US2112705A - Radio circuit for static limitation - Google Patents

Description

UUOH MM HEUUM i Filed Feb. 11, 1935 2 sheets-sheet 2 m T M m u h ll 4)) u I". "I W u l? u m I m "r R 3 s \u M H a 9 ZIHI Jw H\ 4 2 I 1 l I-HI i=2 2\ u w lo M l 2 a I v 1 If? m I H |9 9/v w *9 4 -v\ \9 J 5 9 3 4 6 16 ...w 9 a 9 FIGURE. V

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IN? E R175 0/1775 FREQUENC Y o m 6 1W M wn D R M mm mm L W can: C/RCUIT SPEAKER FIRS 7" TECTOR RAP/0 FRIOUf/W Y INVENTOR DAVID C MCCAA BY GQ/P AT T ORNEY Patented Mar. 29, 1938 UNITED STATES Search PATENT OFFICE RADIO CIRCUIT FOR STATIC LIMITATION David G. McCaa, Lancaster, Pa., assignor to Alan N. Mann, Scarsdale, N. Y., as trustee Application February 11, 1935, Serial No. 6,015

9 Claims.

My invention relates to radio receiving circuits. More particularly it relates to a radio receiving circuit in which thermionic tubes are employed for limiting static impulses to predetermined values. In my co-pending application, Serial No. 6,014 filed February 11, 1935, entitled improvements in Thermionic tube circuits, I have disclosed specific circuits which may be employed as voltage limiting devices. In this application, I shall describe improvements in the aforesaid circuits and novel means in which said circuits may be employed in radio receiving circuits.

One of the objects of my invention lies in improvements in thermionic tube circuits for limiting and amplifying voltage impulses. Another object is to incorporate voltage limiting circuits in a radio receiving system. A further object is to automatically adjust the voltage gain of the re- .ceiving circuits with respect to the incoming carrier currents so that the gate or limiting action .will be fully and automatically utilized. A further object is to provide a voltage limiting system ca- .pable of being extended to sharp, well defined, output limits by exceedingly small input voltages so that the system may be used in that part of the receiving system in which the input voltage ratio is a maximum.

In the accompanying drawings: Figure I represents the basic circuit I employ.

Figure II shows a family of characteristic grid voltage-plate current curves obtained from the circuit of Figure I.

Figure III represents two stages of the arrangement of Figure I.

Figure IV shows the over-all grid voltage-plate current characteristic of the circuit of Figure III.

Figure V illustrates an alternative arrangement to Figure III applied to a radio receiver.

Figure VI is a schematic diagram of a radio receiving circuit embodying Figure III.

Figure VII is a schematic diagram of a superheterodyne receiver in which Figure III is employed at intermediate radio frequency and with automatic gain control.

In Figure I, l represents a screen grid tube in which 2 is the grid, 3 the cathode, 4 the heater, 5 the screen grid, and 6 the anode. The input terminals of the tube and circuit are I and 8. A suitable bias for the grid 2 may be obtained by adjusting the slider of potentiometer 9 which is shunted around battery I0. One terminal of the battery connects to cathode 3 which is grounded at II. The slider of 9 is connected to 8 which is by-passed to 3 by condenser l2,

The negative terminal of B battery [3 is connected to ground H. The positive terminal of I3 is connected to a non-inductive'and relatively high resistance I4 which in turn connects to anode 6. The anode 6 is connected to an output terminal l5 by condenser 16. The remaining output terminal I! is connected to ground I I. A potentiometer I8 is shunted across all or a suitable portion of B battery I3. The slider of potentiometer I8 is connected to screen grid 5. The screen grid may be by-passed to ground by condenser I9 and the B battery may also be bypassed by a condenser 20.

By employing suitable values, by the way of example, tube RCA 224, grid voltage -1.5, heater voltage 2.7, B battery 22 volts, screen grid voltage 6, and. a megohm resistance in the external anode circuit, the unusual characteristic of grid voltage plotted against plate current illustrated in Figure II is obtained. The unusual abrupt change from the sloping character of the curve to the straight horizontal line, represented by angle A, is secured by a novel combination of means. The high external plate resistance and the low B voltage tend to straighten out the characteristic curve and to limit the anode voltages to values which normally insure anode current saturation. The adjustment of the screen grid voltage is a further insurance of anode saturation currents, and aid in positioning angle A, and means of determining the steepness of the slope of the grid voltage plate current characteristic curve. The grid voltage determines the normal position of the grid on the slope of the characteristic curve.

As the grid voltage is adjusted from positive toward negative (see Figure II) the anode current remains constant, until the angle A is reached. At this point the anode current changes very abruptly and as the grid is made more negative the curve is at first a straight line. Finally the curve begins to bend and approaches zero anode current, as the grid is made more and more negative. The theory and operation of Figure I is more fully described in my co-pending application mentioned above.

In Figure III two tubes and circuits, similar to the one illustrated in Figure I, are connected in series. A unipotential cathode tube such as an RCA 224 is represented at 2|. In this tube 22 is the control grid, 23 the unipotential cathode, 24 the heater, 25 the screen grid, and 26 the anode. The control grid is connected to input terminal 21. The other input terminal is 28 which connects to the slider of potentiometer 29. The po- HUGH;

tentiometer 29 is shunted around biasing battery 30. The positive terminal of 30 and the cathode 23 are both grounded at 3|. A by-pass condenser 32 connects 28 to 23.

The negative terminal of B battery 33 is grounded at 3|. The positive terminal of 33 conmeets to one terminal of resistance 34. The remaining terminal of 34 connects to the anode 26. A potentiometer 38 shunting all or part of battery 33 may be used to control the potential of screen grid 25 by means of the slider on 38. The screen grid 25 may be connected to ground 3| by condenser 39. The B battery may be by-passed by condenser 40.

The second stage employs screen grid tube 4|, RCA type 224 by way of example. In this tube 42 is the control grid, 43 the unipotential cathode, 44 the heater, 45 the screen grid, and 46 the anode. A potentiometer 49 is connected across battery 59 to bias the cathode 43 positive with respect to the grid 42. The slider of 49 provides means to suitably adjust the biasing voltage. The negative terminal of 50 is grounded at The slider of potentiometer 49 is connected to ground 5| by by-pass condenser 52.

The negative terminal of B battery 53 is grounded at 5|. The positive terminal of this battery is connected to one end of resistance 54. The remaining terminal of 54 is. connected to anode 46. The anode 4'6 is connected to output terminal 55 by condenser 56. The other output terminal 5'! is grounded at 5|. A potentiometer 58 shunts all or a suitable part of 53. The screen grid 45 is connected to the slider of the potentiometer 58. The screen grid may be bypassed to ground by condenser 59. A by-pass ggndenser 60 may be shunted across B battery The characteristic of each of these separate stages is similar to that illustrated in Figure II. The values given above for Figure I may be employed in the circuits of Figure III. When a varying input voltage is applied between 2'! and 28, the plate or anode current varies in resistance 34 as shown in Figure II. As the anode current in 2| decreases, the anode 26 will become more positive with respect to ground because of less voltage drop in 34. Since 42 is directly connected to 26, as 26 becomes more positive, 42 will also become more positive with respect to ground, overcoming the bias voltage 49. The increasingly less negative, or even positive voltage on 42 with respect to cathode 43, causes the anode current in 4| to reach saturation represented by angle A as the anode current of 2| is falling.

On the other hand as the anode current of 2| approaches saturation, the maximum voltage drop is approached in resistance 34 and anode 26 becomes less positive or approaches its minimum positive charge. The minimum positive voltage on anode 26 also means minimum positive voltage on control grid 42 with respect to ground, because they are directly connected. Since the bias 49 makes the grid 42 negative with respect to cathode 43, the result is an increasing negative charge on 42 which results in decreasing anode current in 4|.

The net result of the changes just described is illustrated by the grid voltage curve of the first tube plotted against the anode current of the second tube as shown in Figure IV. A and B represent the critical angles of tubes 2| and 4|. The positions Y and Z at which angles A and B occur may be determined by the constants chosen,

The actual values of voltage may be of the order given above for Figure I.

I do not intend to be limited to the precise values shown because I have found a wide range of tubes, resistances, and voltages may be used. By the way of preference, the unipotential cathode tube is especially suited to the circuits of my invention because of the sharp angle A which may be obtained by the use of this type of tube. Likewise the triode tubes may be used, but I prefer the screen grid tubes because the capacity between the grid and plate of the triode becomes elTective at high radio frequencies and, unless neutralized, tends to upset the gate or limiting action. This may even be true, to a slight extent, in screen grid tubes, which may then require some neutralization.

If the second tube of Figure III happens to have a characteristic which causes it to draw grid current, (which is shown as Is. in Figure II) when operating near the angle B limit, a load is placed on resistance 34. This may disturb the characteristic illustrated in Figure IV. Although this is only true of certain tubes, and may be overcome by the choice of tubes, or the constants of the circuit, I have been able to entirely eliminate the trouble by the use of a coupling tube.

In Figure V, I have shown a complete radio receiver employing my gate or limiting circuits and in addition I have shown the use of a coupling tube. The coupling tube is inserted between the two stagcs of Figure III to avoid the deleterious efiects of grid current in the second tube and make the first tube independent of the second.

In Figure V, 66 represents an antenna, 61 a primary inductance which is grounded at 68. The secondary inductance 69 is coupled to the primary 61. Variable tuning capacity is connected in parallel to the inductance, through the large capacity of the by-pass condenser 82. The tuned circuit comprising 69, I0, 82 is connected to the screen grid type 224 tube II as follows: One terminal of condenser 18 connects to the control grid 12. The remaining terminal of 10 connects to the unipotential cathode 13. The heater 14 may be energized by batteries or alternating current. The screen grid is suitably biased by an adjustable potentiometer connection.

The control grid 12 is biased negatively by adjusting potentiometer 19 which is shunted across battery 80. The positive terminal of 80 and the cathode 13 are both grounded at 8|. The by-pass condenser 82 keeps the radio frequency currents from flowing in I9 and 89. The negative terminal of B battery 83 is grounded at 8|. The positive terminal is connected to the resistance 84. The remaining terminal of the resistance 84 connects to the anode 1'6.

Potentiometer 88 shunts all or part of battery 83 to provide a suitable bias means for screen grid 15. The screen grid is by-passed to ground by condenser 89. The B battery 83 may be bypassed by condenser 90. Instead of directly connecting tube H with the succeeding tube, I interpose coupling tube 9| which may be unipotential cathode tube; such as, the RCA 227.

The grid 92 of tube 9| is connected to the anode 16 of the preceding tube The unipotential cathode 93 is heated by 94 which may be energized by batteries or alternating current. The cathode is grounded through self-biasing resistances 95 and 96 which produce a normal voltage drop which biases 92 negatively with respect to 93, and

l-UVD unuirinil LHLIIU I H2. This prevents the flow of grid current from grid 92 to cathode 93.

The self biasing resistance 96 may be by-passed by a suitable capacity 91. The resistance 95 is grounded at 98. The B battery 99 is grounded at 98 and its positive terminal is connected to the anode I00.

The coupling tube is connected to the limiter tube I I I. This tube may be an RCA type 224 and is comprised of control grid H2, unipotential cathode H3, heater H4, screen grid H5, and anode H6. The control grid H2 is connected to the junction of resistances 95 and 96. The cathode H3 is connected to the slider of potentiometer H9 which shunts battery I20. The negative terminal of I20 is grounded at I2I. A by-pass condenser I22 connects the cathode to ground.

The negative terminal of B battery I23 is grounded at I2I. The positive terminal of I23 is connected to resistance I24. The resistance in turn is connected to the anode H6. The anode H6 is coupled to the detector circuit by capacity I 26. A potentiometer I28 shunts all or part of battery I23. The slider of I28 is connected to the screen grid H5. A by-pass condenser I29 may be connected between the screen and ground. Likewise I30 may be connected across I23.

The detector circuit may be of any of the circuit arrangements well known to the art. Such circuit is represented by the device within the dotted lines I3I. The circuit illustrated in Figure V, insofar as the gate or limiting action of tubes II and II I is concerned, is the same as Figure III. The coupling tube connections are arranged so as not to affect the phase relations of tube 1| with respect to I I I.

The bias of grid III is chosen to fix the gate width YZ when bias on grid 12 is zero, so that desired incoming modulated carrier currents will vary between Y and Z. The bias on grid 12 is then adjusted to the operating position corresponding with X of Figure IV. This is illustrated by the curve C. It is apparent that C will be faithfully and efficiently repeated as variations in the anode or plate current shown as D. Voltages, represented by static currents or otherwise, exceeding the limits YZ are shown. as E. The effect of E on the anode current appears as F. The dotted line portion of F is cut oif by the gate action.

In Figure V the incoming signal currents received by the antenna are induced in the first tuned circuit. Static currents are likewise set up in the first tuned circuit. The static charges are ordinarily of great amplitude but of short duration. The signal currents, on the other hand, usually are of long duration. When the signal currents are of low voltage, compared with the static voltage, the gate will cut off the excessive amplitude of the static impulses without affecting the signals. The detector circuit receiving the equalized desired and undesired voltages will integrate each of the two effects. Due to the marked difference in duration the energy involved in the reproduced desired signals will far outweigh the undesired static impulses. Although the voltage input at the detector is equal, the sound energy ratio of the signals to the static may be of the order of ten to one to a listener.

Thus my invention makes it possible to receive signals through atmospheric disturbances which may render an ordinary receiver practically useless. Although I have illustrated a relatively simple system operating from batteries, it should control.

beiil'lill iwum be understood that my circuits may be energized entirely from rectified and filtered currents. In Figure VI a tuned radio frequency system I is shown in schematic outline. In this illustration I3I may be the circuit of Figure III or V. The potentiometers I32 and I33 are similar to 29 and 49 or 19 and H9. A conventional tuned or tunable radio frequency amplifier is shown as I34. The detector is I35, the audio amplifier I36, and the loud speaker I31.

I prefer operating the gate circuit in front of the radio frequency amplifier because the static signal ratio appears most favorable at this point.

7 However, I have had excellent results when the gate follows the radio frequency amplifier. In the system of Figure VI, it should be understood that I32 and I33 are usually adjusted to the particular signal strength and gate action desired.

The gate action circuits may be applied to superheterodyne radio receivers. The circuit of Figure III may be applied in front of the inter-- mediate frequency amplifier as shown in Figure VII. The manual operation of the gate controls may be simplified by employing automatic volume Such control may be employed to regulate receiver gain in front of the gate circuit so that all voltage applied to the gate will be subject to the A. V. C. (automatic volume control) action.

In Figure VII the antenna system I38 is coupled in the conventional manner to the radio fre-' quency amplifier I39. The first detector or mixing tube I40 is coupled to the heterodyne oscillator MI and an A. V. C. control circuit I48. The resultant currents, now at intermediate frequency, are fed to the gate circuit I42. This circuit may be that of Figure III. The currents in the output of the gate circuit are amplified by the intermediate frequency amplifier I43. The second detector is represented by I44.

The circuit I48 may include any of the forms of A. V. C. Well known to those skilled in theart. The voltage derived from the rectifier for the A. V. C. is fed by means of conductors I45 to the radio frequency amplifier I39 and detector I40 to control the gain of the amplifier and first detector. The A. V. C. control tends to keep constant the voltages, representing desired signals, applied to the gate circuit I42. Thus the gate opening will be suitable for all desired signals and excessive voltage impulses will be limited as shown in Figure IV. The output signals may be amplified by audio amplifier I46 and reproduced by loudspeaker I41.

I have described several species of circuits which have a gate or limiting action. These circuits have been applied to different types of radio receivers. It will occur to those skilled in the art that the circuits of my invention may be varied and employed in different arrangements.

By way of example, I have found that the A. V. C. action, instead of being applied to the radio frequency amplifier to control its gain, may be applied to effectively and variably operate the biases of the gate tubes. In this manner the gate is automatically adjusted to the signal instead of the signal to the gate. Similar modifications are within the scope of my invention.

I claim:

1. In a device of the character described, a

, pair of screen grid thermionic tubes, means in eluding the screen grid in each of said tubes for .limiting the maximum value of their anode cur- .rents, a single thermionic tube for coupling said ]pair of tubes and connections between said plumlonic tubes, anode circuit resistances for limiting the maximum value of the anode currents of such tubes so as to insure normally constant maximum currents in their anode circuits, a third single thermionic tube for coupling said pair of tubes and connections between said pair of tubes and said third tube so arranged that the anode currents of one of said pair of tubes are rising when the anode currents in the other of said pair of tubes are falling.

3. A device as described in claim 2, including biasing means for normally establishing the anode currents in the anode circuits of said tubes at values less than said constant currents.

4. A radio receiving device comprising a plur ality of radio frequency responsive circuits, means for connecting one of said circuits to an other of said circuits comprising a pair of thermionic tubes each having an anode and a cathode and a control electrode, operating connections for said tubes comprising an anode circuit for each of said tubes including a resistance, coupling means for coupling the output of the first of said tubes to the input of the second of the tubes, a source of anode potential for said tubes and biasing means for each of said control electrodes, the values of the operating connections for said tubes being such as to cause increasing currents to flow in one anode circuit concurrent with decreasing currents in the other anode circuit and being such as to cause said pair of tubes to have an operating graph in which the input voltage of the first of said tubes plotted against the output current of the second of said tubes is represented by a constant current portion, a changing current portion and a constant current portion, and being such as to cause the junctions of said portions to be affected by an input voltage of substantially less than onetenth of a volt.

5. A structure as specified in claim 4, in which each of the said two tubes has a second control electrode and the operating connections for said tubes include means for applying potential to such second control electrodes.

6. A structure as specified in claim 4 which further includes means for preventing grid current in the input circuit of the second of said tubes from afiecting the output circuit of the first of said tubes.

7. A structure as specified in claim 4, which further includes a third tube connecting the output of the first of said pair of tubes to the input of the other of said pair of tubes so that grid current in the second of said pair of tubes does not afiect the first of said tubes.

8. A radio receiving device including several radio frequency responsive circuits, a pair of thermionic tubes, connections between one of said circuits and the input of the first of said thermionic tubes, connections between the output of the second of said tubes and the other of said circuits, and operating connections for said tubes comprising an anode circuit for each of said tubes including a resistance, means for limiting the maximum value of the anode currents in each of said tubes, connections between the first of said tubes and the second of said tubes adapted .to limit the anode currents of said second tube to a maximum and minimum value, a source of anode potential for each of said tubes and means for establishing the normal anode current of the second of said tubes substantially midway be- ,tween the maximum and minimum values, the

values of the operating connections for said tubes being such as to cause said two tubes to have an operating graph in which the input voltage of said first tube plotted against the output current of said second tube is represented by a constant current portion, a changing current portion and a second current portion and being such as to cause the junctions of said portions to be affected by an input voltage of substantially less than one-tenth of 2, volt.

9. In a radio receiving system, a plurality of signal responsive circuits, means for coupling one of said circuits to another of said circuits, said coupling means comprising a pair of screen grid thermionic tubes, and operating connections for said tubes comprising means for adjusting the voltage applied to the screen grid of such tubes normally to limit the maximum value of the anode current of each of said tubes, anode circuits for said tubes including resistances, coupling means for coupling the output of one of said tubes to the input of the other of the tubes, a source of anode potential for said tubes and biasing means for the grids of said tubes, the values of said operating connections for said tubes being such that the output of the second of said tubes is limited to sharply defined maximum and minimum values represented by angular changes in .the characteristic curve and being such that the angular change in such characteristic curve will be affected by changes of input voltage to the first of said tubes of substantially less than one-tenth of a volt.

DAVID G. McCAA.

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