US2078994A - Demodulator - Google Patents

Description

May 4, 1937.

L. 5E. BARTON DEMODULAT OR Filed Oct. 28, 1953 Patented May 4, 1937 DEMODULATOR Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 28, 1933, Serial No. 695,555

7 Claims.

My invention relates to demodulators and it has particular relation to the utilization of diodes as demodulators in radio receivers.

There are many advantages accruing from the use of a diode as a detector in a radio receiver, Such advantages arewell known to those skilled in that art. Y

If a diode is provided with an output circuit constituted by a leak-resistor and a radio fre- 10 quency by-pass condenser and the said output circuit is coupled to the input terminals of an audio-frequency amplier tube in the manner referred to above, distortion will occur when incoming signals have high percentage modulation. Such distortion is occasioned by the fact that the total impedance of the leak and the shunting network to audio frequency currents is less than the ohmic resistance of the diode leak to the direct current resulting from rectication of the carrier. It is, accordingly, an object of my invention to provide a diode-utilization network that shall permit the demodulation, without distortion, of a carrier wave having high percentage modulation. Y f

Another objectioi my invention is to provide an improved method of utilizing a multi-electrode thermionic tube for the combined functions of demodulation and audio-frequency amplification. As mentioned above, distortion occurs when a carrier wave having high percentage modulation is demcdulated in a diode that is coupled to an amplier through a resistor-condenser network for the reason that the impedance of the network to audio frequency currents is less than its resistance to direct currents. This condition. pro-- Vides an audio frequency path to ground which7 by reason of the audio frequency potentials developed tends to pass more peak current than is available from the rectified carrier wave, with the result that the potentials impressed upon the input of the audio frequency amplifier tube are not truly representative of the modulation; In

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order, therefore, to make certain that the current requirements of the system may be met, I impress 45- upon the input circuit of the diode, through a resistor,` additional potential derived either from the commonA anode supply source for the receiver' or from a separate battery. VSuch addi` `tional-potential makes available the direct cur- 5 rentV needed to take care of high percentage modulated carriers and the potentials impressed upon the amplifier tube are not distorted.

The novel features that I consider characteristic of my invention are set forth with .particularity in the appended claims. 'Ihe invention itself, however, both as to its organization' and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of a specific embodiment, when read in connection 10 with the accompanying drawing, in which Fig. l is a diagrammatic View of a portion of a radio receiver of the superheterodyne type including a preferred embodiment of my invention; 15

Fig. 2 is a graph exemplifying the manner in which my invention obviates distortion, and

Fig. 3 is a graph exemplifying certain diode characteristics.

Although my invention is applicable to resistance-condenser networks of known types for the purpose of coupling a diode to an audio frequency amplier, I prefer to illustrate it in connection with a double diode-pentode tube,

wherein demodulation of an incoming carrier 25A wave, audio-frequency amplification of the demodulated signal, and the derivation of automatic volume control potentials are simultaneously accomplished.

In practicing my invention, referring to Figure 30 1 of the drawing, I prefer to utilize a thermionic tube of thecombined double diode-pentode type. This tube is provided with two diode plates l and 3, a common equi-potential cathode 5, a control grid 1, a screen grid 9, a suppressorgrid Il, and an anode I3.` v Y Incoming modulated signals" at an intermediate frequency are impressed between the diode plate l and the cathode ofthe tube, over a circuit including the tuned secondary of an intermediate frequency transformer l5, a leak constituted by a plurality of resistors R1 and Rz, series connected, the rst resistor being shunted by an audioi'frequency and radioY frequency by-pass 45r eld winding L1 of a loudspeaker I6, a potential 50' source exemplified in the drawing by a rectier I1, a tuning-indicator lamp I8, and the primary winding of an audio frequency transformer I9. An audio frequency by-pass condenser C4 is connected in shunt to the rectifier and the eld winding.

Incoming signals impressed between the diode plate I and the cathode of the tube are rectified therein and give rise to a direct current (I) in the leaks R1 and R2, which current has an average value proportional to the amplitude of the carrier and has an audio frequency component the amplitude of which is proportional to the percentage modulation of the said carrier.

The control grid 'I of the tube is connected through a resistor R4 to the movable contact element 20 of a potentiometer device, one end of the resistor element R5 of which is directly con' nected to the junction between the leaks R1 and R2 and the other end of which is connected to the cathode of the tube through a stopping condenser Cs.

The direct current (I) derived from the intermediate frequency carrier appears across the leaks R1 and R2, and the bias applied to the control grid 'I of the tube depends upon the ratio between the two diode leak resistors. The amplitude of the audio frequency potentials iinpressed upon the control grid is determined by the setting of the movable element 20 of the potentiometer. The audio frequency current is shunted across R1 by the condenser C1, so that the control grid I may receive full grid swing at low percentage modulation.

If the diode condenser C2 has a very high impedance to audio frequency compared to the ohmic resistance of the resistors R1 and R2, disregarding, for the moment, the fact that the condenser C1 has low impedance to audio frequency currents, and the presence of the potentiometer and the stopping condenser C3, the impedance of the resistors R1 and R2, from the end farthest from the cathode to the cathode is the same for audio frequencies as for direct current.

Assuming, as mentioned above, that the rectied carrier current has an average value represented by (I) and is owing through the resistors, thevoltage (D. C.) acrossthe said resistors is (I) (R14-Rz) and, if 100% modulation exists, the voltage across the resistors will decrease to Vzero and increase to 2 (I) (R14-R2) during a complete audio cycle. This means that the peak audio voltagel component of the current under the' given conditions is equal to (I) (R14-R2).

Now, assume that the parallel paths constituted by R5, C3, and C1 are added in order that the de modulated signal may be impressed upon the grid 'l of the amplifier portion of the tube and that the said parallel paths for audio frequency have .approximately onehalf the impedance of the a peak audio frequency current will flow in the dition, referring to Figure 2 of the drawing, re-

duces the average diode current (I) to Zero early in the negative part of the audio frequency potential cycle and the current is thereafter cut off until the instantaneous value of the audio frequency current requirement is not greater than (I). This causes distortion on all carriers having more than 331/3% modulation.

It can also be shown that, if the modulation is substantially three times the average current normally available must flow in the diode if distortion is to be obviated.

According to my invention, therefore, I connect the end of the resistor R1 nearest the diode plate tothe anode potential source I'I through a variable high resistor Re. This resistor, preferably, has a, maximum resistance of the order of 5 megohms, which is so high that the total imped- .i

ance of the diode coupling network to audio frequency currents is substantially unaffected.

The increased diode current provided by the connection just referred to only increases the actual diode voltage by a very small amount. That c is, referring to Fig. 3 of the drawing, if the normal diode current is (Io) without the additional direct current potential, a current three times as large, indicated by the point Io, causes the positive potential on the diode plate to only increase a trifle. Therefore, the detector efiiciency is decreased by a negligible amount if the incoming carrier wave has a normal amplitude of several volts.

It should also be apparent, from the drawing, l

that the amplitude of the signal impressed upon the control grid from the diode portion of the tube depends upon the position of the contact element 20. Movement of the element 20, to

control volume, has no effect upon the bias ap plied to the grid since, as before stated, this depends only upon the ratio of R1 to R2.

From the foregoing description of a preferred embodiment of my invention, it will be apparent that the system may be adjusted to handle, with- Y out distortion, signals having modulation percentage in excess of S31/3%.

The amount of extra current supplied to the diode is determined by the resistor Re and the potential of the source I'I. The adjustment of the resistor R5 determines the percentage modu lation which may be efliciently handled, but in general the added current should be sufficient to prevent distortion with 100% modulation.

It will be noted, from the drawing, that a connection extends from the diode plate 3 through resistors R7 and Ra to ground, through which potential may be applied between the diode plate 3 and the common cathode 5 of the tube. This provides automatic volume control potentials| for preceding amplifier tubes in the system, analogous to the system disclosed and claimed in my co-pending application, Serial No. 640,946, led November 3, 1932, and assigned to Radio Corporation of America. Since the provision of As hereinbefore pointed out, through practicing my invention, I am able to utilize a diode for the demodulation of high percentage modulated carrier waves and to impress them, through a network having low impedance to audio frequencies, upon subsequent devices, such as the input terminals of an amplifier tube, or a loudspeaker, without distortion.

Although I have disclosed my invention in connection with a single multi-electrode tube, it is not limited to the specific tube shown and it is, of course, also applicable to receivers wherein the diode and audio frequency amplifier tubes are physically separate and distinct.

Many other modifications will be apparent to those skilled in the construction and operation of radio receiver devices. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination, a modulated signal input circuit, means providing a diode rectifier in connection with said circuit, an output impedance in series with said rectifier and connected with the cathode thereof, volume control means connected in shunt with at least a portion of said output impedance, an electric discharge amplier associated with said diode rectifier and having an input grid connection with said volume control means and having a conductive connection therethrough to said output impedance, and means for applying a positive potential to the anode "end of said diode output impedance.

2. In combination, a modulated signal input circuit, means providing a diode rectifier in connection with said circuit, an output impedance in series with said rectifier and connected withv the cathode thereof, potentiometer means connected in shunt with at lleast a portion of said output impedance, an electric discharge amplier associated with said diode rectifier and having an input grid connection with said potentiometer means and having a conductive connection therethrough to said output impedance 'means for applying a positive potential from one of the amplifier electrodes to said diode output impedance adjacent to the anode end thereof, and means i for varying said potential.

3. In combination, a thermionic tube of the multi-electrode type having a cathode, a control grid, and a plate constituting an amplifier and being provided with a plate which, in conjunction with the said cathode, is adapted to function as a demodulator of the diode type, a connection between the diode plate and the cathode including a resistor-capacitor network, a connection from the network to the control grid, and means for supplying unidirectional positive potential to the anode of the demodulator with respect to its cathode, whereby distortion is prevented in the event that carrier waves having high percentage modulation are impressed upon the tube between the diode plate and the cathode thereof.

4. The invention set forth in claim 3 characterized in that the impedance of the resistorcapacitor network to currents at audio-frequencies is less than its direct current resistance.

5. The invention set forth in claim 3 characterized in that the resistor capacitor network includes a resistor connected conductively between the diode plate and the cathode of the tube and a second resistor and capacitor serially connected in shunt relation to the first mentioned resistor and further characterized in that the impedance of the entire network to currents at audio frequencies is less than the direct current resistance of the said rst mentioned resistor.

6. In combination, a thermionic tube having a cathode, a control grid and a plate constituting an amplifier and being provided with an additional plate which, in conjunction with the cathode, is adapted to function as a demodulator of the diode type, an input circuit for the diode portion of the tube including a leak resistor, a capacitor having relatively high impedance to currents at audio-frequencies connected in between a point on the leak resistor and the cathode, a capacitor having relatively low impedance to currents at audio frequencies connected in shunt to the remainder of the leak resistor, a capacitor having low impedance to currents at audio frequencies and a coupling resistor serially connected between the aforementioned point on the leak resistor and the cathode and a variable connection between the coupling resistor and the control grid whereby maximum grid-swing may be obtained during the demodulation of carrier currents having low percentage modulation.

7. In combination, a modulated signal input circuit, means providing a diode rectier in connection with said circuit, an output impedance in series with said rectier and connected with the cathode thereof, potentiometer means connected in shunt with a portion of said output impedance adjacent to the cathode end thereof, an electric discharge amplier associated with said diode rectifier and having an input grid connection with said potentiometer means and a conductive grid connection therethrough to said output impedance, and means for supplying current at a unidirectional positive potential to the anode end of said diode output impedance with respect .to the cathode end thereof.

LOY E. BARTON.

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