US2153760A - Automatic volume control circuits - Google Patents

Description

April 11, 1939. c. N. KIMBALL AUTOMATIC VOLUME CONTROL CIRCUITS Filed Nov 17, 1937 :5 INFINITE +3 IMPEDANCE [EAMPL/F/ER mm J r #5 v M 5 row;

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(i/A6155 N. KIMBALL BY WW ATTORNEY.

Patented Apr. 11, 1939 PATENT OFFICE AUTOMATIC VOLUME CONTROL CIRCUITS Charles N. Kimball, East Orange, N. J assignor to Radio Corporation of America, a. corporation of Delaware Application November 17, 1937, Serial No. 174,943

13 Claims.

My present invention relates to gain control circuits for radio receivers, and more particularly to automatic volume control circuits for a radio receiving system.

In the past, automatic volume control circuits (AVC) for broadcast radio receivers, that is, receivers adapted to receive signals in a range of 500 to 1500 k. 0., have employed a direct current voltage derived from received signals, and proportional' to the signal carrier amplitude, for reducing the mutual conductance, or gain, of the high frequency amplifier tubes; the latter usually being of the remote cut-off type. There are various disadvantages in such gain control systems,

and it is often desirable to vary the transmission efiiciency of the signal transmission system by controlling the signal transfer between high frequency networks of the receiver.

Now, I have found that the signal transfer between a tuned network of a high frequency amplifier circuit and a following tuned network may be readily varied, in response to changes in signal carrier amplitude, by electric-ally associating with the preceding one of the tuned networks an impedance which varies in some predetermined manner with the signal carrier amplitude. For example, the impedance can be made properly to vary with signal amplitude so that the associated tuned network feeds signal energy to the following networks in a manner such that the audio output level of the receiver is held constant.

The impedance inverting properties of transmission lines of electrical length, equal to odd multiples of a quarter wave length at the frequency for which the line is desired, are well known. By employing a quarter wave length line at the operating I. F. of a superheterodyne radio receiver, the line could be used as an impedance inversion network; and, hence is useful for controlling the signal transfer between a tuned network and. following networks. However, such a line would be impracticable, since physically it could not be tolerated in a radio receiver. However, all pass lattice type networks can be designed so that they act as transmission lines of prescribed electrical length, and, also, have the property of impedance inversion. Such an impedance inversion network employs inductance and capacity related to the terminating impedance of the network so that, as can be readily shown, the input impedance of the network is inversely proportional to the magnitude of the terminating impedance. By associating such an impedance inversion network with a tuned network in a radio receiver, and varying the terminating impedance in magnitude in response to received carrier amplitude change, it is possible to vary the input impedance of the inversion network in a manner such as to vary the signal transfer from the tuned network to the following 5 networks.

Accordingly, it may be stated that it is one of the main objects of my present invention to pro vide an automatic volume control circuit for a radio receiver, wherein the control circuit com- 10 prises a network whose input impedance is a reciprocal function of the terminating impedance magnitude, and which input impedance is electrically associated with a tuned network of the receiver; the magnitude of the terminating im- 15 pedance being varied in a sense such that the signal transfer from the tuned network to the following networks is varied to maintain the audio output level of the receiver substantially uniform regardless of wide variations is received 20 carrier amplitude at the receiver signal collector.

Another important object of this invention is to provide a receiver equipped with a high frequency amplifier feeding ademodulator through a tuned network and a variable impedance net- 5 work of the type whose input impedance is a reciprocal function of the magnitude of the terminating impedance; and the said input impedance being arranged in electrical association with the aforesaid tuned network, the carrier 30 amplitude being employed to determine the effective value of the terminating impedance of the impedance network.

Still other objects of my invention are to improve generally the efi'lciency and reliability of 35 AVG circuits for radio receivers, and more especially to provide AVC circuits, using impedance inversion networks, which are not only reliable in operation, but economically manufactured and assembled in radio receivers. 40

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 drawing in which I have indicated diagrammatically two circuit organizations whereby my invention may be carried into effect.

In the drawing,

Fig. 1 shows a circuit employing the invention,

Fig. 2 illustrates a modification.

Referring now to the accompanying drawing, and specifically to Fig. 1, there are shown only 5 those networks of a superheterodyne receiver which are essential to a proper understanding of this invention. The numeral I designates an amplifier tube between whose input electrodes are impressed signal energy at an operating intermediate frequency (I. F.) and the I. F. may have a value chosen from a frequency range of substantially to 450 k. c. It is not believed necessary to describe the networks of a superheterodyne receiver, which networks would precede the I. F. amplifier I. It is merely necessary to point out that generally such networks would comprise a signal collector feeding a tunable radio frequency amplifier; and the amplified signals being converted to the operating I. F. by a converter network which includes first detector and local oscillator circuits.

The I. F. energy, regardless of how derived, is impressed upon the I. F. transformer 2, whose primary and secondary circuits are each fixedly resonated to the operating I. F. It is to be understood that the primary circuit of transformer 2 may be coupled to the output circuit of the first detector, or it may be coupled to the plate circuit of a preceding I. F. amplifier. The cathode circuit of amplifier I includes the usual grid bias resistor-condenser network 3, and the latter provides the maximum amplification bias for the I. F. amplifier.

The tube I is preferably of a type having a low plate resistance value, and such a tube may be of the triode type. The amplified I. F. signals are transmitted to the demodulator through an I. F. transformer 4 whose primary circuit 5 is fixedly tuned to the I. F., the plate of tube I being connected to a point of desired positive potential through a radio frequency choke coil 6 having a high impedance. It is desired that the primary circuit 5 of transformer 4 have a low C ratio.

The secondary circuit 1 of transformer 4 is resonated to the operating I. F., and its high potential side is connected to the control grid of the detector tube 8; the low alternating potential side of the circuit 1 is established at ground potential. The grounded terminal is connected to the cathode of tube 8 through a path which includes a load resistor 9 shunted by a condenser I II. The condenser III has a magnitude of approximately mmf., and acts as a radio frequency by-pass condenser. The plate of tube 8 is connected to a point of positive potential, and the cathode side of resistor 9 is connected to the input electrodes of the first audio frequency amplifier of the audio frequency network through an audio frequency coupling condenser II. It is pointed out at this time that the detector 8 is of the infinite impedance diode type. Such a detector has, also, been referred to as a degenerative plate circuit detector, because there is a degenerative audio voltage across 9.

Normally, that is in the absence of signals, there is a sufficient voltage drop across resistor 9 to bias the control grid of tube 8 close to cutoff. Hence, with increasing signal amplitude, the cathode side of resistor 9 becomes increasingly positive in direct current voltage. It is not believed necessary further to describe the functioning of the detector circuit, since it has been disclosed and claimed by P. O. Farnham in application Serial No. 8,864, filed March 1, 1935. It is sufiicient to point out at this time that the voltage across resistor 9 bears a substantially linear relation to the signal input voltage impressed on circuit I.

There is disposed in electrical association with the tuned circuit 5 an impedance inversion network generally designated by the numeral I2.

The network comprises a coil I3 connected in series between circuit 5 and the plate of tube I4; and, also, a coil I4 between ground and the oathode of tube I 4'.

The tube I4 functions as the terminating impedance of the network I2; that is, the cathodeplate impedance of tube I4 acts as the terminating impedance connected between coils i3 and I4 ofthe network I2. The cathode of tube I4 is connected to ground by a path including condenser I5 and coil I4. The condenser I5 may have a magnitude of approximately 0.01 mf. The plate side of coil I3 is connected to the ground side of coil I4 through condenser It, while condenser II connects the opposite terminal of coil I3 to the junction of coil I4 and condenser I5.

Condenser I5 isolates the cathode of tube I 4 from D. 0. ground. This is necessary because the cathode is at positive D. C. potential with respect to ground. C15 is 0.01 mi. for it must have an impedance at I. F. which is much less than the lowest value that the Tp of tube I4 might attain. If C15 is too small, with a consequent appreciable impedance at I. F, it affects the operation of the impedance inverting network, since it is a part of the terminating impedance; hence with r of I 4 equal to zero, the terminating impedance is not zero, but is equal to the reactance of C15.

The cathode of tube I4 is connected to a point I8 on the power supply bleeder resistor P; the point I8 is at a positive potential with respect to the grounded end of the bleeder resistor P. The control grid of tube I4 is connected to ground through a path which includes lead 20, resistor 2| and load resistor 9. The condenser 22 is connected between the grid side of resistor 2i and ground, and the condenser cooperates with the resistor 2I to provide an audio frequency filter network which suppresses audio pulsations in the direct current voltage supplied to the grid of tube I4. The tube I4 has a relatively low plate resistance magnitude at zero bias. The plate resistance of tube I and the tuned impedance of the primary circuit 5 are of low magnitudes, and preferably less than 100,000 ohms. Furthermore, the constants of network I2 are chosen so that the Q of the coils I3 and I4 is high.

It is essential that the Q of the coils I 3 and I4 be high because this quantity defines the limits of input impedance, Z, which can be attained in the network I2. With Tp of I4 zero, the input impedance to network I2 is twice the parallel resonant impedance of coil I3 and condenser Ii. The resonant impedance is equal to the quotient of the square of the coil reactance (at I. F.) and the coil resistance, or equal to which equals QwL. The input Z is then ZQwL. Q is defined as the ratio of reactance of a coil, or a condenser, at a given frequency to the A. C. resistance of the coil, or condenser, at the same frequency. It is a measure of the power factor of the element, and is thus a figure of merit. It determines, among other things, the sharpness of selectivity of a tuned circuit in which the element is used. A high Q coil will have a power factor nearer to zero than will a lower Q coil, and will have in a conjunction with a paralleling condenser, ahigher tuned anti-resonant impedance at any frequency for which its Q is higher.

It will be observed that the input impedance of network I2 is effectively in series with the tuned impedance of circuit 5. As explained heretofore, it is possible to vary the magnitude of this series impedance by varying the magnitude of the impedance between the plate and cathode of tube I4. The magnitude of the terminating impedance of network I2 is adjusted by varying the control grid of tube l4, and the latter becomes increasingly less negative in potential as the sig nal carrier amplitude increases. It will be seen that, in the absence of signals, the control grid of tube I4 has a predetermined negative potential with respect to the cathode of the tube. By choosing this no-signal bias for tube It to be substantially cut-off, then the impedance between the cathode and plate of tube I4 is substantially infinite, and, hence, the input impedance of the network I2 will be relatively small in magnitude. The input impedance of network I2 has been shown in dotted lines in the drawing, and is generally denoted by the symbol Z.

It is not believed necessary to enter upon a theoretical demonstration that the magnitude of the input impedance Z is a reciprocal function of the magnitude of the terminating impedance of network I2. It is sufficient to point out that it can be mathematically demonstrated that the magnitude of the impedance Z is equal to the product of the ratio of the inductances of network I2 to the capacities thereof, and the reciprocal of the terminating impedance magnitude. The magnitudes of the inductances and capacities of network I2 are chosen so that network I2 is resonant to the operating I. F.; hence, the magnitude of the input impedance Z is inversely proportional to the magnitude of the impedance between the cathode and plate of tube I4. Coils I3, I4 and condensers I1, l6 are so chosen that network I2 is resonant to the I. F. in the following manner:--Coil I3 and condenser I6 are arranged so that, if they were in the circuit alone, with the Tp of tube I4 infinite, they would constitute a low impedance to the I. F., i. e., they would be in series resonance at the I. F. Coil l3 and condenser ll, if connected in parallel would be anti-resonant (parallel resonant) to the I. F. This also applies to coil I4 and condenser It. With the Tp of tube I4 infinite, coil I4 and condenser I I are in series resonance. In parallel with this arrangement, coil I3 and condenser IIi are also in series resonance. Each branch circuit has an impedance at series resonance equal to the coil resistance. Since the two series resonant circuits are in parallel, the net impedance represented by Z is half the coil resistance of the coils if the latter are identical.

If Tp of tube I4 is zero, coil I3 and condenser I! are in parallel resonance, and, in series with this arrangement, coil I4 and condenser I6 are also in parallel resonance. Hence, the impedance represented by Z is equal to twice the parallel resonant impedance of coil I3 and condenser I1.

As the signal carrier amplitude increases the cathode side of resistor 9 becomes increasingly positive in direct current voltage, and the grid of tube I4 becomes increasingly less negative. Hence, the impedance between cathode and plate of tube [4' decreases. An increase in the magnitude of the series impedance Z results in a decrease of the alternating current voltage developed across the tuned impedance of circuit 5. The limit is reached when the terminating impedance of network l2 becomes zero thereby giving the input impedance Z its maximum value,

and thus producing minimum signal transfer between circuit 5 and circuit I. The input impedance Z can thus be made to be very large or Very small by controlling the bias on the grid of tube It. The voltage division between the circuit 5 and impedance Z is, therefore, a function of the carrier amplitude; the tuned impedance of circuit 5, and the plate resistance of tube 5, are made relatively small compared to the maximum impedance of Z so that the AVG range is wide.

The variation of the cathode to plate impedance of tube I4 is at a rate such that the impedance Z is varied to maintain the signal carrier amplitude substantially uniform at the input circuit 1 of the detector 8. Of course, the network I2 need not be of the specific type shown in the drawing; for example, it may be of any other type which has the property of having its input impedance varied as a reciprocal of the terminating impedance magnitude. Again, the network I2 can be connected so that its input impedance is in series with the controlled tuned network, but precedes it with respect to the plate of the preceding amplifier tube. In general, it is to be understood that my invention covers the electrical association of a network of the type designated by the numeral I2 with a tuned network in a manner such that the signal voltage across the tuned network may be varied in response to a change in magnitude of the terminating impedance of network I2.

The arrangement in Fig. 2 differs from that in Fig. 1 in that the tube i l employs a cathode load network Lo-Co to produce a low terminating impedance for network I2 when the tube I4 is operating above cut-off bias. The condenser 6 isolates the plate of tube I from the cathode of tube It for direct current. The coil Lo and condenser Co are resonant to the I. F.; they are connected between the cathode of tube I 4' and point I 8 on the bleeder resistor P. When tube I4 is cut off the value of the impedance of Lo--Co is QwLo. As soon as the carrier amplitude increases, and the grid of tube I 4 gets less negative, the impedance between the cathode of tube I l and ground is QwLO in parallel with of tube I4. With exceptionally strong signals it is desired that the plate resistance of tube I4 be very low; this degenerative arrangement for tube It attains the latter.

If it is desired to avoid cross-modulation effects on the first R. F. amplifier of the receiver, it may be necessary to use the conventional type of AVG circuit to supplement the present AVC arrangement; such conventional AVC circuit causes the gain of the first R. F. amplifier tube to decrease as the signal carrier amplitude increases.

While I have indicated and described two 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. In combination with a, signal transmission tube provided with a tuned output circuit, an automatic signal transmission control circuit comprising a network provided with a terminating impedance and whose input impedance is a reciprocal function of the magnitude of the terminating impedance, said input impedance being electrically connected with said tuned output circuit, and means to vary the magnitude of said terminating impedance thereby to adjust in an inverse manner the magnitude of said input impedance.

2. In a system as defined in claim 1, said network including a tube whose cathode-to-plate impedance provides said terminating impedance.

3. In a system as defined in claim 1, said network having itsinput impedance in series relation between said tuned output circuit and a point of relatively fixed potential, and said varying means being responsive to signal amplitude at said tuned output circuit.

4. In a system as defined in claim 1, said network including reactances of opposite sign and being resonant to the operating frequency of said tuned output circuit.

5. In combination with a source of electrical waves, a load impedance connected to said source, a network electrically connected with said load impedance, said network being of the type whose input impedance bears a predetermined relation to the magnitude of its terminating impedance, said input impedance being electrically connected with said load impedance in such a manner that a change in the magnitude of said input impedance varies the alternating current voltage developed across said load impedance, and means for varying the magnitude of said terminating impedance.

6. In a system as defined in claim 5, said terminating impedance being provided by the oathode-to-plate impedance of an electron-discharge tube, and said varying means adjusting the magnitude of said cathode-to-plate impedance in response to variations in wave amplitude.

7. In a radio receiver, a signal transmission tube provided with a tuned output circuit feeding a detector, an automatic volume control circuit which comprises a network of the type whose input impedance is a reciprocal function of its terminating impedance magnitude, said input impedance being arranged in electrical connection with said tuned output circuit in a manner such that an increase in said input impedance magnitude causes a decrease in alternating current voltage across said tuned output circuit, and means responsive to an increase in received signal carrier amplitude for varying said terminating impedance magnitude in a sense such that said input impedance magnitude is caused to vary in a direction to maintain the carrier amplitude at the detector input substantially uniform.

8. In a receiver as defined in claim 7, said network including an electron discharge tube whose cathode-to-plate impedance provides said terminating impedance, and said varying means including a connection for varying the gain of said last named tube.

9. In a receiver as defined in claim '7, said network including a tube having a tuned load in its cathode circuit, said load providing said terminating impedance, and said varying means including a connection for varying the gain of the last named tube.

0. In combination with a signal transmission tube provided with a tuned output circuit, an automatic volume control circuit comprising an all pass lattice type network provided with a terminating impedance, the input impedance of the network bearing a reciprocal magnitude relation to the terminating impedance, electrical connections between said output circuit and the said network, and means responsive to variation in the signal amplitude for adjusting the magnitude of the terminating impedance.

11. In combination with a signal transmission tube provided with a tuned output circuit, an automatic volume control circuit comprising an all pass lattice type network provided with a terminating impedance, the input impedance of the network bearing a reciprocal magnitude relation to the terminating impedance, electrical connections between said output circuit and the said network, means responsive to variation in the signal amplitude for adjusting the magnitude of the terminating impedance, said lattice network being resonant to the operating frequency of the output circuit, and said terminating impedance including an electron discharge tube.

12. In a wave transmission system, at least one resonant circuit tuned to an operating wave frequency, a network comprising inductance and capacity in predetermined relation and a terminating impedance, the network input impedance bearing an inverse magnitude relation to the terminating impedance, said network being connected to said resonant circuit and having its input impedance in circuit with the latter, and means, responsive to amplitude variation of transmitted waves, for controlling the magnitude of said terminating impedance.

13. In a wave transmission system, at least one resonant circuit tuned to an operating wave frequency, a network comprising inductance and capacity in predetermined relation and a terminating impedance, the network input impedance bearing an inverse magnitude relation to the terminating impedance, said network being connected to said resonant circuit and having its input impedance in circuit with the latter, means, responsive to amplitude variation of transmitted waves, for controlling the magnitude of said terminating impedance, said network being resonant to said wave frequency, and said terminating impedance comprising a network tuned to said wave frequency.

CHARLES N. KINEBALL.

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