US2388233A - Radio-frequency transmitter - Google Patents

Description

Oct. 30, 1945.

J. N. WHITAKER RADIO FREQUENCY TRANSMITTER Filed Feb. 26, 1944 Seam Mm 2 Sheets-Sheet 1 19/6 FREQUENCY POWER JUPPL Y 5 Yd TE M Jz/aes MW/w art my NTOR RNEY Oct. 30, 1945. J. WH|TAKER 2,388,233

RADIO FREQUENCY TRANSMIT TER Filed Feb. 26, 1944 2 Sheets-Sheet 2 BY mwonusv Patented Oct. 30, 1945 Search teem RADIO -FREQUEN CY TRANSMITTER James N. Whitaker, West Englewood, N. J., as-

signor to The Hammarlund Manufacturing Company, Incorporated, New York, N. Y., a

corporation of New York Application February 26, 1944, Serial No. 524,038

7 Claims.

This invention relates to radio frequency transmitters and more especially to devices for simultaneously switching circuits to diiferent frequencies without detuning. It also relates to switching of high power radio frequency currents without the use of metallic contacts.

In radio frequency transmitting apparatus intended to cover more than one frequency or frequency range, it is general practice to use a multiple contactor switch for increasing or decreasing the inductance in the tuned circuit when it is desired to operate over a frequency range greater than may be obtained by the use of a single fixed inductance and a variable capacitance.

Another method is to use a variable inductor and a variable capacitor. However, in this latter system, it may not be possible to cover the desired frequency range even with the use of a variable inductor, and it again becomes necessary to resort to some sort of a contactor to increase or decrease inductance in the circuit.

Where such changes in inductance are made infrequently, it is possible to accomplish the desired result by physically replacing the inductor with another inductor of the proper value, or by the short circulting of a part of the inductance.

When changes are desired at frequent intervals, and where the loss of time involved would be serious, it has been necessary to employ a multiple contact switch.

A multiple contact switch can be designed and made to operate satisfactorily in radio frequency circuits where the power involved is rather low, and where the frequency is not too high. Howveer, in the case of the higher powered radio transmitters the losses due to contact resistance, and arcing when the contacts are opened become a very serious problem. Another disadvantage in the use of a contactor type switch is that even though a suitable contact may be obtained when the switch is new, continued use and an accumulation of particles of foreign matter soon cause the contacts to wear, thereby introduc-ng higher contact resistances and resulting in unsatisfactory performance.

The device herein disclosed performs switching operations in radio frequency circuits without introducing any of the losses due to contact variations, such as are encountered in the usual type of a multiple contactor switch.

The use of this new invention is also disclosed in combination with several types of radio transmission systems.

The present invention is based on the principle of the relatively low resistance presented by a capacitor to radio frequency currents. In one form it consists of a variable capacitor of special design, having at least one rotating element consisting of several parallel plates, arranged to interleave with two or more corresponding groups of parallel plates so spaced that the rotating element will interleave with only one group of stationary plates at any one time, except possibly while the rotor element is being moved from one position to the next. In order better to illustrate the principles involved in my invention, reference is now made to the drawings, where:

Fig. 1 shows a switching device according to my invention as applied to a single conductor system.

Fig. 2 shows a switching device adapted for a two conductor system.

Fig. 3 schematically shows electrical circuits similar to those employed in Fig. 2.

Fig. 4 shows a typical transmitter embodying therein various forms of my invention,

In Fig. 1 I have shown a source of radio frequency I connected to the rotor 2 of a four position capacitor-switch. The rotor 2 may be rotated either clockwise or counter-clockwise as indicated by the arrows, so that the rotor plates 3 may be engaged with stator plates 4 as shown, or with either of stator plates 5, B or I.

With rotor plates 3 engaged with stator plates 4, as shown in Fig. 1, radio frequency energy from the high frequency generator I will flow through the capacitance formed by the interleaving of rotor plates 3 and 4 and through inductance 8 to ground. This flow of radio frequency energy will produce an electromagnetic field around inductance 8 which will introduce energy into inductance 9 from which it may be directed into a utilization circuit In indicated at arrow points.

We will now assume that it is desired to operate the system on another frequency, and that there is insufficient inductance in inductor 9. We rotate rotor plates 3 until they are interleaved with stator plates 5. The capacity between rotor plates 3 and stator plates 5 is now relatively large, and the capacity between rotor plates 3 and stator plates 4 is negligible. Therefore, radio frequency energy will flow from high frequency generator I through rotor plates 3 stator plates 5 and inductor II where the energy will be transferred inductively to inductor I! from which it may be applied to utilization circuit l3.

Likewise if a still lower frequency is desired, rotor plates 3 may be rotateduntil they are interleaved with stator plates 8, thereby permitting the radio frequency energy to fiow through inductor l4 and by inductive coupling, through inductor I 5 to utilization circuit l6.

If a still greater value of inductance were required, rotor plates 3 would be rotated until they were interleaved with stator plates 1, allowing radio frequency energy to flow through inductor I! and thence by induction, through inductor l8 to utilization circuit l9. Substantially this identical arrangement may also be used where it is desired to transfer radio frequency energy of one frequency to any one of four utilization circuits. In this case, inductors 8, ll, [4 and I! might be identical with each other in value.

In Fig. 2 is shown the use of my invention in connection with a push pull amplifier. I have shown here a conventional neutralized push pull amplifier circuit with the anodes of the amplifier tubes coupled to the usual inductance by means of the device of my invention.

A may be seen by a study of Fi 2, the circuit is arranged for shunt feeding of plate potentials to the amplifier tubes, with my invention acting both as D. C. blocking or isolating capacitors and as a switching device.

The high frequency generating system and the power supply system are irrelevant to my invention and therefore are shown as a box, with connections to appropriate parts of the power amplifier circuit.

Referring to Fig. 2, radio frequency potentials of opposite phase are applied to grids 20 and 2| of push- pull amplifier tubes 22 and 23 by means of connections 24 and 25. Cathode heating power is applied to the cathodes of the tubes by means of connections 26-21 and 28-49.

Anode D. C. potential is applied to the tubes by means of connection 30 through R. F. isolating chokes 3| and 32. The amplifier is neutralized in the conventional manner by means of variable capacitors 33 and 34. The anode circuits are tuned to resonance with the desired frequency by means of variable capacitor 35.

The operation of a neutralized push pull amplifier is well known by those familiar with the art, and therefore need not be described in further detail.

Referring again to Fig. 2, I have illustrated a form of my present invention which replaces a conventional two pole four position contactor type band switch. This device consists of two rotor plates, or groups of rotor plates 36 and 31, insulated from each other electrically but mechanically coupled rigidly together in order that they may be rotated as a unit, around a central shaft or bearing 46.

Stator plates or groups of stator plates are suitably mounted around the periphery of the circle described by the rotation of the rotor plates. This arrangement is such that the rotor and stator plates will interleave with each other to form complete capacitors, when the rotor plates are at rest in the proper pos";ion. M is shown a conventional s ush ull neutralized g inductor a rent anode potential by means of capacitors 41 and 4B and is resonated at the desired frequency by mean of variable capacitor 50.

Comparison of Fig. 2 and Fig. 3 reveals that, when employing the device of my invention, the capacity formed by the interleaving of rotor plates 33 and 31 with stator plates 38 and 39 respectively (Fig. 2) performs the same electrical fed function as do capacitors 41 and 48 (Fig. 3), i. e. as isolation and coupling capacitors.

Referring to Fig. 2 let us assume that it is not possible to resonate inductor 5| at the desired frequency by the adjustment of tuning capacitor 35. Assume also, that inductor 52 has more inductance than inductor 5| and that the desired frequency is lower than when inductor 5| was being used. In order to place inductor 52 in the circuit, rotor plates 36 and 31 are rotated until they interleave with stator plates 40 and 4| respectively. If still another value of inductance is required, rotor plates 36 and 31 may be rotated still further until they interleave with stator plate 42 and 43 respectively, placing inductor 53 in the circuit, or until they interleave with rotor plates 44 and 45 respectively, placing inductor 54 in the circuit.

Reference is now made to Fig. 4 wherein I schematically illustrate a complete and improved radio transmitter circuit incorporating the above described invention, together with several other novel switching arrangements.

Figure 4 illustrates a crystal controlled pentode oscillator, and three consecutive frequency multiplier stages incorporating pentode tubes, followed by a pentode power amplifier. Means are provided whereby it is possible to apply the radio frequency energy from the oscillator or any one of the multiplier stages to the grid circuit of the power amplifier stages. Additional means are provided for rendering inoperative all unused frequency multiplier stages in such a manner that the capacity loading of a previous stage is not disturbed. Means are also provided for adding capacity loading equal to that produced by the power amplifier input system, to all stages including the oscillator which are in use but which are ahead of the stage to which the power amplifier is connected.

When the oscillator output is utilized as excitation for the power amplifier, the power amplifier input circuit will cause some capacity loading of the oscillator plate tank circuit. Therefore, when the oscillator is used to excite the power amplifier, no fixed compensating capacity is connected across the oscillator plate tank circuit.

Assume now that it is desired to excite the power amplifier from the first frequency multiplier circuit and the input of the power amplifier is transferred from the oscillator plate tank circuit to the first multiplier plate tank circuit. Because the capacity represented by the amplifier input circuit has been removed from the oscillator plate tank circuit, said oscillator plate tank circuit will require retuning or the addition of a fixed compensating capacity equal to the capacity of the amplifier input circuits.

One element of this improved radio transmitting circuit is the use of a single dial control for tuning of the oscillator and frequency multiplier stages, and in connection with this element I have introduced a device for automatically connecting a suitable compensating capacity across the oscillator plate tank circuit when the amplifier input circuit is not connected thereto. A similar arrangement is also provided for each of the multiplier stages except the last one, in order that the output of any multiplier stage may be utilized without the necessity of retuning the plate tank circuits.

In the power amplifier plate circuit, I show four separate inductors capable of being resonated at the frequency of the oscillator, or at the output frequency of one of the three multipliers. These inductors are connected into the power amplifier plate circuit by means of the capacity switching device shown in Figs. 1 and 2 and earlier described in this specification.

All switching devices related to the changing of frequencies are mechanically connected together as indicated by dotted lines. to provide a single control for frequency changing, and to assure the proper combination of connections for any given frequency.

In the oscillator and frequency multiplier stages, means are provided for individually ad- Justing the inductance in each plate tank circuit. Likewise provision is made for individually adjusting the fixed capacity across each inductance. Further means are provided for varying the capacity across all oscillator and multiplier tank circuits simultaneously, to provide tuning of all oscillator and multiplier stages by means of a single control dial or other similar device.

The plate tank circuit of the power amplifier is separately tuned, since variations in the utilization circuit are likely to react on the plate tank circuit tuning, requiring re-adjustrnents of this circuit only.

In Fig. 4 I also show a conventional power supply system. and a metering system by means of which it is possible to observe the value of plate current drawn by each tube inthe radio frequency circuit without disturbing the circuit in any way, and with the used of a single milliampere meter. Thus in the circuit shown, one meter is used where four would normally be required, thereby effecting an appreciable saving in space and cost.

The metering system consists of a conventional milliampere meter connected to the two movable arms of a two pole multiple switch. One set of contacts of the switch are connected to the plate supply system, while the others connect to shunts which are in series with the plate circuit of each tube. It is thereby possible to obtain an indication of the plate current drawn by each tube by rotating the switch control dial (or knob) to the desired position.

The operation of the complete transmitter shown in Fig. 4 is as follows:

Quartz crystal '55 and grid biasing resistor 56 are connected between control grid 51 of a beam type pentode vacuum tube 58 and ground. A resonant circuit consisting of an inductor capable of limited variations 58, a trimmer capacitor 60 and a main tuning capacitor 6| is connected in the circuit between the anode 62 and the oscillator power supply. A fixed capacitor 63 provides a short radio frequency path between the low end of the plate tank circuit and ground.

A screen voltage dropping resistor 64 is connected between the oscillator plate supply and the screen grid 65 of the oscillator tube to reduce the D. C. voltage applied to the screen grid to a value below that which is applied to the plate. Bypass capacitor 66 provides a short radio frequency path between screen grid 65 and ground.

Cathode biasing resistor 61 and bypass capacitor 68 are connected between cathode 69 and ground, completing the oscillator circuit.

Oscillations of the crystal 55 are introduced and maintained by regeneration through the tube in the usual manner, when the oscillator plate tank circuit is properly resonated.

Radio frequency energy from the oscillator plate tank circuit is applied to the grid of frequency multiplier tube ll through D. C. blocking capacitor 12, and to contactor 18 of switch 14 for is desired to operate on the fundamental frequency of the oscillator. When the power amplifier arid circuit is not connected to the oscillator plate tank circuit, a capacitor 18 equal to the capacity of the power amplifier input circuit is connected between contact 13 of switch 14 and round through contact 16 of switch TI. This arrangement insures that the oscillator plate tank circuit will not be thrown out of resonance when the power amplifier input circuit is switched in or out of the oscillator plate tank circuit.

Grid 10 of frequency multiplier tube II is connected to ground through radio frequency choke l8 and grid biasing resistor 19. Screen grid 80 connects to the high voltage plate supply through screen voltage dropping resistor 8|. Radio fre quency potentials appearing on the screen grid 80 are bypassed to ground through capacitor 82. Anode 83 connects to the high voltage plate supply through a plate tank circuit consisting of inductor 84 which is capable of limited variations in inductance, trimmer capacitor 85 and main tuning capacitor 86. Capacitor IOI serves to bypass radio frequency energy to ground completing the R. F. circuit back to cathode 81. This circuit is normally resonated at a harmonic of th oscillator frequency. Cathode 81 connects to ground through cathode biasing resistor 88 and through contact I30 of switch I29 and is bypassed directly to ground by capacitor 88, completing the first multiplier circuit.

Radio frequency energy from the first multiplier circuit is applied to grid 80 of second multiplier tube 8I through grid blocking capacitor '82. R. F. energy from the first multiplier is also applied to contact 83 of power amplifier input selector switch 14. Capacitor 84 is connected between selector switch 14, contact 98 and contact 95 of selector switch 11. The value of this capacitor is made equal to the input capacity of the power ampliher, and it is switched into the circuit when the input circuit of the power amplifier is switched out, in order that the tuning of the first multiplier plate tank circuit will not be disturbed by such switching.

Grid of second multiplier tube 9| connects to ground through R. F. choke 96 and grid biasing resistor 81. Screen grid 98 connects to the high voltage anode supply through screen grid voltage dropping resistor 88. Capacitor I00 acts to bypass all R. F. screen potentials to ground.

second multiplier is passed through grid blocking apacitor H0 to grid I, of third multiplier tube 2. A connection is also made from the second multiplier plate tank circuit to contact I I3 of selector switch H, to supp y excitation to the power amplifier. The capacity of compensating capacitor Ill is equal to the capacity of the power amplifier input circuit, and is connected between contact N3 of selector switch 14 and contact N5 of selector switch 11 in such a manner that it is connected into the circuit to compensate for the amplifier input capacity, when the power am lifier use as excitation for the .power amplifier when it 'll!v input is not connected to the second multiplier plate tank circuit, thereby maintaining resonance of the second multiplier plate tank circuit.

Grid III of third multiplier tube H2 is connected to ground through R. F. choke H8 and grid biasing resistor II'I. Screen grid II8 connects to the high voltage plate power supply through screen voltage dropping resistor II9.

Capacitor I24 short circuits all R. F. screen potentials to ground.

Anode I2I connects to the high voltage plate power supply through the third multiplier plate tank circuit consisting of inductor I22 which is variable between certain limits, trimmer capacitor I23 and main tuning capacitor I24. Capacitor I24 completes the R. F. circuit to ground and thence back to cathode I25.

Cathode I25 connects to ground through cathode biasing resistor I26, and contact I32 of selector switch I29. Cathode bypass capacitor I21 completes the circuit of the third multiplier stage.

The plate tank circuit of the third multiplier sta e connects to contact I28 of selector switch I4 to supply excitation to the power amplifier. Since this is the last multiplier stage in the circuit, and since it is rendered inoperative when it is not being used to excite the power amplifier, no compensating capacitor is usually required to maintain resonance in its plate tank circuit when the power amplifier input circuit is not connected thereto.

Selector switch I29 acts to remove the cathode to ground connections of all unused frequency multiplier stages. Selector switches 14, I1 and I29 are ganged together so that all may be operated simultaneously as a unit.

In Fig. 4, I have shown all the switching devices pertaining to frequency changing in their respective positions for operation in the highest frequency range of the transmitter.

Although this system may be operated at any radio frequency, let us assume for the purpose of explanation that we are using a 2 megacycle crystal for frequency control. Also, while it is also possible and practical to operate each multiplier stage at three to four times the frequency of the preceding stage, let us assume that we are doubling the frequency in each multiplier stage. Assume that the oscillator circuit generates R. F. energy at a frequency of 2 megacycles. The first multiplier doubles this frequency, and produces R. F. energy at a frequency of 4 mc. The second multiplier doubles the frequency again, and produces R. F. energy at a frequency of 8 me. The third multiplier again doubles the frequency and produces R. F. energy at 16 mo. The 16 me. energy from the third multiplier is applied through contact I28 01 selector switch 14 and through grid blocking condenser I83 to control grid I33 of amplifier tube I34.

Grid I33 of amplifier tube I34 connects to ground through R. F. choke I35 and grid biasing resistor I36. Screen grid I31 connects to the high voltage plate supply system through screen grid voltage dropping resistor I38. Screen grid bypassing capacitor I39 provides a short path to ground for any R. F. potentials appearing in the screen grid circuit. Cathode bypass capacitors I40 and I rovide short R. F. paths between the cathode I42 and ground. Resistor I and R. F. bypassing capacitor Ill form a cathode biasing system.

Anode D. C. potential is applied to anode I43 through R. F. choke I44. Variable capacitor I45 resonates the plate tank circuit at the desired I frequency.

The special output switching device described earlier in this specification acts both as a blocking capacitor to prevent the flow of D. C. potentials from the anode supply flowing through the plate tank inductor to ground, and as a switch for selecting any one of four plate tank inductors.

We have assumed that the power amplifier is being excited with R. F. energy at a frequency of 16 mo. Therefore inductor 8 is of such a value that it may be resonated at 16 me. by the adjustment of variable capacitor I45 when rotor plates 3 and stator plates 4 of the special switching device are interleaved.

Inductor 9 is inductively coupled to plate tank circuit inductor 8 and therefore is capable of coupling energy out of the plate tank inductor 8 and supplying it to the utilization circuit I0.

It is to be noted that the three selector switches I4, 11, and I29, and the switching device for inductance selecting, are mechanically coupled to a common actuating shaft in such manner that they may be operated by a common controlling knob, dial, or other suitable device, so that all switching functions necessary in changing from one frequency range to another may be accomplished simultaneously.

Assume now that it is desired to operate in the next lower frequency range which will be 8 mc., if we continue to assume the frequency arrangement previously described. The dial controlling the various switching devices is rotated until contact I32 of selector switch I29 is open circuited, rendering the third multiplier stage inoperative by opening the D. C. circuit between cathode resistor I26 and ground; contact II5 of switch 11 is open circuited, removing compensating capacitor I I4 from the second multiplier plate tank circuit; contact I28 of switch I4 is open circuited from the power amplifier input circuit and contact II3 of switch 14 is connected to the input circuit of the power amplifier; and the rotor of the special inductance changing device in the power amplifier circuit reaches a position where rotor plates 3 interleave with stator plates 5, placing inductor II in the power amplifier plate tank circuit.

The oscillator and first multiplier plate tank circuits have not been disturbed and therefore do not require retuning. Likewise, the second multiplier plate tank circuit will not require retuning because the capacity of the amplifier input circuit which has now been added is exactly compensated for by the removal of compensating capacitor II4 from the circuit. The only retuning possibly necessary will be the readjustment of variable tuning capacitor I45 in the power amplifier plate tank circuit.

The plate tank circuit of the third multiplier stage will no longer be resonant because the capacity of the power amplifier input circuit has been removed therefrom. However, this does not effect the operation of the system, since the third multiplier is now inoperative.

If it is desired to operate on 4 mc., the .oupling or switching device is moved around one step further, opening the circuit between contact I3I of switch I29 and ground, rendering the second multiplier stage inoperative; opening the circuit between contact and ground in switch 11 thereby removing compensating capacitor 94 from the second multiplier plate tank circuit; connecting contact 93 to the power amplifier input circuit, thereby applying 4 mo. R. F. energy to the power amplifier circuit; and placing inductor I4 in the power amplifier plate tank circuit by the interleaving of rotor plates 3 and stator plates 6. The power amplifier plate tank circuit may now be resonated by the adjustment of variable capacitor I45 if necessary.

As in the previous paragraph, where the 8 mo. operation was described, the oscillator plate tank circuit remains undisturbed and therefore does not require returning. The capacity of the power amplifier input circuit has been added to the second multiplier plate tank circuit, but since compensating capacitor 94 has been removed from this circuit, resonance has been maintained, and no retuning is required.

Both the capacity represented by the power amplifier circuit and compensating capacitor III are now disconnected from the second multiplier plate tank circuit, which throws this circuit completely out of resonance. However, this circuit has already been rendered inoperative and whatever may happen to it is irrelevant.

If it is desired to operate on a frequency of 2 me. (which we are assuming is the fundamental frequency of the crystal controlled oscillator) the switching device is rotated one step further to the fourth and last position.

Contacts I32, I3I and I30 of selector switch I 29 are now all open, rendering all three multiplier stages inoperative. Contacts H5, 95 and I6 of selector switch 11 are open, removing compensating-capacitors Ill, 94, and I from the second multiplier, first multiplier, and oscillator plate tank circuits respectively. Contact I3 of selector switch I4 is connected to the power amplifier input circuit, applying 2 me. R. F. energy to the power amplifier input. The capacity of the power amplifier input circuit has been added to the oscillator plate tank circuit, but since capacitor 15 (which is of exactly the sam capacity as that of the power amplifier input circuit) has been removed, the oscillator plate tank circuit will not require retuning. The fact that all multiplier circuits may now be non-resonant is immaterial, since they have already been rendered inoperative by the open circuiting of contacts I30, I3I and I32 of selector switch I29.

Inductor I1 is now connected into the power amplifier plate tank circuit by the interleaving of rotor plates 3 with stator plates 1 of my special inductance switching device.

The power amplifier plate tank circuit is now resonated by an adjustment of variable capacitor I45.

While the oscillator frequency specified for illustrative purposes was 2 megacycles, and the output of each successive multiplier stage was specifled as twice the excitation frequency applied thereto, such values were purely illustrative and not limiting. since the oscillator frequency might be any feasible frequency in the R. F. spectrum, where quartz frequency control plates may satisfactorily be employed, and each multiplier stage might be operated on some multiple other than the second multiple of the excitation frequency, applied to the grid circuit of its associated vacuum tube.

Likewise the power amplifier may be operated as an additional frequency multiplier, or as an ag plifier on some frequencies and as a multiplier on other frequencies.

Attention is now directed to a special adaptation of my invention whereby it is possible to obtain single dial control of the tuning of several radio frequency circuits, each operating on a different frequency from the other, but each of Search team the diil'erent frequencies being harmonically related to the other frequencies.

In the development of the radio transmitting system described herein, I have found that radio frequency circuits may be resonated at harmonically related frequencies by the use of variable capacitors of equal value, mechanically ganged together for single dial control. For instance, the first circuit may be resonated at 2 megacycles, the second at 4 mc., the third at 8 mc., etc., and that as the tuning progresses, and the first circuit resonates at 3 mc., the second will resonate at 6 mo. and the third at 12 mo. etc., over the full variable range of the tuning capacitors. To accomplish this result, I first assure that all variable capacitors used for main tuning capacitors are of equal value for any given adjustment from zero to maximum capacity. Secondly, by means of trimmer capacitors, I assure that all external capacities incidental to the circuits are exactly equal. Next I assure that each inductance is of the correct value to resonate at the desired frequency with the capacity present (which includes the stray circuit capacities, tuning capacity, and trimmer capacity). To obtain a more critical adjustment of inductance than may be readily obtained by adding or subtracting turns from the coil or by changing the diameter of the coil, and to correct for variations in inductance due to normal manufacturing tolerances, I include some means for continuously varying the inductance over a limited range. This means may consist, for example, of a copper disc adjustably located within the magnetic field of the inductor, a powdered iron core adiustably located within the magnetic field of the inductor, or a sectional inductor arranged in such a manher as to provide for a variation in the mutual coupling between sections.

Such an arrangement is indicated in the oscillator and frequency multiplier stages of the transmitter circuit shown in Fig. 4.

Referring again to Fig. 4, the crystal controlled oscillator and the three following frequency multiplier stages are arranged for simultaneous tuning. Variable capacitors BI, 86, I05 and I24 are the main tuning capacitors each of identical capacity. The rotors of these main tuning capacitors are connected together mechanically, and are operated in unison by a common dial (not shown). Trimmer capacitors 60, 85, I04 and I23 are separately adjustable, and once they are adjusted, they remain unchanged. Likewise inductors 59, 84, I03 and I22 are separately ad- Justable, and are only adjusted during the initial alignment, and thereafter remain fixed at the value originally obtained in the initial ad- Justment. Each succeeding inductor has an appreciably lower inductance than the inductor used in the preceding stage.

Since the value of capacitance remains identical for each stage, and since the inductance is not the same in each stage, it will be understood that the frequency range of each tuned circuit will be at wide variance with that of every other tuned circuit in the system because of the difference in the ratio of inductance to capacity.

For instance, if 50 m. m. f. of capacity will resonate the first circuit to 2 megacycles, and if 50 m. m. f. of capacity will resonate the second circuit to 4 mc., and if the main tuning capacities were changed so that the first circuit will resonate at 3 mc., then the second circuit will resonate at 6 mo. In other words, if the proper value of inductance is chosen for each circuit, and the variation in capacity is held identical for each circuit. the change in the resonant frequency will be proportionate in the two circuits. If the resonant frequency of the first circuit is changed from 2 me. to 3 mc., the change is only 1 mc., but represents a change of 50%. At the same time, the resonant frequency of the second circuit has changed from 4 me. to 6 mc., representing a change of 2 me. or twice the change in frequency of the first circuit, but the change in frequency of said second circuit is 50% of its original frequency, therefore, the change in the two circuits are exactly proportional.

With the arrangement above shown and described it is possible and practical to arrange several tuned circuit having a common tuning control, so that each of said tuned circuits will be resonant at a multiple of the resonant frequency of a preceding circuit, and to maintain indicating the anode current drawn by each tube in the radio frequency circuit. This is accomplished by the use of a two pole multiple position switch and a conventional meter, with separate meter shunts connected in appropriate parts of the circuit. These shunts are connected to the contacts of the switch in such a way that the meter is connected across each shunt in succession, in accordance with the rotation of the meter selector switch.

Referring to Fig. 4, I85 is a conventional current indicating instrument, shunted by a capacitor I48 to prevent damage to the instrument by stray R. F. currents. The positive pole of the instrument connects to one movable arm I41 of the selector switch, and the negative pole of the instrument connects to the other movable arm I48 of the selector switch.

Meter shunt I48 is connected in series with the anode supply of the oscillator tube. The end of this shunt toward the tube connects to contact I58 of the selector switch, and the power supply end of the shunt connects to contact II of the selector switch. To obtain an indication of the anode current drawn by the oscillator tube 58, selector switch arms I48 and I41 are rotated until they are in contact with contactors I58 and I5I respectively. Any anode current drawn by the oscillator tube will cause an IR drop across meter shunt I48 and will produce a corresponding indication in meter I 85.

Meter shunt I52 is connected in series with the anode supply to the first multiplier. One end of shunt I52 connects to contact I53 and the other end connects to contact I54 of the selector switch. Anode current drawn by first multiplier tube 1| will cause an IR drop across shunt I52 which will cause meter I85 to indicate the anode current drar'n by tube 1| when movable arms I48 and I41 of the selector switch are in contact with contactors I53 and I54 respectively.

Meter shunt I55 is connected in series with the anode supply of second multiplier tube 8|. Contactors I58 and I51 of the selector switch are also connected to the two ends of shunt I55. Any anode current drawn by tube 8I will cause an IR drop across meter shunt I55 which may be applied to meter I85 by rotating selector switch arms I48 and I41 until they contact contactors I58 and I51 respectively.

Meter shunt I58 is connected in series with the anode supply to third multiplier tube II 2. The opposite ends of this shunt also connect to contactors I58 and I88 of the meter selector switch. Any anode current drawn by tube II2 will cause an IR drop across shunt I58 which may be indicated by instrument I85 when the movable arms I48 and I41 of the selector switch are in contact with contactors I58 and I88 respectively.

Meter shunt I8I is serially connected in the anode supply circuit of the power amplifier tube I34. Opposite end of this shunt are also connected to contactors I82 and I83 of the meter selector switch. Any anode current drawn by the power amplifier tube I34 will cause an IR drop across meter shunt I8 I. When the movable arms I48 and I41 of the meter selector switch are in contact with contactors I82 and I88 respectively, any IR drop appearing acros meter shunt I8I will be indicated by instrument I85.

Since a separate metering instrument is used for each stage in normal practice, I have effected a. saving of four instruments by using only a singl instrument, a selector switch, and five meter shunts.

The remainder of the circuit shown in Fig. 4 illustrates a conventional power supply system consisting of a main power switch I84, main protective fuses I85 and I88, anode supply fuses I81 and I88, cathode heating transformer I88, anode supply switch I12, anode supply transformer I13, rectifier tubes I14 and I15, rectifier cathode heating transformer I18, filter reactors I11 and I18, filter capacitors I18 and I88 and voltage dividing resistors HI and I82. This power supply system is well known to the art and therefore requires no further explanation.

I claim:

1. In a radio transmitting system, a device for simultaneously resonating a plurality of circuits operating at harmonically related frequencies by a single dial control, including a plurality of mechanically coupled variable capacitors having substantially equal values of capacity throughout the range of variation thereof and means for connecting each capacitor across a predetermined one of the respective inductors of said circuits, whereby change of capacity of said circuits will still maintain the harmonic relation therebetween.

2. In a radio transmitting system, a device for simultaneously resonating a plurality of circuits operating at harmonicalluelated frequencies by a single dial control, including a plurality of m chanically coupled variable capacitors having substantially equal values of capacity throughout the range of variation thereof and means for connecting each capacitor across a predetermined one of the respective inductors of said circuits, whereby change of capacity of said circuits will still maintain the harmonic relation therebetween and means for adjusting the values of said inductors so that the harmonic relation between said circuits may be maintained when said capacitors are equal in value.

3. In a radio transmitting system comprising an oscillator, one or more freguency multipm stages and an amp er a device for resonating 7 the oscillator an frequency multiplier stages to from the oscillator to the multiplier circuits and from one multiplier circuit to another, said means including a plurality Of switches mechanically coupled together, at least one of said switches acting effectively to add and subtract capacity when said input is transferred from one circuit to another, so as to maintain resonance.

4. In combination, a push pull amplifier of the electronic tube type including two output anodes, means for feeding energy to said anodes and means for withdrawing energy therefrom and selectively feeding said energy to a predetermined circuit while blocking the flow of direct current through said withdrawal means, said withdrawal means including two conductors, each connected at one end to a respective anode, and a switching device including two movable members electrically discrete but mechanically coupled so as to move together, and a plurality of sets of stators, each set having two individual stators arranged so as electro-statically to be coupled respectively with said two movable members, upon rotation of said movable members, each of said conductors being connected at the other end to a respective movable member of said switching device, whereby said movable members function to switch the withdrawn energy and simultaneously function as blocking condensers with respect to said anodes.

5. A multi-irequency radio transmitter including a plurality of resonant circuits each including an inductor and a capacitor, single control means for altering the tuning of each circuit to Search Ream a single predetermined frequency harmonically related to the frequency or each other of said circuits, said control means comprising a plurality of variable condensers having substantially identical electrical constants, each condenser constituting one of said capacitors, and mechanical coupling'means for simultaneously varying all said condensers to an equal extent, and means for keeping all said circuits in resonance when said variable condensers are changed in capacity values, said last means comprising a trimmer condenser connected to at least one variable condenser, and so adjusted that stray external capacities incidental to each circuit are kept substantially identical in all said circuits and also comprising a mechanically operable element acting to adjust the value of at least one of said inductors so that the respective values of the inductors are such that resonance is obtained at a frequency harmonically related to the resonant frequency of at least one other inductor;

6. A transmitter according to claim 5 in which said inductor adjusting element comprises a disc or conducting material located within the electromagnetic field or said inductor and rotatable with respect thereto.

7. A transmitter according to claim 5 in which said inductor adjusting element comprises a core of suitably comminuted iron located within the electro-magnetic field of said inductor and movable with respect thereto.

JAMFB N. WHITAKER.

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