US2268872A - Variable frequency oscillation generator - Google Patents

Description

Jan. 6, 1942. R HEWLETT 2,268,872

VARIABLE FREQUENCY OSCILLATION GENERATOR Filed July 11. 1939 FIG.2.

Mill/1M 1?. HEWLETT ATTORNEY Patented 6, 1942 UNITED STATES PATENT OFFICE VARIABLEgStIHJ-QTION williamlmralonltgllflifsaallgnorto Hewlett-radar! Oarnpany,!aloflto, Calif, a

ea-alating eo-partnaabip andDavldPaeiu-i Application July 11; 19:9, Serial No. 3,301

(hims- My invention relates to variable frequency osciilation generators and more particularly to uscillation generators wherein the frequency is determined by means of resistance capacity net works.

Oscillation generators capable of tuning over a wide range of frequencies are useful for many purposes, for examp1e,as test generators in place of beat frequency generators. In general such oscillation generators have been provided with a variably timed resonant circuit to maintain the oscillations at a frequency to which the tuned circuit is adjusted.

According to my invention, 1 provide an oscillation generator with a variable resistance capacity network forming a positive feedback path for the purpose of determining the frequency of oscillation.

According to another feature of my invention an oscillation generator with capacity frequency determining ts and with a negative feedback circuit for reducing distortion caused in the amplifier circuit; is provided.

According to a further feature of my invmtion I provide an automatic amplitude control arrangement in a negative feedback path of an oscillation generator to maintain the amplitude oi" the generated oscillations constant deaiite changes in the supply voltages for the tubes.

Other advantages and features of my invention will be apparent from the particular description thereof made in connection with the accompanying drawing, in which Fig. 1 shows a diagrammatic circuit of the preferred form of an oscillation generator in accordance with my'invention, and

Fig. 2 shows curves for the purpose of illustrating the operation of the circuit.

In Fig. l is shown an oscillation generator cornprising two amplifying tubes I. and II, coupled together by resistance capacity means in a known manner. The voltage supply sources and the cathode heating supply are omitted for simplifying the circuit. The anode of tube i. is coupled to the input grid of tube ii and the anode of hrbe II is then connected to a circuit which provides a feedback path through a coupling condenser l2 and series connected resistance R1 and variable capacity C1, and a second circuit comprising R:

and variable condenser C1, to the input of tube ments. In practical construction, however, it is oscillations in the circuit. By properly proportioning the value of the resistance capacity networks B1, C1, 13:, C3, the desired frequency may be obtained. If R1, and 01, respectively, equal R2, and C3, the ratio of the voltage Eb at point D to the voltage 1?. at point a varies with frequency in a manner similar to a resonance curve. In Flg.2thiscurveisshowninsolidlineswith voltage ratios of F4. and Eb plotted as ordinates tive frequency. It can be seen that at zero phase shift, the maximum of the E, Es ratio occurs. If the various condensers and resistances are not made equal as they were for plotting this curve,

"the maximum would occur at some other point and a symmetrical curve would not be obtained.

However, the principles of my invention apply to the circuitregarrlless of equality of these elegenerally dsirahle to make the separate elements qua As shown in Fig. 1, frequency shift in the oscillation generator is accomplished by adjusting variable capacities C1 and C2. 11; is clear, however, that since the resistances as well as the capacities serve to control thefrequency of the oscillator, either the resistances alone or both the rmistances and capacities may be adjustable for varying the tuning.

For the tape of oscillators illustrated in Fig. 1,

40 it is n in order to obtain satisfactory operation that the amplifiers comprising tubes I II and II have phase shift independent of changes in supply, etc and furthermore, there should be provided some means for controlling the amplitude of the oscillationsso that they do not exceed the range over which tubes I. and II will operate as class A amplifiers. A constant amplifier I. The circuit including B1, C1, B2, C2, provides aregenerative or positive coupling between the output and input circuits of the amplifiers fl and I I, so that feedback occurs suflicient to maintain phase shift is necessary to insure a constant frequency, because the phase angle, of the transfer impedance of the resistance capacity network from point a to point D varies only slowly with frequency. Accordingly, a small change in amphiier phase shift such as might be produced by a variation in voltage supply requires a comparatively large change in the frequency of operation to produce a compensating phase shift in the resistance capacity coupling system.

In the circuit of Fig. 1, I provide a negative feedback arrangement for stabilizing the amplifier. This feedback is made from the output of amplifier ll over coupling condenser l2 and the resistance R3, R4 to the cathode of tube l0. The potential drop across the resistance produces a negative feedback to the input of amplifier III. This negative feedback operates in a manner known per se to reduce the amplification of the system and at the same time to stabilize the amplifier gain in magnitude and phase with respect to variations occurring within the amplifier, such as tube changes, temperature variations and changes caused by fluctuation in the voltage supplies to the electrodes of the amplifier system.

Amplitude control to prevent the oscillations from building up to such a large value that distortion occurs, is obtained according to my invention by non-linear action in the amplifier circuit. In order to produce this. non-linear variation, I provide for resistance R3 a small incandescent lamp, or similar device in which the resistance increases rapidly with increased current fiow, the lamp being heated by the plate current of the tubes f and H or by an auxiliary means, so such a temperature that its resistance will vary rapidly with a small change in current. Thus, when the oscillation amplitudes tend to increase, the temperature of the lamp R: increases with a resulting increase in resistance thereby causing a greater negative feedback, thus reducing the amplification. Similarly, as the oscillations decrease in amplitude, the current through the lamp is reduced permitting the lamp to cool with an accompanying decrease in resistance and reduction of the negative feedback, thus increasing the amplitude of the generated oscillations. As a result the system operates at substantially a constant amplitude which is preselected to be below the value at which grid current flows. As a result no distortion of the wave form takes place.

While I have described the amplitude control system in connection with my particular resistance capacity tuned oscillation generator, it is clear that this feature of my invention is similarly applicable to various types of oscillation generators, wherein frequency stabflization is obtained by means of negative feedback. Since the lamp itself responds relatively slowly to current changes it tends to average out the voltage peaks and does not cut of! the peaks of the generated waves, maintaining the amplitude constant without producing distortions in wave form.

In the construction of oscillation generators in accordance with my invention, it is generally convenient to make the resistance R: equal to R2, and capacity C1 equal to C2. Under these conditions the frequency of oscillation is zaap. It will be noted that this frequency is inversely proportional to the capacity instead of being inversely proportional to the square root of the capacity, as in the case of the tuned circuit arrangement. Accordingly, the frequency may be varied over a much wider range, utilizing the same type of condensers than in the case of oscillation generators utilizing a tuned circuit feedback. A frequency range of 10 to 1 can be easily obtained on a single dial in accordance with this system. If other ranges are desired, decimal multiplying factors can be obtained by changing resistances R1, R2, in decimal values.

An experimental arrangement employing a 5 four gang broadcast condenser with the sections connected in parallel in pairs, so that one pair corresponds to Cl, and the other pair corresponds to C2, has been found capable of covering the frequency range of 20-20,000 cycles in three subdivisions, namely, 20-200, 200-2000 and 2000- 20,000 cycles, by employing three sets of resistances. The output voltage on this circuit was found to be constant within approximately over the entire frequency range, while it was found that the output waves had a distortion in the order of 25% only. The frequency stability was such that only a negligible frequency shift in the order of less than .1% with quite a large variation in voltage supply occurred. It can, therefore, be seen that an oscillation generator in accordance with my invention is capable of covering an extremely wide frequency range and maintaining substantially pure sine wave output throughout the entire range.

While I have illustrated my invention merely by showing a preferred embodiment thereof, it is clear that many variations thereof may be made by those skilled in the art within the teachings of this disclosure. The particular connections of the amplifier'circuit may be varied considerably and as above stated, the capacities and/or resistances may be varied in. order to change the tuning. Although I have shown an ordinary filament lamp as the volume control resistor, any known arrangement may be utilized for this purpose, for example, these lamps used for such purpose are generally termed ballast lamps. V

What I consider to be my invention is defined in the accompanying claims.

What I claim is: l

1. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude, vacuum tube means having control, anode and cathode elements, and a frequency determining electrical network connected to the vacuum tube means for generation of electrical oscillations, said network including a substantially non-inductive positive feed back path and also adegenerative circuit including a path for oscillatory current of said generator, oscillatory current flow through the last named path serving'to reduce the amplitude of oscillations generated, said path in said degenerative circuit having an impedance variable in response to variations in the amplitude of the oscillatory current. of the generator to tend to maintain the amplitude of oscillations constant.

2. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude. vacuum tube means having control, anode and cathode elements, and a frequency determining electrical network connected to the vacuum tube means for generation of electrical oscillations, said network including a substantially non-inductive positive feed back path and also a negative feed back path for oscillatory currents, oscillatory current flow through the last named path serving to reduce the amplitude of oscillations generated, said negative feed back path including a resistance element which increases its resistance with an increase in oscillatory current 5 flow through the same, thereby serving to automatically stabilize the amplitude of oscillations generated and to maintain the amplitude of oscillations constant.

3. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude,

vacuum tube means having control, anode and cathode elements, and an electrical network con nected to the-vacuum tube means for generation of electrical oscillations, said network having a non-inductive frequency determining positive feedback path including resistive and capacitative elements variable for varying the frequency of operations, and also having a separate noninductive negative feed back path having an impedance variable in response to variations in the output current of the generator, thereby serving to automatically stabilize the amplitude of oscillations generated and to tend to maintain the amplitude of oscillations constant.

4. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude, vacuum tube means having control, anode and cathode elements, and an electrical network connected to the vacuum tube means for generation of electrical oscillations, said network having a non-inductive frequency determining positive feed back path including resistive and capacitative elements variable for varying the frequency of operations, and also having a separate noninductive negative feed back path including a resistive element connected to pass anode current through the same and said element being adapted to increase its resistance with an increase in oscillatory current flow through the same, thereby serving to stabilize the amplitude of oscillations generated and to maintain the frequency of thesame constant.

5. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude, at least two vacuum tubes each having control,

, anode and cathode elements, and a frequency determining electrical network connecting the tubes in cascade, said network including adjustable substantially non-inductive reactive means forming a path for positive feed back of oscillatory energy from said second tube to said first tube whereby oscillations are generated at a selected frequency, and means forming a path for negative feed back of oscillatory energy from said second tube to said first tube whereby energy transferred by said path tends to reduce the amplitude of oscillations generated, said last means including an impedance connected to pass anode current for the first tube and adapted to increase its resistance with an increase in flow of oscillatory current through the same.

6. In an oscillation generator for producing sustained electrical oscillations of a desired substantially constant frequency and amplitude, at least two vacuum tubes each having control, anode and cathode elements, a non-inductive frequency determining electrical network connecting the tubes in cascade, said network including means forming a path for positive feed back ofoscillatory energy from said second tube to said first tube whereby oscillations are generated, and means forming a path for negative feed back of oscillatory energy from said second tube to said first tube whereby energy transferred by said pathtends to reduce the amplitude of oscillations generated, said last means including a resistance element connected to pass oscillatory anode current for the first tube and oscillatory anode current from said second tube and which 7 increases its resistance with an increase in oscillatory current flow through the same thereby serving to stabilize the amplitude of oscillations generated.

WILLIAM R. HEWLETT.

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