US2240450A - Piezoelectric crystal apparatus - Google Patents

Description

Aprifi 29, W41. J. M. WOLF-SKILL PIEZOELECTRIC CRYSTAL APPARATUS Filed Jan. 29. 1940 vvvvvvv qmwup V VV VIII Patented Apr. 29, 1941 2,240,450 PIEZOELECTRIC CRYSTAL APPARATUS John M. Wolfskill, Eri

Electric Company,

Pa., assignor to Bliley Erie, Pa., a partnership composed of F. Dawson Bliley and Charles Collman Application January 29, 1940, Serial No. 316,269

6 Claims.

This invention relates to a continuously variable frequency device and more particularly to apparatus for combining oscillations from a step by step constant frequency crystal oscillator with oscillations from a continuously variable crystal oscillator so as to obtain a continuous frequency variation over a wide range and maintain all the advantages of crystal control during this variation.

An object of this invention is to provide a small compact quartz crystal holder, holding a number of crystal elements which are consecutive in frequency and which may be successively engaged by means of a switch to provide step by step frequency'control of a crystal oscillator.

Another object is to provide a method of combining or mixing the frequency of the step by step controlled crystaloscillator with that of a continuously variable crystal oscillator so as to eliminate as much as possible the products of modulation from the desired output frequency.

A further object of this inventionis to provide an oscillation generating apparatus employing piezoelectric elements or other electromechanically vibratile elements for controlling the frequency or frequencies produced, selected ones of said elements having predetermined frequencies forming an arithmetical progression and another of said elements having means associated therewith for varying its frequency over a range equal approximately to the numerical value of the common diiference of said progres-, sion, the frequencies of said elements forming said progression being mixed or heterodyned with the frequency of the variable element so that continuously variable frequency oscillations that are frequency stabilized by said elements are obtained. 7

In the communication art it is becoming practically a necessity to have some means by which the frequency of a transmitter may be varied readily from time to time. This is particularly true in the amateur field where it is desirable to change frequency from one portion of the band to another in order to avoid interference from other stations. Various methods have been devised for varying the controlling frequency. These include electron coupled and other selfexcited types of oscillators as well as narrow frequency range crystal controlled oscillators. Electron coupled and self-excited oscillators, however, are not as stable as crystal controlled oscillators and have certain other disadvantages. Electron coupled oscillators can be madefairly stable if all the factors which might cause aadvantages of a self-excited oscillator because frequency variation or frequency change are taken into account. Problems in the design of suchinstruments are many and, at best, they do not approach the stability of a good crystal frequency controlled oscillator.

The variation in frequency obtainable to date with a single quartz crystal by present methods known to the art is relatively small and although it does not, in itself, satisfy the needs in this direction, it may be employed as described further in this specification, to solve the problem. A method of continuously varying the frequency of quartz crystals over a relatively narrow frequency range is described in my Patent No. 2,079,540. The invention described in this patent relates to a wedge type adjustable air-gap employed in a single or multiple crystal holder and the variation of the gap controls the frequency variation of one or more crystals in succession.

Other methods of obtaining step frequency variation include those of using a number of individual crystals, connected to a tap switch for rapid switching from one crystal to another. Such arrangements, however, do not have the it is often desirable to use a frequency which is somewhere between the several step crystals on hand. There was, therefore, a need in the art for a method of obtaining a much wider continuous frequency variation possessing all the advantages of crystal control and making a Very easy change of frequency possible.

The present invention makes use of a small compact step by step crystal controlled oscillator combined with a relatively narrow range variable frequency crystal controlled oscillator arranged so that the step by step controlled oscillator is employed to switch step by step from one frequency to the next while the variable frequency crystal controlled oscillator is arranged to be varied and combined with the selected step by step frequency so as to produce a continuously variable controlled or stabilized frequency. The crystal preferably employed in the variable frequency crystal controlled oscillator is of the high activity type, and as a result a wider percentage frequency variation may be obtained with it. It has, however, a high frequency temperature drift, but this is of little consequence because the circuit in which the crystal is used excites the crystal very lightly, causing very little temperature rise.

An example of the manner in which the continuous frequency coverage can be accomplished is as follows. It is assumed that the frequencies of the step by step crystals form a sequence each of which differs from the preceding by kilocycles. The frequency of the oscillator controlled by this step by step crystal unit is combined with that of another oscillator controlled by a single variable frequency crystal which, in itself, has a continuous frequency variation of 15 kilocycles. These two frequencies are fed into a mixer circuit of some type and either the sum or the difference frequency amplified and filtered from the undesired modulation products. It can readily be seen that practically any frequency variation desired may be obtained by this method, its variation being limited only by the number of crystals which can be used in the step by step unit. In actual practice, it is found desirable to use the difference frequency between the two oscillators because this enables a much higher frequency to be used for the continuously variable crystal element. Since the variation obtainable with any one element is a certain percentage of the frequency of the crystal, the higher the frequency of the crystal, the higher will be the variation obtainable by means of airgap variation.

In the step by step frequency'unit it is desirable to select crystals of fairly low frequency,

'so that for the selected step frequency difference from one element to the next of 15 kilocycles, a considerabledimensional change is obtained between the various crystals. Way close tolerances can be obtained in the actual frequency of these units.

Assuming that it is desired toproducean os-- cillator having a continuous frequency variation of 150 kilocycles from 3700 kilocycles, ten crystals are selected for the low frequency oscillator having'frequencies-from 1600 kilocycles to 1465 kilocycles in steps of 15 kilocycles. The output of this oscillator is heterodyned with a continuously variable frequency crystal oscillator whose minimum frequency is 5300 kilocycles and has a variationof plus 15 kilocycles, the difference between these two oscillators then gives a continuous frequency variation from 3700 kilocycles to 3850 kilocycles. These and other features of this invention are more fully described in the following specification and illustrated in the drawing in which briefly, the sole figure is a circuit diagram of the oscillator apparatus. 7

Referring to the drawing reference numeral 10 designates the vacuum tube of the first oscillator circuit which is controlled by the step by step crystal unit 1?. This oscillator is of the untuned type so that the crystal is the frequency determining element, and requires no other adjustment for oscillation over a wide frequency range.

The crystal unit H is shown with four crystal elements l8, I9, and 2! each having one electrode connected to a common wire also connected to the grid H of tube It. A selector switch 22 connected to the condenser 23 is employed for selecting the crystal element desired for use. In practice different numbers of crystal elements may be employed and the frequencies thereof may differ by a common difference, such as, 15 kilocycles, or if desired, several groups of crystal elements may be employed in the unit ll, one group containing a number of crystal elements having frequencies differing by a common difference so that said frequencies are spaced substantially uniformly over a given band and another group containing a number of crystal 'ele ments likewise having frequencies differing by In this 7 the same or some other common difference but extending over another band more or less widely separated from the aforesaid given band.

The contact arm of the switch 22 is coupled through the condenser 23 to the anode 13 of the tube H). Positive potential from a source of anode current supply, such as, a motor generator, batteries, rectified alternating current and the like, is applied to the aforesaid anode through a choke coil 24 and a potential reducing resistor or impedance 45. A by pass condenser 25 is connected between the coil 24 and the cathode [2. The resistor I5 is employed connected between the grid ii and cathode I2 to provide suitable bias potential for the grid and a condenser I6 is connected across this resistor to facilitate production of oscillations by the tube.

The cathodes of all of the tubes IE, 23 and 42 are indirectly heated by heaters such as is, associated with tube 16, connected to suitable batteries or low voltage transformers. If the cathode and heater are sufficiently electrically insulated from each other one transformer secondary winding may be employed for the heaters of all of the tubes provided, of course, that they all require the same voltage. In any case however they may be energized from separate secondary windings of the samev transformer which may be the same transformer as is used .for supplying the anode current.

The output of the oscillator tube ill is fed into the mixer grid3l of. a mixer tube 28 which is similar to a 6L7 type tube, through the coupling condenser 25. The crystal oscillator circuit of 1 the tube 28including the first grid 30, the screen 32, and the cathode 29 form a circuit such as described and claimed in'my patent application Serial No. 296,676 filed Sept. 26, 1939. This type of oscillator requires no adjustment for oscillation over a wide frequency range, and because of the high amplification factor of a tube employed, the crystal current is exceptionally low. This is a desirable feature, because the crystal used has a relatively high frequency temperature drift since it is cut at an angle to obtain high activity, and substantial frequency variation by means of the air-gap adjustment. Naturally, the lower the crystal current through the crystal, the smaller will be the temperature rise, and consequently the frequency drift during oscillation. 1

The tube 28, being of the 6L7 type, employs two control grids 30 and 3| shielded-from each other by the screen 32 which is connected to the chassis I00, that is grounded for high frequencies through the condenser 38. A resistor 21 is con nected between the grid 3! and ground id!) to provide an external or return path "to the cathode '29 which is connected to ground I09 through the'high frequency choke coil 36 shunted by the condenser 31. The variable air gap crystal holder and crystal element 34 shunted by the: grid resister 35 are connected between the grid 35 and ground I00. a

" -A pcsitive potential-is applied to the screen 32 through the vOItagereducing resistor or impedance 39 which is connected to the anode current supply positive terminal; The anode current supply designated by the terminals 'B and -l-B "may be of the type described in Patent No.

1,251,377 of Hull and White.-'

I The screen 32 thusactsiias an anode for the crystal} oscillator circuit of the tube 28, and if desired a tuned or tunable circuit may becom Patent No; 1,195,632 of nected to this electrode 32 between it and the condenser 38 as is described in my aforesaid application Serial No. 296,676.

The output circuit of the mixer tube consists of a tuned tank 40 consisting of inductance 4| shunted by condensers 4 2 and 43 connected to the anode 33 and tuned to the desired output frequency which, in this case, is the difference frequency between the two oscillators. Coupled to this output circuit through a condenser 46 is an amplifier tube 41 with another tuned tank 59 consisting of inductance 60 huntedby condensers 6| and 62 in the plate circuit tuned to the, same output frequency. The tube 47 may be of the 6P6 or 1852 or similar type employing cathode 48, controlgrid 49, screen grid 50, suppressor gridl and anode 52. Grid resistor 53 is connected between the grid 49 and ground I00 and grid bias resistor 54 shunted by condenser 55 is connected between the cathode 48 and ground I00. The screen 50 is connected to the positive terminal of the anode current supply through resistor 51 and around said resistor are also connected by-pass condensers 56 and' 58 as shown. An additional by-pass condenser 44 is connected between the tank 40 and ground.

The tuning of the two tank condensers 43 and 62 should preferably be coupled mechanically so that their resonant frequency tracks, the tuning can then be accomplished by means of a single control. The tuning condensers 43 and 62 themselves should be of such a size that with their maximum capacity variation the resonant frequency of the tank circuits remains within the desired band. 'The two tanks can be so designed that fixed capacities 42 and 6| are used in addition to the adjustable condensers. The prime reason for having the tunable elements cover only a definite band of frequencies is to eliminate the possibility of tuning these output tanks to the wrong modulation products; that is, if their range was wide enough, it would be possible to tune to, say, the second harmonic of the 1600 kilocycles oscillator 28 which is used in the example. In other words, if the tank tuning was broad enough, the output might possibly be tuned to 3200 kilocycles which would not be the desired frequency. However, if the tuning control varies the resonant frequency of these tank circuits between 3500 kilocycles and 4000 kilocycles, it is obvious that the modulation products are completely filtered out from. the desired frequency. Generally, the tuned circuits are broad enough to cover several hundred kilocycles so that it will not be necessary to retune these tanks as the frequency of the crystal oscillator I0 is changed, however this could be accomplished by mechanically coupling the switch 22 to the condensers 43 and 62 and making the stationary contacts of switch 22 each wide enough so that the switch arm could be moved continuously with the condenser control and would connect the next crystal into circuit after said arm and condenser were moved sufficiently so that the tanks would be adjusted to the beat frequency of said next crystal. In the example given where 150 kilocycle variation is obtained, one tuning of the Control on the tank circuits is satisfactory over this range. If maximum output is required to drive a succeeding stage, however, it may be desirable to retune these tanks slightly to their exact resonance, but in any case, only several tunings would be necessary in the 150 kilocycle range.

As examples of the wide continuous frequency variation obtainable over selected frequency bands reference is made to the following tables in which the first column is the frequency range of the variable frequency crystal 34, the second column is the frequency of the crystal selected in unit I! and the third column is the difference frequency:

' Step by step Variable frequency crystal frecrystal quency Output frequency (5300 kc. to 5317 kc.) 1600 kc. 3700 kc. to 3717 kc. (5300 kc. to 5317 kc.) -1585 kc 3715 kc. to 3732 kc. (5300 kc. to 5317 kc.) l570 kc 3730 kc. to 3747 kc. (5300 kc.- to 5317 kc --1555 kc 3745 kc. to 3762 kc. (5300 kc. to 5317 kc.) -1540 kc 3760 kc. to 3777 kc. (5300 kc to 5317 kc.) 1525 kc. 3775 kc. to 3792 kc. (c300 kc to 5317 kc 1510kc 3790 k(3.'t0 3807 kc. (5300 kc to 5317 kc -1495 kc .3805 kc. to 3822 kc. (5300 kc. to 5317 kc -1480 kc 3820 kc. to 3837 kc. (5300 kc. to 5317 kc. 1465 kc. 3835 kc. to 3852 kc.

To change the frequency range of the output frequency, all that is necessary is to change the frequency of one crystal; for example, if the 5300 kilocycle crystal is changed to 5100 kilocycles the following range of output frequencies which are continuous for 152 kilocycles is obtained.

(5100 kc. to 5117 kc.) l600 kc 3500 kc. to 35=l7 kc.

'100 kc. to5117 kc.) 1585 kc 3515 kc. to 3532 kc. (5100 kc. to 5117 kc.) -1570 kc 3530 kc. to 3547 kc. (5100 kc. to 5117 kc.) 1555 kc 3545 kc. to 3562 kc. (5100 kc. to 5117 kc.) -l54:0kc 3560 kc. to 3577 kc. (0100 kc. to 5117 kc.) -l525 kc. 3575 kc. to 3592 kc. (5100 kc. to 5117 kc.) 1510 kc. 3590' kc. to 3607 kc. (5100 kc. to 5117 kc.) 1495;kc. 3605 kc. to 3622 kc. (5100 kc. to 5117 kc.) -1480 kc. 3620 kc. to 3637 kc. (5100 kc. to 5117 kc.) -1465 kc. 3635 kc: to 3652 kc.

Of course, if desired, the amplifier tube 41 can be used as a harmonic multiplier, in which case the first tank circuit 40 connected to the mixer tube 28 is tuned to the difference frequency, and the multiplier tube tank circuit 59 to, say, the second harmonic of this frequency. Additional amplifier tubes may be connected to the tank 59 by meansof the inductance coupling coil 63 or capacitative coupling may be used. If relatively low power is desired the coil 63 may be coupled to the antenna.

In all the examples given above, the variation of the variable air-gap frequency crystal 34 was 15 kilocycles. In actual practice, it is possible to obtain as much as 17 kilocycles or 18 kilocycles variation with a 5300 kilocycle crystal. Although theoretically this is not necessary, a slightly wider frequency variation in this unit allows for a wider tolerance in the 15 kilocycle steps of the step by step unit II. If all the crystals in the complete unit I! were ground to their theoretically exact frequency, 15 kilocycles would be all the variation that would be required in the variable air-gap unit 34, but since it is impractical to grind the step by step crystals to a closer tolerance than possibly 500 cycles, it is desirable to have the variable air-gap unit cover a slightly wider variation than the frequency separation between the step by step crystals. This makes it possible to have complete coverage over the entire frequency range without too close a grinding tolerance on the fixed crystals of unit I1.

Another desirable feature of the invention is that the step by step crystals may be made up in a unit covering a definite range, say 1465 kilocycles to 1600 kilocycles, and then it is possible to shift to any desired working or output frequency having the same variation simply by changing the frequency of the crystal 34. In other words, by changing one crystal (the continuously variable one) another kilocycle variation may be obtained in some other portion of the frequency spectrum.

Another feature of the invention is that the step by step crystal unit gives rough frequency selection from one crystal to the next and the variable air-gap crystal unit gives Vernier adjustment of the frequency between the rough adjustments.

All examples are given simply to illustrate the invention, any other ranges can be used to cover other parts of the frequency spectrum and it is of course obvious that changes in various details described may be made without departing from the spirit and scope of the invention as defined by the claims.

What I claim is as follows:

1. A variable frequency oscillation enerator, comprising: a variable frequency piezoelectric crystal controlled oscillation generator including a vacuum tube and a plurality of piezoelectric crystal elements ground to different frequencies spaced by a predetermined substantially uniform frequency interval over one or more frequency hands, a second piezoelectric crystal controlled oscillation generator including a vacuum tube, a piezoelectric crystal and a holder for said last mentioned crystal for varying the frequency of said last mentioned crystal by an amount great enough to at least cover the aforesaid frequency interval of said plurality of crystals in said first mentioned oscillator, and means connected to said oscillators for producing beat frequency oscillations from the oscillations generated by said first mentioned oscillation generator and said second oscillation generator, said beat frequency oscillations being continuously variable over wide ranges by selecting crystal elements from said plurality of crystal elements one after the other and varying the frequency of said second oscillation generator substantially throughout its range each time a new crystal element of said plurality of crystal elements is selected.

2. A variable frequency oscillation generator as set forth in claim 1 further comprising: filter means connected to select the desired beat frequency and for suppressing the undesired frequencies accompanying said desired beat frequency.

3. A variable frequency oscillation generator as set forth in claim 1 further comprising: a

tuned circuit connected to receive the desired beat frequency from said oscillation generators, an amplifier connected to said tuned'circuit and a second tuned circuit connected to the output of said amplifier, and means for varying said first and second tuned circuits only through a limited frequency range to cover the range of variations only of said beat frequency to remove undesired frequencies accompanying said beat frequency.

4. A variable frequency oscillation generator, comprising: a vacuum tube oscillator, a plurality of electromechanically vibratile elements having different frequencies spaced by a predetermined substantially uniform frequency interval over one or more frequency bands, means for selectively connecting said electromechanically vibratile elements to said vacuum tube oscillator for producing oscillations controlled by the selected electromechanically vibratile element, a second electromechanically vibratile element frequency controlled vacuum tube oscillator for producing continuously variable frequency oscillations, the frequency variation of said second electromechanically vibratile element controlled oscillator being such as to at least cover the aforesaid frequency interval, a coupling circuit for connecting said first and second oscillators together to produce beat frequencies therebetween, a circuit tunable to said beat frequencies, and connections for impressing oscillations from said oscillators on said tunable circuit.

5. A variable frequency oscillation generator as set forth in claim 4 further comprising: an amplifier connected to said tunable'circuit and a second tunable circuit connected to the output of said amplifier, and means for varying the frequency characteristics of said first and second tunable circuits only through a limited frequency range to cover the range of variations of said beat frequency to remove undesired frequencies accompanying said beat frequency.

6. 'A variable frequency piezoelectric crystal controlled oscillation generator, comprising: a piezoelectric element frequency controlled oscillator for selectively producing a plurality of relatively low frequency oscillations spaced apart by a substantially uniform frequency interval, a second piezoelectric element frequency controlled oscillator for producing oscillations having a continuously variable frequency, variable over a range equal to the above mentioned frequency interval, said second oscillator being adapted to generate oscillations of frequencies considerably higher than the aforesaid relatively low frequency, connections between said first and said second oscillator for mixing the oscillations produced and for producing piezoelectric crystal controlled oscillations the frequency of which may be continuously varied between predetermined wide limits by selecting said relatively low frequency oscillations and each time such selection is made varying the frequency of said second oscillator' over its range so as to produce a beat frequency which may be varied over a Wide range as the range of said relatively low frequencies is covered.

JOHN M. WOLFSKILL.

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