US2294117A - Automatic volume control circuit - Google Patents

Description

Aug# 25, 1942 R. L. HoLLlNGswoR-rH .2,294,117

AUTOMATIC VOLUME CONTROL CIRCUIT Filed June 4, 1940 EE HOLLINGSWORTH BY 77% M TTOHNE Y Patented Aug. 25, 1942 ire-D STAT-ss enr-snr ofrFl-f-ce to Radio Corporation o tion of Delaware f America, a v.corpora- Application June 4, 1940, Serial'No.,338,'680

(Cl. .Z50-20) Claims.

My present invention relates generally to automatic gain control circuits, and more particularly to automatic volume control circuits of the vtype adapted for use in receiving signals in the ultra-short wave band.

Automatic gain control arrangements which are in use at the present time in receiving sig- `nals in the 4-30 megacycle (mc.) band have several operating limitations. In the first place, the rate of vcompensation is the same for ascending and descending signal carrier levels with the result that on the ascending portions of the fading periods blasting effects are had. In other words, during the rapid fading which is experienced in the ultra-short wave band, the conventional type of automatic gain control circuit `produces a rate of compensation which is not wherein the action of the control circuit approaches that which is had in short wave communication when employing two-set diversity reception for telephone communication, and the essential and outstanding characteristics of the novel control circuit comprising instantaneous fading compensation without reducing the low audio frequency response, and the AGC time constant network being so chosen as to cause smooth receiver gain control action thereby substantially eliminating blasting effects on the ascending portion of the rapid fade.

Another important object of this invention is to provide an automatic volume control circuit for a short wave receiver wherein the volume control circuit is provided with a device of unidirectional conductivity, and a time constant network whose magnitude is sufficiently large to provide substantially smooth fading compensationffor rapid fading without effect on the lower audio frequencies of the modulation range.

Still other objects of my present invention are to improve generally the eiiciency and reliability of automatic gain control circuits for short wave receivers, and more especially to provide an automatic gain control circuit for short Wave receivers which is simply and economically `assembled inthe receiver.

The novel features-Which I believelto be characteristic of my invention are set forth in :particularity in theappendedclaims; the vinvention itself, however, `as to both -its organization and method of operationwill best be understood :by reference to the following description taken in connection with the drawingin which Ihave indicated diagrammatically Va circuit organization whereby my invention may be-carried into effect.

In the drawing:

Fig. 1 shows a short wave superheterodyne receiver embodying the present invention,

Fig. 2 graphically shows the effect of the invention on the lower portion of the audio range,

Fig. 3 graphically illustrates the manner in which the present invention provides smooth fading compensation.

Referring now to the receiving system shown in Fig. 1, it is to be understood that the latter may be a superheterodyne receiver of the type shown employed in short Wave telephony. The receiving band in such a receiver may be of the order of 4-30 mc., and the receiver will be provided with a dipole I* for collecting the short wave signals. The antenna may be of the usual grounded antenna type if desired, or it may even be a radio frequency line capable of distributing signals in the specied signal range. The signal collector device is coupled to the tunable input circuit 2 of the radio frequency amplifier 3, and the amplier may comprise one or more stages of tunable modulated carrier amplification. The amplified modulated carrier signals are impressed upon the tunable input circuit 4 of a following rst detector 5, and the output circuit of the latter is designated by numeral 6.

It will be understood that circuit 6 is resonated to the operating intermediate frequency (I. F.). The latter may be chosen from a range of 'l5-450 kc. The numeral 1 denotes the usual tunable local oscillator which produces oscillations of a frequency constantly diiering from the frequency'of the signals impressed'upon the rst detector 5 by the value of the I. F. The numeral 8 designates the customary Yand conventional tuning control mechanism which mechanically couples the adjustable elements ofthe various tuning devices of the tunable circuits of networks 3, 5 and 1. The I. F. amplifier 9 may comprise one or more stages of I. F. amplication, and the input circuit I3, coupled to the detector ,output circuit 6, is Xedly-tunedito the I. F. value.

The I. F. output circuit II is coupled to the input circuit I2 of the second detector I3. The latter is shown as being of the diode type, and has its anode connected to the high alternating potential side of the input circuit I2. The low potential side of input circuit I2 is connected to ground by a load resistor I4, while the cathode of diode I3 is also grounded. There is developed across resistor I4 rectied voltage which is derived from the modulated carrier signals at I. F. The audio frequency component of the rectified I. F. currents is fed to a succeeding audio frequency network through condenser I5. 'Ihe audio network may comprise one or more stages of audio amplication, and the last stage may be followed by any desired type of reproducer. The direct current voltage component of the rectified I, F. currents is utilized in two paths now to be described.

One of these paths comprises a so-called noise silencer device. The device comprises an electron discharge tube I 6 whose input grid I'I is connected to the anode end of resistor I4. The plate I8 of tube I6 is connected to ground through a path which includes the direct current source I9 and the resistor 20, it being noted that the plate I8 is connected to the positive terminal of source I9. The cathode of tube I6 is at ground potential. Hence, when normal electron iiow takes place from the cathode to the plate I8 of tube I6 direct current voltage is developed across resistor 20.

The magnitude of the direct current voltage developed across resistor 2|] depends upon the bias of grid I'I with respect to the cathode of tube I 6. The bias of grid I I in turn is dependent upon the direct current voltage developed across resistor I4 due to rectication of the I. F. signal currents. The second path which utilizes the direct current voltage developed across resistor I4 comprises a device of uni-directional conductivity, such as diode 2I. The cathode 22 of the diode 2I is connected through resistor 23 to the anode end of resistor I4. The cathode 22 is also connected through resistor 25 to the ungrounded end of resistor 2i). The anode 30 of diode 2I is connected to ground through the resistor element 3l, the latter being shunted by condenser 32. The anode 30 has direct current voltage connections 40 and 4I to the stages of the receiver whose gain is to be controlled in dependence on the magnitude of the signal carrier. In other words, the connections 40 and 4I are the automatic volume control (AVC), or automatic gain control, which is utilized to vary the gain of each of the controlled stages in such a sense that the carrier amplitude at the detector input circuit I2 is substantially uniform over a wide range of carrier amplitude variation at the signal collector I.

In the absence of received signals there is substantially no direct current voltage developed across resistor I4. Consequently, the grid I1 of tube I6 has minimum negative bias. This results in maximum space current now through tube IB with a resulting large direct current Voltage developed across resistor 20. Since cathode 22 of diode 2| is connected to the end of the resistor 20 which becomes increasingly negative with respect to ground as the voltage drop across resistor 20 increases, it follows that anode 30 of diode 2I will assume a positive potential with respect to cathode 22. In that case diode 2| is conductive, and, therefore, the controlled stages 3 and 9 will have negative bias applied thereto.

In other words, assuming that the signal grids of each of the radio frequency and intermediate frequency amplifiers are connected to connections 40 and 4I, then these signal grids will be established at a high negative bias when the drop across resistor 20 is a maximum. As a result the gain of controlled stages 3 and 9 will be greatly i reduced, and the electrical noise impulses, which audio frequencies.

normally would be detected at I3, will not be heard due to the greatly reduced gain of ampliers 3 and 9.

However, as the received signals increase in amplitude, direct current voltage is developed across resistor I4. This causes the negative bias on grid I1 .to increase thereby reducing the voltage developed across resistor 20. A point is soon reached where the diode 2| is non-conductive. This means that the gain of each of stages 3 and 9 is restored to optimum. As the received signals increase in amplitude above a predetermined weak carrier amplitude the direct current voltage developed across resistor I4 increases in magnitude, and eventually completely cuts off tube I6 so that no voltage is developed across resistor 20. However, since the cathode 22 is connected through path 23 to the anode end of resistor I4, the cathode 22 becomes negative with respect to the diode anode 30 with the result that the diode 2| again becomes conductive, and permits the increasing negative bias derived from resistor I4 to be applied over connections 40 and 4I as an AVC bias. Resistors 23 and 25 function as filter resistors, and may each have magnitudes of approximately 200,000 ohms.

The network 3I-32 has a large time constant, and resistor 3I may have a value of approximately 1 megohm whereas condenser 32 may be chosen from a range 0.1 to 1.0 microfarad. The large time constant network 32-3I permits very fast AVC action to occur without reducing the low As a matter of fact there is had an accentuation of the lower audio frequencies. In Fig. 2 there is shown (curve A) the reduction of the lower audio frequencies, up to about 100 cycles, when the time constant network 32-3I has a small time constant value. The curve B denotes the accentuation of the lower audio frequencies up to about 100 cycles, caused by the utilization of the large time constant network in conjunction with the diode device 2|. This compensation of low audio frequency curtailment, and indeed accentuation of the lower audio frequencies, is of advantage in receiving short Wave telephony signals since the quality of these received signals is maintained substantially higher than has been thought hitherto possible.

There is also secured smooth control action because of the provision of a reasonable amount of hold over in the time constant network 32-3I. In Fig. 3 the full line curve shows an illustrative rapid fading period. The dash line surve, designated Prior art AVC, illustrates the type of fading compensation which has been secured in the prior art when using a fast acting AVC circuit. The dash line curve designated Effect of large time constant illustrates the smooth AVC action which is secured with the present invention, and such smooth rapid fading compensation is provided by the combined actions of the diode 2I and the large time constant network 32-3I. Automatic gain control systems in use at the present time have a rate of compensation which is the same for ascending or descending signal carrier levels.

On the ascending part of a fade, the present invention permits the number of negative electrons to increase very rapidly in the time constant network 32-3l through a one-Way circuit thereby positively controlling the upper halves of the carrier magnitude increase. If a conventional gain control system were allowed to operate as fast as in the present case, reduction of the audio frequency compensation would result. In the present case, as has been previously pointed out, the magnitudes of the time constant elements are increased so that there is produced a considerable amount of audio compression in the receiver audio output. The instantaneous action of the AVC circuit through unidirectional device 2| permits the condenser 32 to act in the manner of a low audio response equalizer across the diode output, but only through that period of time when the flow of electrons will be increased through the series feed tube, i. e., on sudden carrier level increases or on modulation level increases. A degree of compression with improved carrier control greatly improves the overall equality of the set when listening in on short wave,'

long distance circuits.

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:

l. In an automatic volume control circuit for a radio receiver, a rectifier having a signal input circuit and a load element across which is developed rectified signal voltage, and automatic volume control path connected to said load element which includes a series device of uni-directional conductivity, said device being rendered conductive upon development across said load element of rectified voltage whose magnitude eX- ceeds a predetermined amplitude, and a relatively large time constant network operatively associated with said series device for rendering the automatic volume control fast and relatively smooth, a second device having a second load element in circuit therewith for developing a direct current voltage, means for applying said direct current voltage to said rst device thereby rendering the latter conductive for signals less than a predetermined minimum amplitude, and means responsive to the voltage developed across said rst load element for rendering the second device ineffective.

2. In a radio receiver provided with -a detector adapted to develop direct current voltage whose magnitude is proportional to the received signal carrier amplitude, an automatic gain control connection to said detector, said connection including a diode in series therein, said diode being rendered conductive upon the development of said voltage to a value above a predetermined magnitude, means responsive to said voltage when less than said predetermined magnitude for rendering said diode conductive independently of said developed voltage, a time constant network of relatively large magnitude operatively associated with said diode.

3. In a radio receiver of the type including a modulated carrier signal amplifier arranged in cascade with a detector, means operatively associated with said detector for deriving a uni-directional voltage from detected modulated carrier signals, an automatic gain control connection between said detector and said amplifier, said connection including a device of uni-directional conductivity which is so constructed, and so arranged in said connection, that the device becomes conductive in response to said uni-directional voltage exceeding `a predetermined amplitude, means responsive to said voltage decreasing below said amplitude for rendering said device conductive thereby to bias said ampliiier, and said connection including a large time constant network for rendering the volume control action relatively fast but relatively smooth.

4. In a radio receiver of the type including a modulated carrier signal amplifier arranged in cascade with a detector, means operatively associated with said detector for deriving a uni-directional voltage from detected modulated carrier signals, an automatic gain control connection between said detector and said amplifier, said connection including a device of uni-directional conductivity as the sole coupling element between the detector and amplifier, said device being so constructed, and so arranged in said connection, that the device becomes conductive in response to said uni-directional voltage exceeding a predetermined amplitude, means responsive to Said uni-directional voltage in excess of said predetermined amplitude for rendering said device conductive, said last means being rendered inef- `fective in response to an increase in uni-directional voltage substantially above said predetermined amplitude, and said connection including a large time constant network for rendering the volume control action relatively fast but relatively smooth, and the constants of said time constant network being so chosen that the low modulation frequency amplitudes are substantially accentuated.

5. In a radio receiver of the type including a modulated carrier signal amplifier arranged in cascade with a detector, means operatively associated with said detector for deriving a unidirectional voltage from detected modulated carrier signals, an automatic gain control connection between said detector and said amplifier, said connection including a device of uni-directional conductivity which is so constructed, and so arranged in said connection, that the device becomes conductive in response to said uni-directional voltage exceeding a predetermined amplitude, and said connection including a large time constant network for rendering the Volume control action relatively fast but relatively smooth, a device adapted to produce a second uni-directional voltage, means for applying said second voltage to said device of uni-directional conductivity thereby rendering the latter conductive when the second voltage has an amplitude above a predetermined value, and means responsive to said first uni-directional voltage for rendering the said second uni-directional voltage device ineffective.

R, LEE HOLLINGSWORTH.

Images