US1962392A - Electric tube frequency multiplier - Google Patents

Description

June 12, 1934. R ARMON 1,962,392

ELECTRIC TUBE FREQUENCY MULTIPLIER Filed Sept. 30. 1932 WITNESSES:

ATTOR EY INVENTOR (56 A. Kai a/7 IV. Harmon Patented June 12, 1934 UNITED STATES PATENT OFFICE Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application September 30, 1932, Serial No. 635,544

6 Claims.

My invention relates to frequency multipliers and particularly to frequency multipliers of the vacuum tube type.

One of the most common frequency multiplier circuits utilizes a vacuum tube operated with the plate current at its saturation value, so that a flat topped wave containing the desired harmonies is obtained. It has been found that when ordinary radio receiving tubes are employed in such a circuit the life of the tubes is very short because of the large value of grid current. There is no way of avoiding the large value of grid current as the grid must be swung positive in order to saturate the tube. As a result, the only way to increase the life of a vacuum tube, which is operated at saturation for frequency multiplication, is to make the tube of more rugged construction. A transmitter tube, for example, will give satisfactory service, but its use is objectionable because it is much more expensive than a receiving tube.

An object of my invention is to provide a frequency multiplier circuit which will give a large harmonic output when utilizing a radio receiving tube and in which the radio receiving tube will have a long life.

More specifically, an object of my invention is to provide a frequency multiplier circuit which takes advantage of the peculiar characteristics of a screen-grid tube.

In practicing my invention, I employ a screengrid tube and connect it into a circuit so designed that the plate voltage varies periodically above and below the screen-grid voltage at the frequency to be multiplied. Harmonics of this frequency appear in both the plate and the screengrid circuits. The above-mentioned voltage variation is produced by means of an anti-resonant circuit in the plate circuit of the screen-grid tube, this anti-resonant circuit being tuned to the frequency of said voltage variation.

Other features and advantages of my invention will appear from the following description taken in connection with the accompanying drawing, in which Figure 1 is a circuit diagram of a frequency multiplier constructed in accordance with my invention;

Figs. 2 and 3 are circuit diagrams of modifications of the circuit shown in Fig. 1; and

Fig. 4 shows some of the characteristic curves of a screen grid tube.

Referring to Fig. 1, the frequency multiplier comprises a screen-grid tube 10 having a cathode 11, a; heater filament 12, a control-grid 13, a

screen-grid l4 and an anode 15. A voltage of the frequency to be multiplied is impressed across the control-grid 13 and the cathode 11 through a transformer 16 having a primary winding 17 and secondary winding 13.

v The control-grid 13 is given a negative bias by means of a biasing battery 19 which has its positive terminal connected to the cathode 11, and its negative terminal connected to the control-grid 13 through the transformer secondary 18.

The anode or plate is maintained at a positive potential by means of a B-battery 20, which has its negative terminal connected to the cathode 11. Its positive terminal is connected to the plate 15 through inductance coils 21 and 22 which are connected in series. The inductance coil 22 is shunted by a variable condenser 23 of such capacity that the anti-resonant circuit 24 which it forms with the inductance coil 22 may be tuned to the frequeny j.

The inductance coil 21 is shunted by a variable condenser 25 of such capacity that the antiresonant circuit 26 which it forms with the inductance coil 21 may be tuned to a harmonic of the frequency f. This harmonic frequency may be designated by n where n is any integer.

The screen-grid 14 is maintained at a positive potential by means of a connection to an intermediate point on the B-battery 20. A shunt condenser 27 is connected across the portion of the battery 20 supplying the screen-grid potential. A shunt condenser 28 is connected across the entire B-battery 20. The cathode 11 is connected to ground.

As will be more fully explained hereinafter, when a voltage having the frequency f is impressed upon the input circuit of the screen-grid tube 10, the anti-resonant circuit 24 causes the plate voltage to vary above and below the screengrid potential, these variations being at the frequency f. This so distorts the plate current of the screen-grid tube that it contains a strong harmonic frequency of the value 711. Since the antiresonant circuit 26 is tuned to this harmonic frequency, there is a large voltage across it at the harmonic frequency which may be impressed upon any suitable amplifier 29.

In Fig. 1, the amplifier 29 is shown inside the rectangle. It comprises a vacuum tube 30 having a cathode 31, a heater filament 32, a control.- grid 33 and an anode 34. The cathode 31 is con nected to ground. The control-grid 33 is negatively biased by means of a battery 35 having its .positive terminal connected to the cathode 31.

The negative terminal of the battery 35 is connected to the control-grid 33 through an inductance coil 36. The inductance coil 36 is shunted by a variable condenser 37 having the proper value for tuning the resulting anti-resonant circuit to the frequency 11]. The anode 34 is maintained at a positive potential by means of the B-battery 38, which has its negative terminal connected to the cathode 31 and its positive terminal connected to the anode 34 through the primary winding 39 of a transformer 40. The secondary winding 41 of the transformer is connected to the output circuit.

The frequency multiplier is coupled to the amplifier 29 by means of a coupling condenser 42 which is connected between the upper terminal of the anti-resonant circuit 26 and the controlgrid 33 of the amplifier tube 30.

The circuit shown in Fig. 2 is the same as that shown in Fig. 1, except that the anti-resonant circuit tuned to the frequency 12 has been put in the screen-grid circuit instead of in the plate circuit. This anti-resonant circuit is formed by the inductance coil 43 and variable condenser 44. In this modification, as in the arrangement shown in Fig. 1, the input circuit of the amplifier 29 is coupled to the upper terminal of the antiresonant circuit tuned to the frequency 12 through a coupling condenser 42.

The circuit shown in Fig. 3 differs from that shown in Fig. 2, mainly in that a third antiresonant circuit 45 has been added. This third anti-resonant circuit comprises an inductance coil v 46 and a variable condenser 47 connected in series in the plate circuit with the anti-resonant circuit 24. The anti-resonant circuit 45 is tuned to the frequency mf which is a harmonic of the frequency nf (117, being any integer having a value greater than n) The input circuit of the amplifier 29 is connected to the upper terminal of the anti-resonant circuit 45 through a coupling condenser 42.

The circuit of Fig. 3 dilfers from that shown in Fig. 2 in another respect. The control frequency f is impressed upon the screen-grid circuit through a transformer 48 instead of upon the control-grid circuit. The control grid 13 is biased negatively by means of a battery 49 having its positive terminal connected to the cathode 11 and its negative terminal connected to the controlgrid 13 through a grid-leak resistor 50. In Figs. 1, 2 and 3, similar elements are indicated by the same reference numerals.

The operation of my frequency multiplier will be more clearly understood by referring to the curves shown in Fig. 4. In taking data for these curves, the voltage on the control-grid was not varied. The tube voltages were so adjusted that the circuit had a negative-resistance characteristic although such a characteristic is not necessary for good operation. The screen-grid voltage was held constant at the voltage indicated by the vertical line Z. The plate voltage was then varied from a value below the screen grid voltage to a value above it. It will be noted that, as the plate voltage was increased from the zero value, the plate current first increased and then decreased to a Zero value and reversed direction. It again came to a zero value and reversed direction, increasing very rapidly in value until the plate voltage reached a point just beyond the screen-grid voltage. From this point on the increase in plate current was small as the plate voltage was increased. The reversal of the current is caused by secondary omission from the plate which gives the circuit a negative resistance. This reversal of the current is not necessary for satisfactory operation of my circuit but, as explained hereinafter, it may be desirable.

It will be noted that the curve for the screengrid current is the reverse of the curve for the plate current. The important feature about this curve is that the screen-grid current decreases very rapidly in value as the plate voltage approaches the screen-grid voltage, and then de creases slowly in value as the plate voltage is further increased.

Attention is called to the fact that these curves do not show the screen-grid tube being operated with a plate current at saturation. When a screen grid tube is operated as an amplifier in the ordinary manner, it is operated along the straight portion of the plate current curve between the points A and B. My invention takes advantage of the non-uniform change in the plate current and the screen-grid current when the plate volt age is changed in the region of the screen-grid potential.

The maximum harmonic output may be obtained by adjusting the circuit to obtain the negative resistance characteristic shown in Fig. 4 and by then making the voltage swing on the control grid great enough to make the instantaneous plate voltage decrease to a value within the negative resistance portion of the curve. For example, the control grid voltage might be such as to vary the plate voltage between the points M and N. It has been found that with the tube adjusted as shown in Fig. 4, there will be a sudden increase in the harmonic output when the control grid voltage reaches a certain value. This is the value which makes the plate voltage swing a certain distance into the negative resistance range of the characteristic curve.

It Will be noted that the anti-resonant circuit, which is tuned to the harmonic frequency 12 may be placed in either the plate circuit or the screengrid circuit. The reason for this will be apparent from the curves, since they show that the screengrid current has the same non-uniform characteristic in the region of the screen-grid potential as the plate current.

Since the anti-resonant circuit tuned to the frequency nf may be placed in the screen-grid circuit, it is possible to obtain a current of the frequency 112 in the plate circuit.

While'the control frequency f may be impressed upon the screen-grid circuit, as shown in Fig. 3, this is not desirable as a general rule, since a smaller voltage of the frequency may be utilized if it is impressed upon the control-grid circuit. For this reason, the preferred form of the circuit employing an anti-resonant circuit tuned to the frequency m) is the one with a transformer in the control-grid circuit, as shown in Figs. 1 and 2.

In order to teach more clearly those skilled in the art how to practice my invention, I shall give specific values for one frequency multiplier circuit. Using a screen-grid UY-224 tube, the screen-grid voltage and the plate voltage may be 90 and 135 volts, respectively. The biasing voltage for the control grid may be 4 volts. It will be assumed that an alternating voltage having a frequency of 5 kilocycles and a value of 2 volts is impressed across the control-grid and cathode.

The inductance coil of the anti-resonant circuit f has a value of millihenrys, and is shunted by a condenser of the proper value to make the circuit anti-resonant at 5 kilocycles. The efiective impedance of this circuit at 5 kilocycles with a coil of good design is approximately 200,000 ohms. Its impedance at 25 kilocycles is approxi mately 300 ohms.

It will be assumed that the anti-resonant circuit M is tuned to the fifth harmonic, that is, to 25 kilocycles. The inductance coil has a value of 10 millihenrys, and is shunted by a condenser having the proper value to make the circuit antiresonant at 25 kilocycles. At 25 kilocycles, this anti-resonant circuit has an impedance of approximately 100,000 ohms. At 5 kilocycles, it has an impedance of approximately 300 ohms.

The shunting condensers 27 and 28 may have a value of .01 microfarads.

The gain of the tube equals Since the plate impedance of the USE-224 tube is 200,000 ohms, and since ,u. is 400, the gain for the above circuit is 133. Thus it will be seen that if two volts is impressed upon the input circuit, it is more than sufficient to swing the plate voltage over a wide range above and below the screen grid potential of 90 volts.

It will be apparent that various modifications may be made of my invention without departing from the spirit and scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are imposed by the prior art and are set forth in the appended claims.

I claim as my invention:

1. A frequency multiplier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, an input circuit connected to said control-grid and said cathode, means for maintaining said second-grid positive with respect to said cathode, means for maintaining said anode positive with respect to said cathode, the relative values of said potential maintaining means being such as to distort the input circuit potential variations as they appear in the anode circuit, two anti-resonant circuits connected in series with each other and with said anode and said last means, one of said anti-resonant circuits being tuned to a harmonic of the frequency to which the other anti-resonant circuit is tuned, and an output circuit coupled to said one resonant circuit.

2. A frequency mult'plier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, an input circuit upon which a voltage having a certain frequency is impressed, said circuit being connected to said control-grid and said cathode, means for maintaining said secondgrid positive a fixed amount with respect to said cathode, means for maintaining said anode positive with respect to said cathode, an anti-resonant circuit connected between said anode and said cathode, and tuned to a harmonic frequency of said certain frequency, an output circuit coupled to said anti-resonant circuit, and means for varying the potential of said anode above and below the potential of said second-grid at said certain frequency.

3. A frequency multiplier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, means for impressing a voltage having a certain frequency across said cathode and said control-grid, a second-grid circuit including means for maintaining said second-grid positive with respect to said cathode, an anode circuit including means for maintaining said anode positive with respect to said cathode, an antiresonant circuit in one of said circuits, said antiresonant circuit being tuned to a harmonic of said certain frequency, an output circuit coupled to said anti-resonant circuit, and means for causing the anode potential to vary above and below the second-grid potential at said certain frequency.

4. A frequency multiplier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, means for impressing a voltage having a certain frequency across said cathode and said control-grid, a second-grid circuit including means for maintaining said second-grid positive with respect to said cathode, an anode circuit including means for maintaining said anode positive with respect to said cathode, an anti-resonant circuit in one of said circuits, said anti-resonant circuit being tuned to a harmonic of said certain frequency, an output circuit coupled to said anti-resonant circuit, and means for causing the anode potential to vary above and below the second-grid potential at said certain frequency.

5. A frequency multiplier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, means for impressing a voltage having a certain frequency across said cathode and said control-grid, a second-grid circuit including means for maintaining said second-grid positive with respect to said cathode, an anode circuit including means for maintaining said anode positive with respect to said cathode, an anti-resonant circuit in said second-grid circuit, said anti-resonant circuit being tuned to a harmonic of said certain frequency, an output circuit coupled to said anti-resonant circuit, and means for causing the anode potential to vary above and below the screen-grid potential at said certain frequency.

6. A frequency multiplier comprising a tube having a cathode, a control-grid, a second-grid, and an anode, means for impressing a voltage having a certain frequency across said cathode and said control-grid, a second-grid circuit including means for maintaining said second-grid positive with respect to said cathode, an anode circuit including means for maintaining said anode positive with respect to said cathode, an

anti-resonant circuit in said second-grid circuit,

said anti-resonant circuit being tuned to a harmonic of said certain frequency, an output circuit coupled to said anti-resonant circuit, and an anti-resonant circuit in said anode circuit tuned to said certain frequency.

RALPH N. HARMON.

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