US2376667A - Automatic tuning of transmitters - Google Patents

Description

. MaY'ZZ, 1945- v o. B. CUNNINGHAM ET Al. 2,376,667

AUTOMATIC TUNING OE TRANSMITTERS Filed March 29, 1943 2 Sheets-Sheet l moe/ma ggz m HNR 7' /Y TEN/V14 *L T L nventors Qttorneg AUTOMATIC TNING OF TRANSMITTERS Patented May 22, 1945 2,376,667 l AUTOMATIC TUNING OF TRANSMITTERS f Oliver B. Cunningham, Haddoniield, Harry Sussman, Pensauken Township, Camden County, and Samuel Gabin, Merchantville, N. J., assignors to Radio Corporation of America, a corporation of Delaware Application March 29, 1943, Serial No; 480,928 y (cl. 25o-17) Claims.

This invention relates to automatic tuning of radio transmitters and more particularly to changing the frequency of operation of a multistage transmitter by means of a single control.

The usual radio transmitter comprises a master oscillator, one or more amplifiers, and a power amplifier. The output circuit of thepower amplier is coupled through resonant circuits to an antenna which is resonated at or near the frequency of operation of the transmitter. The tuned circuits preceding the final power amplier stage may be arranged for single control tuning adjustment by means of mechanical gauging similar to that used in broadcast receivers. However, the power amplifier and antenna circuits cannot be controlled in this manner because of the wide variation in resistance and reactance of the antenna with variations in frequency. For this reason the adjustment of the frequency of operation of a transmitter is somewhat cumbersome, requiring the separate tuning of at least three different circuits: the exciter or oscillator andamplier circuits, the power amplifier tank circuit and the antenna circuit. In addition to this the coupling between the antenna and the power amplifier must be readjusted for each change in the frequency.

It is the principal object of the present invention to provide an improved method of and means for tuning automatically the circuits following the master oscillator including the power ampler output circuit and the antenna of a radio transmitter.

Other objects will become apparent to those skilled in the art upon consideration of the following description with reference to the accompanyingdrawings, of which Fig. 1 is a schematic circuit diagram of a preferred embodiment of the invention, Fig. 2 is a schematic circuit diagram of a phase detector employed in the circuit of Fig. 1, and Fig. 3 is a modified embodiment of the inventiom j Referring to Fig. 1, a power amplifier stage I is coupled through. a'transformer 3 to an antenna circuit including an adjustable loading inductor 5; The primary of the transformer 3 is tuned by meansofvariable capacitors 'Iv ands connected in parallel. The secondary circuitfis' tunedby'a variable capacitor II and the variableinductor 5. The capacitors 1 and II are mechanically ganged, as indicated by the dash line I3, to the tuning control of the driver amplifier and preceding stages of the transmitter. The capacitor 9 is f mechanicallyfconnected to a reversible motor similar motor Il. The secondary of the transformer 3 is movable with respect to the primary to permit variation of the coupling and is me- Cil I5 andthe inductor 5 is similarly connectedto av 55 chanically connected to a reversible motor I9. A phase detector 2I is connected to the grid and anode circuits of the power amplier I. Theoutput of the phase detector is connected to an amplier 23. The amplifier 23 is arranged to control the energization 'of "the motor I5 fi'om a power supply, not shown.

A second phase detector 25 is connected to the anode circuit of the power amplifier I and to a capacitor 2l included in the ground circuit of the secondary of the transformer 3. An amplifier 29 similar to the amplifier 23"is connected to one group of xed contacts 33 of a relay 3|. The movable contacts 35 of the relay 3l are connected to the motor Il. A second group of fixed contacts 3l is connected to the power supply circuit. A rectifier 39 is connected across the capa'citor 2l and to the energizing coil 4I ofthe relay 3|.

A resistor 43, bypassed by a capacitor 45, is included in the cathode circuit of the amplifier I. The resistor 43 is connected through a resistor 4l to the input circuit ofv an amplifier 49. The output circuit of the amplifier 49 is connected to the motor I9. A D.C. source 5I is connected through' a variable resistor 53` and a meter 55 across the resistor 4l.

Referring to Fig. 2, each of the phase detectors 2| and 25 comprise a pair'of diodes 51 and 59 connected between a center tapped inductor 6I and a center tapped resistor 63. The input circut 6l is connected to a, phase inverter 69. The output of the phase inverter 69 is applied across the inductor El. In order to prevent variations in phase shift through the phase inverter with variations in frequency, the load resistor 'I.I is of very low resistance. This may provide a gain through the phase inverter of less than unity. The diode 59 is connected to the input circuit 6l through an adjustable voltage divider 'I3 to permit balancing of the output of the phase inverter. A capacitor 'I5 is provided between the input circuit 6'l and the control 'grid of the phase inverter tube 69. The capacitor 15, in conjunction with the resistance of the voltage divider 13, provides substantially a 90 degree phase shift of the input to the circuit 61.

The operation of the phase Shifters shown in Figr2 is as follows:

When the voltage applied to the input circuit between the center taps of the elements 6I and B3 is positive at the same instant that the anode of the diode 51 is positive, and that of the diode 59 is negative, the diode 51 will conduct during one-half of each cycle and the diode 59 will not conduct. This causes the upper end of the resistor 63 to become positive with respect to the lower end. Conversely when the voltage between the center taps or the voltage across the inductor 6| is reversed, the lower end of the resistor 63 becomes positive. When the two voltages are 90 degrees out of phase the Voltage across the upper half of the resistor 63 is equal vand opposite to that across the lower half so that the resultant output voltage v.is zero. The phase shifting network in the input circuit of the phase inverter 69 causes the device to provide' zero output when the two input voltages are in phase or 180 degrees out of phase, and maximum output when the two voltages are 90 degrees out of phase, with a polarity depending upon which of the two input voltages leads the other.

The operation of the system of Fig. l is as follows:

The power amplifier circuit is tuned approximately to resonance by means of the capacitor 1 through the mechanical connections I3 to the tuning elements of the preceding stages. The antenna circuit is similarly rbrought to a rough approximation of its proper adjustment by means of the capacitor I|. The phase detector 2| responds to the voltage in the grid andanode circuits of the tube I, providing direct current output related in its magnitude and in its polarity to deviation of the output voltage from a phase 180 degrees different from that of the input voltage. This D.C. voltage controls the amplifier 23, which controls the power to the motor I5. Thus the motor is energized to run in the proper direction to make the anode voltage exactly 180 degrees out of phase with the grid voltage by adjustment of the capacitor 9. Under this condition the plate current and the plate'voltage of the power amplifier are in phase, and the tank circuit is properly tuned.

The phase detector compares' the tank circuit voltage with the voltage across the capacitor 21. Since at the beginning of the adjustment cycle there may be insufficient voltage appearing at the capacitor 21, due to drastic mistuning of thev antenna circuit, 'the motor I1 is normally connected through the contact 31 of the relay 3| to the power supply. This causes the motorv I1 to run continuously, driving'the inductor 5 to change the antenna circuit tuning until some voltage does appear across the capacitor V21. When this occurs the relay 'winding 4I is energized through the rectifier 39, transferring the connections of the motor l1 to the output circuit of the amplifier 29. The motor I1 is then operated under the control of the phase detector 25 to adjust the antenna circuit to resonance.

The` coupling between the primary and secondary of the transformer 3 is adjusted by the motor I9 in response to the output of the amplier 49. The voltage drop across the cathode resistor of the power amplifier I is proportional to the amplifier plate current. The D.C. source 5I provides a current through the resistor 41, causing a voltage drop which opposes the voltage across the resistor 43. The drop in the resistor 41 is adjusted to a predetermined value corresponding to the rated platev current of the amplifier I by means of the variable resistor 53 and the meter 55. Thus when plate current of the amplifier is greater than the proper value, a positive voltage is applied to the input of the amplifier 49, and when the plate current is less than the rated value, a negative voltage is ap.- plied to the ampliiier 49.

For example, the tuning of the power amplifier plate circuit may be controlled by Varying either the capacitance, as shown, or the inductance. Magnetic coupling to the antenna circuit is described in the illustrative embodiment of Fig. l, but any coupling circuit with an adjustable mutual impedance may be used. Any known phase detector circuit may lbe substituted for that shown in Fig. 2, depending upon particular design requirements. In the above describedA system, the power amplifier is tuned approximately by gang connections to the exciter, and the antenna circuit control searches for rough tuning. It will be apparent to those skilled in the art that either of these methods may be applied to any of the tunable circuits, and that the rough tuning may be omitted providing the phase detectors are designed to operate over the necessary range. The rotation of the motor I9 is controlled in such a direction as to adjust the` coupling of a transformer 3 to the correct value.

It is to -be understoodl that the above described adjustments are made concurrently so thatif thereis any interaction, each is altered slightly to make the necessary correction until the proper phase and current relations are secured. The motors I5, I1 and I9 may be interconnected with' the'respective amplifiers 23, 29 and 49 in known manner to prevent huntingf or overshooting and reversal.

For clarity in description independent motors and servo amplifiers have been illustrated. However a single motor may be used with clutch connections to the various tuning elements for sequential adjustment of several stages. Refer'- ring to Fig. 3, a single motor 11 is mechanically connected as indicated by the dashline 19 to a plurality of electro-magnetic clutches 8|, 83, 85 and 81. The clutch 8| is connected to the exciter tuning control, the clutch 83 is connected to the power amplier output circuit tuning control, the clutch 85 is connected to the antenna coupling control and the clutch 81 is connected to the antenna tuning control. Phase detectors 89, 9| and 93 are connected to the exciter power amplifier and antenna circuit respectively, and a D.C. comparison circuit 95 is connected to the power amplifier, as in the arrangement of Fig. "1. The output circuits of the phase detectors 89, 9|, 93 and the comparison circuit 95 arecon# nected to a multiple circuit switch 91 in the input circuit of a servo amplifier 99. A second multiple circuit switch I0| is mechanically ganged with the switch 91 and connected to the clutches 8|, 83, 85 and 81. A D.C. source`|93 is connected to the movable contact of the switch IUI. f

As the D.C. source |03 'is connected through the switch IUI to one of the magnetic clutches, for example, the clutch 8|, the input circuit of the amplifier 99 is connected through the switch 91 to the corresponding control device, in this case the phase detector 89; The motor" will then operate in the same manner as the system of Fig. 1 to adjust the tuning of the exciter stage.' Upon completion of the adjustment of 'the exciter tuning the switches 91 and I 0| are moved to their next position, engaging the clutch 83 and con'- necting the phase detector 9| to the servoy amplifier. This operation is continued down the line, each clutch being engaged in turn until all-the ldjustments vare completed. The switches4 vand IUI may be operated manually, or automatically by means of a stepping magnet |05 constructed similarly to the stepping switch used in automatic.

telephone exchanges. In the system of Fig. 3, the magnet |05 is energized from the D.C. source |03 through a relay |01. The actuating coil of the relay |01 is connected to the output of the servo amplifier 11. Ihus as long as there is any output from the amplifier 99 the contacts of the relay |01 are held open and the magnet |05 is deenergized. When adjustment of any stage is completed, the output of the servo amplifier falls to zero and the relay |01 is deenergized, connectng the sourcei|03 to the magnet |05 and actuating the switches 91 and |0| through one step to connect the next following stage of the transmitter. A limit switch may be operated by the stepping magnet 85 to deenergize the magnet after the adjustment of the last stage is completed.

We claim as our invention:

1. In combination with a radio transmitter including a power amplifier, driver amplifiers, a frequency determining oscillator and an antenna circuit, at least one variable reactance element in the plate circuit of said power amplifier, 'a motor arranged to vary the reactance of .said element, a source of energy for said motor, control means connected between said source and said motor for controlling said motor in response to the diilerence in phase between the input and the output of said power amplifier, at least one variable reactance element and a further reactance element in said antenna circuit, a second motor arranged to vary said antenna circuit variable reactance element in response to the difference in phase between the output voltage vof said power ampliiier and the voltage appearing across said further antenna circuit reactance element, and a third motor arranged to vary the coupling between the output circuit of said power amplifier and said antenna circuit in response to the deviation of the plate current of said power amplifier from a predetermined value.

2. In a radio transmitter including an output l tube, a power output circuit, an antenna circuit coupled to said output circuit, and mechanically operable means for varying the coupling between said circuits, a motor connected to said mechanically operable means, and plate current responsive means connected to said output tube for controlling the energization of said motor to increase said coupling in response to the deviation of the plate current of said tube below a predetermined value, and to decrease said coupling in response to deviation of said plate current above said predetermined value. y

3. In combination with a radio transmitter including a plurality of adjustable resonant circuit elements, independent means responsive to the tuning of each of said elements, an electric motor, independent clutch means connected between said motor and each of said adjustable elements, a control amplier connected to energize said motor, multiple circuit multiple position switch means connected between the input circuit of said control amplifier and each of said tuning responsive means so as to connect, in each position, said input circuit to a respective one of said tuning responsive means, a second multiple circuit'multiple position switch connected between a source of energy and each of said clutch means so as to connect, in each position, said source of energy to a respective one of said clutch means, and a mechanical connection between said switches, whereby upon actuation of said switches said resonant circuit elements are connected in sequence to said motor while said tuning responsive elements are connected in the same sequence to said control amplifier.

4. A radio transmitter comprising a plurality of cascaded stages, each including at least one tunable circuit element and means responsive to the tuning of said element, a control ampliiier including an input circuit and an output circuit, a multiple position switch in said input circuit for sequentially and selectively connecting said tuning responsive means to said input circuit, a motor connected to said output circuit to be energized thereby in accordance with the input to said amplifier, selectively operable clutch means connected between said motor and each of said adjustable resonant elements, and multiple position switch meansfor selectively and sequentially engaging said clutches.

5. The invention as set forth in claim 3 including means for actuating said switches step by step from one position to another, a source of energy for said actuating means, and means responsive to output from said control ampliiier to disconnect said actuating means from said source of energy, whereby y said switches are actuated only upon deenergization of said motor.

OLIVER B. CUNNINGHAM.

HARRY SUSSMAN.

SAMUEL GUBIN.

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