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Publication numberUS2289183 A
Publication typeGrant
Publication dateJul 7, 1942
Filing dateMay 22, 1940
Priority dateMay 22, 1940
Publication numberUS 2289183 A, US 2289183A, US-A-2289183, US2289183 A, US2289183A
InventorsBarnes John L, Ehret Robert J
Original AssigneeRca Corp
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Modulation system
US 2289183 A
Abstract  available in
Images(1)
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Claims  available in
Description  (OCR text may contain errors)

Patented July 7, 1942 MODULATION SYSTEM Robert J. Ehret, Collingswood, and John L. Barnes, Merchantville, N. J., assignors to Radio Corporation of America; a corporation of Delaware Application May 22, 1940, Serial No. 336,550

2 Claims.

This system relates to frequency modulation systems, and more particularly to an improved frequency modulator of the type in which the control frequency of a piezo-electric crystal element is varied.

One of the principal problems which confronts the designer of a frequency modulated transmitter is that of providing effective frequency modulation without sacrificing the necessary stability of the average or unmodulated carrier frequency. This is particularly true in the design of high frequency transmitters which utilize a relatively low frequency oscillator and series of frequency multipliers, since undesired changes in the average oscillator frequency are multiplied by the same factor as the desired step-up of the average frequency.

The use of a modulated crystal controlled low frequency oscillator has been proposed, in which the control frequency of a piezo-electric crystal is modulated, by varying the air gap between the crystal element and one of the electrodes.

crystal by an electro-magnet adjacent to the diaphragm which is energized by the modulating voltage. The crystal is connected in a conventional oscillator circuit and stabilizes the average frequency of the oscillator in the usual manner.

Variations in the air-gap,however, produce vari- 3 ations in the oscillator frequency which can be utilized in accordance with well established'practice toenergize or excite a transmitter, for example. Such an arrangement is described in Patent No. 1,933,735, issued November 7, 1933, to A.

Hund, for example.

It has been found that rather small variations in the oscillator frequency are produced by the systems of the prior art. This may be explained by the fact that the diaphragm does not vibrate as a whole :at all frequencies but breaks up into different modes of vibration for different frequencies in a manner which is similar to the production of standing waves on a vibrating string.

Thus, when one portion of the diaphragm is moving toward the crystal another portion may be moving away from the crystal, so that the effective change in the gap is minimized.

nent magnet as a core.

In addition it is well known that a magnetically operated diaphragm produces double frequency components unless a fixed magnetic flux is employed to pull or bias the diaphragm away from its neutral position. This is, of course, the reason that the electromagnets of all telephone receivers, headphones, and the like, employ a perma- If such an arrangement be utilized to eliminate frequency doubling in the crystal modulator it will be appreciated that the pull on the diaphragm will give it a curved shape so that the effective air gap .at the center is increased. To obtain the maximum frequency variation for a given diaphragm movement, however, it is essential that the spacing between the crystal and the diaphragm be as small as possible. It will be appreciated, therefore, that for a given diaphragm movement it is not possible to obtain as great a change in frequency with the biased magnetic diaphragm as with one which is not so biased, and yet the biasing is necessary to reduce distortion.

Accordingly it is the principal object of this invention to provide a frequency modulated oscillator having a stable average frequency. Other objects include the provisionof an improved crystal modulator of the variable air gap type; the provision of improved means for driving the movable electrode of a crystal modulator; the provision of a light, economical crystal modulator suitable for use in portable equipment; and the provision of a variable air gap crystal modulator of improved operating characteristics.

In brief, the foregoing objects are accomplished, in accordance with the present invention by providing the crystal with a substantially inflexible spaced electrode which is mounted on an electromagnetic or electrodynamic motor mechanism, the electrode being movable bodily toward and away from the crystal. Such an electrode need not be rigidly supported at the edges, as does a stretched diaphragm, and consequently will present less impedance to the driving mechanism. Its surface will not be distorted by wave patterns, nor is it necessary to distort the electrode to overcome second harmonic generation. As a result the spaced electrode may be smaller than the diaphragm, and the unmodulated Or normal air gap may be reduced, thus increasing the modulating range of the device. In another embodiment of this invention the driving mechanism for the spaced electrode is another piezo-electric element of the pick-up type, such as a Rochelle salt crystal, or the like.

This invention will be better understood from the following description when considered in connection with the accompanying drawing, and its scope is indicated by the appended claims. Similar reference numerals refer to similar elements throughout the drawing.

Referring to the drawing, Figure 1 is a schematic drawing of an electromagnetic modulator; Figure 2 is a section view of a modulator assembly, Figure 3 is a schematic drawing of an electrodynamic modulator; and Figure 4 is a schematic drawing of 1a piezo-electric modulator.

Referring to Fig. 1, a quartz crystal 5 is mounted on a base plate 1 which may be any conventional crystal holder. A spaced electrode 9 is positioned adjacent the upper face of the crystal, and is rigidly mounted on and supported by an arm I I which is, in turn, connected to a soft iron magnetic armature l3, so that movement of the armature produces a like movement of the electrode 9 in a direction perpendicular to the surface of the crystal.

The magnetic armature I3 is pivotally mounted at a point l5 intermediate its ends between the opposed faces of a pair of U-shaped pole pieces I! and I9 which are oppositely magnetized by a permanent or electromagnet 2|. A deflecting coil 23, shown in cross-section between the pole pieces I! and 19, produces a magnetic field which coincides with the armature axis so that the armature is magnetized by currents flowing through the coil. It will be appreciated that the effect of the two similar poles of the. two pole pieces will have a cumulative pull on the magnetized armature to cause it to rotate in one direction or the other, depending on the direction of the current in the deflecting coil. The pivotal mounting I5 is suiliciently stiff to prevent excessive movement of the armature. Consequently, the spaced electrode 9 will vibrate at a frequency corresponding to the frequency of the energizing current applied to the input terminals.

The spaced electrode 9 is grounded and the base plate 1 connected to the grid of a thermionic oscillator tube 25, or the connections may be reversed, if desired. The anode of the oscillator tube 25 is connected to an oscillatory circuit 21 and a source of anode potential 29 in the conventional manner. A grid leak 3| supplies operating bias for the tube.

It has been found that increased frequency modulation will result from a given spacin and -movement of the top electrode when the crystal is connected in parallel with a fixed capacitor 33. This result is apparently contrary to that which would normally be expected, since the effective change of capacity is reduced by the shunt capacity. The value of this capacitor may be as high as several hundred micro-microfarads for a crystal operating on a fundamental of 7 inc, but its actual value for maximum frequency shift will depend upon the particular oscillator circuit.

The cross-sectional drawing of a crystal modulator unit illustrated in Fig. 2 shows the constructional details of an actual device corresponding to the schematic illustration of the preceding figure. The crystal 5 is mounted on a base plate 1 and clamped at the edges by means of a retaining ring 35. By the term base plate is herein meant the fixed electrode of the crystal, which may comprise a conductive mounting plate which is also an electrode, or a conductive material plated on the crystal which is then mounted on a non-conductive support. The base plate 1 is mounted on an adjusting screw 31 for setting the air gap between the crystal and the spaced electrode 9. The screw 31 is threaded into a collar 39 which is mounted in the bottom of a cuplike porcelain container 4| which forms the main body of the crystal holder. Electrical connection to the fixed electrode is made by means of a pin 43 set in the porcelain, and connected to the collar 39 by means of a lead 45.

The top of the porcelain cup is covered by an apertured plate 41, through which extends the driving member H which connects the magnetic armature 13 to the spaced electrode 9. The driving member is held in position by a stiff wire 49, one end of which is connected at right angles to the driving member II at or near the electrode, the other end being connected to the top plate 41. The wire 49 functions to maintain the electrode parallel to the crystal, throughout its range of movement. In the absence of such a guiding wire the electrode 9, arm H and armature l3 would tend to rotate about the axis I5 and the electrode would not remain parallel to the crystal, which is an undesirable condition. This lead or wire 49 also provides an electrical connection to the spaced electrode of the crystal. A mounting pin 5| is electrically connected to the top plate 41 by a screw 53. Various materials may be used in the construction of these posts, but preferably materials having a low temperature coefficient should be used. Metals of differing coefficients may also be utilized to compensate for undesired expansion of the elements due to changes in temperature.

The top plate of the crystal holder is fixedly connected to the U-shaped pole piece I! of the driving mechanism. For convenience, the deflecting coil is wound in two serially connected sections and 51 separated by insulation to provide a space for the armature pivot which is, for example, a resilient spring strip 59, the lateral cross section of which is shown. The pole pieces I! and I9 are mounted on supporting members 6| and 63 which are fastened to the ends of a highly magnetized permanent magnet 65.

The arrangement illustrated in Fig. 3 utilizes an electrodynamic drive mechanism similar to that employed in loudspeakers. It comprises a permanent magnet 61 and a voice coil 69 mounted in the intense magnetic field of the magnet by means of a flexible spider H. The usual cone is replaced by a metallic electrode 9 which is adjacent the surface of a piezo-electric crystal element 5. The crystal is mounted on a base plate I, as before. It is to be understood that the crystal holder may be of the type illustrated in detail in Fig. 2, although, for the sake of simplicity, constructional features well known to those skilled in the art have been omitted. It' will be understood that the crystal may be connected to any oscillator to control its average frequency, which frequency will be modulated by a signal voltage applied to the wire coil through the input terminals.

A unique driving mechanism is illustrated in Fig. 4, wherein one piezo-electric element is used to control the frequency of an oscillator, while a second-piezo-electric element is used to drive the spaced electrode of the first, and thus modulate the oscillator frequency.

The first piezo-electric element 5 is mounted in the usual manner on a base plate 1 which is, for example, insulatingly supported on a member 13. Immediately above the first crystal, a second piezo-electric element 15 is mounted. This element may be of the type commonly used in vibration pickup devices and loudspeakers such a a quartz or a Rochelle salt crystal. It is mounted at one edge 1'! on the common supporting member 13. The second or motor piezoelectric element 15 is provided with two foil electrodes 19 and 8|, as is well known. The motor element electrode between the adjacent faces of the two elements is grounded, and constitutes an electrode for the frequency controlling element 5. A modulating voltage is applied between the two foil electrodes 19 and 8! with the result that a physical vibration is set up by the motor element which varies the resonant frequency of the frequency controlling element, and thus modulates the oscillator frequency.

While the illustration shows the grounded foil of the motor element functioning as an electrode for the oscillating crystal, it is to be understood that separate elements may be used, if desired. That is, the two crystal elements may be located some distance apart and connected by suitable coupling means, but, for simplicity, the arrangement shown is preferred.

The present invention has been described by means of several specific embodiments, but there are, of course, many modifications which may be made without departing from the spirit of this invention. Its scope, therefore, is only limited by the prior art and the spirit of the appended claims.

We claim as our invention:

1. In a variable air gap crystal holder, the combination including a fixed electrode, a piezoelectric crystal mounted in a fixed position with respect to said electrode, a movable electrode adjacent said crystal whose position with respect to said crystal produces a change in the frequency characteristic of said crystal, an armature, said movable electrode being movable with said armature, armature driving means responsive to a modulating impulse, and means for increasing the frequency change for a given movement of said movable electrode comprising a condenser connected in shunt to said crystal.

2. The method of increasing the range of frequency modulation of a variable air gap crystal which includes the steps of utilizing said crystal to generate controlled frequency oscillations, varying the air gap of said crystal to modulate the frequency of said 5 oscillations, applying shunt capacitance across said crystal, and adjusting the magnitude of said capacitance to a value which substantially increases the frequency change for a given change of said air gap.

' ROBERT J. EHRET.

JOHN L. BARNES.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2458987 *Jul 18, 1945Jan 11, 1949Motorola IncPiezoelectric crystal unit
US2484636 *Sep 26, 1947Oct 11, 1949Bell Telephone Labor IncModulation system
US2513899 *Feb 25, 1948Jul 4, 1950Ferranti LtdPiezoelectric multiplying device
US2532038 *Mar 27, 1945Nov 28, 1950Sebouh DickranMethod and apparatus for producing electrical waves of predetermined formation
US2661622 *Jul 22, 1947Dec 8, 1953Hartford Nat Bank & Trust CoElectric vibration pick-up
US2702354 *Feb 28, 1952Feb 15, 1955Astatic CorpContact microphone
US3019397 *Sep 3, 1958Jan 30, 1962Bendix CorpPressure responsive device combined with positive feedback oscillator circuit
US3329909 *Feb 21, 1966Jul 4, 1967Blonder Isaac SApparatus for producing mechanical oscillations suitable for controlling sweep circuits and other devices
US3949246 *Dec 23, 1974Apr 6, 1976The United States Of America As Represented By The Secretary Of The ArmyPiezoelectric bimorph controlled variable capacitor
DE1112581B *Apr 1, 1959Aug 10, 1961Philips NvSchwingkondensator
DE1121219B *Jan 2, 1958Jan 4, 1962Siemens AgSchwingkondensator
Classifications
U.S. Classification332/139, 331/158, 310/318, 310/350, 331/162, 331/178
International ClassificationH03J7/02, H03J7/04, H03C3/28, H03C3/00
Cooperative ClassificationH03J7/04, H03C3/28
European ClassificationH03J7/04, H03C3/28