US2144224A - Automatic volume control circuits - Google Patents

Description

Jan. 17, 1939. w. RLKOCH 2,144,224

AUTOMATIC YOLUMECONTROL CIRCUITS Filed June 30, 1937 3 Sheets-Sheet l Allzl NEG@ Allw S.

ar Q n saws 7am/0J /vo swg v INVENTOR W/NF EL R. KOCH BY TTRNEY IIIIIIIH Jan., l7, 1939. 'w. R. KOCH 2,144,224

AUTOMAT'IC VOLUME CONTROL CIRCUITS Filed June 30, 1937 S'SheetS-Sheet y2 All EGG Jam. 17, 1939. w. R. KOCH 11448224 AUTOMATIC VOLUME CONTROL CIRCUITS Filed June 30, i957 3 sheetssheet :5

Patented Jan. 17, 1939 UNITED STATES AUTOMATIC VOLUME CONTROL CIRCUITS Winfield R. Koch, Mei'chantville, N. J., assignor to Radio Corporation of America, a. corporation of Delaware Application Jlme 30,1937, Serial No. 151,102

10` Claims.

My present invention relates toautomatic volume control circuits for radio receivers, and more particularly to improved volume control circuits adapted to regulate the gain of cascaded signal transmission tubes in a graduated manner.

Automatic gain control has been applied, in the past, to the cascaded signal transmission tubes of a radio receiver; the control being graduated in such a manner that the tubes handling the most signal energy have had the least control bias applied to them. The purpose of such progressively decreasing control action is to prevent distortion by overloading on strong signals of the transmission tubes following the first radio frequency amplier tube. However, such control action results in a poor signal-hiss noise ratio when `receiving weak signals. 4Since the subsequent high frequency transmission tubes have very little AVC bias applied during weak'signal reception, hiss and noise originating in the output circuit Vof the rst transmission tube are greatly amplified.

Accordingly, it may be stated that it is one V of the main objects of my present invention to provide a novel method of, and means for, automatically controlling the gain of a plurality of cascaded signal transmission tubes, wherein the gain control of the tubes increases sequentially `-for weak signal reception, while the gain controldecreases sequentially for strong signal reception, thereby eiectively preventing the reproduction of hiss and noise when weak signals are received and preventing overloading when strong signals are received.

`Another important object of my invention is to provide in a radio receiver of the type employing a graduated automatic volume control ar- 4rangement which acts to apply control bias to J'the cascaded receiver tubes in progressively decreasing amounts, an arrangement for permitting such decreasing control action only when the received signals are above a desired amplitude; the gain control of the receiver tubes being 15 graduated in a reverse sense for the reception of signals below a desired amplitude.

Another object of the invention may be stated to reside in the provision of a superheterodyne "receiver employing an automatic volume control circuit (AVC) which acts sequentially to bias in a progressively increasing sense at least two signal transmission tubes prior to the second detector during weak signal reception, whereas the same tubes are controlled in the reverse sense vfor strong signal reception.

(Cl. Z50- 20) Another object of my invention is to provide an AVC network for a superheterodyne receiver, which network biases the second I. F. amplifier, first I. F. amplifier and radio frequency amplifier in succession for reception of weak signals, whereas thesestages are biased in the reverse order upon the received signals exceeding adesired intensity.

Still other objects of my invention are to improve generally the operating eiciency of radio receivers of the AVC type, and more especially to provide novel graduated AVC circuits which are reliable in operation, and economically manufactured and assembled in receivers.

The novel features which I believe to be characteristic of my invention are set forth in particularity 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 connection with the drawings in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried intoeffect.

In the drawings: 2;,

Fig. 1 is a circuit diagramof a superheterodyne receiver employing the invention,

Fig. 2 illustrates a modification of the AVC circuit of Fig. 1,

Fig. 3 graphically shows the operation of the AVC circuits of Figs. 1 and 2,

Fig. 4 shows a receiver employing still a different form of AVC circuit,

Fig. 5 is a graphic analysis, similar to Fig. 3, of the operation of the AVC circuit in Fig. 4,

Fig. 6 is a circuit diagram of still another modification,

Fig. 7 is argraphic analysis of the AVC operation in the circuit of Fig. 6.

Referringvnow to the accompanying drawings, wherein like reference characters in the diierent figures designate similar circuit elements, there is `shown in Fig. 1 a superheterodyne receiver of conventional construction; a signal collector A feeding a tunable radio frequency amplifier I, a converter 2, first and second I. F. amplifiers 3 and l and a second detector (not shown) all in cascade. The second .detector will, of course, feed audio signals to an audio amplifier network which is followed by a reproducer. Each of the cascaded stages is of well known and familiar construction; each amplifier tube has a tuned input circuit which is coupled to the output circuit of Ya preceding tube. Thus, converter 2 has the 55 n .a (A) tunable input circuit 5 thereof coupled to the output circuit of radio frequency amplier I.

The amplier I has a tunable signal input circuit 6 coupled to the collector A. The oscillation tank circuit 'I has its variable tuning condenser 'I arranged for unicontrol adjustment with the variable condensers of the signal input circuits 5 and 6. The dotted line 8 represents the unicontrol tuner which varies Athe position of the condenser rotors. The frequency range of circuits 6 and 5 (which may be, for example, 500 to 1500 k. c.) differs from the frequency range of the tank circuit 1 by the value of the I. F. ('75 to 450 k. c. may be the range from which the I. F. value is selected). The converter output circuit 9 is coupled to the input circuit I IJ of the rst I. F. amplifier 3; the output circuit II of the latter is coupled to the input circuit I2 of the second I. F. amplifier 4. Circuits I3-I4 couple the plate circuit of amplifier 4 to the second detector. Each of the resonant circuits 9-III; II--I2; I3-I4 is tuned to the operating I. F.; hence, the tuning thereof is iixed to the desired I. F. Value.

The converter 2. may be of the combined local oscillator-first detector type using a 6A'7 tube if desired; however, separate local oscillator and rst detector tubes may be utilized. Each of amplifiers I, 3 and 4 includes a self-bias resistor in its cathode circuit; thus numerals I5, I6, I'I denote the grid bias resistors disposed in the space current paths of tubes I, 3 and 4 respectively. These bias resistors are of progressively decreasing magntudes; the reason for such design will be given at a later point. The tubes I, 2, 3, 4 are energized from a direct current source P; the latter is illustrated as a bleeder resistor whose terminals are connected across the lter output of the usual rectiiied 60 cycle current source. `The screen and plate electrodes of the signal transmission tubes are tol be understood as being connected to points of appropriate positive'potential on the bleeder P; an intermediate point of the latter is established as ground potential reference point.

The AVC circuit for the receiver comprises a diode I8 having a load resistor I9 connected between its` electrodes. Signal energy is impressed on the diode by the connection, through condenser 20, from the I. F. circuit I4 to the cathode side of resistor I9. The direct current voltage component of rectified I. F. energy is impressed on the amplifier 2| the latter being normally biased beyond cut-oit'. The cut-off bias is secured by connecting the control grid of tube 2| to thenegative terminal of resistor P through a path including resistors 22 and I9. The cathode of tube 2| is connected to a less negativefpotential point 23 on resistor P; the voltage between point 23 and the negative terminal of resistor P is more than suiicient, in the absence of signals to cut off the plate current oW through amplifier 2l. Since the grid of tube 2| is connected to the cathode side of load resistor I9, the impression of I. F. energy on the diode I8 causes the cathode side of resistor I9 to become increasingly positive until the positive potential overcomes the normal cutoi bias.

The plate of AVC amplifier 2| is connected to point 24 on resistor P through a path including resistor 25 and lead 26; an audio bypass condenser 21 being connected between the grid and cathode of tube 2|. The plate side of resistor 25 is connected by AVC leads to the control grids of tubes I, 3 and 4.v Thus, lead 28 is connected to the aforesaid control grids by lter resistors 297,29' and 29". Of course, the AVC lead may be connected to the signal grid of the converter, if desired. The normal bias on the controlled tubes is provided by connecting the bias resistors I5, I6 and I'I to points of respectively different negative potentials on bleeder P. Resistor I5 is connected to point 3| on the bleeder by lead 30; resistor I6 is connected to point 32 by lead 33; lead 26 connects resistor I'I to point 24. When no signals are impressed on the receiver the normal positive potential of the cathode of tube I will be equal t0 the voltage drop across resistor I5 less the voltage between points 3| and 24 on resistor P.

The drop across resistor I less the potential difference between points 32 and 24 determines the normal cathode potential of tube 3; while the drop across resistor I'I provides the bias for tube 4. The voltage relations are so chosen that the leffective negative biases on the control grids of the controlled tubes are the same; that is, the tubes I, 3 and 4 have equal transconductance for signals of less than a desired amplitude. This initial equality is secured, despite the non-uniform magnitudes of the self-bias resistors I5, I6, I'I, by the tap adjustments of the cathode leads on bleeder P.

A pair of diodes 34 and 35 are employed to reverse the sequence of gain control action when signals of a relatively strong intensity are received. Diode 34 has its cathode connected to the grid side of resistor 29'; its anode being connected by lead 36 to point 3'I on the bleeder P. Thus, the diode 34 is normally biased to be nonconductive; point 31 being at a negative potential with respect to point 24 lto which the diode cathode is connected. Diode 35 is similarly biased by connecting its cathode to the grid side of resistor 29 and connecting the diode anode to the point 38 through lead 39; point 38 being at a negative potential with respect to point 24. These two diodes 34 and 35 are rendered conductive only when the AVC bias developed across resistor 25 overcomes the biases on the diodes. Since the biases are progressively less on diodes 34 and 35, it follows that diode 35 will be rendered conductive prior to diode 34.

In the circuit of Fig. 1, the AVC action does not commence on any of the transmission tubes I, 3 and 4 until the cut-oli bias on the AVC amplier 2 IV is exceeded by the signal amplitude; that is, by the drop across resistor I9. This explains why, in the three curves of Fig. 3, the initial portions of the curves have the same eiective bias before the control action commences. The progressively decreasing delay for the AVC action is secured by means of the cathode resistors of decreasing size. Considering the second I. F. amplier 4, its cathode resistor I'I is of a relatively low value, and, therefore, the potential drop thereacross is relatively small. This potential drop supplies the initial bias for the control grid of the amplier. As the AVC bias on the control grid of the second I. F. amplifier 4 increases When Weak signals are received, the drop across the cathode resistor I'I is varied by a small amount since the resistor is relatively small in magnitude. This means that the effective control grid bias applied to the amplifier 4, for gain control, increases at a fast rate, because the opposing potential drop across the cathode bias resistor changes by such a small amount that it does not prevent the I. F. ampliiier 4 from using practically the entire developed AVC bias.

However, when the signals have attained an intensity of a predetermined amount above the Weak signal intensity, the diode 35 will be ren- 75 dered conductive, the cathode of the diode being connected through the AVC lter resistor 29" to the plate of the AVC amplifier ZI. When the diode 35 is rendered conductive, current flows through the lter resistor 29, and the resulting potential opposes the AVC bias to an extent sufficient to prevent further appreciable increase of negative bias on the control grid of tube 4.

In the case of the rst I. F. amplier 3, it will be seen that the sloping middle portion of the second curve in Fig. 3 signifies a less rapid increase of control grid bias. This follows because the cathode resistor I5 has a larger magnitude than the cathode resistor II of the second I. F. amplifier. In turn, this means that the increasing AVC bias for Weak signal reception is opposed by a larger potential variation across cathode bias resistor I5. The diode 34 in this case produces the flattening effect ofthe characteristic curve, because the diode becomes conductive when the AVC bias becomes sufliciently strong to overcome the initial cut-off bias of the diode. Hence, in Fig. 3 the middle curve shows the control grid bias for tube 3 increasing at a less rapid rate than in the case of tube 4; and

eventually becoming constant in value.

In the case of the amplifier I, the cathode resistor I5 is larger in magnitude than the following two cathode resistors, and, therefore, there Awill be a larger variation in voltage drop thereacross to oppose the increasing AVC bias. In this case it is not desired to use a limiting diode, because it is necessary to have the AVC action on the first signal transmission tube greater than the AVC on the following tubes for strong signal reception. The top curve in Fig. 3 shows the control grid bias increasing at a slower rate, but maintaining the increase even after the control biases on the tubes 3 and 4 have become co-nstant. l

The circuit in Fig. 2 is similar in action to that of Fig. 1. Instead of using cathode bias resistors y of different magnitudes, the cathode resistors i5'-I-Il remain equal in Value. However, to secure the sequentially increasing AVC rates for the cascaded stages. there are disposed resistorsand 4I of progressively decreasing magnitudes in the screen grid leads of tubes I and 3. It is to be noted that the screen grid l. lead of the second I. F. amplifier4 actually has zero added resistance. The screen grid leads 42, 43 and 44 are connected to points of progressively lower positive potential on the bleeder resistor. For example, the screen resistor 4i] of the ampli- `iler I is connected to the most positive point 45 on the bleeder;` the rst I. F. amplifier screen` resistor 4I is connected to a point 46 which is of less positivev potential; and lead 44 is connected to point 4l of stillless potential. The points 4l, 46 and 45 are sochosen that the leffective screen potential of each amplifier is the same inthe absence of AVC bias being developed across resistor 25. `Vihen the voltage across load resistor I9 (not shown in Hg. 2) exceeds the cut-off bias on tube 2I, then the AVC bias across resistor 25 acts on the control grids of the ampliers I--3-4 to reduce the space current flow thereof.

Since the screen grid leads progressively decrease in resistance vaiue, the Variation in potenf tial across resistor 45 will be greater than that across resistor 4I and still greater than that in lead 44. Hence, the gain of the amplifiers I, 4 will vary at progressively fasterrates;r the gainof tube 4 will decrease much more quickly than that of tube 3, 4and the gain of the latter will dei tive with respect to its anode.

crease at a faster rate than that of tube I. However, when the signals get suiiiciently strong to cause diodes 35 and 34 to become successively conductive, then the gain reduction order will be reversed. As explained previously, the diode 35 will become conductive when its cathode is nega- Since point 38 is less negative than point 3l it follows that the cut-off bias on diode 35 is less than that applied to diode 34. It will be noted that resistor 25 is connected to the intermediate ground potential point on bleeder P b-y lead 25. Also, resistors 4E) and 4I may be made variable in magnitude to permit a control over the rate of gain variation in tubes I and 3. The curves oi Fig. 3 represent the operation of the circuit in Fig. 2, it being understood that the ordinates in the latter case would represent the gain of each amplifier. When weak signals are received the gains of the amplifiers are sequentially reduced, whereas for strong signal reception the gain reduction order is reversed.

In Figs. 1 and 2 the controlled tubes have been shown as having the same initial gains; the `order of gain variation being dependent upon the use of resistors (either in the cathode circuit or screen circuit) ,of different magnitudes. In Fig. 4 there is shown a modification wherein the order of gain variation for weak signals is dependent upon a sequentially decreasing delay of the AVC bias applied to thecontrol grids of amplifiers I, 3 and 4. For strong signals, however, the diodes 35 and 34 are employed to reverse the gain reduction sequence. The bias resistors I5', I6' and Il are of equal magnitude; the resistor II is connected by lead 5D to the intermediate ground potential point of the bleeder resistor P. A lead 5I connects resistor I 6 to a more negative potential point 52 on the bleeder, while lead 54 connects resistor I5 to a point 53 on the bleeder which is more negative than point 52.

In the absence of received signals the normal gain of each of the controlled tubes is determined by the voltage drops across the bias resistors. It will be seen that the control grid of amplier I is connected to the negative side of resistor l5' through a resistor tapped at an intermediate point, and divided into a pair of series sections 55 and 56. The control grid of amplier 3 is connected to the negative side of bias resistor I6 through resistor sections 5l and 58;

while the control grid of amplifier 4 is connected to the grounded lead 5I) through a path which includes the AVC lter resistor 59, lead 5t', AVC lead 6 I, resistor 25. There is associated with each of resistor sections 5S and 55, diodes 5?; and ,63 respectively. The anode of diode 52 is connected to the junction of resistors 55 and 56, while the cathode thereof is connected to the AVC lead 6I. The diode 53 has its anode connected to the junction of resistors 51 and 53, while its cathode is connected to the lead 6I.

It will be observed that thediode 62 normally has its anode negatively biased with respect to its cathode, because its anode is connected to the point 53 on the bleeder P and the diode cathode is connected to the ground potential point. The diode 53 similarly has a normal cutoif bias, but this cut-off bias is less than that applied to diode 52, because the anode of diode 63 is connected to point 52 on the bleeder which is less negative than point 53. The diodes 35 and 34 are connected in circuit with ampliers 4 and 3 respectively in the manner previously de-f scribed. The anode of diode 35 has a cut-off bias applied to it, in the absence of signals which Yis equal to the potential between the ground point on bleeder P and point 52. The diode 34 has a cut-off bias applied to it which is equal to the potential difference between points 53 and 52 on the bleeder. The AVC bias developed across resistor 25 determines the conductivity of each of diodes 62, 63, 34 and 35.

The progressively decreasing delay of the AVC action is secured in this case by the following means: Considering the second I. F. amplifier II, when the AVC bias is produced, the gain of the amplifier 4 is reduced until the limiting diode 35 becomes conductive; the lowest characteristic in Fig. 5 flattens out. In the case of the first I. F. amplifier 3, the AVC bias does not begin to cut the grain down until a greater predetermined AVC bias intensity is attained. The AVCr bias begins to affect the amplifier 3 when the auxiliary diode 63 connected to the resistor 58 becomes conductive. 'Ihis will occur when the AVC bias becomes greater than the voltage between point 52 and ground. The diode 34 cornes into play when the anode side of the resistor 58 in the auxiliary diode circuit becomes sufficiently negative to render the limiting diode 34 conductive which will occur when the AVC voltage becomes greater than the voltage from point 53 to ground thereby flattening the AVC voltage characteristic in Fig. 5. In

the case of the amplifier I, there is no limiting diode, but the auxiliary diode 62- acts to produce AVC bias when it is rendered conductive by an AVC voltage greater than the voltage between point 53 and ground. It is to be noted that the cut-off bias of the auxiliary diode 62 is chosen so that the AVC action for the amplifier I is delayed to a greater extent than in the case of the amplifier 3 and the second I. F. amplifier 4. Fig. 5 shows the control grid bias changes as signal strength varies; the AVC bias on amplifier 3 is not effective until the diode 34 has become conductive.

Comparing Figs. 3 and 5, it will be seen that the circuit arrangements of Figs. 1 and 2 in reality provide a progressively decreasing rate of change of the AVC bias, whereas the circuit in Fig. 4 sequentially delays the AVC bias from being applied to the controlled tubes. From a generic viewpoint, in both cases there is a progressively increasing gain reduction action on the controlled tubes for vweak signals, whereas for stronger signals the gain reduction action is progressively decreased,

In Fig. 6 there is shown a modification wherein the ampliiier I has the control grid thereof con nected to the cathode bias resistor I5 through a normally conductive diode 10. This means that the AVC bias will not affect the control grid of amplifier I until the diode I0 is rendered nonconductive. 'I'he amplifier 3 has a normally conductive diode 1I connected between the control` grid thereof and the negative side of the cathode bias resistor I6. The cut-off bias on this diode 1I is less than the cut-off bias on the diode 1Q. With proper circuit design the diode 'II will ben come non-conductive at a smaller AVC bias value than that required to make diode 'I0 nonconductive. It will be noted that the AVC bias is progressively smaller for stronger signals, because of the intermediate tap of the I. F. amplifier control grid on the AVC bias resistor 25.

Fig. 7 shows that, as in the case of the other arrangements, for weak signals the AVC bias progressively increases on the cascaded transmission tubes, whereas for stronger signals the AVC bias progressively decreases. Specifically, of course, the diodes I0 and 'II function as devices for blocking the AVC action on ampliers I and 3; the blocking being released sequentially from the I. F. amplifier to the amplifier I. However, the tap 80 on the bias resistor 25 assures less AVC bias on the amplifier 3. Hence, the curves in Fig. '7 are shown as having different slopes, the slope for the amplifier I being, of course, steeper.

While I have indicated and described several systems 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 organizations 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. A method of operating a radio receiver of the type including a plurality of cascaded signal transmission tubes, which includes sequentially decreasing the gain of said tubes from the last of the tubes to the first one, and sequentially decreasing the gain of said tubes from the iirst of said tubes to the last one as the signal strength increases.

2. A method of operating a radio receiver of the type comprising a plurality of cascaded tubes which includes transmitting signal energy through said tubes, deriving a control bias from received signal energy, applying the control bias to said tubes from the first to the last tube in a progressively increasing sense for weak signal reception, and reversing the sense of application of the control bias for strong signal reception.

3. A method of automatically controlling the gain of at least two cascaded signal transmission tubes which includes automatically controlling the gain of the tubes in a sense such that the gain of the tubes is reduced in a sequential manner from the second tube to the iirst tube, and reversing the gain reduction sequence as the signal strength increases.

4. In a superheterodyne receiver including a plurality of cascaded signal transmission tubes, and an automatic control circuit for progressively reducing the gain of the transmission tubes as the signal strength increases, additional means responsive to the automatic volume control means for reversing the gain reduction sequence when weak signals are received.

5. In a radio receiver of the superheterodyne type including a radio frequency amplier and an intermediate frequency amplifier, an automatic gain control, means responsive to said gain control which reduces the gain of the intermediate frequency amplifier to a greater degree than that of the radio frequency amplifier for a predetermined range of weak signal intensities, and additional means responsive to said gain control which reduces the gain of the radio frequency amplifier to a greater degree than that of the intermediate frequency amplifier for signal intensities greater than a predetermined value.

6. In a multi-stage amplifier of the type com prising a plurality of cascaded tubes, an automatic volume control circuit for said tubes providing progressively decreasing gain control from the last to the first of the tubes in response to weak signal reception, and means operatively associated with said control circuit for reversing the order of said gain control in response to reception of signals above a predetermined amplitude.

7.111 a radio receiver, at least two carrier wave amplifiers arranged in cascade, an automatic gain control arrangement for said amplifiers, said arrangement predominantly reducing the gain of the second amplilier in response to carrier amplitudes below a desired intensity level, and predominantly reducing the gain of the first amplier in response to carrier amplitudes above said level.

8. In a superheterodyne receiver of a type comprising at least a radio frequency amplifier and at least two intermediate frequency ampliers all arranged in cascade, means establishing each of the ampliers at the same normal gain,

means for reducing the gain of said ampliers in a progressively decreasing manner from the last to the rst amplifier during weak signal reception, and means for further reducing the gain of at least the first of the ampliers and to a greater extent than the following amplifiers during strong signal reception.

9. In a wave transmission system including at least three cascaded amplifiers, an automatic gain control circuit for said amplifiers, means for delaying gain reduction of said amplifiers by said control circuit in a progressively increasing order from the last to the first of said amplifiers in response to weak signal reception, andmeans preventing further gain reduction by said control circuit of the last two amplifiers, While gain reduction of said first amplifier continues, in response to strong signal reception.

10. In a wave transmission system comprising at least two cascaded ampliers, an automatic gain control circuit for said amplifiers, means delaying gain reduction of said ampliers by said control circuit in a progressively increasing order from the second to the rst amplifier in response to wave amplitude increase up to a predetermined amplitude, and additional means causing gain reduction of said amplifiers by said control circuit in a progressively decreasing order from the first to the second amplier for Wave amplitudes above said predetermined amplitude.

W'INFIELD R. KOCH.

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