US2025595A - Modulation circuit - Google Patents

Description

Patented Dec. 24, 1935 UNITED STATES PATENT OFFICE MODULATION CIRCUIT vania Application January 23, 1934, Serial No. 707,912

3 Claims.

Our invention relates to modulation circuits and it has particular relation to means for preventing over modulation in transmitting circuits.

An object of our invention is to provide means that will automatically limit the modulation voltage in radio transmitting circuits to a predetermined value.

Another object of our invention is to eliminate theinterruptions that occur in the circuits of broadcasting stations, due to an abnormal increase in the grid and plate voltages and currents in the modulator and power amplifier stages.

Other objects of our invention will become evident from the following detailed description taken in conjunction with the accompanying drawings in which:

Figure 1 is a diagrammatic-view of a system including a preferred form of our invention;

Fig. 2 is a diagrammatic view of an equivalent circuit illustrating effect of the various components of the circuit of Fig. 1;

Mai loss.

- power amplifier stage.

Fig. 3 is a graph disclosing the region in which the modulation suppressor of Fig. 1 operates; and,

Fig. 4 is a graph disclosing the permissible voltage in the modulator circuit.

The records of broadcasting stations disclose that a frequent cause of interruptions of programs is due to over modulation. This condition of over modulation causes abnormal grid and plate voltages and currents in the modulator and power amplifier stages, with the result that there may be flash-backs that damage the tubes of the circuit. To prevent damage to the tubes of the circuit, an overload relay is provided that will interrupt the circuit and prevent damage to the tubes. While the tubes of the circuit are thus protected by the overload relay, yet the interruption is highly undesirable generally and particularly if each interruption causes a finan- It is practically impossible to foresee when any performer or performers before the microphone may cause an increase of sound volume to produce an excessive voltage in the audio stage and cause over modulation in the Manual means cannot, of course, act quickly enough to prevent damage or interruption in case the sound volume at the microphone is suddenly increased. Accordingly, it is an object of our invention to provide means for automatically limiting the voltage of the modulation circuit when the audio frequency voltage exceeds a prescribed value, as for instance,

the one corresponding to modulationv in the output stage.

In Fig. 1 is diagrammatically illustrated the circuit of a broadcasting transmitter circuit and a preferred form of over modulation suppressor applied to the audio section of this circuit.

The sound to be reproduced in the receivers tuned to the station is picked up by the microphone HB and the electrical circuit therethrough passes through the studio audio amplifier stage I l tothe tube [2 which may be of the U'Xw210 type. This tube is connected to the transformer l3 at the primary points labeled A and B. The transformer is connected to the standard telephone line which at present has a. resistance value of 500 ohms. The portion of the circuit up to this point We shall call the studio audio input. An attenuator i4 maintains the proper resistance in the line and across it, and it is well-known in the art. A transformer l5 connects this to the conventional amplifier tube I6 through a resistance ll. The output of this audio circuit is then connected to a suitable modulator l8'and radio frequency power amplifier l9, wherein a radio carrier frequency is provided and this carrier frequency is modulated by the audio frequency produced in the circuit just described. The signal is then broadcast from the antenna 20.

While the above description concerns a radio broadcasting station circuit, it is apparent that the invention might be applied to similar forms of circuits involved in the modulation of one circuit by another, but which are not broadcast from an antenna.

A preferred form of our invention'includes a shunt circuit across the audio stage or line and preferably between the attenuator and the transformer l5. This shunt connection 30 is connected preferably to the primary winding of a transformer 3! having its secondary connected to two tubes 32 and 33. The ends 34 and 35 of the secondary are preferably connected to the anode 36 of the tube 32 and the anode 31 of the tube 33 respectively. The midpoint 33 of the secondary of this transformer 3| is connected to the cathode 39 of the tube 32 and the cathode 40 of the tube 33. The grid ll of the tube 32 and the grid 52 of the tube 33 are connected to their respective anodes through condensers 43 and 44.

A negative bias is placed upon each of the grids by the respective batteries 45 and 46 or other source of voltage. These'batteries are connected between the midpoint 38 of the transformer 3| and the connections to the grids of both of the tubes. Chokes 41 and 48 isolate the grids at audio frequency from ground.

The tubes 32 and'33 may be of any suitable form having suitable static characteristics. While one of these tubes would act to prevent over modulation during one-half cycle, it is preferred to have the two tubes connected in this pushpull arrangement so that the suppressor would be effective for all portions of the sine wave voltage.

It is preferred to use the tubes of the type known as UX250 in this over modulation suppressor. In further describing the effect of the suppressor upon the audio stage, certain static characteristics of these tubes will be considered andalso the parameters of the standard circuit will be mentioned. It will be recognized, of course, that the invention is not limited to the specific tube used nor to the specific values of the standard circuit, nor to the particular values of current, voltage, transformer ratios, etc., mentioned in the following description.

It is preferred to use a highly negative bias on the grids 4| and "of the tubesof the over modulator suppressor circuit. In our specific ex- .ample, we have taken this voltage. on these tubes as -95 volts. The plate voltage on these tubes will, of course, depend upon the voltage across the audio line, and this will, in turn, depend upon the input voltage of the audio stage. Reference is made to the electrical equivalent circuit in Fig. 2 where E1 is the voltage from the circuit including the audio amplifier studio stage and the tube The transformer I3 has a reflected impedance of 500 ohms into the circuit, illustrated at 50 in Fig. 2. This reflected impedance, of course, comes .from the prior parts of the circuit. The attenuator resistance of 500 ohms is illustrated at 5|. The current through these two is labeled I1. There are then two branch circuits 52 and 53 and the voltage across these two a branch circuits is E2.

resistance of the attenuator is 500 ohms.

This voltage E2 isthe output voltage of the circuit and we are primarily interested in the effect on this output voltage E2 of an increase in the input voltage E1. The various values given below have been calculated from equations and from the ordinary vacuum tube static curves, in

.this case for the UX-250 tube.

,For-% modulation we will assume that E: equal 7.5 volts peak and that the transformer 3| of the overmodulation suppressor has a one to twenty ratio. The values of the tube are taken as E =peak plate voltage;

I peak plate current; and

E =peakgrid voltage.

The Mu of the tube is taken as 3.8.

E2= of Ep since the coupling ratio is 20 to l, or '10 to 1, considering the central tap connection. In is equal to E2 divided by 500 since the I3 equals 10. I 'from the transformer ratio of 10 to 1: I1=I2+I3 which follows from the sum of the branch currents. E1=E2+1000 I1. The thousand comes from the reflected impedance 50 of 500 ohms and the line resistance 5| of 500 ohms.

It will be noted that the plates of the two suppressor tubes are connected across a high ratio transformer 3| and have no direct'current supplyvoltage.' The grids are coupled to the plates by the condensers 43 and 44 so that any change of plate voltage is also impressed on the grids. The grids'of the tubes are biased highly negative by a battery 45 .or 46 and influence of negative bias on grid will be counterbalanced by the positive voltage on the plate due to 100% mod- E E; I, I: I; I 10001 E; E 1E 000 000 0 000 0 0 0 360 360 O00 000 005 005 5 2. 5 7. 5 -27() 235 000 000 O10 010 10 5. 0 15. 0 170 -120 000 000 015 015 15 7. 5 22. 5 76 -l 004 040 016 056 56 8. 0 64. 0 -57 +23 010 100 017 117 117 8. 5 125. 5 38 +47 O18 018 198 198 9. 0 207. 0 19 +71 It will be noted from the above that the control voltage E0 is negative for values up to 7.5 volts of E2. Above this point, the control voltage becomes positive and the tubes will function by initiating an electronic current that will make the over modulation suppressor circuit operate and introduce a variable resistance that will prevent an over modulation of the voltage in the audio stage. This is illustrated by the curve in Fig. 3 in which the values of E1 are plotted against the values of E2. It will be noted that the output voltage rises linearly until the 100% modulation voltage is reached and then the curve levels off very sharply. The control voltage of the tube against the current in the plate circuit of the tube of the over modulation suppressor is also plotted in Fig. 4. It will be noted, from the values illustrated by the points from A to B, that the control voltage permits of no current in the plate circuit of the tube, but the moment that the control voltage is positive the value of the plate current in the tube rises very sharply.

Between the limits A and B on the curve in Fig. 4 the audio amplitudes are entirely unaffected. In other words, the output voltage will a be directly proportional to the input voltage up ,to' the 100% modulation, as disclosed in the curve in Fig. 3. However, the moment over modulation occurs, the current through the tubes will be automatically started and the modulator and osoillation voltage will be kept below the value at which there is danger of interruption of performance. Other tubes with different characteristics can, of course, be used and the various constants of the circuit changed to suit particular condi tions. Q I

Although we have shown and described certain specific embodiments of our invention, we are fully aware that many modifications thereof are possible. Our invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

We claim as our invention: 1. A transmitting arrangement having in combination a circuit for providing a carrier frequency, a circuit modulating the carrier frequency of said first-mentioned circuit and a circuit connected in shunt to said second mentioned circuit and comprising a grid controlled electron discharge tube having an anode electrode, a negative bias on the grid of said tube, means for coupling said gridand anode whereby the voltage I of said second circuit will be simultaneously impressed upon said grid and anode electrodes. 2. In combination, a source of variable potential, a load circuit connected thereto for operation at potentials belowfa predeterminedmaxlmum, a pair of electron discharge clevices'con-v nected 180 degrees out of phase with each other and effectively in shunt to said load circuit, each of said devices comprising a cathode and at least two cold electrodes one of which cold electrodes is disposed between said cathode and the other cold electrode, a biasing potential on at least one of the cold electrodes of each of said devices, said biasing potential being of such a value as to preclude operation of said electron discharge devices at impressed potentials below the value of said above referred to maximum potential, and means for applying potentials derived from said variable source to both of said cold electrodes of each device simultaneously.

3. In combination, a source of variable potential, a load circuit connected thereto for operation at potentials below a predetermined maximum, a pair of electron discharge devices connected 180 degrees out of phase with each other and effectively in shunt to said load circuit, each of said devices comprising a cathode and at least two cold electrodes one of which cold electrodes is an intermediate electrode disposed between said cathode and the other cold electrode, a biasing potential on each of said intermediate electrodes, said biasing potential being of such a value as to preclude operation of said electron discharge devices at impressed potentials below the value of said above referred to maximum potential, and means for applying potentials derived from said variable source to both of said cold electrodes of each device simultaneously.

HENRY N. KOZANOWSKI.

HARRELL V. NOBLE.

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