US2256931A - Piezoelectric crystal oscillator - Google Patents

Description

Sept. 23, 1941.

J. M. wom-SKILL 2,256,931

PIEZOELECTRIC CRYSTAL OSC ILLATOR Filed Sept. 26, 1939 glwucnion Patented Sept. 23, 1941 HEzoELEoTmc CRYSTAL osoiLLA'roR John M. Wolfskin, Erie, Pa., assignoito Bliley Electric Company, Erie, Pa., a. partnership Icomposed of F. Dawson Bliley and Charles Collman Application September 26, 1939, Serial No. 296,676

Claims. v(531, Z50-36) This invention relates to oscillator circuits in general and more particularly to oscillator circuits for use with piezo-electric quartz crystals. This application is a continuation in part of my application SerialNo. 256,781, filed Feb. 16, 1939. g

An object of this invention is to provide a circuit for use with harmonic crystals so that the crystal may be made to oscillate readily at its higher odd harmonics, such as the fifth, seventh, ninth, eleventh, etc.

Another object is to obtain ultra-high frequencies directly from a single stage or single tube piezo-electrical crystal oscillator by employing a novel form of feedback in the circuit so that oscillations will readily start on these higher harmonies of the crystal.

A further object of this invention is to provide a piezo-electric crystal oscillator circuit adapted to sustain oscillations which bear a harmonic relation to the fundamental frequency of the crystal and in which the power output at the harmonic frequency is comparable to the power output obtainable at the fundamental frequency.

Another object of this invention is to provide a piezo-electric crystal oscillator circuit adapted to sustain oscillations which bear a harmonierelation to the fundamental frequency of the Piezoelectric crystal and in which oscillations of useful amplitude of a frequency of 110 megacycles or more may be produced.

' Another object of this invention is to provide aV piezo-,electricV crystal controlled oscillator in which the frequency of the oscillations generated correspond to the odd harmonics of the piezo-electric crystal and in which the various od'd harmonics at least as high as the eleventh may be readily selected by adjusting the plate tank circuit of the oscillator tube and vigorous crystal control of the oscillations obtained as the tank circuit is tuned through each of the aforesaid harmonics.

In accordance with this invention I provide a piezo-electric crystal controlled oscillation generator adapted for producing high and ultra high frequency oscillations which are piezo-electric crystal frequency stabilized. Frequency stability isvery desirable when working With the high` and ultra high frequency oscillations. By employing my invention frequency stability is obtainable with these frequencies practically as readily as with lower frequencies where the piezo-electric crystal is employed on its fundamental frequency.

In the present invention I propose employing a three electrode oscillation generator tube to .tube crystal oscillator. v

seventh, mc. on the'ninth, .eleventh'harmonie which the piezo-electrccrystal, adapted for harmonic operation, is connected. TheY circuit of this three electrode tube is provided with-means for kregulating the-amount of feedback to enable the circuit to4 start generating oscillations readily on the desired crystal harmonic. This feedback control is preferably variable inasmuch as-more 'feedback is desired lat the higher order har- Multiple grid tubes may also be used with this same form of feedback, but it is desirable that the input capacity of the tube between grid and cathode be small, and since this is generally smaller in triodetubes than in multi-grid tubes, the V-triode usually gives improved performance. By `making use -of this invention, it is possible to obtain oscillation of the crystal on as high as the ninth and eleventh harmonic, and consequentlyjobtainultra-high frequency output directly from `the plate tank circuit of the single As an example-of the high frequency obtainable 'from such a combination-of harmonic vcrystal and the circuit-of this invention, it is possible touse a'harmonic crystal with a fundamental frequency of 10 mc. (which is very practical for manufacture) and obtain output of 70 mc. onthe or V mc.on the Other and further objects and features of this Vinvention will be apparent to those skilled in the art to which it relates from the following specication and the claims;

vInthe drawing, briefly, Fig. 1 shows a three electrode tube circuit which may be employed in accordance withthis invention; Fig. 2 illustrates a preferred form-of crystal Vcontrolled oscillation generator circuit for producing high'and `three element-vacuum tube 5 is -connected with its -grid electrode 8 to the crystal holder electrode 6a and the cathode 5 to the holder electrode 6. The piezo-electric crystal 2 in this case is preferably of the type described in Patent No. 2,7157,- 808 issued to me landthe type described in my application Serial No. 256,781, led Feb. 16, 1939,

Afor Piezo-electric crystal apparatus. A gri-d leak resistor 'I is connected between the grid and the cathode. The anode I is connected to the oscillatory circuit II consisting of the inductance I2 and the variable condenser I3. A source of anode current supply I4 shunted by a high frequency by-pass condenser I5 is connected to the oscillatory circuit II an-d to the cathode 9. This circuit is a triode crystal oscillator circuit with the plate tank circuit Il tuned to the harmonic on which output is desired. In this circuit the feedback for starting oscillations is only that in the tube itself. This is oftentimes not adequate for starting of oscillations on the high harmonics, such as the seventh and ninth, and additional feedback must be added. However care must be taken not to select a tube 4having high internal capacities because these will reduce the high or ultra-high frequency potentials to such an eX- tent that the circuit will be ineffective.

General practice in the art when attempting to increase the feedback for fundamental crystal oscillators is to insert a capacity Cp between the grid 8 and the plate I0 of the vacuum tube 5.

VA small additional capacity Cp may be added between the grid 8 andthe -anode IE! for the purpose of starting the circuit in oscillation under the control of the crystalV 2 and this capacity may be removedafter the generation of oscillations is started. Or this capacity may consist of a small variable condenser which may be increased toV start the oscillations and then decreased to increasevthe eiiiciency or effectiveness of the circuit.

In Figure 2 is shown a' form of feedback circuit which is very effective in causing the crystal to oscillate on the desired high odd Aharmonic. Here the electrode Gfof the crystal`2 is 'connected to the center tap of a capacity diquently they are connected between the tank circuit II and the cathode Sl of the tube 5.

The cathode in this case is grounded to the metal frame or cabinetfof the oscillator.V The element I in this case is preferably a choke coil although it may be a high impedance resistance. In either case it is a high impedanceV at the working frequencies. cuitv is such that when the plate Vtank circuit IIv is tuned away from any oddv harmonic of 'the crystal 2, the crystal willV oscillate on its fundamental frequency by virtue of the fact that it will oscillate as a Colpitts oscillator, the crystal essentially being `connected between the grid 8 and plate ID, and condenser I1 acting long as the plate tank circuit II Vis tuned to a 1,7

frequency remote from any of the odd harmonies of the crystal. As soon as the plate tank circuit tuning approaches the various odd harmonics, the crystal changes from a fundamental mode of oscillation to a harmonic mode, oscillating on the one vcorresponding to the plate The operation of the cirrgb As the plate tank vcircuit II is tuned,

oscillation will take place.

tank frequency. This is often accompanied by a slight shift in what would normally be the odd harmonic frequency if the multiplication took place in a tube multiplier. That is, the harmonic frequency when the crystal is oscillating as a harmonic oscillator is not always an exact multiple of the fundamental frequency.

The amount of feedback now maybe controlled by varying the condenser I1, and feedback can be increased to such a point where self- This condenser is not exceptionally critical, a value of mmf. being'a representative value for an ordinary high frequency triode tube. This, of course, depends on the type of tube employed, and cannot be definitely specified. It also depends on the order of harmonic used. Generally, the higher the harmonic on which it is desired to make the crystal oscillate, the smaller the capacity I'I will have to be; that is, the feedback must be increased. 1

The circuit shown in Fig. 3 operates' on somewhat the same principle as that shown in lig.l 2 and also may be used to obtain high harmonic output. Here the cathode S Yof the tube is at a radio frequency potential with respect to the metal frame or cabinet since the high impedance choke coil I8 is connected between the cathode 9 and the frame ground connection 22. The electrode 6 of the crystal 2 is also grounded at 22. In Fig. 2 the cathode- 9 is at ground potential, and one crystal terminal 6 is ata radio frequency potential determinedby the potential drop across condenser' I'I. This is the main difference between these two circuits, the performance otherwise lbeing lessentially the same. The crystal will again oscillate' on the fundamental as long as the plate tank'circuit tuning is remote from any of the odd harmonics as described in connection y'with Fig. I2.

Electric oscillation in" the circuit of Fig'. 3 takes place by virtue of the fact' that the circuit again takes the form of a Colpitts oscillator in which Ythe impedance of the plate tank Vcircuit II and bypass condenser I5i s small, at the fundamental frequency, and the crystal is essentially connected between the grid 8 and plate IQ, condenser I'Ia serving as the necessary capacity reactance inthe plate circuit 'for the condition of oscillation. The condenser ila is decreased to increase feedback and as was pointed out above more feedback is'usually necessary as the order of the harmonic is increased. The choke I8 is of a' high impedance and may consist of several spaced sections Vof honeycomb type windings. For purposes of analysis of the fundamental frequency, the circuit can be put in the form of Fig. 4 in which the crystal electrode 6 is connected to the plate II) through the impedance IIa of the tank circuit. This yim'- pedance IIa is assumed to be low at frequencies other than the harmonics of the crystal 2. Furthermorefor purposes of observing the' operation of the circuit shown in Fig. 3 the ground connection 22 is moved up to the cathode 9 as indicated at 2|, in Fig. 4. Thi's'leaves a capacitative reactance in the'plate circuit which is essential` for oscillation in this type ofV circuit; The choke I8 Vprovides a direct 'current path betweenA the source of current supply I4 and the cathode and isolates the pointr'a, that is, the cathode from point b, the negative terminal of the battery at radio frequencies.

It is not necessary in both these -circuits `that the crystal actually oscillate'at the fundamental frequency. However, it is a good criterion for adjusting the proper amount of feedback so that oscillations will take place at the harmonic frequencies of the crystal when the plate tank cir'- cuit is 'tuned to the various odd harmonics. AIt must be born in mind`here that in thepresent invention actually two different forms of oscillators are incorporated in one circuit,v and that although the circuit approaches Athe Colpitts form when the crystal is oscillating on its fun# damental, this is not the case when the crystal circuit functions as a harmonic oscillator. The most desirable application of the circuit is to produce ultra-high frequency crystal controlled oscillations. v

In cases where a crystal is used which is not adapted for harmonic oscillation, that is, for instance, when an ordinary X-cut crystal is used, the circuit will oscillate on the crystals fundamental frequency regardless of where the plate tank circuit istuned. It is then possible to tune the plate tank circuit to both the odd and even harmonics of the :crystal frequency and obtain harmonic output, but in this case the tube simply acts as a multiplier circuit and the crystal does not oscillate at its harmonic frequencies. The output then on these various harmonics is very materially reduced. This form of operation is entirely different than when a harmonic crystal is used, the crystal actually oscillating on its odd harmonics and giving output equivalent to the output which might be obtained if a fundamental crystal could be made at these ultra-high frequencies.

The vacuum tube may be of the multiple grid type. However, in the case of the three element tube it is very desirable to employ a vacuum tube having low internal capacities between the electrodes thereof and for this as well as other reasons type 955, SEG', 6J5G and RK34 tubes are preferred. In the case of three element vacuum tubes of the type illustrated in Figs. 1, 2, 3 and 4 the capacity Cg between the cathode and the grid should not exceed three or four micro-microfarads at the harmonic frequency on which the crystal is oscillating. A tube should be selected in which the grid to cathode capacity is as small as possible. It is true that in the circuit shown in Fig. 1 the capacity between the grid and anode is employed for feedback coupling and is desirable especially on the high harmonics. However a limited capacity can be added between the grid and the anode externally of the tube as shown in Fig. 1 and this capacity should preferably be variable. Adding capacity between the grid and anode is recommended only in the case of Fig. 1 when a tube in which this capacity is exceedingly small is selected. 'I'he circuit arrangements shown in Figs. 2, 3 and 4 do not require the addition of this capacity directly between the grid and anode and for this among other reasons these circuits are preferred. In the case of all of the circuits described it is Very desirable to maintain the capacity between the cathode and the grid at a very low value. This is an important consideration when high and ultra high frequency electrical oscillations are produced in the crystal because the capacitative reactance of even small capacitiesis relatively small at these frequencies and if a substantial capacity exists between the cathode and the grid a relatively large radio frequency current may fiow through this capacity causing an additional load to be placed upon the crystal and thereby reducing the radio` frequency potential of the crystal to va point that itis insufiilcient properly to -drivethe vacuum tube at the desiredharmonic frequency of the crystal. Y

' Theoscillatory circuit I'I is adjusted substantiallyf t'the harmonic frequency* of the crystal desired either byvarying the inductance I2 or the capacity I3until this'circuit is resonant substantially to the harmonic'frequency desired. In the case of the circuit shownin Figs. 1, 2, 3 and 4 the ratio of the capacity tothe inductance in the circuit II should be high for proper operationof the crystal on a harmonic.

In Vprac`ti `:e it may -b'e desirable to employ a variable condenser for thecapacitor I5 Figs. 1, 2 and 3 and this may be done without diiculty. Furthermore it may b e desirable to adjust the condensers v I3 and I1 Fig. 2 with a single control and this may be accomplished by using Various mechanical linkages; lThe condensers I3 and I'Ialig. 3 also may be controlled together. It is also obvious lthat theI electron discharge devices, audions or thermionic tubes 5 shown in Figs. l. 2, 3 and 4 'may be provided with indirectlyheated cathodes. f

It will be observed v'that I have described various features of an embodiment of my invention in detail, however, I dov not desire to limit my invention to the exact details set forth except insofar as those detailsmay be defined by the claims. n j f What vI, claim" is as fol1ows:`Y

1.'Piezo`electric crystal oscillation generator apparatus, comprising: an electron discharge device having cathode, grid and plate, a piezo-electric crystal adapted to oscillate on its odd harmonics, holder electrodes for said piezo-electric crystal, one of said electrodes connected to said grid, an output circuit including a tunable circuit having one terminal connected to said plate, said output circuit being adjusted to correspond substantially to the frequency of an odd harmonic of said piezo-electric crystal, a capacitative-voltage-divider regeneration control connected between the other terminal of said tunable circuit and said cathode for maintaining said plate circuit predominately capacitatively reactive, and means for connecting the other electrode of said piezo-electric crystal to a point on said voltage divided so that a substantially high impedance at the generated frequency is connected between said other crystal electrode and said cathode.

2. Piezo-electric crystal oscillation generator apparatus, comprising: an electron discharge device having cathode, grid and plate, a piezoelectric crystal adapted to oscillate on its fundamental frequency or odd harmonics thereof, a tunable circuit having one terminal connected to said plate, said piezo-electric crystal having an electrode connected to said grid, a capacitative-voltage-divider regeneration control connected between the other terminal of said tunable circuit an-d said cathode for maintaining said plate circuit predominately capacitatively reactive, and means for connecting the other electrode of said piezo-electric crystal to a point on said voltage divider having a relatively high radio frequency potential corresponding to the generated frequency.

3. Piezo-electric crystal oscillation generator apparatus in which energy outputs corresponding to odd harmonic frequencies of the piezo-electric crystal are obtainable which correspond substantially to energy outputs obtainable when fundamental frequency, comprising: an electron discharge device having cathode, grid and plate electrodes, a piezo-electric crystal having an electrode connected to said grid, an oscillatory circuit connected at oneterminal to -said plate, capacitativewoltage-divider regeneration control means comprising at least one condenser connected between said oscillatory circuit and said cathode, means for grounding sai-d cathode at generated frequencies, and connections-for-connecting another electrode of said piezo-electric crystal to an intermediate point on said capacitatiVe-voltage-divider regeneration control of said piezo-electric crystal, said one condenser having a substantial impedance at the generated frequency. Y

4. Piezo-electric crystal oscillation generator apparatus, comprising: anA electron discharge device having cathode,` grid and plate, a piezoelectric crystal adapted to oscillate on its fundamental frequency or harmonics thereof, a tunable circuit having one terminal connected to said plate, said piezo-electric crystal having one electrode connected to said grid, capacitative divider means including a pair of condensers in series connected between said cathode and the other terminal of said tunable circuit, and connections for connecting the other electrode of said piezo-electric crystal to said capacitative divider means between said series connected.v condensers, the one ofsaid pair of condensers connected between said cathode and said piezothe' piezo-electric crystal is oscillating on its electric crystal having a substantial impedance to the generated frequency for applying some of the' generated oscillationsv to said crystal so that said piezo-electric crystal will oscillate on high order odd harmonic frequencies when said tunable circuit is tuned to one of said high order odd harmonic frequencies or onits fundamental frequency when said tunable circuit is tuned between odd harmonic frequencies of said piezo-` electric crystal. l.

5. Piezo-electric crystal oscillation generator. apparatus, comprising: an electron discharge device havingcathode, grid and plate electrodes, a tunable output circuit connected to said plate electrode, a piezo-electric crystal adapted to oscillate on its harmonics having one electrode connected. to said grid electrode, means for tuning said tunable output circuit to one of the harmonics of said crystal, and capacitative divider means including av pair of condense'rs connected in series, means for connecting said capacitative divider means between said tunable output circuit and said cathode, said cathode being grounded, and means for connecting the other electrodeof said crystal to said capacitative divider between said condensers, said condenser connected between said cathode and said last mentionedcrystal electrode having an 4iinpedance sufficiently large to impress a regenerative potential on said crystal at the generated frequency. i

K JOHN M. WOLFSKILL.

Images