US2382615A

US2382615A - Oscillator tuning system

Info

Publication number: US2382615A
Application number: US443244A
Authority: US
Inventors: Hugh L Donley
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-05-16
Filing date: 1942-05-16
Publication date: 1945-08-14
Anticipated expiration: 1962-08-14

Description

.A 1945' H. DONLEY OSCILLATOR TUNING SYSTEM Filed May 16, 1942 FPEgu/Ewcr Meal/Lawma- 800.905

INVENTOR ATTORNEY Patented Aug. 14, 1945 OSCILLATOR TUNING SYSTEM Hugh L. Donley, Collingswood, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application May 16, 1942, Serial No. 443,244

6 Claims.

This invention relates to a frequency modulation system, and in particular to a method of and means for varying the frequency of an oscillator without causing appreciable amplitude variation.

If the frequency of an oscillator is varied by changing solely one of the reactances of the oscillator tank circuit, the impedance of the tank circuit is altered and the amplitude of oscillation thereby altered in addition-to the change of frequency. This may be seen from the expression for the impedance of a tuned circuit; namely,

where r is the effective series resistance of the tank circuit and L and C are its inductance and capacity, respectively. From this expression, it may be seen that if either L or C is varied, the impedance of the tank circuit will vary unless a corresponding change in rr is produced substantially in proportion to the change in inductance if inductance L is varied, or inversely proportional to the change in capacity if the capacity C is varied.

The present invention is based on an appreciation of these principles which are so employed by me that I am able to change the frequency of the oscillator without producing any material change in the amplitude of oscillation. From a broad aspect, I accomplish this result by producing reactance variations in the oscillator tank circuit and simultaneously therewith producing variations of effective resistance which tend to maintain the amplitude of oscillations more constant than in the case where only the reactance of the oscillator tank circuit is varied.

The following is a description of the invention accompanied by a drawing whose single figure illustrates, by way of example only, one embodiment of the invention.

Referring to the drawing in more detail, there is shown one way by which the frequency of an oscillator can be varied over a desired range without varying the amplitude of oscillation. The oscillator comprises a vacuum tube I having a tank circuit 2 of inductance and capacity coupled between the grid and anode. For varying the frequency of the oscillator there is provided I acontrol vacuum tube 3 whose grid is coupled to the anode of the oscillator tube through a blocking condenser, as shown, and whose anode is coupled to a parallel tuned circuit 9 composed of a coil 4 and a condenser 5. Tuned circuit 9 is tuned to a frequency above the frequency of the'oscillator I.

Coil 4 is wound around a ferromagnetic core, preferably magnetite in particle form, and the ends of this core are positioned in a magnetic path constituted by a U-shaped laminated iron aflair which receives its exciting flux from a modulator coil 6, as shown. Modulating coil 6 has its terminals coupled to a low frequency modulating source, such as a. source of speech waves. The lengths of the magnetite core and its surrounding coil 4 are much longer than their diameters in order to enable as much change as possible in the permeability of the core with change in exciting flux.

A battery I in series with modulator coil 6 and of suitable value provides a direct current bias on the core, thusassuring an operating point on the substantially linear portion of the radio frequency volts-permeability characteristic.

The reason that magnetitein powdered or comminuted form is preferred for the core is because it is an easily saturated material, although other comminuted ferromagnetic materials may be used.

The anode circuit 4, 5 of the control tube 3 possesses both resistance and inductive reactance, as a consequence of which theeffective admittance between grid and ground has both a positive susceptance component and a negative conductance component. The former component affects chiefly the frequency of oscillation while the latter afiects chiefly its amplitude. I have found that if the anode tuned circuit of the control tube l consists of a parallel tuned circuit whose natural frequency is above the mean frequency of the oscillator and whose natural frequency is varied by varying its effective inductance, that under suitable conditions of adjustment the variation of the aforesaid negative conductance is so related to the variation of the aforesaid susceptance as to tend to maintain the oscillator tank impedance substantially constant in magnitude and hence the generated oscillations substantially constant in amplitude. The best adjustment of the natural frequency of the tuned circuit 4,5 is readily determined experimentally and will be found to be substantially higher than the oscillator frequency. The natural frequency of tuned circuit 4, 5 should not be too close to the oscillator frequency, nor too far away from the oscillator frequency to materially reduce the frequency shift desired from the oscillator. The experimental procedure is, of course, to observe the amount of amplitude modulation produced as the anode circuit tuning is adjusted and to set the adjustment at the point which gives the least amplitude modulation.

By applying modulating potentials to modulator coil 6, I am able to change the permeability of the core as a result of which there is a corresponding change in the inductance of coil 4'. This change in permeability, in effect, changes t e tuning of the circuit 4, 5 and this is re- Y I be lower than the Q of the oscillator tank 2.

The Q of circuit 4, I must, of course, be a reasonable one which will not unduly load the oscillator and which will give a good saturation. By having a reasonably low Q for tuned circuit 4, I, it is possible to obtain a greater percentage frequency shift in the oscillator for the same power input to the modulating coil compared to a high Q for tuned circuit 4, 5. By inserting more magnetite material in the core it is possible to obtain greater permeability but this will reduce the Q of the circuit on account of the larger losses in the core.

In one embodiment tried out in practice, the oscillator mean frequency was fifty megacycles. The control tube used was an RCA 6J5 vacuum tube. The Q of the oscillator tank was 225. The Q of timed circuit 4, I changed approximately from 100 to 150 with the core in and out of the coil 4, the inductance changing by the ratio 1.5 where the core gave an initial eifective permeability of approximately 2.

Although the illustrated embodiment of the invention has been described with particular reference to varying the capacity of the oscillator tank, obviously the invention is not limited to such an arrangement since, if desired, modifications of the arrangement are readily devised in which the inductance of the oscillator tank can be varied and simultaneously therewith the eifective resistance to maintain the ratio of effective inductance to resistance constant. Other changes may readily suggest themselves to those skilled in the art without departing from the spirit and scope of the invention. For example, the grid of the control tube can be inductively coupled by a small coil to the oscillator tank instead of being capacitively coupled, as shown.

The term angular veloci employed in the appended claims is deemed to include both frequency and phase modulation.

What is claimed is:

1. A high frequency oscillator comprising a vacuum tube having an anode, cathode and grid, and an oscillatory circuit coupled between the anode and grid, a connection from the cathode to a point on said oscillatory circuit intermediate the ends thereof, means for varying the frequency of the generated oscillations with a minimum of amplitude variation comprising a control vacuum tube also having anode, cathode and grid electrodes, a connection from the grid of said last tube to the anode of said'oscillator tube, a parallel tuned circuit connected between the anode and cathode of said control tube, said tuned circuit having a coil surrounding a ferro-magnetic core and ,a condenser in shunt to said coil and being tuned to a frequency higher than the frequency of the generated oscillations, and means for varying the permeability of said core in accordance with low frequency modulating potentials.

2. In combination, an oscillator having a frequency controlling tank circuit of reactants and eifective resistance, a, vacuum tube having a grid coupled to said oscillator and an anode coupled to a resonant circuit whosenatural frequency is higher than the'frequency of oscillations produced by said oscillator, said resonant circuit including an inductance coil surrounding a ferromagnetic core, and means for varying the per- 7 meability of said core in accordance with 1811 1 modulations to thereby reflect variations in admittance upon said tank circuit.

3. In combination, an oscillator having a frequency controlling tank circuit of reactance and effective resistance and having a particular value of Q, a vacuum tube having a grid coupled to said oscillator and an anode coupled to a resonant circuit of a lower value of Q whose natural frequency is higher than the frequency of oscillations produced by said oscillator, said resonant circuit including an inductance coil surrounding a ferromagnetic core, a magnetic path for supplying exciting flux to said core, and means for varying the permeability of said core in accordance with signal modulations to thereby reflect variations in admittance upon said tank circuit, said means including a modulating coil in circuit with said path and coupled to a source of low frequency modulating energy.

4. A frequency modulation system comprising a high frequency oscillatonhaving a tuned circutt of reactance and effective resistance, a vacuum tube having a grid coupled to said oscillator and an anode coupled to a resonant circuit whose natural frequency is higher than the frequency of oscillations produced by said oscillator, said resonant circuit including an inductance coil surrounding a ferromagnetic core, and a source of low frequency modulating energy for varying the permeability of said core.

5. A frequency modulation system comprising a, high frequency oscillator having a frequency controlling tank circuit of reactance and effective resistance and having a particular value of Q, a vacuum tube having a grid coupled to said oscillator and an anode coupled to a resonant circuit of a lower value of Q whose natural frequency is higher than the frequency of oscillations produced by said oscillator, said resonant circuit including an inductance coil surrounding a ferromagnetic core, a, magnetic path for supplying exciting flux to said core, and means for varying the permeability of said core in accordance with signal modulations to thereby reflect variations in admittance upon said tank circuit, said means including a modulating coil in circuit with said path and coupled to a source of low frequency modulating energy, the lengths of said inductance coil and said ferromagnetic core being appreciably greater than their diameters.

6. A frequency modulation system comprising a high frequency oscillator having a tuned circuit of reactance and effective resistance, a vacuum tube having a grid coupled to said oscillator, said vacuum tube also having an anode, and means coupling said anode to a circuit tuned to a frequency higher than the frequency of oscillations of said oscillator, said last circuit including a coil wound upon a magnetisable core, and means for supplying modulating energy for varying the permeability of said core.

HUGH L. DONLEY.