US2103457A - Frequency control line and circuit - Google Patents

Description

Patented Dec. 28, 1937 UNITED STATES PATENT OFFICE Clarence W. Hansell, Port Jefferson, N. Y., ass gnor to Radio Corporation of America, a corporation of Delaware Application June 21,

8 Claims.

This invention relates to low loss tuned circuits, and particularly to transmission line sections for frequency control and circuits therefor.

It is known that a tuned section of a transmission line serving as a low power factor circuit can be used to stabilize the frequency of an oscillator, and can be used to replace ,piezo-electric crystals in transmitters operated with relatively high power output and at very high frequencies.

The form of tuned line circuit best suited for frequency control is one made up of two concentric conductors, with the outer conductor completely enclosing the inner one. This form of line is.relative1y easy to construct and is completely shielded. The power factor of the line as a resonant circuit is not increased by radiation or coupling to surrounding objects and circuits. If desired, the outer conductor may be utilized as a means of mounting and support for tubes and other circuit elements. Copper is one of the most satisfactory materials from which to construct the line for ordinary applications but aluminum or aluminum alloys may be used where weight is an important consideration.

Except for determining the frequency, the length of the line is an important consideration, from the standpoint of electrical performance, only in respect to the temperature rise and the total oscillatory energy which may be stored in the line. If large diameter conductors are used the energy storage rating will usually be great enough so that the required length will be only a quarter or a half wave for the frequency at which the line is to be operated.

The degree to which a line can be made the predominating element in determining the frequency of an oscillator is proportional to the amount of oscillatory energy which may be maintained in it with a given amount of power. Therefore the quality or figure of merit for the line may be taken as the ratio of oscillatory energy to power loss. This ratio is popularly known as the Q of the line.

Tests and theory indicate that lines made up of straight tubular conductors have a temperature coefficient of frequency variation corre sponding fairly closely to the mechanical temperature coefficient of linear expansion for the material of which the line is made. {So long as both conductors are made of the same material and have the same temperature. the ratio of their diameters and therefore the electrical constants per unit of length do not change with temperature. To a reasonable degree the change in frequency With change in temperature can be 1935, Serial No. 27,679

considered as due only to change in length. The approximate temperature coefficient of linear expansion and the resistivities for materials which may be used advantageously in the construction of lines are as follows:

In practice, lines used for frequency control are also subject to frequency variations due to unequal heating of inner and outer conductors. This effect is most evident in lines used with relatively large power dissipation and low frequencies and causes a temporary frequency drift while the line is warming up. The effect can be made small by using large dimensions and heavy material in the line. It is not very important in oscillators operated at 50,000 kilocycles or higher with power levels obtainable from commercially available tubes, at the time of this patent application.

One of the simplest and most effective means for reducing the temperature coefficient of frequency variation is to hold the length of the inner conductor constant regardless of temperature.

One of the objects of the present invention is to provide a novel method of and apparatus for reducing the temperature coefficient and at the same time shortening the line. In general, this object is achieved by making use of the difference in the temperature coefficients of expansion of copper and aluminum to vary the capacity of a compensating condenser in a direction tending to compensate for variation in length of the inner conductor.

Another object is to provide a single tube oscillator circuit which is especially adaptable for use with frequency control transmission lines.

Other objects and features will appear from a reading of the following detailed description which is accompanied by a drawing wherein:

Fig. 1 shows one form of transmission line of the-invention with the oscillator of the invention, and

Fig. 2 shows another form of line in accordance with the invention.

Referring to Fig. l in more detail, there is shown a tuned circuit in the form of a concentric transmission line comprising an outer aluminum conductor I and an inner copper conductor 2, both connected to opposite plates of a compensating condenser 3. It is preferred that the length of the inner conductor 2 be a quarter wave less the shortening brought about by the condenser 3, although it will be obvious that the line may also be an equivalent half wave long electrically, equivalent to two quarter wave sections in series, in which case it is most readily applicable to a push pull oscillator.

Any change in temperature, will cause conductors l and 2 to change their lengths, but due to the differences in their temperature coefficients the lengths will vary differently and thus change the capacity of the compensating condenser 3, which will vary in a direction tending to compensate for variation in length of the inner conductor. This arrangement also tends to compensate for the higher temperature rise of the inner conductor due to losses in the line.

As an example of the operation of the line of Fig. 1, assuming an increase in temperature, there will be occasioned a greater increase in length of the outer aluminum conductor than in length 7 of the inner conductor 2 due to the difference in their tem perature coefficients. Consequently the plates of condenser 3 will befurther separated and a,

reduction in capacity effected in such direction as to compensate for the variations in length of the conductors.

In Fig. 2 the materials of the line elements are reversed from that shown in Fig. 1. Fig. 2 shows the outer conductor made of copper and the innor conductor made of aluminum;

In order to accomplish the same compensating effect in the structure of Fig. 2, as accomplished in Fig. 1, the movable plate of the compensating condenser is now shown below the fixed plate. As the length of the aluminum line increases, the space between the condenser plates increases and the capacity decreases, which is equivalent to shortening the line. We thus need the proper relation between physical lengths, capacities and spacing to obtain zero temperature coeflicient of frequency variation for the electrical natural period of the line. The method of compensation will be apparent from what has been said in connection with Fig. 1.

Returning now to a consideration of the circuit of Fig. 1, there is shown in connection with line I, 2 a controlled oscillator circuit which is readily applicable for use with the tuned concentric line. This circuit comprises an electron discharge device 4 provided with a condenser 5 for controlling the effective capacity between the anode 6 and the grid '1. In operating the circuit, the condenser 5 is adjusted to give about the smallest feedback from anode 6 to. grid 7 which can be used to make the oscillator work eificiently. Any excess feedback reduces the ability of the line I, 2 to stabilize the frequency of the oscillator. The circuit functions with the regeneration controlling condenser 5 set either above or below the capacity value required for a balance but one adjustment or the other will be preferred, depending upon the ratio of the effective resistance in anode and grid circuits and the frequency. The resistance that the invention is not limited solely to lines having copper and aluminum materials since other materials possessing desirable temperature coefficients may also be used in accordance with the principles of the invention.

What I claim is:

1. A tuned circuit comprising inner and outer conductors connected together at one of their adjacent ends and having different temperature coefiicients of linear expansion, a condenser within said outer conductor comprising a plate attached to said outer conductor and a plate attached to said inner conductor, said plates being spaced apart and movable relative to each other, whereby changes in the resonant frequency of said tuned circuit, due to a change in temperature and lengths of said conductors, are substantially compensated for by a variation in capacity of said condenser.

2.'A tuned circuit comprising an outer conductor made of aluminum and an inner conductor made of copper, both conductors being connected together at one of their adjacent ends, and a compensating condenser comprising a fixed plate attached to the inner surface of said ductorsfis substantially compensated for by a change in capacity of said condenser.

3. A tuned circuit comprising'an outer con 7 ductor made of copper and an inner conductor made of aluminum, both said conductors being connected together at one of their adjacent ends, and a condenser comprising a fixed plate attached to the inner surface of said outer conductor and a plate attached to said inner con.-

ductor whereby any tendency toward change in the resonant frequency of said tuned circuit, due to changes in length of said conductors, is substantially compensated for by a change in casaid inner conductor, whereby changes in the resonant frequency of said tuned circuit, due

to a change in length of said conductors, are

substantially compensated for by a variation in capacity of said condenser.

5. In combination, a tuned circuit comprising inner and outerconductors, an electron dis-,

charge device oscillation generator having a grid, anode and cathode, a radio frequency connection from said grid to a point intermediate the ends of said inner conductor, a connection from said cathode to said outer conductor, a feed-back cir cuit including an inductance in series with a variable regeneration condenser between said anode and grid, a source of energy having its positive terminal connected to said inductance at a point intermediate the ends of said inductance and its negative terminal connected to said cathode, said condenser being adjusted slightly off that value required to balance the grid,- anode inter-electrode capacity, whereby there is obtained less feedback coupling between said anode and said grid than ordinarily would be provided'by said inter-electrode capacity.

6. A radio frequency oscillator comprising a single vacuum tube having a grid, anode and cathode, a feedback path comprising an inductance coil in series with a regeneration control condenser connected between said anode and grid, a source of potential whose positive terminal is connected to said inductance at a point intermediate the ends of said inductance and whose negative terminal is connected to said cathode, an output circuit tapped to points on said inductance which are symmetrically located on opposite sides of said first point, and an input circuit for controlling the frequency of said oscillator connected between said grid and anode, said input circuit comprising a tuned circuit in the form of a concentric line having inner and outer conductors, a connection from said outer conductor to said cathode and a capacitive connection from a point intermediate the ends of said inner conductor to said grid.

7. In combination, an oscillation generator comprising an electron discharge device having an anode, cathode and grid, a frequency control circuit comprising a concentricline having inner and outer conductors coupled together at one of their adjacent ends, a connection from said outer conductor to said cathode, a connection from a point intermediate the ends of said inner conductor to said grid, an output circuit including reactive means connected between said anode and said grid for incompletely balancing the interelectrode transfer of energy between said anode and grid.

8. A tuned circuit comprising an inner and an outer conductor electrically coupled together at substantially one of their adjacent ends and having different temperature coeincients of linear expansion, a capacitor having a plate attached to each of said conductors, said plates being spaced apart and movable relative to each other, whereby changes in the resonant frequency or said tuned circuit, due to a change in temperature and lengths of said conductors, are substantially compensated for by a variation in capacitance of said capacitor.

CLARENCE W. HANSELL.

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