US2288434A - Automatic gain control system - Google Patents

Description

June 30, 1942. w. E. BRADLEY 4 AUTOMATIC GAIN CONTROL SYSTEM Filed March 1 4, 1941 Patented June 30, 1942 2,288,434 AUTOMATIC GAIN corrrnor. sYs'rsM William E. Bradley, Northampton. Pa, aasignor to Phllco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application March 14, 1941, Serial No. 383,454

10Claims.

This invention relates to automatic gain control devices for use in receivers of modulated carrier wave signals. The device of the invention is particularly adapted for use in superheterodyne receivers wherein the second detector output is lower than usual and, therefore, cannot be used directly for automatic gain control purposa. The invention provides an unusually large variation in control voltage for a given variation in signal level without the use of additional means for amplifying the control signal. More specifically, means are provided whereby a control voltage may be developed which is substantially equal to the peak-to-peak amplitude 01 the modulated carrier wave signal from which the control is derived.

Objects of the invention are to provide: An improved automatic gain control system adapted to maintain the average amplitude of a signal constant; an improved automatic gain control system adapted to maintain the average carrier constant in a system in which the average amplitude of the signal varies; an improved automatic gain control system adapted to maintain the average signal amplitude constant in a system in which the average carrier level varies, as in a television system; an improved delayed automatic gain control system; an improved volume compressor for use in a system in which the average carrier, level is constant; and an improved means for deriving an automatic gain control voltage from a signal transmission channel without placing any appreciable additional load on the channel due to the presence of the deriving means.

The invention will be described with reference to the accompanying drawing, in which Fig. l is a diagram of a circuit embodying the various features of the invention; and

electrical signalling system employing a modulated carrier wave signal.

In Fig. l the signal channel has been generally represented as comprising a source 01' signal I, a variable gain amplifier 2, and a signal utilization means 3. The source may be regarded, for the present, as comprising the radio frequency amplifier and converted stages of the receiver,

the signal provided by the source being of inter-- I mediate frequency. The variable gain amplifier would accordingly be the intermediate frequency stages; and the utilization means would include the second detector, video amplifier stages, and finally the picture reproducing device. The standard television signal is one in which, before transmission, the average carrier level varies,

while the average amplitude of the signal peaks is essentially constant. In the course of transmission, the signal may be so modiiied that the average amplitude is no longer constant when the signal is received. To restore the signal to its initial form in which the peaks are of substantially constant amplitude, it has been found convenient to obtain a control voltage which varies with the synchronizing signal peaks 0! the composite television signal. This voltage may be obtained by peak detection of the modulated carrier wave television signal where nega-- tive modulation is employed. or by peak detection of the detected signal regardless oi what the polarity of modulation may have been. The control voltage thus derived may be utilized to control the gain of the system. However, devices heretofore used for this purpose have the disadvantage that the control voltage magnitude is limited to something less than half the peakto-peak amplitude of the modulated carrier wave signal. By the present invention there are provided means for obtaining a control voltage, the magnitude of which may approach the overall peak-to-peak amplitude of the modulated signal level. This will be explained more fully hereinafter. A second diode space discharge device 9 has its cathode directly connected to the anode of diode 4 (through switch i0) and has its anode grounded through a load resistor ii by-passed by condenser I 2 when the switch I! is in position a.

Referring to the operation of the circuit Just described, the diode I will draw current on the positive signal peaks. The condenser 1 is made sumcientiy large efiectively to by-pass currents of carrier frequency so that a D. C. voltage proportional to the positive peaks of the modulated carrier wave signal will appear across the resistor t. This voltage will add to the modulated carrier signal voltage appearing on the anode of the diode 3 to yield a modified signal whose positive peaks will have been brought to a substantially common level. This will be understood more clearly by reference to Fig. 2 where at A there is represented the modulated carrier wave signal derived from the output of the amplifier 2. At B there is represented the leveled signal which will appear at the anode of the diode 6. It will, of course, be understood that the extent to which the carrier frequency peaks are leveled will depend upon the rapidity with which their amplitudes vary and upon the magnitude of the time constant of the impedance in the cathode circuit of the diode G. In the present instance, where the television signal may have horizontal sy chronizing signal peaks risingv above the normal level of modulation at intervals of approximately ninety microseconds and where the time constant of the circuit comprising condenser l and resistor ii is of the order of magnitude of this interval, only the synchronizing signal peaks themselves will be leveled in the manner illustrated at H in Fi 2.

The diode 9 will draw current only on the negative peaks of the leveled carrier wave signal and there will tend to be developed across the resistor a voltage proportional to the negative peaks of said signal. It isdesired, oi course, that the volt= age so developed should follow only the negative synchronizing peaks and that it should not be subject to the more rapid variations of the video modulation. Accordingly, the condenser i2 is chosen to give to the circuit ii, 02 a time constant large by comparison with the intervals between synchronizing pulses. With the switch 06 in position d, the voltage developed across re-= sistorfll will be supplied to the amplifier 2 to control the gain thereof.

It was observed that the D. C. bias applied to the cathode of the diode 41 might serve to determine the amount of delay in the development of gain controlling voltage since, by maintaining the cathode at a positive potential with reference to ground, the diode may be prevented from drawing current until the carrier signal peaks exceed the bias voltage. Obviously, if no delay is desired the cathode of the diode 3 may be connected directly to ground by closing the switch 85. An alternative method of obtaining delay is by providing a battery or other suitable bias source 'in'the connection between the anode of the diode 0 and the cathode of the diode 9. In the circuit of Fig. 1 this may be done by openhis the'switch iii to insert the battery 10. In this case, although the diode l is permitted to draw current as soon as any signal is present, the diode 0 cannot conduct until the negative signal peaks fall below a certain level.

a sence systems and at g for the system in accordance with this invention.

Where, as in the case just described, the system is adjusted to operate on the peaks only of the modulated carrier wave signal, the load placed upon the main signal channel during such intervals may be objectionable. More specifically, in the circuit of Fig. 1 as above described, the presence of the large condenser l2, which is necessary to give a large time constant for the combination M, ii, will permit a large current to flow in the diode 9 during the occurrence of synchronizing pulses. This will produce a marked reduction in the amplitudes of the synchronizing pulses by comparison with the overall signal am= plitude. This deleterious efiect may be avoided by the insertion of resistor ill in series between the anode of the diode 9 and the condenser l2 to limit the current through the diode. It will also be desirable to provide a suitable by-pass condenser lii for currents of carrier frequency. This alteration may be made in the circuit of Fig. l

From the foregoing description of the operavantage is clearly illustrated in Fi 2 where the change in control voltage for a given increase in signal strength is designated at f for the prior art by placing the switch I 3 in position b.

The system as above described is adapted to maintain the periodically recurrent peaks of a modulated carrier television signal substantially constant. In the case of an audio modulated signal, however, it is generally not desired to maintain the signal peaks constant except where volume compression is sought. Rather, it is desired to maintain the average carrier level constant. By suitable modification, the system in accordance with the invention may be adapted to do this while retaining the advantages heretofore set forth. For this purpose there is provided a load impedance connected directly to the anode of the diode 9 having a time constant adapted to permit the voltage thereacross to follow the carrier wave envelope. In the circuit of Fig. 1 such load impedance comprises the resistor is and the condenser 263 connected to the diode when the switch it is in position 0. In order to obtain a control voltage corresponding to the average value of the carrier envelope, an integrating circuit comprising the resistor Zl and the condenser 22, having a relatively longer time constant than the load impedance 89, 2@ is connected thereacross. The integrated voltage is supplied to the amplifier a? to control the gain thereof, with switch id in position 6. Such voltage will tend to maintain the average carrier level constant.

In the circuits above described and under the circumstances as set forth, the following values of circuit elements were used successfully. However, it should be understood that the invention is not thus limited and that such values are subject to wide variation depending upon the application to be made of the invention:

Resistor l l 150,000 ohms Condenser i 2 10 mlcrofarads Resistor 4 ll=l5,000 ohms Condenser H3 25 micro-microfarads Resistor 6:3300 ohms Condenser l =0.01 microfarad Condenser 28:250 microfarads Resistor l 9: 100,000 ohms Resistor 2 l 1 megohm Condenser 22:0.05 microfarad Although the invention has been described with reference to certain specific embodiments thereof, it is to be understood that it is susceptible of embodiment in other physical forms. Accordingly, the scope of the invention is sub- Ject only to the limitations imposed by the following claims.

I claim:

1. In an electrical signalling system, a source of a modulated carrier wave signal, a variable gain amplifier for said signal, apparatus for developing a control voltage proportional to the peak amplitudes of said carrier wave signal, said apparatus comprising means responsive to carrier peaks of one polarity and separate means responsive to carrier peaks of opposite polarity, means for modifying the responsiveness of at least one of the aforesaid means thereby substantially to reduce its tendency to modifythe envelope of said carrier wave signal, and means for utilizing said control signal to control the gain of said amplifier.

2. In an electrical signalling system, a source of a modulated carrier wave signal, a variable gain amplifier for said signal, apparatus for developing a control voltage proportional to the peak amplitudes of said carrier wave signal, said apparatus comprising means responsive to carrier peaks of one polarity for establishing a direct current voltage which varies substantially in accordance with said peaks and means re-' sponsive to carrier peaks of opposite polarity for establishing with reference to said first voltage a second voltage such that the instantaneous difference between said voltages is substantially proportional to the instantaneous peak-to-peak amplitude of said carrier wave signal, means for modifying the responsiveness of at least one of the aforesaid means thereby substantially to re- 1 duce its tendency to modify the envelopeof said carrier wave signal, and means for utilizing said second voltage to control the gain of said amplifier.

3. In an electrical signalling system, a source of a modulated carrier wave signal, said signal having periodically recurrent modulation peaks, a variable gain amplifier for said signal, apparatus for developing a control voltage proportional to the overall amplitude of said modulation peaks, said apparatus comprising means responsive to modulation peaks of one polarity and separate means responsive to modulation peaks of opposite polarity, means for modifying the responsiveness of at least one of the aforesaid means thereby substantially to reduce its tendency to modify the envelope of said carrier wave signal, and means for utilizing said control sig-,

nal to control the gain of said amplifier.

4. In an electrical signalling system, a source of a modulated carrier wave signal, a variable gain amplifier for said signal, apparatus for developing a control voltage proportional to the peak amplitudes of said carrier wave signal, said apparatus comprising a rectifying device coupled to said amplifier, said device being responsive to carrier peaks of one polarity to develop a unidirectional voltage, a second rectifying device, means for combining modulated carrier signal from said amplifier with said unidirectional voltage for supplying said combined signal to said second device to develop a second unidirectional voltage, means for modifying the responsiveness of said second device thereby substantially to reduce its tendency to modify the envelope of said carrier wave signal, and means for utilizing said second voltage to control the gain of said amplifier.

5. In an electrical signalling system, a source of a modulated carrier wave signal, a variable gain amplifier for said signal, apparatus for developing a control voltage proportional to the overall amplitude of said carrier. wave signal, said apparatus comprising, a serially connected non-linear device andimpedance connected to said amplifier, a second serially connected nonlinear device and impedance connected to said first device, said secondimpedance comprising a resistor shunted by a relatively large capacit ance, a resistor serially connected between said capacitance and said first-mentioned non-linear device and adapted to decrease the efiective load placed upon said amplifier at high frequencies, and a connection for supplying the voltage developed across said second impedance to control the gain of said amplifier.

6. In an electrical signalling system, a source of a modulated carrier wave signal, a signal channel adapted to transmit said signal, a variable gain amplifier in said signal channel, a first current path connected to said signal channel and including means responsive to one polarity of said signal, a second current path in shuntwith said first path and including means responsive to the opposite polarity of said signal, means for modifying the responsiveness of at least one able gain amplifier in said signal channel, a first current path connected to said signal channel and including a unidirectional conducting device responsive to one polarity of said signal, a second current path in shunt with said first path and including a second unidirectional conducting device arranged in opposition to said first device so as to be responsive to the opposite polarity of said signal, means for modifying the re-.

sponsiveness of said second device thereby substantially to reduce its tendency to modify the envelope of said carrier wave signal, and a connection between said second path and said variable gain amplifier for controlling the latter.

8. In an electrical signalling system, a source of a modulated carrier wave signal, a signal channel adapted to transmit said signal, a variable gain amplifier in said signal channel, a unilateral conducting device connected to said channel and responsive to one polarity of said signal, an impedance in series with said device, said series-connected device and impedance forming one current path, a second current path in shunt with said first path and comprising a second unilateral conducting device in opposed relation to said first device, whereby said second device is responsive to the other polarity of said signal, means in series with said second device to modify its responsiveness thereby substantially to reduce its tendency to modify the envelope of said carrier wave signal, and a connection between said second path and saidvarlable gain amplifier for-controlling the latter.

9. In an electrical signalling system, a source of a modulated carrier wave signal having periodically recurrent modulation peaks of opposite polarity, a signal channel adapted to transmit said signal, a variable gain amplifier in said signal channel, means for deriving a portion of the signal energy from said channel, .means for modifying the derived signal so as to level the modulation peaks of one polarity, means for deriving from said modified signal a control voltage whose instantaneous amplitude is a measure of the amplitudes of the opposite polarity peaks relative to the leveled peaks, means for modifying the operation of said last-named means thereby substantially to reduce its tendence to modify the envelope of said carrier wave signal, and means for applying said control voltage to said variable gain amplifier to control the output signal of said channel.

10. In an electrical signalling system, a source of a modulated carrier Wave signal, said signal having periodically recurring peaks of modulation substantially exceeding the normal pli= tude of said signal, a signal channel adapted to transmit said signal and including a variable gain amplifier, detector means coupled to thea connection between said second path and said variable gain amplifier for controlling the latter, and means for'limiting the current drawn by at least one of said current paths thereby to prevent substantial modification of the envelope of said modulated carrier wave by the action of said one current path.

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