US2470738A - Temperature compensated crystal unit - Google Patents

Description

May 17, 1949. H. M. BACH TEMPERATURE COIPENSATED CRYSTAL UNIT 4 Sheets-Sheet 1 Filed Jan. 30, 1945 xQIMDUwI 2. lnrur Rucumu:

(AT T.)

FIG 1 FIG. 2

May 17, 1949. H. M. BACH 2,470,733

TEMPERATURE COIPENSATED ems-m1. uu'rr Filed Jan :50, 1945 4 Sheets-Sheet 2 FIEOUENCY May 17, 1949. H. M. BACH 2,470,733

TEMPERATURE COMPENSATED CRYSTAL UNIT Filed Jan. 30, 1945 4 She'ets-Sheet 3 Flq. 4-

Flli. 7

Patented May 17, 1949 TEMPERATURE COMPENSATED CRYSTAL UNIT Henry M. Bach, Lawrence, N. Y., assignor to Premier Grystal Laboratories, Incorporated,

New York, N. Y.

Application January 30, 1945, Serial No. 575,236

15 Claims.

This invention relates to temperature compensated crystal units, and more particularly, to a method and means for controlling the frequency drift of crystal units.

A main object of this invention is to provide an improved method and means for limiting the frequency drift of crystal units resulting from changes in ambient temperature to a very narrow range.

A further object of this invention is to provide a novel method of restricting the frequency drift of crystal units caused by changes in ambient temperature to meet close frequency drift tolerances hitherto unobtainable without the use of constant-temperature ovens.

A further object of this invention is to provide a novel structure for restricting the frequency drift of crystal units resulting from changes in ambient temperature to a narrow band of frequencies close to the nominal frequency of the crystal without the employment of special energydissipating equipment, such as heaters.

Further objects of the invention will appear from the following description and claims, and

-' from the accompanying drawings, wherein:

pensating means in accordance with this invention.

Figure 5 is a graph similar to Figure. 3 but illustrating the temperature-frequency characteristic of a crystal unit of different orientation than the unit of Figure 3, employing the method of temperature compensation of this invention.

Figure 6 is a schematic diagram of the crystal unit of Figure 5 provided with temperature'compensating means in accordance with this invention.

Figure 7 is a schematic diagram of a modification of the crystal unit of Figure 3 provided with temperature compensating means in accordance with this invention.

Figure 8 is a graph illustrating-the overlapping eflect which is obtained when conventional thermostatic switches are employed in the structure of this invention.

One of the important electrical considerations which determines the oscillation frequency of a piezo-electric crystal operating in the usual type of crystal oscillator circuit is the input reactance as viewed from the terminals of the crystal unit.

, For a given crystal unit there is a fairly definite range of input reactances over which the crystal and input reactance as a network will permit oscillations to be sustained in a particular oscillator circuit.

It is a well known fact that a crystal unit designed to operate at a given frequency in one circuit will be oil frequency when used in another circuit having a different input reactance by an amount which may be as much as several hundred cycles. It is also found that by placing a variable reactance, such as a variable air-dielectric trimmer condenser across the terminals of a crystal unit, the oscillation frequency may be adjusted by varying the shunt reactance, and hence, the input reactance to a desired value. Ordinarily, this procedure is employed in frequency standards to match the crystal oscillator frequency, or some multiple thereof, against a primary standard of frequency.

It is well known in the art that crystal units, even of the so-called zero coemcient orientations such as the AT, BT, and the like, drift appreciably over the wide range of ambient temperatures now encountered in practical usage, especially in portable ground equipment and in airborne equipment. It has therefore been necessary, where close frequency tolerances are specified, to enclose these crystal units in constanttemperature ovens, usually maintained close to a fixed temperature by a heater element requiring a source of current therefor.

To further illustrate this problem, under present methods of manufacture a BT cut crystal unit can drift plus or minus approximately 0.02% from nominal frequency over a temperature range of from -50 centigrade .to centigrade. Sucha crystal unit would not be satisfactory for use in certain types of equipment, such as transmitters employing high multiplication factors for example, unless the frequency drift was held to a much lower variation, say, approximately plus or minus 0.005% over the temperature range. To accomplish this reduction in drift by the method of the prior art it would be necessary to enclose the unit in a constant temperature oven.

It is a prime purpose of this invention to dispense with the need for such an oven by taking advantage of the frequency-reactance characterlstic of the crystal unit in conjunction with the frequency-temperature characteristic thereof.

Referring to the drawings, Figure 1 is a graph which by way of example shows the typical variation in frequency of an oscillating crystal, connected in a conventional circuit, such as the tuned plate circuit of Figure 2, when the input reactance X, viewed from the crystal terminals, is varied from an initial value X1 to a final value X: with the ambient temperature remaining constant at a value To.

As shown in Figure 2, the variation of crystal input reactance X may be obtained by varying the capacitance of a condenser Cx connected across the crystal. At an initial setting of condenser Cx the capacitance will be such as to present an input reactance X1 across the crystal and the frequency will be a value F1; As the capacitance of the condenser Cx is reduced, input reactance X increases in accordance therewith and the frequency increases in accordance with the frequency-reactance characteristic line I of Figure 1. Thus, at a reactance value X: the frequency will be E, and at a reactance value K1 the frequency will be F3. As above stated, a fre= quency variation of the order of several hundred cycles may thus be obtained.

Referring to Figure 3, a set of curves ii, i2 and i3 is disclosed representing the variation in frequency of a typical B'I' cut crystal unit between temperature values T1 and T2 for input reactance values X1, X2 and X3, respectively. As is well known, such characteristic curves for the ET cut crystal unit are in the form of parabolas, the frequency values at the extreme ends of the temperature range being lower than the values at the intermediate portion of the range.

By using appropriate values for the input reactances the temperature characteristics may be raised or lowered with respect to each other, and as many characteristics may be obtained as there are settings of input reactance.

Let it be assumed that the normal input re actance without added shunt capacitance across the crystal is a value &. Then the frequency will normally vary over the temperature range T1 to T2 between a value F and a value Fm the value F occurring at the cold end and at the hot and of the range. Let it further be assumed that the desired frequency drift tolerance is such that the frequency-temperature characteristic of the crystal unit is required to be held between the values Fe and Far, as represented by the respective horizontal lines 2i and 3d in Figure 3.

In accordance with the method of this invention, the crystal unit is shifted from its original temperature characteristic 8% at an input reactance value of X: to a new temperature characteristic ii at a reduced input reactance value of X: at a point on characteristic is below toler= ance limit line 20 so that it will remain within the specified frequency tolerance. Another shift can be made further on in the temperature range to a still further reduced input reactance X1 having the characteristic ii, and the process is reversed in the hot region so that the actual frequency-temperature characteristic is as shown by the full line curve of Figure 3. Itis clear from Figure 3 that the frequency will be thus held between the specified tolerance limits Fe and FM.

Figure 4 shows a schematic arrangement of one means of carrying out the above method. Connected across the crystal unit is a first shunt circult comprising a capacitor C2 and a pair of ther- 4 mostatic switches D and A. At the starting temperature T1 switch D is closed and switch A is open. Switch A is adapted to close at a temperature TA, shunting capacitor C: across the crystal, thereby shifting the crystal characteristic from curve l3 to curve l2. A second shunt circuit is connected across the crystal comprising a capacitor Ca and a pair of thermostatic switches C and B. At the starting temperature T1 switch C is closed and switch B is open. Switch B is adapted to close at a temperature Ts, above temperature TA, thereby shifting the crystal characteristic from curve I! to curve Ii. Switch C is adapted to open at a temperature To, shifting the crystal characteristic from curve I back to curve i2, and switch D is adapted to open at a temperature To, shiftingthe crystal characteristic from curve 52 back to its original characteristic i3 at the hot 'end of. the range.

The process may be reversed, starting at temperature T2 where switch elements 0 and D are open and A and B are closed. Switch D will close at temperature To and shift the crystal characteristic to curve i2, and switch C will close at temperature To, shifting the crystal characteristic to curve 8 i. Continued reduction in tempera ture causes switch B to open at temperature Ts, restoring the crystal to curve i2, and further reduction of temperature to TA causes switch A to open, putting the crystal back on its original characteristic curve [43.

It is thus seen that by controlled automatic reunits may be stabilized within close limits, and" the degree of stability substantially depends on the number of thermostatically controlled reactance stages employed.

For example, in Figure 5 a series oi frequencytemperature characteristics for a typical-AT cut crystal is shown, having input reactances 2Q, X2 and X1 respectively. Curve 33 is the characteristic at reactance X1, curve 32 is the charac= teristic at reactance &' and curve 35 is the characteristic at reactance X1. Assuming that it is required to stabilize the frequency between F? and FM, as before, starting from a frequency F1 at the cold end, a circuit is employed, as shown in Figure 6, wherein a first shunt branch contains capacitance C2 and thermostatic switch E, closed at temperature T1 and adapted to open at temperature Ts, anda second shunt branch contains capacitance Ca and thermostatic switch F, closed at temperature T1 and adapted to open at temperature Ts. Starting at the cold end tem-= perature T1, the crystal remains on characteristic curve 3% until temperature Ts isreached, whereupon switch E opens and the crystal shifts to characteristic 32. Upon further increase of temperature, a value T? is reached which opens switch F and shifts the crystal to characteristic 33. It will be seen from Figure 5 that the frequency is thus maintained between the specified values F1 and Fm. As in the case of the BT crystal above considered, the stabilizing action is reversible.

Instead of employing shunt reactance units, series elements may be employed. In the embodiment represented by the schematic diagram of Figure 7, a series capacitance Cs is employed proac'zonss vlded with'a thermostatic shunting switch 55. The crystal is ofthe BT orientation so that in the hot or cold region it is necessary to raise the characteristic. Hence, switch II is provided with a first bimetal switch arm ll cooperating with a switch contact 52 and a second bimetal switch minimum-in the intermediate temperature region,

as requiredto hold the frequency drift of the BT crystal tea reduced excursion. Obviously, other parameters and arrangements The input reactance is therefore thereof than shuht or series capacitance may be employed to controlthe input reactance of the crystal unit for temperature drift compensation.

The method of this invention contemplates the use of either capacitance or inductance arranged in networks adapted to provide specific characteristics as required. Furthermore, continuous variation of capacitance or inductance responsive to temperature variation may be employed, by

" using spiral wound thermostat members, for example, connected so as to rotate variable condenser shafts orto actuate movable inductor cores,

to provide the desired compensation.

The thermostatic switches may be included in the crystal holder together with the reactance elements and may be provided with appropriate adjusting means, such as adjustable screw contacts which are known in the art for setting the temperature values at which the circuits controlled th'ereby are opened or closed.

Consideration must be given to the fact tha the usual bimetal'thermostat will overlap by a few degrees in its temperature response in one direction of temperature change with respect to its temperature response in the other direction of temperature change. Therefore, the adjustment of the contact points must be such that the overlapping region, as indicated by the shaded area in Figure 8, will be located between the upper frequency limit line H and the lower frequency limit line 42. Thus, for increasing temperature, the frequency characteristic will move along line 3 to the right until point U is reached, whereupon the bimetal thermostatic switch will shift the characteristic to line 44, whereas, for decreasing temperature the characteristic will move along line 44, to the left until point L is reached, whereupon the switch will shift the characteristic to line 43. Between the temperatures T1. and To the crystal may be on either characteristic 43 or 44. The shaded area may, of course, be reduced by making the thermostatic switch more sensitive.

A further simplified embodiment of this invention which would be effective to reduce the frequency variation of the conventional BT cut oscillator plate would consist of designing the plate for compliance with the reduced drift tolerances at the intermediate portion of the frequency-temperature characteristic and providing a shunt capacitance across the plate with a pair of thermostatic switch elements in series there- 4 with. One switch element .would be adapted to ment would be adapted to open adjacent the high end of the range. This would lift the drooping portions of the BT characteristic adjacent the temperature extremes to bring the plate within frequency tolerance at said extremes. It is of course understood that both thermostatic switches would remain closed over the intermediate portion of the temperature range.

Although methods have been described above for changing the input reactance of the crystal unit to compensate for frequency changes due to temperature changes, itis apparent that since any'changes in frequency may, in general, be compensated for by inverse changes in electrical loading of the oscillating'crystal, the compensation may be accomplished by changing the resistance component of the crystal input impedance instead ,of .the reactance component. Therefore, it is contemplated that this invention includes thermostatic control of shunt or series resistances as well as shunt or series reactances, reactance networks, or networks including both resistance and reactance. This invention further contemplates the application thereof to crystal units of the air-gap, pressure-mounted, plated and other known types of mountings of oscillator plates.

While certain specific embodiments of the method and means of compensating for crystal frequency drift caused by temperature change have been disclosed in the foregoing description, it will be understood that various modifications within the spirit of the invention will occur to those skilled in the art. Therefore, it is intended that no limitations be placed on the invention other than as defined by the ,scope of the appended claims. v

What is claimed is:

1. The method of limiting the frequency variation with temperature of a piezo-electric vibratory plate comprising the steps of decreasing the input impedance of the plate when the frequency rises above a first upper limit value as a result of temperature changes, and increasing the input impedance when the frequency drops below a second lower limit value as a result of other temperature changes.

2. The method of limiting the frequency variation with temperature of a piezo-electrio vibratory plate having a frequency-temperature characteristic which would normally cause the frequency to exceed given limits of variation,.comprising the steps of decreasingthe input impedance of the plate when the frequency rises toward the upper limit of variation, and increasing the input impedance when the frequency decreases toward th lower limit of variation. 1

3. The method of limiting the frequency variation with temperature of a piezo-electric vibratory plate having .a substantially linear -frequency-temperature characteristic over its specifled temperature range, comprising the steps of increasing the input impedance of the plate when the frequency decreases toward the lower limit of frequency variation, and decreasing the input impedance of the plate when the frequency rises toward the upper limit of frequency variation.

4. The method of limiting the frequency variation with temperature of a piezo-electric vibratory plate having a frequency-temperature characteristic which would normally cause the frequency to exceed specified limits of variation over the temperature'rang'e, comprising the steps of decreasing the input reactance of theplate when'the frequency rises toward the upper limit of variation, and increasing the input reactance of the plate when the frequency decreases toward the lower limit of variation.

5. An oscillator circuit comprising a piezoelectric oscillator unit and an oscillation generator whose frequency is controlled by the frequency of the piezo-electric unit, said piezo-electric unit having a frequency-temperature char-' acteristic normally causing a frequency variation over a specified temperature range, means for limiting the frequency variation with temperature to a range between an upper limit of frequency 'and a, lower limit of frequency comprising means responsive to temperature changes for increasing the input impedance ofthe circuit with respect to the piezo-electric unit when said characteristic decreases toward said lower limit and decreasing said input impedance when said characteristic rises toward said upper limit.

6. A piezo electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequency variation with temperature of the plate between an upper limit of frequency and a lower'limit of frequency, comprising means responsive to itemperature changes for increasing the impedance of the crystal unit when said characteristic drops toward said lower limit and decreasing the impedance of the crystal unit when said characteristic rises toward said upper limit.

7. An oscillator circuit comprising a piezo=elec= tric oscillator plate and an oscillation generator, said plate being connected to said generator so as to control the output frequency of the generator in accordance with the oscillation frequency of the plate, the frequency of the plate having a normal variation over a specified temperature range, means for limiting the frequency of the plate to a specified variation over said specified temperature range less than said normal variation comprising means responsive to changes in temperature normally causing a decrease in frequency for increasing the input impedance of the oscillation generator with respect to said plate when the frequency decreases toward the lower limit of said specified variation,- and means re= sponsive to changes in temperature normally causing an increase in frequency for decreasing the input impedance of the oscillation generator with respect to said platewhen the frequency rises toward the upper limit of said specified variation.

8. In an oscillator circuit comprising a piezoelectric oscillator plate and an oscillation generator, said plate being connected to said generator so as to control the output frequency of the generator in accordance with the oscillation frequency of the plate, the frequency of the plate having a normal variation over a specified temperature range, means for limiting the frequency of the plate to a specified variation over said specified temperature range less than said normal variation, comprising a network connected in said oscillation generator, said network com-- prising means responsive to changes in temperature normally causing a decrease in frequency of said plate for increasing the input impedance of said generator with respect to said plate when the frequency of the plate decreases toward the lower limit of said specified variation, and further means in said network responsive to changes in temperature normally causing an increase in frequency of said plate for decreasing the input impedance of said generator with respect to said plate when the frequency of the plate rises toward the upper limit of said specified variation.

9. A piezo-electric crystal unit comprising avibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequency change with temperature of the plate to a value less than said variation between an upper limit of frequency and a lower limit of frequency, comprising a circuit connected to the plate, said circuit comprising an impedance element and thermostatic switch means, said thermostatic switch means being adapted to operate said circuit so as to increase the impedance of the crystal unit responsive to a temperature change which normally causes the frequency-temperature characteristic to, approach the lower limit of frequency variation, and tofurther operate said circuit so as to decrease the impedance of the crystal unit responsive to a temperature change which normally causes the frequency-temperature characteristic to approach the upper limit of frequency variation.

10. A piezo-electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequency change with temperature of the plate to less than said normal variation, comprisin a normally closed circuit connected across the plate, said circuit comprising an impedance element and thermostatic switch means, said thermostatic switch means being adapted to open said circuit so as to increase the impedance of the crystal unit responsive to a temperature change which normally causes the frequencytemperature characteristic to approach the lower limit of frequency variation.

11. A piezo-electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequencychange with temperature of the plate to less than said normal variation, comprising a normally open circuit connected across the plate, said circuit comprising an impedance element and thermostatic switch means, said thermostatic switch means being adapted to close said circuit so as to reduce the impedance of the crystal unit responsive to a temperature change which normally causes the frequency-temperature characteristic to approach the upper limit of frequency variation.

12. A piezo-electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means. for limiting the frequency change to less than said normal variation, comprising a normally open first circuit connected across the plate, said first circuit comprising a first impedance element, and a normally closed second circuit connected across the plate, said second circuit comprising a second impedance element, and thermostatic switch means adapted to open said second circuit so as to increase the impedance of the crystal unit responsive to a temperature change which normally causes the frequency-temperature characteristic to approach the lower limit of frequency variation and being further adapted to close said first circuit so as to decrease the impedance of the crystal unit responsive .to a temperature change which normally causes the frequencytemperature characteristic to approach the upper limit of frequency variation.

13. A piezo-electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequency change to less than said normal variation, comprising an impedance element connected in serieswith said plate, and means responsive to a change in temperature normally causing said frequency-temperature characteristic to approach the upper limit of frequency variation to shortcircuit said impedance element.

14. A piezo-electric crystal unit comprising a vibratory piezo-electric plate having a frequencytemperature characteristic normally causing a variation in frequency of the plate over a specified temperature range, and means for limiting the frequency change to less than said normal variation, comprising an impedance element connected in series with said plate. a shunt circuit across said impedance element, and means responsive to a change in temperature normally causing said frequency-temperature characteristic to approach the lower limit of frequency variation to open said shunt circuit.

15. In combination, a piezo-electric crystal element having a natural frequency of vibration subject to variation with temperature, a capacitive impedance element, switching means for connecting said impedance element in parallel circult relation with said crystal element thereby to determine the frequency of vibration of said crystal element, and thermal responsive means arranged to actuate said switching means to disconnect said capacitive element upon a predetermined variation in temperature either above or below a predetermined mean temperature.

HENRY M. BACH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,994,228 Osnos Mar. 12, 1935 2,137,304 Parkin Nov. 22, 1938 2,264,764 Koerner Dec. 29, 1941

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