US2171243A - Frequency control system - Google Patents

Description

Patented Aug. 29, 1939 UNITED STATES PATENT OFFICE 2,171,243 FREQUENCY CONTROL SYSTEM Lewis J. McKesson, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 14, 1937, Serial No. 142,612 4 Claims. (01. 171-327) This invention relates to a new and novel bility with crystal control consistent with satismethod of frequency control compensation for factory output, despite tuning and voltage controlling the frequency variations caused by changes in the circuits, an optimum air gap is electric elements. condition can be maintained at all times irrespec- An object of this invention is to provide an tive of ambient or circuit element temperature improved method of compensation for frequency changes. variations caused by temperature variations of This invention will best be described and unthe circuit elements employed for frequency derstood by referring to the accompanying drawcontrol systems, such as quartz crystals. ing, in which:

Another object of this invention is to provide Fig. l is a typical oscillator circuit using a an improved form of piezo-electric crystal holder thermostatic element of my invention for frein which the crystal element is interposed bequency compensation; tween, a first and second electrode, the frequency Fig. 2 is a plan view of my improved frequency variation caused by temperature changes being control system; and controlled by a bimetallic strip which is elec- Fig. 3 is a sectional elevation partly in section, trically connected to one of the electrodes in the section being taken on lines 3-3 of Fig. 2.

such a manner as to cause variation in capacity Referring now in detail to Fig. 1 of the drawbetween the other electrode of a frequency conmg, a quartz crystal I is shown interposed betrol system. tween metallic crystal electrodes 2 and 3, elec- A feature of this invention is that the fretrode 2 being maintained at an equivalent ground quency control system is provided with a physipotential. Mechanically supported by and elec- ;5 radio transmitters is through the use of a piezoan output condenser I3 and plate supply voltage electric element acting as a mechanical vibrating I4. The cathode I5 is bypassed with the tuned element which, because of its mechanical and circuit bya condenser I6. A variable condenser electrical properties, is able to maintain an os- I1 is provided to feed back a small amount of in accordance with the temperature. well known to the art.

There are several means of maintaining con- Figs. 2 and 3 show a detail of construction of stant frequency now in use, but such means are a, piezo-electric crystal holder employing the ments associated with the crystal. element 4 which is rigidly connected at one end In order to obtain the greatest frequency stato the lower electrode by stud 24, nuts 25 and 55 two electrodes on 2 washers 26. Fine adjustment is made by means of the screw 21. This bimetallic plate forms a capacitive element with the upper electrode 3, the value of which will change in either a positive or negative manner with temperature; this capacitance change with temperature will change the frequency of oscillation of the crystal. By changing the capacity of the bimetallic plate to the upper electrode it is possible to vary the temperature coefficient of the entire unit in either a positive or negative manner over a large range. In actual practice, I have found that this adjustment is such that the frequency change caused by the capacitance change can be made equal and opposite to the frequency change due to the quartz element and its associated circuit; the result is then found to be a zero frequency variation for any temperature variation within the normal range of ambient temperatures encountered in practically all radio transmitter circuits.

Although only one form of frequency variation is possible, since the principle utilized in this invention is to vary the capacity through the mechanical movement of the bimetallic element 4, which movement is proportional to the temperature, a large number of methods of accomplishing this result can be devised, for example, inductance, capacitance or resistance values may be varied in similar manners to obtain similar results. Also, many circuit variations are possible and Fig. 1 is intended to show by example only how the application of the bimetallic element is used. Some circuits employ more than the piezo-electric element and this invention is applicable to such circuits to vary the capacitance between any two or more electrodes. Consequently, no attempt will be made to enumerate such modifications in this application and the invention is not to be limited, except as to such limitations as are clearly imposed in the appended claims.

What is claimed is:

1. A piezo-electric crystal holder comprising a fiat metallic base member acting as a first electrode, a metallic plate forming a second elec trode located above said first electrode and insulatingly secured thereto by two support members, a crystal interposed between said first and second electrodes, and a third electrode in variable capacitance relationship with said second electrode and electrically connected with said first electrode.

2. A piezo-electric crystal holder comprising a metallic base member acting as one electrode for said crystal holder, .a crystal mounted on said metallic base member and secured in position by two pins located in said base, a second electrode insulatingly supported located above said crystal by a pair of spacing members, a pair of bushings and binding means between said second electrode and said bushings, and a third electrode in variable capacitance relationship with said second electrode and electrically connected to said first electrode.

3. A pieZo-electric crystal holder comprising a base member acting as one electrode for said crystal holder, a crystal mounted on said base member, a second electrode located above said crystal, a third electrode comprising a bimetallic strip in variable capacitance relationship with said second electrode and electrically connected to said first electrode, and threaded means in said base member for adjusting said third electrode with respect to said second electrode.

4. A piezo-electric crystal holder comprising a base member acting as one electrode for said crystal holder, a crystal mounted on said base member, a second electrode insulatingly supported on said base member and located above said crystal, a third electrode comprising a bimetallic strip in variable capacitance relationship with said second electrode supported and electrically connected to said first electrode by a stud-like member, and means for adjusting said third electrode with respect to said second electrode.

LEWIS J. MCKESSON.

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