substantially cathode potential. The space current flow through resistor 25 is a maximum, since the resistor is in the plate to cathode current stream of tube l1. However,

. as the signal amplitude exceeds a predetermined level the voltage drop across resistor IB'increases and renders the grid of tube llincreasingly negative. This results in a decrease in space current fiow through resistor 25, and the voltage across the latterdecreases until the cathode actually reverses in polarity with respect 'to the grounded anode I6. When the cathode of tube |1 attains a negative potential with respect to the anode Hi there will be a flow of current through resistor 24. Thevoltage across resistor 24, of course, increases directly with the increase of carrier amplitude. Hence, it is only necessary to connect the grids of thetubes tobe controlled in gain to the resistor 24 in order to secure the 1 AVG action.

Thus, the signal control 'gridcircuits of the radio frequency amplifier, converter and intermediate amplifiers and |3 are connected through the heavy line leads designated AVC to the anode side -of resistor 24. The resistorcondenser network 26 functions to suppress ripples in'the bias voltage applied through the AVC leads; and the latter leads also include ripple suppression resistors at the points of con- Each of nection to the aforesaid-grid circuits. the gain-regulated tubes includes the usual grid bias resistor network for establishing maximum gain, or sensitivity, condition in the absence of AVG action; the latter existing when. signals becathode of each of the selectivity control tubes is grounded through a grid bias resistor network. The plate impedances of tubes 30 and 3| are varied by varying the negative biases of the grids thereof; when the received signal amplitude is a minimum these biases are a minimum. and are produced by the bias resistors in the cathode circuits of tubes 39, 3|. In such a case the shunting effect of tubes 30 and 3| on the networks 35 and |-9 respectively is a maximum, and these networks have relatively sharp, single peak resonance curve characteristics. The characteristics are broadened double peaked curves when the shunting effects of tubes 30 and 3| are removed.

The shunting effect of each of tubes 30 and 3| is regulated by the networks now to be described. From the circuit 9 the intermediate frequency signal energy is transmitted through condenser H to amplifier 8. The latter amplifies the signal energy prior to rectification thereof, the rectification being performed to secure a variable negative bias for the tubes 30, 3|. The amplified signal energy is impressed upon the diode rectifier electrodes of multiple duty tube 50 through a relatively sharp network comprising tuned circuits 5| and..52. The circuit 5| is disposed in the plate circuit of tube 8, while circuit 52 is connected between the anode 53 of tube 50 and the cathode thereof; the load resistor 54, properly by-passed, is included in the cathode tide of circuit 52. Each of the circuits 5| and 52 is tuned to the operating intermediate frequency,

and they are magnetically coupled at M2 so as to impart a sharp resonance curve characteristic to the signal channel feeding the diode anode 53.

The curve S in Fig. 2 illustrates the nature of the resonancecharacteristic of the sharp channel feeding tube 50; this curve is contrasted to curve B in the figure. The selectivity of circuits 35 and 7-9 depends on the-fidelity desired for weak signals. M2 is made as sharply selective as possible without making the receiver too difficult to tune, or keep in tune. In general, this should result in havingMz somewhat sharper than circuits 35 and '|.9.

The circuit elements associated with tube 55 follow very closely those associated with tube H. The anode 53, grounded through load resistor 24, is normally negative with respect to the voltage of the cathode of tube 50, and it will be observed that the cathode is connected by adjustable resistor 25' to a point which is about 150 volts, with respect to ground. The control grid of tube 50 is connected to the anode side of resistor 54, and functions to reduce the space current flow through resistor 25 as the signal amplitude increases. When the space current flow through resistor 25' has been reduced to a point such that the cathode potential is negative with respect to the grounded anode 53, then current will flow through resistor 24.

tube '88 through The anode side of the latter resistor 2 is connected by a lead 60 (denoted by the symbol 3G and 3!. The resistor-condenser filter 2'3 is used to prevent any pulsating current component from being transmitted through lead 69. It will now be seen that an increase in received signal amplitude above a desired value results in the impression of negative bias upon the grids of tubes 39 and 3! sufficient in magnitude to increase the plate impedances thereof to a point such that the networks 3-5 and 1-9 are broadened.

It will be noted that the tubes 58 and 31, although used for different purposes, function with their associated circuits in the same manner. Circuits of this type delay regulation of the gain of tubes to be reversal action. Such gain control circuits are disclosed and claimed in application Serial No. 640,946. filed Nov. 3, 1932, by L. E. Barton. Further, the selectivity control tubes and their functions have been disclosed and claimed in application Serial 1932, by G. L. October 15, 1935.

In addition to the automatic regulation of the Beers, Patent No. 2,017,523,

selectivity of the intermediate frequency circuits, the tube 56 acts to vary the conductivity of the tone control tube 10. The latter, as disclosed by G. L. Beers in U. S. P. 1,961,329 of June 5, 1934, has its plate connected to the grid side of resistor 22 by a condenser 1i, and a choke coil 1?. is disposed in the plate voltage supply line of the tube. The grid is negatively biased by the usual grounded cathode bias resistor, and variable bias is applied to the grid by a lead 13 (also designated by the letters ATC to show that it is the automatic tone control lead). through lead 69, to the anode sideo-f resistor 24. The function of the control tube 19 is to vary the flow of audio currents, through condenser ii, to ground. When the bias applied to the grid of lead 13 increases due to signal increase, then the control tube offers more impedance to audio currents flowing through condenser iito its plate thus causing less attenuation of the higher audio frequencies through resistor 22.

The operation of the various control networks (gain, selectivity and tone) will now be explained, special attention being directed to the effect of the sharp and broad channels (M2 and M1). It

will beobserved that a separate intermediate frequency channel is provided to the selectivity and tone control rectifier tube 59, and that this channel is maintained sharp so that no broadening of the selectivity curves can occur until the signal istuned in to exactly the middle ofthe band. The amplifier 8 which feeds the sharp intermediate frequency channel is not regulated in bias by the AVG network. The significance of this resides in the fact that full selectivity control will be obtained when the signal is tuned in, and

the AVG action, obtained through the AVG tube I1, is replaced by the control action obtained through the selectivity control tubes 30 and 3|.

Assuming now that the receiver is being tuned to a desired carrier station, there willlbe a change in the plate to cathode impedances of tubes 30, 35, and 1B which are under the control of tube 59. The negative bias impressed on the gridsof each of these three control tubes will increase controlled by a polarity- No. 588,909, filed January 26,

of the tone control tube The lead 13 is connected as the tuning of the receiver approaches the desired carrier frequencyof theaccepted band of the station desiredto be received. The two selectivity control tubes and the tone control tube are operated by the signal through the-sharp intermediate frequency channel M2. As the signal is tunedin to this channel the grids of the selectivity control tubes become more negative and the plate impedance of these tubes increases. The" intermediate frequency transformers 'associated with these selectivity control tubes predominate in the double peaked effect as the shuntingeffect of these tubes is removed, and the selectivity curves of these transformers broadens out. Thetone control tube'oifers more impedanccs to audio currents flowing through the condenser ii to its plate'as the negative grid bias thereof increases thereby causing less attenuation of the higher. audio frequencies through the resistor 22.

Of course, when the receiver is mistuned the attenuation of the higher audio frequencies is a maximum because the control tube 10 offers minimum impedance to the flow of the higher audio currents through condenser 1!. In this. way for a received signal amplitude which is less than a predetermined value the audio output of the receiver will include a minimum of higher audio frequencies, and this means in turn that noise and other sounds in the higher'audio frequency range will be effectively suppressed. Thus simultaneously'with thecontrol of the radio frequency selectivity of the receiver there is secured at higher audio frequency attenuation control which cooperates with the selectivity control to render reception of weak signals efficient.

The AVC system operates in the usual manner to maintain the signal amplitude at the input cirusual automatic selectivity control systems, the

circuits will stay broad because enough signals will go, through the intermediate frequency transformers to keep the selectivity control tubes functioning. On the other hand with the present type ofsharp automatic selectivity control systems, only by accurate tuningwill the full frequency rangeof the receiver be obtained.

Instead of separate tubes to secure selectivity control and volume control action ordinary diodes could be used in place of the duo-diode triodes shown. The diodes in such case could be combined with the-preceding signal amplifier,.and adiode pento-de tube could be used for this pur-- pose. However, by using the tubes 50 and I1 as shown, another desirable characteristic is obtained. By adjusting the resistor. 25' in the cathode circuit of the selectivity control tube 50,

the signal strength at which the receiver begins, to broaden out can be adjusted easily. For example, as the signal increases the set can be made to broaden out at first. Then as the signal increases further, the AVG action will aid in volume controlling, and the set will not be broadened out to the full extent. On the other hand the adjustment to resistor 24 can be made so.

that the AVG action will operate first as the signal increases, and then, after a certain signal strength, the selectivity control will start to broaden out the receiver.

This present arrangement permits a more flexible automatic selectivity control as well as one having superior operating characteristics. The variable resistor 25 can be employed for varying the degree of delay of the AVG action. The higher the voltage value of the negative end of resistors 25, 25' the greater will be the gain secured in the triode sections of tubes l1 and 50, and the more complete will the controls be. Of course, it will be understood that the specific type of selectivity, tone and gain control networks shown herein are simply illustrative in nature. Those skilled in the art are fully aware of networks adapted to perform similar functions, but which networks are specifically different from those shown in the present case.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination, in a receiver, a source of high frequency energy including an amplifier for the energy, said amplifier being provided with an input circuit tuned to a desired signal frequency, means responsive to an increase of said energy above a predetermined intensity level for decreasing the selectivity of said amplifier input circuit, a transmission channel tuned to said desired fre quency coupling said last means and the output of said amplifier, said channel having a relatively sharp resonance curve characteristic, a demodulator, a transmission channel coupling the demodulator input to the output of said amplifier, said last channel having a resonance curve characteristic which is substantially broader than 'the characteristic of said sharp channel, and means responsive to said intensity level increase for reducing the gain of said amplifier.

2. In combination, in a receiver, a source of high frequency energy including an amplifier for the energy, said amplifier being provided with an input circuit tuned to a desired frequency, means responsive to an increase of said energy above a predetermined intensity level for decreasing the selectivity of said amplifier input circuit, a transmission channel tuned to the desired frequency coupling said last means and the outputof said amplifier, said channel having a relatively sharp resonance curve characteristic, a demodulator, a transmission channel tuned to said desired frequency coupling the demodulator input to the output of said amplifier, said last channel having a resonance curve characteristic which is substantially broader than the characteristic of said sharp channel, and means responsive to said intensity level increase for reducing the gain of said high frequency amplifier, an audio frequency utilization network coupled to the demodulator output, and means controlled by said selectivity control means for variably attenuating the higher audio frequencies in the demodulator output.

3. In combination, in a superheterodyne receiver, a source of intermediate frequency energy including an amplifier for the energy, said .am plifier being provided with an input circuit tuned to the said intermediate frequency, means responsive to an increase of said energy above a predetermined intensity level for decreasing the selectivity of said amplifier input circuit, an intermediate frequency transmission channel coupling said last means and the output of said amplifier, said channel having a relatively sharp resonance curve characteristic and being tuned to said intermediate frequency, a demodulator, an intermediate frequency transmission channel coupling the demodulator input to the output of said amplifier, said last channel having a resonance curve characteristic which is substantially broader than the characteristic of said sharp channel, and means responsive to said intensity level increase for reducing the gain of said intermediate frequency amplifier, said selectivity control means including an electron discharge tube having its plate to cathode impedance effectively in shunt with the tuned input circuit of said amplifier.

4. In combination, in a superheterodyne receiver, a source of intermediate frequency energy including an amplifier for the energy, said amplifier being provided with an input circuit tuned to the said intermediate frequency, means responsive to an increase of said energy above a predetermined intensity level for decreasing the selectivity of said amplifier input circuit, an intermediate frequency transmission channel coupling said last means and the output of said amplifier, said channel having a relatively sharp resonance curve characteristic and being tuned to said intermediate frequency, a demodulator, an intermediate frequency transmission channel coupling the demodulator input to the output of saidamplifier, said last channel having a resonance curve characteristic which is substantially broader than the characteristic of said sharp channel, and means responsive to said intensity level increase for reducing the gain of said intermediate frequency amplifier, and an intermediate frequency amplifier coupling the output of said first amplifier and the input of saidsharp transmission channel.

5. In combination, in a superheterodyne receiver, a source of intermediate frequency energy including an amplifier for the energy, said ampli fier being provided with an input circuit tuned to the said intermediate frequency, means responsive to an increase of said energy above a prede termined intensity level for decreasing the selectivity of said amplifier input circuit, an inter mediate frequency transmission channel coupling said last means and the output of said amplifier, said channel having a relatively sharp resonance curve characteristic and being tuned to saidintermediate frequency, a demodulator, an inter- 'mediate frequency transmission channel coupling the demodulator input to the output of said amplifier, said last channel having a resonance curve characteristic which is substantially broader than the characteristic of said sharp channel, and means responsive to said intensity level increase for reducing the gain of said intermediate frequency amplifier, and independent intermediate frequency amplifiers arranged to feed intermediate frequency energy to said sharp and broad channels respectively.

6. In a receiver for radio frequency signals, including means for automatically adjusting amplification of the receiver with signal strength, and including means for automatically adjusting the frequency response characteristic of the receiver with signal strength, means for restricting the operation of the automatic adjustment of the frequency response characteristic to a small range of mis-tuning of the receiver, means for restricting the operation of the amplification adjusting means to a relatively wider range of mis-tuning of the receiver, said small range being substantially independent of influence by the automatic adjustment of said frequency response characteristic.

'7. A radio receiver comprising selectivity control means, means for controlling the sensitivity of the receiver, independent devices electrically associated with each of said means for automatically actuating them in response to amplitude variations in received signals, said devices having resonant input circuits tuned to an operating signal frequency, the input circuit of the device associated with the sensitivity control means having a relatively broader resonance curve characteristic than the input circuit of the other device.

8. A radio receiver comprising selectivity control means, audio frequency fidelity-control means, means including a signal-tuned input circuit having a relatively sharp resonance curve characteristic for automatically actuating said two control means, means including a signaltuned input circuit responsive to changes in strength of incoming signals for automatically regulating the sensitivity of the receiver, said last means input circuit having a relatively broader resonance curve characteristic than said first characteristic.

9. A radio receiver comprising selectivity control means, means for controlling the sensitivity of the receiver, independent devices electrically associated with each of said means for automatically actuating them in response to amplitude variations in received signals, said devices having resonant input circuits tuned to an operating signal frequency, the input circuit of the device associated with the sensitivity control means having a relatively broader resonance curve characteristic than the input circuit of the other device, and the time constants of the said two control means being so related that the sensitivity control means operates faster than the selectivity control means.

10. A radio receiving system comprising selectivity control means, audio frequency fidelity control means, means including a signal-tuned input circuit for simultaneously actuating said two control means, receiver sensitivity control means, and means including a signal-tuned input circuit independent of said actuating means for operating said sensitivity control means in response to amplitude variations in received signals, said two input circuits having substantially diflerent resonance curve characteristics.

11. In a signal transmission system including a signal transmission network, a demodulator coupled to said network, and a demodulated signal utilization network, means for automatically varying the frequency response characteristic of the system in response to amplitude variations in signals, a network coupling said last means to the network preceding said demodulator, and said coupling network having a resonance curve characteristic which is substantially sharp with respect to the resonance curve characteristic of the said preceding network coupled to the demodulator.

WINFIELD R. KOCH.

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