US2039663A - Receiver manual and automatic gain control circuits - Google Patents

Description

May 5, 1936. H A. ROBINSON RECEIVER MANUAL AND AUTOMATIC GAIN CONTROL CIRCUITS Filed No v. 5, 1954 q -w T whllnmwl l I! W x (v \m Kg mw n U HNQN mw m g 07 "Q Iv. W w [M g M 3 m 3 3 W v *QNNQQ gm Patented May 5, 1936 UNITED STATES PATENT OFFICE RECEIVER MANUAL AND AUTOMATIC GAIN CONTROL CIRCUITS of Delaware Application November 3, 1934, Serial No. 751,317

7 Claims.

My present invention relates to circuit arrangements for selecting automatic or manual gain control in a radio receiver, and more particularly to circuit arrangements of this type which are adapted for airplane radio receivers.

One of the primary objects of the presentinvention is to provide a simple and efficient arrangement for permitting the selection of automatic or manual volume control for radio receiving equipment wherein a simple switching arrangement is utilized, and the same physical volume control is employed in both positions of the switching arrangement. I

Another important object of this invention is to provide a circuit arrangement for gain control of radio receivers wherein the simplicity of the switching, and the requirement of a single volume control potentiometer for both the manual and automatic functions are important additions.

- Another object of the invention is to provide a volume control device for a radio receiver which is capable of being manually adjusted, and which functions as a sensitivity control by varying the bias on the control grids of the radio frequency and/or intermediate frequency amplifier tubes, and wherein the same volume control functions to adjust the audio output level when a simple selector switch is utilized to place the receiver under the control of an automatic volume control circuit.

Still another object of the invention is to provide a simplified circuit arrangement for selection of automatic or manual volume control, the arrangement being particularly applicable on radio receiving equipment designed for reception of telephone or other modulated signals involving an uninterrupted average carrier amplitude where AVC is desirable, with quick change to manual control for reception of C. W., or modulated signals, in which the carrier is interrupted periodically.

Still other objects of the invention are to improve generally the simplicity and efliciently of circuit arrangements for selecting automatic or manual volume control in radio receivers, and to particularly provide such circuit arrangements which are not only efficient. in operation, but economically constructed and assembled in radio 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 drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, which represents a typical communication re- 5 ceiver particularly adapted for aircraft use, the receiving system generally embodies an R. F. amplifier, a first detector, a local oscillator, an I. F. amplifier, a combined second detector and AVC tube, and. an A. F. amplifier. The circuits associated with each of these successive stages are conventional in nature, and for this reason these circuits are shown in such a manner that those skilled in the art will readily understand their functions, but they will not be described in any detail. The superheterodyne receiver shown is to be understood as being applicable not only to aircraft receivers, but to numerous commercial receivers which are usually employed for C. W. and modulated reception.

In general, the purpose of the present invention is to control the gain of the receiver automatically, or manually, and to provide a simple device for switching the receiver from manual to automatic gain control, and, furthermore, to provide two types of manual gain control; one functioning when the AVG network is operated, and the other functioning in the absence of AVG operation. These two types of manual gain control utilize, in the interests of simplicity, a common manually adjustable potentiometer. The switching ar rangement and the manually adjustable potentiometer are both disposed in a control box which is connected. to the receiver proper by means of a shielded cable; the control box, additionally, being connected to a power supply unit (not shown) through a shielded power cable.

The essential circuit features of the superheterodyne receiver will now be described, it being noted again that circuit details are omitted because they do not constitute a part of the present invention. The receiving system is adapted to operate in a frequency range of 2000 to 6600 kilocycles, and utilizes exponential pentode tubes of the 39 type in the R. F. and I. F. amplifier stages, as well as in the first detector stage. The multiple function tube used in the second detector network is of the type, while the A. F. amplifier tube is a power pentode of the 89 type. The second detector tube 85 includes a diode anode 86 which is connected by lead 81 to the signal grid circuit of R. F. amplifier tube 39, and to the signal grid circuits of first detector tube 39 through resistor R2 and lead 88.

The lead 88 is also connected to the signal grid 55 circuit of the I. F. amplifier 39". When the oathode of tube 85 becomes negative with respect to the anode 85, then the signal grids of the Com trolled tubes 39, 39', and 39" are simultaneously rendered increasingly negative with respect to the cathodes of these tubes. This results, of course, in a reduction in the gain of the controlled tubes, and a consequent decrease in the volume of the receiver. The automatic volume control arrangement shown is incorporated in the receiver to reduce the effects of fading which is prevalent in the frequency range in which thereceiving system shown operates. The automatic volume control arrangement additionally minimizes the frequent adjustment of the volume control when approaching or leaving the airport transmitters.

The tuning condensers of the radio frequency amplifier, first detector and local oscillator have their rotors arranged for mechanical uni-control, as shownin dotted lines, and. the tuning control may be of the remote control type, with the control unit mounted on the control box. Additionally, the wave changing switches designatedby the numeral 10 may be mechanically uni-controlled, and adjusted from the control box. The control box is compact, and can be mounted in any small, though easily accessible, place in an airplane. For remote control tuning flexible tuning shafts are supplied,'and are easily fitted to have the tuner-adjusted from the control box. The flexible power cable and receiver cable :which run to the control box are fitted with removable plugs and receptacles These plugs and receptacles are conventionally represented in the drawing, and it is believed that their representations are self-explanatory.

The male plug on the control box receives a female plug on the power cable connecting to the power supply unit, while the receiver cable terminates in a male plug fitting thefemale receptacle on the control box. The flexible tuning shafts are'supplied with aslotted end fitting which engages with a pin on the tuning control shaft (or the band change switch) in the control box. Short slotted'shafts are furnished for interconnecting the tuning and band change controls on the control box (not shown to preserve simplicity of drawing) with their respective shafts in the receiver proper. These various control constructions are well known to those skilled in the art,

' and it is believed that the aforegoing brief description will be suflicient for the purpose of this application. t

Thepower supplyunit maybe of any well known type, and, for example, a vibrator-rectifier unit may be employed. In such a construction a full wave vibrator breaks the primary current, and the'correspondingly induced secondary voltage is rectified by an 84 type full wave rectifier. A suitable filter removes the ripple and radio frequency interference. If a dynamotor unit is employed as the power supply unit, a suitable filter is utilized in connection with the generator of the unit for reducing the commutator ripple and radio frequency interference. Both of these types-of'power unit can be supplied for operation from a 6 volt battery, as well as the usual 12 voltairplane battery. In general, the power supply unit'should be located fairly distant from the receiver antenna lead, and as near the airplane battery as possible. The antenna A should be located as far as possible from the motors and other source of electrical interference. The antenna is a standard airplane vertical pole antenna, andthe receiving system employs a good short, direct, low resistance ground to the frame of the airplane. I

The cathode of the second detector tube 85 is connected to the B terminal of the control box and power supply through a path which includes resistor R15 and lead 5'. The grid of tube 85 is connected to the anode side of resistor R11, and for this reason becomes increasingly negative in potential with respect to the cathode. of the same tube as the received signal amplitude increases.

As the signal grid of tube 85. becomes more and more negative, the space current flow through resistor R15 decreases, with the result that the cathode of tube 85 becomes negative with respect to anode 86. Thus, when anode 86 becomes positive with respect to the cathode of tube 85 a flow of electronic current occurs through the AVG network including resistors R2 and R3 and the signal grids of the controlled tubes become increasingly negative with respect to their cathodes. In other words, up to a predetermined intensity of received signal amplitude the anode 86 is essentially negative with respect to the cathode of tube 85. When this predetermined signal amplitude level is exceeded the voltage of anode 86 becomes positive with respect to the cathode of tube 85.

Received signals, collected by antenna T, are impressed upon the R. F. amplifier tube 39. This tube, of course, has a normal signal grid bias which is produced by the usual cathode circuit resistor. The amplified signal energy is then impressed upon the signal input circuit of the first detector tube 39', and local oscillations are impressed upon the cathode coil L1 disposed in L.

the cathode circuit of the first detector tube. The local oscillator is a triode of the 31 type, and its circuits are too Well known to require any detailed explanation. The coil in the grid circuit of the oscillator is coupled tothe cathode coil L1.

The beat frequency energy, at the operating intermediate frequency, is transmitted to the amplifier 39 through the tuned coupling network M1, and the amplified intermediate frequency energy is impressed upon the tuned diode detector circuit of tube 85 through the tuned coupling network M2. There is impressed upon the signal grid of tube 85 both direct current and audio frequency componentsof the detected signal energy, and the audio component is transmitted through coupling network M3 to the audio amplifier 89. The audio frequency component is amplified, and then impressed through audio frequency transformer T1 upon anydesired type of reproducer.

The switch S in the control box is the volume control selection switch, and is of the doublepole, double-throw type. One arm of this switch is grounded, while the other arm is connected to a tap P which is adjustable along resistorRmz. One pair of selection switch contacts are contacted by the arms of the selection switch for the operation of the AVG network, and this position of the selection switch, as Well as the contacts, is represented by the reference character A. The other pair of contacts are designated by the reference character M. One of the contacts A is connected through lead I00 and resistor R3 to the AVG leads 88 and 81. The other contact of this pair of contacts A is connected by the condenser C100 to the telephone jacks. One of the pair of contacts M is connected to the contact adjacent condenser C100, while the remaining contact of this pair is connected to AVC lead 88.

The resistor R102 is connected between the telephone jacks and the plate circuit of the audio amplifier 89. One end of resistor R102 is grounded, and a resistor R14 connects the telephone jacks and the secondary winding of transformer T1 to the -B side of resistor R15.

When the switch S is thrown to the M position, the gain of the receiver can be manually adjusted by moving tap P. This adjustment varies the negative bias voltage of the radio frequency amplifier, first detector and intermediate frequency amplifier. When the switch S is thrown into the A position, or the AVG position, adjustment of the tap P varies the audio output level of the receiver. In other words, adjustment of tap P in this A position of the selector switch does not affect the negative grid bias voltages of the tubes 39, 39' and 39".

Considering the circuit details of the two positions of the volume control selector switch, it is first pointed out that in the M position of selector switch S the audio output circuit iscom pleted through the capacitor C100 to ground. The arm of the tap P is, in this position of the selector switch, connected to the grid return leads of the tubes 39, 39 and 39" through the radio frequency filter resistors R1, R2 and R0. The negative control voltage is obtained at the volume control through the cable lead 1 and the audio filter resistor R14 which is connected to the negative (ungrounded) terminal of the B supply (180 volts). In this position of the volume selector switch the end of resistor R3 is disconnected from ground, thus preventing the functioning of the AVG network.

When the switch S is thrown into the A position, the AVG position, the switch grounds resistor R3 and completes the circuit across which the AVG anode 86 of tube 85 can develop negative potential depending upon the signal amplitude. This position of the selector switch provides automatic gain control of the radio frequency amplifier, the first detector and the intermediate frequency amplifier. The tap P is then connected through the capacitor C100 to the output telephone jacks, and adjustment of the tap along resistor R102 varies the audio volume level. In this position of the selector switch S movement of the tap P along resistor R102 does not result in any variation of the negative grid bias of tubes 39, 39' and 39". The capacitors C100 and C25 are capacitors which readily transmit the audio frequencies, and act to keep the B voltage from shorting through the output transformer secondary 296. Filter resistor R14 is required to isolate the audio output circuit from the relatively low impedance by-pass to ground on the -B supply.

It is believed that the operation of the circuit arrangement including the present invention will be clear from the aforegoing description and the circuit diagram of the drawing. The low impedance head telephones are inserted in the jacks, and the receiver energized by means of the On-Off switch in the control box. The dial lamp in the control box shows by its brightness whether the filaments of the tubes are up to their operating temperature. Assuming that the band change switch has been thrown to the desired frequency band, the receiver is tuned rather carefully because of the extreme degree of selectivity which is provided in this receiver. When the receiver is being tuned the selector switch S is thrown to the AVG position, and the tap P can then be adjusted to vary the audio level, and a suitable volume level may be easily selected. This is used particularly for the receptionoftelephone-or other modulated signals involving an uninterrupted voltage carrier amplitude. When it is desired to receive C. W., or.

modulated signals in which the carrier is interrupted periodically, it is only necessary to throw the switch S to the M positionand control the gain, or sensitivity, of the receiver by'varying the tap P.

' While I have indicated and described a system amplifier and an'output device, means for automatically controlling the gain of said radio frequency amplifier, means including a manually adjustable resistance for controlling audio transmission through said receiver, means including a manually adjustable resistance for controlling the sensitivity of said radio frequency amplifier, switch means adjustable to one position to render said automatic gain control and said audio transmission control means operative while rendering said sensitivity control means inoperative and adjustable to a second position to render said automatic gain control and said audio transmission control means inoperative while rendering said sensitivity control means operative, both said manually adjustable resistances consisting of a common variable resistor.

2. In a radio receiver, the combination with a radio frequency amplifier, a detector, an audio amplifier and an output device, means for automatically controlling the gain of said radio frequency amplifier, means including a manually adjustable resistance for controlling audio transmission through said receiver, means including a manually adjustable resistance for controlling the sensitivity of said radio frequency amplifier, switch means adjustable to one position to render said automatic gain control and said audio transmission control means operative while rendering said sensitivity control means inoperative and adjustable to a second position to render said automatic gain control and said audio transmission control means inoperative while rendering said sensitivity control means operative, the manually adjustable resistance of said audio control means being the adjustable resistance included in the radio frequency sensitivity control means.

3. In a radio receiver, the combination with a radio frequency amplifier, a detector, an audio amplifier and an output device, adjustable means for controlling the sensitivity of said radio amplifier, adjustable means for controlling the ratio of the output level at said output device to the input level at said detector, and switch means adjustable to alternative positions to render either of said adjustable means operative and the other inoperative, and both of said control means including a common adjustable resistor.

4. In a radio receiver, the combination with a radio frequency amplifier, a detector, an audio amplifier and an output device, an adjustable means for controlling the sensitivity of said radio amplifier, adjustable means for controlling the ratio of the output level at said output device to the input level at said detector, both said adjustable means comprising a common variable resistor, and switch meansadjustable to alternative positions to render either of said adjustable :means operative and the other inoperative, a control box remote from the two amplifiers and detector, and said aifiustable means and switch means being disposed in said control box.

5. In combination with a radio frequency amplifier, a detector, and an audio frequency am-- plifier, manually operable means for controlling the volume of the output of the audio amplifier, automatic means for controlling the volume of the audio amplifier irrespective of the amplitude of incoming signals, manually operable means for controlling the sensitivity of the radio frequency amplifier, both of said manually operable means having a variable impedance in common, and manually operable means for selecting the type of volume control desirable.

6. In combination, in a radio receiver of the type including a radio frequency amplifier, a first detector, an intermediate frequency amplifier, a second detector, an audio frequency amplifier, and an output device, manual sensitivity control means for varying the signal grid bias of the radio frequency and intermediate frequency amplifiers and the first detector, manual volume control means for varying the audio frequency output level in the audio frequency amplifier, both said manual control means including a common variable resistor, and switch means for selecting the type of manual volume control desirable.

7. In combination, in a radio receiver of the type including a radio frequency amplifier, a first detector, an intermediate frequency amplifier, a second detector, an audio frequency amplifier, and an output device, manual sensitivity control means for varying the signal grid bias of the radio frequency and intermediate frequency amplifiers and the first detector, manual volume control means for varying the audio frequency output level of the audio frequency amplifier, both said manual control means including a common variable resistor, switch means for selecting the type of manual volume control desirable, and an automatic gain control network, operatively associated with said manual audio volume control, adapted to be rendered operative when the manual audio volume control is rendered operative.

HARRIS A. ROBINSON.

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