US1744835A

US1744835A - Constant-frequency oscillator

Info

Publication number: US1744835A
Application number: US58166A
Authority: US
Inventors: Russell S Ohl
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1925-09-22
Filing date: 1925-09-22
Publication date: 1930-01-28
Anticipated expiration: 1947-01-28

Description

Jan. 28, 1930.

R. S. OHL

CONSTANT FREQUENCY OSCILLATOR Filed Sept. 25, 1925 !NVENTOR [3. 5. O/Ll/ ATTORNEY Patented Jan. 28, 1930 UNITED STATES PATENT orrlcs aossnnr. s. can, or new You, n. 2, micron 'ro 1mm mnrrronn um um:-

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This invention relates to means fer reducing electrical oscillations and part1 rly to a system of that type characterized by means to maintain substantially constant the frequency of the oscillations produced thereby.

In the production of electrical oscillations by means of vacuum tube devices, dlfiiculty has been experienced in keeping constant the frequency of said oscillations owing to the fact that the constants of the vacuum tubes affect the magnitudes of the tuned circuits, which in turn control the frequency. As is well known, these constants, such as the plate impedance of a vacuum tube, are not the same during the operation of the tube as when the tube is in non-operated condition, due to changes within the tube itself as, for example, filament emission, gas pressure changes within the tube, and variations of plate voltage.

Various ways have been suggested heretofore to maintain constant the frequency of such oscillators or to maintain constant the frequency in the circuit supplied by such oscillators. One of the methods consists in using a loose coupling between the oscillator and the antenna circuit. Another method consists in the use of a master oscillator, and a third method in the use of a crystal to control the frequency of a vacuum tube oscillator, which frequency was then used in producinga harmonic frequency, the harmonic being used as the radio frequency.

None of the schemes enumerated absolutely prevents a change of carrier frequency, al though the master oscillator system eflects an improvement over the single oscillator tube, and to a greater extent the crystal control system is better than the master oscillator system. These systems, however, possess in herent defects, the most important being that they are diflicult to apply to the higher radio frequencies of the order of a million cycles or more because of the number of tuned circuits involved.

This invention resides in a system for the production of electrical oscillations, which is characterized by two tuned circuits, one of which determines the frequency and the other drawing, of which Figure 1 shows schematically a simple form. of embodiment of the invention and Figs. 2 and 3 are modifications of the form shown in Fig. 1 to produce frequencies within the voice or the wire carrier range. v

In the arrangement shown in Fig. 1, the tube 1, which forms part of the oscillator, has its output circuit connected with the antenna 2, but the said output circuit may be connected with a wire line as in Figs. 2 and 3. Connected with and forming part of the output circuit is a resonant circuit made up of the inductance 3 and the variable condenser 4. The said inductance has its midpoint connected to ground through the condenser 5. A balancing condenser 6 is effectively connected between the grid and the plate of the oscillator tube 1. A resistance 7, which may be of the order of 5,000 ohms, is connected between the grid and the grounded filament of the said tube. A source of plate potential 8 is con nected with the midpoint of the inductance 3 to apply potential to the plate of the tube 1. The tube 9, which forms part of the amplifymg circuit, has its plate connected with the grid of the oscillator 1, the connection including a condenser 10 whose function is to keep the plate potential applied by the source 8 to the plate of the tube 9 from afl'ectin the grid of the tube 1. The output circuit 0 the tube 9 is connected across the grounded resistance 11 so that the potential to ground across this. resistance is applied directly to the grid of the oscillator. The input circuit of the amplifier 9 is connected with a resonant control circuit comprising an inductance 12 and a variable condenser 13. These elements are so put together as to constitute a mechanically rigid tuned circuit of extremely low loss, which is electrically shielded from outside disturbances by means of the shield 14, and furthermore is rendered practically free from effects of changes intemperature. The inductance of this tuned circuit, which has its 10.

midpoint grounded, has two taps, on each side of the center of the coil about a small fraction viz about one-tenth of the total number of turns from the center. One of the ta s is connected with the grid of the amplifier; the other is connected through the balancin condenser with the plate of the amp er, as shown. This prevents the resonant control circuit from oscillating of its own accord and it also raises the in ut impedance of the amplifier at the radio requencies involved. One side of the inductance 3 connected with the output side of the oscillator 1 and forming art of the resonant circuit connected thereis connected with the grid of the amplifier 9, the connection includin a small variable condenser 16. Ener is t us fed back from the free end of the inductance in the output circuit of the oscillator tube throu h the small variable condenser 16 to the ri of the am lifier tube. This coupling con enser contro s the magnitude of the oscillations fed back to the amplifier.

The method of operation of the system described above will probably be quite obvious from the foregoin description of the parts of the circuit an the functions that they perform, so that little further explanation should be necessary. Oscillations produced by the tube 1 will cause the resonant circuit 34 to resonate at the frequency to which it is adjusted. A part of the energy at this freuency will be fed back through the conenser 16 to the conductor connecting the grid of the amplifier 9 with one of the taps of the inductance 12 of the resonant circuit 12-13. This will cause the latter resonant circuit to resonate at the frequency to which it is adjusted. This frequency will then be 0 impressed upon the amplifier 9, which in turn will impress an amplified voltage of this frequency upon the input side of the oscillator 1 and thus the process will be repeated. The major portion of the ener of the frequency generated will be transmitted throu h the condenser 17 to the antenna 2 for ra iation therefrom. Only a small portion of the energy goes back'through the condenser 16 to the resonant circuit. It will therefore be seen that the system described depends not on the stability of the vacuum tubes but upon the stability of the mechanically rigid resonant circuit 1213. In other words, variations in the constants of the tubes as the operate will not produce any substantial e ect upon the to chanical viewpoint but is also shielded from extraneous sources of interference and is maintained at a substantially constant temrature. It is also to be noted that the feedhiick condenser 16 which, for example, may

6; have a capacity of about 25 microfarads, may

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produce a slight effect upon the tuning of the resonant circuit 1213, but inasmuch as the ratios of the

capacities

16 and 13 and the value of the coupling involved are low the effect of such coupling is negligible. nce the system is set 1n operation to produce a definite frequency it Wlll continue to do so.

The arrangements shown in Figs. 2 and 3, in which the same reference numerals have been used to indicate similar parts, are intended to show the application of the invention to systems for the production of oscillations within the range of frequencies applicable to wire carrier systems. Thus, in Fig. 2

the output circuit of the oscillator 1, which includes the inductance 3 and the condenser 4, is coupled to an output circuit 20 by means of the mutual inductance between the winding 3 and the winding 28 connected with the output circuit. The resonant control circuit 12-13, which is protected by the shielding surface 14, is connected to ground through the midpoint of the inductance 12 and is also connected with the grid circuit of the amplifier 9, the connection including a condenser 27. As in Fig. 1, a feed-back connection is established from the inductance 3 in the output circuit of the oscillator to the resonant circuit, the connection including a condenser 16. This feed-back circuit is connected to the input of the first am lifier at a point between the condenser 27 an the grid of the amplifier 9. Another amplifier 21 is inserted between the amplifier 9 and the oscillator 1 and the output of each amplifier is impressed across a resistance such as 11 and 23, to which a source of plate potential 24 is also connected.

Condensers

10 and 29 are placed in the grid circuits of the amplifier 21 and the oscillator 1 in order to protect the rids of these tubes from the effect of the pfite batter 24.

the arrangement shown in Fig. 3, the resonant circuit in the output side of the 0s cillator 1 is connected with the resonant control circuit 1213 by means of a connecting circuit including the resistance 25 and the condenser 26. As in Fig. 1, the feed-back coupling between the resonant circuit in the output side of the oscillators of Figs. 2 and 3 is of such magnitude as to reduce to a minimum the energy fed back to the control circuit. In this manner the effect of external changes on the frequency determined by the resonant control circuit 12-13 is greatly reduced.

While this invention has been disclosed in a particular form it is apparent that it is caable of embodiment in other and different orms within the spirit and scope of the appended claims.

What is claimed is: 1. In a system for producing electrical oscillations of constant frequency, the combination with a vacuum tube oscillator having tween the output of the said oscillator and .nected with the in a resonant circuit connected with the output side thereof, of an amplifier connected with the input side of said oscillator by a resistance coupling, a sharply tuned resonant circuit shielded from external influence conut side of the said amplifier, and means to eed back a small amount of energy form the said first resonant circuit to the said second resonant circuit.

2. In a system for producing electrical oscillations of constant frequency, the combination with a vacuum tube oscillator having its output circuit tuned to the desired frequency, of a control circuit sharply tuned to the desired frequency, shielding means to protect the control circuit from external interference, a condenser coupling between-the saidfirst tuned circuit and the said control circuit to feed back a small amount of energy to energize the said control circuit, and means to connect the said control circuit 'to the input circuit of said oscillator, the said connecting means including a unilateral device preventing undesired reaction between the said oscillator and the said control circuit, and a resistance coupling between the said oscillator and the said unilateral device.

3. In a system for-producing electrical oscillations of constant frequency, the combination with an oscillator having a tuned output circuit, of'an amplifier, means to connect said amplifier to the input side of the said oscillator, the said connection including a resist-.

ance coupling, a second tuned circuit mechanically rigid and shielded fromexternal interference connected with the input side of the amplifier, and a condenser coupling bethe second tuned circuit.

4. In asystem for cillations of constant requency, the combination with an oscillator having a tuned output circuit, of a frequency control circuit sharply tuned, mechanically rigid and shielded from external interference, means to couple the output circuit of the said oscillator with the said frequency control circuit, and means to connect the said frequency control circuitto the input of the said oscillator, the said connecting means comprising a unilateral device and a resistance coupling between the said device and the said oscillator.

In testimony whereof, I have signed my name to this specification this 21st day of September 1925. 1

RUSSELL S. OHL.

producing electrical os-