US2108899A - Automatic volume control - Google Patents

Description

Feb. 22, 1938. H. o. PETERSON AUTOMATIC VOLUME CONTROL! Filed Oct. 50, 1954 2 Sheets-Sheet 1 N M R m Y $3 32 W E DI w \x L m m T P am r H m Y u mfi fig m B l WESQ w \Q m S Feb. 22, 1938. O H. o. PETERSON AUTOMATIC VOLUME CONTROL 2 Sheets-Sheet 2 Filed Oct. 50, 1954 INVENTOR HAROLD 0.? ERSON BY fiLg ATTO RN EY Patented Feb. 22, 1938 UNITED STATES AUTOMATIC VOLUME CONTROL Harold Olaf Peterson, Biverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application October 3 0, 1934, Serial No. 750,634

4 Claims.

This invention relates to automatic volume control systems and particularly to such a system as is suitable for operation on interrupted carrier wave signalling circuits. 5 The conventional types of automatic volume control circuits used on broadcast receivers depend upon the strength of the incoming carrier wave to maintain continuously a proper adjustment of the gain in the receiver so as to produce an approximately constant output level. Such an automatic volume control system is not well suited for operation on a signal which may be periodically interrupted for considerable lengths of time. When operated on such a signal, the 5 usual kind of automatic volume control system brings the receiver gain up to a maximum value during the intervals of notransmission. Such an arrangement may bring the noise level in the output up to an objectionable value and may require transmitter operation for an appreciable length of time when transmission is resumed before conditions of receiver gain return to a normal condition. These difficulties are overcome in the present invention which discloses a circuit a particularly adapted to signalling by telegraphic codes in systems wherein the telegraphing is achieved by an on-and-oif keying of the transmitter.

A feature of the invention lies in the provision n of a capacitor which is adapted to be charged by incoming signals through a network which has high conductivity while signals are coming in but low conductivity in the absence of signals, whereby automatic volume control voltage can be rapidly built up during a short period of the incoming signal but dissipated. much more slowly during even prolonged absences of signals. In this manner there is achieved the desirable result of maintaining the gain in the receiver substantially constant at its last self-determined value during periods of no transmission, such as are normally encountered in the handling of telegraphic communication.

A better understanding of the invention may be had by referring to the following detailed description, accompanied by drawings wherein: Figs. 1 and 2 show two different embodiments thereof as applied to receiving circuits.

In the arrangement of Fig. 1, the signal received on the antenna A is amplified by a multistage radio :frequency amplifier .RFA and passed into a heterodyne detector unit HD wherein the signal is heterodyned by a local oscillator .O to produce an intermediate frequency. This intermediate or new frequency is amplified in the selective intermediate frequency amplifier, the output from which is passed through a transformer I into a rectifier I. Some output from the intermediate frequency amplifier is also passed into a second heterodyne detector unit H'D' wherein the 5 new frequency is caused to heterodynewithenergy from another oscillator O to produce an audio frequency output which is passed on through transformer 6 to excite the grids G G of electronic devices 4 and 5 respectively.

The rectified signal passing through rectifier I also passes as a unidirectional current through resistor 2. The circuit is so arranged that the rectifier l ordinarily does not pass current in the absence of signal, although it will be appreciated that there may be a slight amount of current in the circuit in the absence of signal due to a direct current potential generated in most vacuum tubes by the emission velocity of the electrons leaving the cathode. The IR drop thus produced across resistor 2 is normally used as a source for automatic control voltage to be passed back over the AVG lead to the grids of the amplifier circuits. Although the AVG (automatic volume control) lead has been herein shown connected to the grids of the radio frequency amplifier stages, it will be understood that of course it may be connected to the intermediate frequency stages if so desired. Electronic devices 4 and 5 have their grids so biased as to be conducting only at times when signal voltage appears in the secondary windings of transformer 6. When the signal voltage does appear in the secondary windings of transformer 6, electronic devices 4 and 5 become conducting and allow an equalization of the voltages appearing across resistor 2 and capacitor 8; in other words, capacitor 8 is allowed to assumea potential equal to the drop across resistance 2. This is because the upper terminal of condenser 8 is in effect connected to the upper A0 terminal of resistance 2 through a relatively low resistance when electronic devices .4 and 5 become conducting.

The voltage across capacitor 8 is the bias Voltage for the radio frequency amplifier tubes REA, and there is therefore achieyeda normal automatic volume control action .as long as there is signal voltage present in the secondary windings of transformer 6. When the signal is interrupted, there is no longer signal voltage excitation at the grids of electronic devices 4 and 5 and these devices at once become non-conducting. This leaves no conducting path except resistor 3 for the discharge of condenser 8. Resistor 3 may be made sufficiently high to make the rate of discharge for condenser 8 very low.

By placing tubes 4 and 5 in opposed relation as shown, there is obtained an AVC voltage, when signals appear on the grids G G which varies in accordance with variations in signal strength.

Fig. 2 illustrates an embodiment suitable for operation on a diversity receiving system, comprised of three independent receivers with outputs combined. The method of operation is similar to that described in connection with Fig. 1. The outputs of the three diversity receivers #1, #2 and #3 are delivered through transformers 53, I4 and 55 respectively, to rectifiers H), H and 12. These outputs combine in the common resistor I6. The IR drop in resistor I6 is connected to the grid of the tone keyer circuit in a manner normally utilized in diversity receivers and as described in United States Patent 1,874,866, granted August 30, 1932 to Harold H. Beverage et al. The same IR drop characterized by resistor i6 is also utilized to obtain automatic volume control action through a bank of electronic devices i1, i8, i9, 20, 2| and 22. These electronic devices receive excitation from the three diversity receivers through respective transformers 23, 24, 25 and are thereby made conducting only during periods of time when signal voltage appears in the secondary windings of these transformers. Of course, individual electronic devices will be made conducting by signal voltages in their associated secondary windings. By this action the IR. drop in resistor It and the voltage across condenser 26 are kept equal during periods when signal voltages appear in transformers 23, 24 or 25, or any one of them. The voltage across condenser 26 is conducted to the grids of the amplifier stages, thereby controlling the gain of such amplifier stages.

In the system of this Figure 2, no one of the receivers comprising the diversity receiving systern will be any more sensitive than is required by the one receiving the strongest signal; that is, no one receiver will have a greater sensitivity than the others.

It is to be understood that the invention is not limited to the precise arrangement of parts shown, since various qualifications can be made without departing from the spirit and scope of the invention. For example, itis possible to provide a' great number of means for causing the signal to excite the grids of electronic devices, which will cause automatic volume control action to become operative. As an illustration, this excitation might be obtained through a special tone keyer in the normal telegraph receiver, or it might also take the form of direct current voltage obtained from the output of the first tube in a tone keyer.

I What is claimed is:

' '1. In a receiving system, an amplifier in the path of the received signals, an oscillator and heterodyne detector in circuit with said path for producing audio frequency energy in accordance with the incoming signals, a rectifier also in circuit with said path for producing unidirectional current, a resistance connected across said rectifier through which said unidirectional current passes, an audio frequency transformer having a primary winding and two secondary windings, said primary winding being connected to the output of said heterodyne detector, two electron discharge devices each having an anode, cathode, and a control electrode, the cathode and control electrode of each device being individually connected to a secondary winding of said audio frequency transformer, a lead connecting the oathode of each device to the anode of its associated device, means for biasing the control electrodes of said devices to render the devices conducting only upon the presence of signal voltage in the secondary windings of said audio frequency transformer. a condenser one terminal of which is connected to a point of relatively fixed radio frequency potential and another terminal of which is connected to said amplifier for controlling the gain thereof, a connection from said last terminal to one of the leads joining the cathode of one device and the anode of the other device, a connection extending between the other lead joining the anode of said one device and the cathode of said other device to a point on said resistance. 2. In a receiving system, an amplifier in the path of the received signals, an oscillator and heterodyne detector in circuit with said path for producing audio frequency energy in accordance with the incoming signals, a rectifier also in circuit with said path for producing unidirectional current, a resistance connected across said rectifier through which said unidirectional current passes, an audio frequency transformer having a primary winding and two secondary windings, said primary winding being connected to the output of said heterodyne detector, two electron discharge devices each having an anode, cathode, and a control electrode, the cathode and control electrode of each device being individually connected to a secondary winding of said audio frequency transformer, a lead connecting the cathode of each device to the anode of its associated device, means for biasing the control electrodes of said devices to render the devices conducting only upon the presence of signal voltage in the secondary windings of said audio frequency transformer, a condenser, one terminal of which is connected to a point of relatively fixed radio frequency potential and another terminal of which is connected to said amplifier for controlling the gain thereof, a connection from said last terminal to one of the leads joining the cathode of one device and the anode of the other device, a connection extending between the other lead joining the anode of said one device and the cathode of the other device to a point on said resistance, and another resistance shunting said last two connections.

3. In combination, in a diversity receiving sys-' tem, a plurality of separate receivers each including an amplifier arrangement in the path of the received signals, an oscillator and a heterodyne detector in circuit with each of said amplifier arrangements for producing beat frequency energy, a beat frequency transformer for each receiver having a primary and a secondary winding, said primary winding being in circuit with the output of its associated heterodyne detector, an electron discharge device for each receiver, each device having an input and an output circuit, said input circuit being connected across the secondary winding of its associated beat frequency transformer, and said output circuits of all electron discharge devices being connected together, means for biasing said input circuits to render said devices non-conducting in the absence of signal voltages in said secondary windings, a condenser connected to said output circuits and to all of said amplifier arrangements for controlling the gain thereof whereby no one of the receivers in said system will be any m sensitive than is required by the one receiving the strongest signal.

4. In a receiving system, a radio frequency amplifier having gain control means connected thereto, a heterodyne detector in the path of output energy from said amplifier, an intermediate frequency amplifier having an input circuit connected to said detector and an output circuit, a second heterodyne detector adapted to receive signaling energy from the output circuit of said intermediate frequency amplifier, an electron discharge device included in said gain. control means and operable under control of output energy from the second said heterodyne detector to be rendered conductive in the presence of signals and relatively non-conductive in the absence of signals, a capacitor and a resistor in circuit with said electron discharge device, and means for charging said capacitor rapidly during periods of signal reception while permitting said capacitor to discharge slowly through said resistor during periods of cessation of signals thereby to render said gain control means suitably eifective 10 upon said radio frequency amplifier.

HAROLD OLAF PETERSON.

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