US2096829A - High frequency oscillator - Google Patents

Description

Oct, 26, 1937.

A. H. TURNER HIGH FREQUENCY OSCILLATOR Filed July 31, 1935 2 Sheets-Sheet l A. H. TURNER HIGH FREQUENCY OSCILLATOR Oct. 26, 1937.

Filed July 31, 1955 I 2 Sheets-Sheet .L usual/m may K9 3 Patented Oct. 26, 1937 UNITED STATES PATENT OFFICE to Radio Corporation of America,

tion of Delaware Application July 31, 1935, Serial No. 33,955

9 Claims.

My invention relates to oscillators. Specifically, my invention is an improved high frequency oscillator having substantially constant frequency over a wide range of plate and heater potentials. Y

one object of my invention is an improved ultra high frequency oscillator. v 3 Another object is an oscillator circuit having substantial constant frequency over a wide'range of plate voltage-and heater potentials. I

--Another object is to provide a high frequency oscillator whose voltage output is relatively constant within the tuning frequency range.

A still further'object is to combine, in a single oscillator, the characteristics of constant frequency and a constant coupling output circuit.

7 Other objects will appear from the following specification and claims.

Figure I is a schematic diagram of a conventional oscillator,

Figure II is a diagram of an oscillator employing a tracking condenser,

Figure III is a diagram of an oscillator embodying one form of my invention,

Figure IV is a modification of Figure III,

Figure V is a graph showing the variation in output frequency with changes in plate voltage,

Figure VI is a graph showing the variation in output frequency with changes in heater voltage,

Figure VII is a graph showing the variation of 'grid'an'd plate current with frequency, and

Figure VIII is a graph showing the variation of frequency with varying plate voltage for difierent values of grid leak resistance.

The circuit illustrated in Figure I is a vacuum tube oscillator employing a tuned grid circuit and tickler' feedback. This circuit arrangement is well known to those skilled in the art. A circuit of this type has objectionable characteristics. 40 The oscillatory voltages across the tuned circuit will be substantially reduced as the frequency'is decreased. If used in a heterodyne receiver, the oscillator will not track with tuned'preselector circuits unless special variable condensers are employed. The greatest difiiculty is experienced at high frequencies where the frequency cannot be stabilized to any substantial degree. A further undesired variation in oscillator frequency takes place with changing plate and heater potentials. This latter effect becomes very serious at high frequencies/ I am aware of numerous arrangements for overcoming these dilficulties. Some of these arrangements are satisfactory at broadcast frequencies but are not applicable at ultra high frequencies of the order of 30 megacycles upward. Some of the theory of oscillators applicable to low frequencies is far from rigorous at ultra high frequencies. My present invention minimizes and compensates the deleterious effects usually experienced in high frequenc'y'oscillator circuits. A 1 3 In Figure II is illustrated a modified form of oscillator similar to Figure I. In Figure II, a grid inductance l is connected through grid 'condenser 3 to grid 5 of thermionic tube 1. The 10 lower end of the grid inductance is connected to plate inductance 9 which is connected to plate II. The junction of the grid and plate inductances is coupled through tracking'condenser l3 to the cathode l5 which is grounded. A variable tuning capacity I! is connected from the upper terminal of the grid inductance to the cathode and ground. A grid leak resistance I9 is connected between grid and cathode. The B battery 2! f has its negative terminal grounded and its positive terminal connects to the upper end of the plate inductance through the radio frequency-choke 23. The B battery may be by-passed by a suitable capacity 2 5. The cathode I5 is heated by filament 27. Inductances l and 9 are preferably mutually coupled.

Although the circuit of Figure II maybe ade justed by properly choosing the value of l3 to track with tuned circuits in a superheterodyne receiver or the like, it still has'objectionable char 0 acteristics at frequencies'of theorder of 30,megacycles and upward. In Figure V the curve A represents the eifect of varying the plate voltagejo n the frequency output of the circuit of Figure II. 5 It is customary to supply the plate voltage from a rectified filtered source of power derived from a commercial sixty cycle volt. supply. Such sources have been known to Vary as much as twenty-five percent in the case of poorly regulated lines and as much .as thirty percent in different sections of the country. Such variation wouldhave a serious eifecton the oscillator of asuperheterodyne. v

A similar variation in a tuned grid [circuit oscillator, although in the opposite direction, is I found in Figure VI, curve C. In this curve the effect of varying heater voltage in terms of frequency change is shown. The curves E, andF, of Figure VII, illustrate the variation of grid '50 and plate currents with frequency in the case of a tuned grid circuit oscillator. Since the effective output of oscillatory current is proportional to the grid current, the large variation of output in the oscillatory current of a. tuned grid circuit oscillator is objectionable from a constant current standpoint. I

. The foregoing examples of variation in the frequency output of a tuned grid circuit oscillator apply especially when the 1 frequency range is from about 30 megacycles and upward. Although the curves are based on observationsof the characteristic of an RCA type 955-Acorn type tube,

' it should be-understood that similar frequency .variations are present in other types of tubes connected to similar circuits.

Many variable factors make it diillcult to mathematically treat the tubesand'associate circuits to derive equations which explain the variations. Among the important variable factors at ultra high frequencies is the recently recognized variation caused by changes of space charge and electron velocity within the tube with changing. plate and'filament potentials. This variation ap:v

pears to alter the efiective'interelectrode capacitieswithin the tube. V Somevariable factors-may be minimizedand otherscompensated.

One important efiect at high frequencies, is the variation'in the effective grid, cathode and/or grid plate capacity with variations in plate circuit' impedance. primarily- ;due to-a decrease in the-effective grid to cathode'capacity, If the oscillatory circuit is 6 inserted in the plate circuit, variations in effective capacity between grid and cathode will have little-eifect, because the tuned plate circuit primarily determines the frequency.

r Figure III is a diagram of a tuned plate circuit oscillator: An inductance3l is connected through grid condenser 33 to grid 35 of tube 31. The lower end of. 3! is connected to inductance 39. The lower terminal of39 is connected to plate 4|. The

junction'of 3land39'is connected through track-1 ing capacity to cathode 45 which is grounded. A variable tuning capacity 41 is connected'between; ground and the lower terminal of inductance 39.; A grid leak 49 is connected between fandthe tuning' of theplate'circuit of the latter. -W'hile 'a comparison of, diagrams indicatesa similarity, a comparison of graphs, Figures V, VI

and. VII; discloses substantialjdifierences. ,In' Fig! N we V the curveB shows a substantially constant frequency withvarying plate voltage. A'comparison of A1 and B indicates a very'great. im-

provement. In like manner curve D of Figure VI shows that fIOIIL'T volts to 5.3 volts or heater potential the frequency.is practically unchanged. Curve C" shows a substantial variation of iregquency with decreasing heater voltages beginning, at.6.3 volts. The" curves shown in Figure VII indicate that the radio frequency output which accompanies curve G is moreconstant than thatindicated by curve E; Because of the shape of curve G the variations in output with ire- "quency may be compensated by various types of compound coupling linking the oscillator output and the work circuit. Such compound coupling keeps the applied voltage constant with changes in oscillator frequency.

. 'While the principal gain in frequency stability illustrated by curves'B and D of Figures V and The increase infrequency is VI, respectively,is due place of grid circuit tuning; the results are not entirely due to this. change. bility indicated by curve B is due to compensating 'means I employ. The circuits of the several figures, when employing a grid leak resistance of the order of one hundred thousand ohms, show an increase in frequency Withincreasing plate potentials. I have observed the efiect of lowering the grid leak resistance is to cause a change in the frequency characteristic with changing plate voltage which over a portion of the range is opposite to the usual frequency increasing tendency.

The result of such observation is found in Figure 7 VIII. The curves J and K with 100,000 ohm grid leak resistances show a rising frequency charac-.

teristic with increased plate voltages. The curves L and-M show a rising frequency characteristic at low voltages and a reversal of slope for higher voltages. By a suitable choice of operating volt age and grid resistance, the" ordinary rising fre- Part of the sta-" to plate circuit tuning in 7 quency characteristic withincreasing plate volti- I age, maybecompensatedaln ciu'ves L-anddMa valueof grid leak resistance is. employed which approaches the optimum value for a constant frequency output with increasing; plate voltages;

Figure IV is a diagram showing a modification or Figure-III} Thegrid circuit inductance is connected through gridcondenser 63' and leak 65 to grid 61' of tube. The lower terminal so L of 6| is connected to cathode 1i and ground. The I plate'cir'cuit inductance 13isconnected'between the plate 15 and the positive-terminal of B battery l'l. ;The B battery has; its negative terminal grounded. The variable tuning capacity 8I- is connected between ground andthe junction of inductance 13* and plate 15. The tracking condenser 83 is connected "between theylowerlterminal of 6| and the upper, terminal'of 13;. The

condenser 83 also acts as a by-pass for battery I1.

The essential'difierencebetweenthe circuits shown in Figures III and IV is that in the latter the.- grid leak resistance is serially connectedto the grid circuit and has a smaller loading efiectin-thiS position. The lesser loading effect of thegrid resistanceis an advantage in a number of circuit arrangements t Y Although my invention may be applied to any oscillator, it is primarily designed for a high frequency oscillatorand is particularly suited. to a superheterodyne receiver.

The tracking condensers i3; and43 of Figures II and III are suit able for aligning the oscillator with respect to the preselector circuits. The combination of the mutual inductive coupling and the capacity of these condensers increases the coupling between V the grid and plate circuits at the lower frequencies, and tendsto-keepthe' output oscillations constant in amplitude throughout the frequency range. In many cases; I find the tracking condenser capacity must 'be large with respect'to 1 the tuning capacity. This large ratiois not the optimum for'impressing' a constant voltage on the 'workcircuit. In such cases, I employ, a

compound coupling between the oscillator and first detector or work circuit. The compound coupling keeps the impressed voltageonthefirst,

detector substantially constant as the oscillator is: tuned throughout the frequency range.

By way offexample Ihave employed the following constants in the .circuit of'Figure'III: Grid circuit inductance two turns number I! enameled copper wire onv 1 diameter form. Plate circuit inductance four turns number Is enameled copper wire on lf'e diameter form.

2,096,829 Both the grid and plate circuit inductances are a continuous winding of six turns spaced about one-eighth of an inch on a common form. The tuning condenser is variable from five micromicrofarads to twenty micromicrofarads. The common condenser is eight hundred micromicrofarads. The grid condenser is thirty-five micromicrofarads and the grid leak about 10,000 ohms. An Acorn tube RCA 955, is preferred. A plate voltage of volts may be used. The rated heater voltage of 6.3 volts is employed.

The circuits described cover arrangements of my invention which give substantially constant high frequency outputs with varying plate and heater potentials. The amplitude of the output current is substantially constant over the tuning range, and the voltage applied to a work circuit may be made uniform by employing the above mentioned compound coupling. The circuit diagrams, curves, and constants are given merely by way of example. I do not intend to limit my invention to the precise arrangements shown because other obvious arrangements within the scope of my invention will occur to those skilled in the art.

I claim:

1. In an ultra high frequency oscillator, a thermionic tube having grid, cathode, and plate electrodes with relatively low interelectrode capacities; means for moving electrons from cathode to plate whereby the effective interelectrode capacities are varied; means for tuning the plate circuit throughout a range of ultra high frequencies; an inductance serially included in the grid circuit; means connected in said grid circuit hav ing an impedance which establishes an increas ing, then a decreasing frequency characteristic in said oscillator with increasing plate voltage; and means mutually coupling said grid and plate circuit, whereby the effect of said moving electrons on the frequency characteristic of said oscillator is substantially reduced.

2. In an ultra high frequency oscillator, a thermionic tube having grid, cathode, and plate electrodes with relatively low in'terelectrode capacities; -means for moving electrons from cathode to plate whereby the interelectrode capacities are apparently varied; means for tuning the plate circuit throughout a range of ultra high frequencies; an inductance, and grid condenser serially connected between grid and cathode; means connected in said grid circuit having an impedance which establishes an increasing then a decreasing frequency characteristic in said oscillator with increasing plate voltage; and means mutually coupling said grid and plate circuits, whereby the effect of said moving electrons on the frequency characteristic of said oscillator is substantially reduced.

3. In an ultra high frequency oscillator, a thermionic tube having grid, cathode, and plate electrodes with relatively low interelectrode capacities; means for moving electrons from cathode to plate whereby the effective interelectrode capacities are varied; means for tuning the plate circuit throughout a range of ultra high frequencies; an inductance, and grid condenser serially connected between grid and cathode; means mutually coupling said grid and plate circuits, whereby the effect of the apparent changes in inter-electrode capacities on the frequency of characteristic is minimized; and a grid leak resistance connected between grid and cathode having a value which gives said oscillator a frequency characteristic which increases then decreases with increasing plate voltage for compensating the frequency variation caused by the apparent change in interelectrode capacity.

4. In an ultra high frequency oscillator, a thermionic tube having grid, cathode, and plate electrodes with relatively low interelectrode capacities; means for moving electrons from said cathode to plate whereby the effective interelectrode capacities are varied by said moving electrons; means for tuning the plate circuit throughout a range of frequencies; an inductance and grid condenser serially connected between grid and cathode; a grid resistance effectively connected between grid and cathode and having an ohm-i0 resistance which establishes a frequency characteristic which increases then decreases with increasing plate voltage; and means mutually coupling said grid and plate circuits, whereby the effect of said moving electrons on the interelectrode capacities is minimized as to frequency changes in the oscillator.

5. In a device of the character of claim 1, a fixed capacity common to the grid and plate circuits.

6. In a device of the character of claim 1-, a fixed capacity serially included in the tuned plate circuit and common to the grid and plate circuits. 7

7. The method of generating ultra high frequency currents of constant frequency in a thermionic tube having control grid, cathode, and plate electrodes; a resonant output circuit; a grid circuit coupled to said output circuit by means of mutual capacitance and mutual inductance; and a source of power subject to variations whereby the movement of electrons in said tube is afiected, which comprises generating a potential difference across said resonant output circuit, inducing in said grid circuit from said output circuit by means'of said mutual capacitance and said mutual inductance a voltage in proper phase to sustain said ultra high frequency currents, and limiting the impedance of said grid circuit to a value which establishes a frequency characteristic in said oscillator which increases then decreases with increasing plate voltage and is substantially uninfluencedby apparent capacity changes caused by said movement of electrons, whereby constant ultra high frequency oscillations are maintained regardless of the variations in said power source.

8. The method of generating constant ultra high frequency oscillations in a thermionic tube having input and output circuits and avoiding the effect of apparent capacity changes in said tube on said frequency which comprises resonating the output circuit of said tube to the desired frequency, generating voltages across said output circuit, feeding from said output circuit voltages to said input circuit having capacitive and inductive components to sustain said resonant output voltages, and substantially limiting the efiects of said apparent capacity changes by limiting the impedance of said input circuit to a value which will have an increasing and decreasing efiect on frequency over a range of plate potentials whereby a constant ultra high frequency oscillation is generated regardless of said apparent capacity changes.

9. The method of generating constant ultra high frequency oscillations in a thermionic tube having input and output circuits coupled by mutual inductance and mutual capacity, and avoiding frequency change caused by variations in the power supplied to said tube and changes in cacomprises resonatingthe output circuit of 7 said tube, generatingvoltages in 'said output circuit by feeding back voltages through said mutual 7 coupling to said input circuit, amplifying input voltages in saidoutput circuit, and adjusting the impedance of said input circuit to a value which establishes an increasing and decreasing frequency characteristic in said'oscillator with increasing plate voltage and'which reflects substantially no reactive component on said resonant output circuit regardless of said power and 5 capacity variations.

ALFRED H1

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