US2144921A - Automatic volume control - Google Patents

Description

Jan, 24, 1939.

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s HUNT 2,144,923

AUTlTM-TIC VOLUME CONTROL Filed Dec. 16, 1937 DEG-PLAT? EMU/I705).

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lNVENTOR SEYMOUR HUNT vf ww,

ATTORNEY Patented Jan. 24, 1939 UNIT-ED STATES PATENT OFFICE AUTOMATIC VOLUDIE' CONTROL *Application December 16, 1937, Serial No.'180,070

Claims.

' My: present invention. relates to automatic volume control circuits for radio receivers, and more particularly to an AVC circuit employed with an infinite impedance diode detector circuit.

There has been disclosed in the past a detector circuit of the infinite impedance diode type. Such a detector is essentially a triode employing a cathode load resistor which is not bypassed for audio frequencies and thereby developingan audio voltage which is impressed in degenerative sense between the tube input electrodes. The useful-audio voltage is tapped'off from the oathode load'resistor. "The constants of 'sucha detector circuit are chosen to provide a practically linearrelation between signal input and audio output voltages. When it is desired to derive an AVC bias from such a detector, difficulties-are encountered. For example, itziisi found that the voltage polarity -at the cathode load: resistor is in the wrongsense-for :proper "AVC action. .It has required auxiliary: tubes and connections to provide an AVC"bias'of correct polarity.

It maybe' stated to be one of the main objects of my present invention to provide anAVC circuit for-a detector *of'the infinite impedance diode "type, whereinthe "AVC biasiis derived directly "from the "detector circuit and in "the correct polarity for gain control.

Another important object of my. invention is to 30 provide a tdetectorricircuit'of the multi-grid tube type; .certain of 1 the grids being employed ina degenerative plate rectification circuit, and-at leastone of the grids being employed as the anode of a carrier rectifier developing AVC bias.

:Anothenobject of rnyinvention is to secure concurrent degenerativeplaterectification, car- --rieramplification and rectified amplified carrier voltage in astage. employing but a single tube.

Still.other objects. of the invention are to im- --40.prove generally thesimplicity and elficiency of radioreceivers employing degenerative plate circuit: detectionand utilizing AVC; and more espe- -cially it isean object. of my invention to provide :suchmadio receiversin :an economical and re- 145 liable manner.

areceiver, of the superheterodyne type; the receiver employs a degenerativeplate circuit detector. This type of detector has, .also, been termed an infinite impedance diode detector. Its use is advantageous in a receiver of the high fidelity type; the detector has .a substantially linear relation between-signal input and. output voltages. Of course, the detector can be used'in a :tuned radio frequency amplifier type of receiver. For the-purpose of illustration, let it be assumed that thedetector tube l is in the second detector stage of a superheterodyne receiver. .The usual andconventional networks precede the tube. Thus, the usual signal collector will feed signals, say in the broadcast band of 550 to 1500 k. c., toa tunableradio frequency amplifier; the latter feeds amplified signals to a tunable first detector which is fed by a tunable local oscillator with local oscillations differing in frequency by the operating 1. F. The latter may have a value of between '75 to 450 k. c. The I. F. output energy of the first .detectoris impressed upon the .I. F. transformer 2 whose primary and secondary circuits are each resonated to the I. F.

The I. F. amplifier network includes a tube 3, say of the pentode type; the tube cathode is connected to groundby a bias resistor 4, the latter being-shunted by-the I. F. bypass condenser 5. The tuned secondary circuit of transformer 2 is connected between the control grid of tube'3and the cathode thereof; a condenser E, of low impedance to I. F. currents, establishes the low potential. end of the secondary circuit at ground potential. The plate'of tube 3 is connected to a source of. positive potential through the primary coil of the I. F; transformer l. The primary and secondary circuits of transformer l are each resonated to the operating I. F.

The amplified I. F. energy is impressed between the input electrodes of tube l; the signal grid 8 of the tube is connected to the'high potential end of circuit 1, and the cathode of the tube is connected to ground through a load resistor 9. The low potential end of circuit 1' is grounded, and a condenser Ill, of low impedance to I. F. currents but of high impedance to audio currents, is con nected betweenthe groundedend of the input circuit 'l'and the cathode end of resistor 9. There is thus developed'audio voltage across resistor 9, and. the audio voltage is impressed between the input electrodes of tube l in degenerative phase. The electrode l l of tube 1 functions as the plate of thetriode detector; the electrode I i being connected to a point of proper positive potential, and

I. F. bypass condenser I2 being connected to ground from the lead to electrode I I.

The tube I, which may be of the pentode type, includes the plate electrode I3 and the auxiliary electrode I4. In a pentode type tube the electrode I4 is the suppressor grid; the electrodes 8, II and M would then be grids disposed in the electron stream from the cathode. The cathode, grid 8 and plate I3 function as an amplifier for the I. F. carrier component of the signal energy impressed on tube I. For this reason the I. F. tuned circuit I5 is connected in the plate lead of tube I; the plate I3 may be established at the desired positive potential from any desired current source. The electrode I4 cooperates with the cathode to provide a diode rectifier; for this function the load resistor I 6 is connected between electrode I4 and the cathode end of resistor 9. An I. F. bypass condenser I! is shunted across resistor I6. The amplified I. F. carrier component is impressed upon the diode rectifier circuit by means of a condenser I8; the latter coupling the plate I3 and electrode I4.

The signal grid of amplifier tube 3 is connected to ground through a path including the coil of input circuit 2'; alternating current filter resistor 20; lead 2|; resistor I6 and resistor 9. Direct current voltage developed across resistor I 6 is, in this way, caused to vary the gain of tube 3. Audio voltage developed across resistor 9 is utilized in a subsequent audio network; the variable tap 30 and audio coupling condenser 3| feeding a desired value of audio voltage to the following audio amplifier network. The latter may employ one or more amplifier tubes, and will terminate in an audio reproducer.

The maximum gain of tube 3 is established by choosing the values of. resistors 4 and 9 so that in the absence of received signals the grid of tube 3 is biased negatively by a desired voltage. For example, if the voltage across resistor 4 were 23 volts in the absence of signals, then the magnitude of resistor 9 would be chosen to develop 20 volts across the latter. Hence, the control grid of tube 3 would have -3 volts applied thereto in the absence of signals. The drop across resistor 9 is sufiicient to bias grid 8 close to the cut-off point of the plate current flow of tube I in the absence of received signals. Accordingly, the drop across resistor I6 is practically zero for such signal value.

However, when signals are received the grid 8 is rendered less negative; this causes audio voltage to be developed across resistor 9. As stated above, the relation between signal input to grid 8 and audio voltage output across resistor 9 is a linear one. The direct current voltage across resistor 9 increases as the signal input increases in value. However, the former is opposed by the voltage developed across resistor I6. As the I. F. carrier amplitude increases, the amplifier electronic section, including cathode, grid 9 and plate I3, amplifies the carrier component. The impedance of tuned circuit I5 and the value of resistor I6 are so chosen that the direct current voltage across resistor I6 increases at a faster rate than the voltage across resistor 9. The amplified carrier component is impressed upon diode rectifier anode I4; this causes the anode end of resistor I6 to assume an increasingly greater negative value. As a consequence the control grid of tube 3 becomes more negative, and the gain of. tube 3 is reduced. By way of illustration it is pointed out that resistor 9 may have a value of 100,000 ohms, whereas resistor I6 may be given a magnitude of 0.5 megohm. The rate of decrease of the gain of tube 3 is such that the carrier amplitude at circuit I is maintained substantially uniform regardless of wide carrier amplitude variation at the antenna circuit.

Of course, the AVG lead 2| may be connected to signal transmission tubes preceding the tube 3. For example, the signal grids of the radio frequency amplifier tubes and the first detector tube may be connected to the AVG lead. It, also, will be observed that the grid I4, by virtue of its connection to the negative side of resistor I6, may have a tendency to reduce the space current flow to resistor 9. If such a tendency becomes pronounced, the voltage reduction across resistor 9 is in the correct direction. This will be clear when it is realized that a reduction of the space current flow through resistor 9 means that the direct current voltage opposing the negative voltage across resistor I6 will diminish. The practical effect of this action is to increase the negative bias on the control grid of tube 3. It will now be realized that the network including tube I provides degenerative plate circuit detection; I. F. carrier amplification; and I. F. carrier rectification for AVC purposes. These three functions are performed by a single tube, and without the utilization of any complicating 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:

1. In combination with a signal amplifier tube, an electron discharge tube provided with a cathode and a plurality of cold electrodes, a network associatedwith a cathode and certain of said cold electrodes for providing a degenerative plate rectification detector, a second network associated with said cathode and certain other of said cold electrodes for providing a signal carrier amplifier, a third network associated with said cathode and at least one of said cold electrodes providing a rectifier for the amplified signal carrier energy, and a connection between the third network and said signal amplifier tubeior automatically regulatilng the gain of the latter in response to variations in the rectified carrier energy. v

2. In a detector network, a tube provided with a cathode, a cold electrode, and three intermediate auxiliary electrodes, a network electrically associated with said cathode and two of said auxiliary electrodes for providing a signal detection circuit, a second network electrically associated with said cathode, cold electrode and one of said two auxiliary electrodes for providing a signal carrier amplifier circuit, a third network electrically connected between the third of said auxiliary electrodes and said cathode for providing a rectifier circuit for the amplified carrier energy, and reactive means for impressing amplified carrier energy upon said rectifier circult.

3. In a detector network of the degenerative plate rectification type, a tube provided with a cathode, plate and at least three grids arranged in succession between the cathode and plate, a

modulated signal carrier input circuit connected between the cathode and the first of said grids,

a load resistor connected in the space current path of said tube and being unbypassed for audio currents, a network resonant to the frequency of said input circuit, said network being connected to the plate of said tube and providing a signal carrier amplifier output circuit, a second load resistor connected between the third of said grids and a point on said first resistor which assumes a positive potential as the signal carrier amplitude increases, means impressing carrier energy in said amplifier output circuit upon said third grid, and means for deriving from said second load resistor a direct current voltage.

4. In a detector network, as defined in claim 3, the magnitude of said second resistor being sufiiciently greater than that of the first resistor whereby said derived voltage increases at a greater rate than direct current voltage developed across said first resistor.

5. In combination with a signal transmission tube of the type having a self-bias resistor disposed in its cathode circuit, a detector tube provided with at least a cathode, a signal input grid, a positive electrode, and output electrode and an auxiliary electrode, a signal input circuit coupled between the detector cathode and said input grid, a load resistor connected between the cathode and ground, said load resistor being unbypassed for audio currents, a connection including a second resistor between the point on said load resistor which assumes an increasing positive potential when the signal intensity increases, and the input electrode of said transmission tube, the relative magnitudes of said selfbias resistor and said load resistor being such that the direct current voltage across the former exceeds that developed across the latter, a signal carrier output circuit connected to said output electrode, said auxiliary electrode being connected to a point on said second resistor which assumes a negative potential as the signal carrier intensity increases.

SEYMOUR HUNT.

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