US1975603A - Crystal controlled oscillator circuits - Google Patents

Description

Oct. 2, 1934. c. W. HANSELL 1,975,603

CRYSTAL CONTROLLED OSCILLATOR CIRCUITS Filed March 14, 1932 'INVENTOR CLARENCE W. HANSELL ATTORNEY Patented Got. 2, 1934 g V CRYSTAL CONTROLLED OSCILLATOR OIECUETS Clarence W. Hans-ell, Port JeffersornN. Y., as- Signor to Radio Corporation of America, a corporation of Delaware Application March 14, 1932, Serial No. 598,566

Claims. (Cl. 250-36) In Fig. 3 of Cady United States Patent the tube which is'often suflicient to set up spu- 1, i72,583 there is described an efiicient crystal rious oscillations independent of the crystal in controlled oscillator.. However, when this cir- Cadys original circuit, particularly in oscillators cult is used it is found that the crystal does not designed to work at rather high frequencies.

5 have the maximum possible effect in controlling By inserting the low impedance between grid 60 the frequency of the oscillations and there is an and filament of the tube I greatly reduce the voltundesirable tendency for spurious oscillations to age which can be set up on the grid, by a given develop, the frequencies of which are not convalue of alternating current voltage on the anode, trolled by the crystal. through any undesired coupling such as that In effect the crystal in the circuit shown in through the anode to grid dielectric capacity. As 65 Cadys Fig. 3 is a narrow band pass filter, through a result the probability of spurious oscillations which the energyfed back to the grid to sustain taking place, which are not controlled by the crysoscillation-s must pass, and it is intended that tal, is very greatly reduced. I sufficient energy be fed back to sustain oscilla- At the same time the A. C. voltage which can tion only at frequencies near the resonant fre be transferred from anode to grid through the 70 quency of the crystal. Since, in Cadys circuit, dielectric capacity between the crystal electrodes the grid of the tube and the grid leak resistance is also greatly reduced and spurious oscillations have inherently a very high impedance, the crysfrom this source of coupling are also prevented. tal can not effectively reduce the feed back to If the desired feed-back through the crystal is the grid at undesired frequencies unless these considered it will be noted thatthe effect of this 75 frequencies are rather far removed from the resfeed back is also reduced sothat somewhat greatonant frequency of the crystal. The crystal acts or coupling between coils 18 and 2 2 in Fig. l is like a series tuned electrical circuit, shunted by required. However, the reduction in feedback the dielectric capacity between the electrodes, through the crystal is much less for the resonant ,25 and so long :as .the grid icircuit impedance is high frequency than for frequencies somewhat off res- 0 the resonance curve of'frequency versus feedonance'for the crystal. Consequently, by propback to the'grid is very much broader than the erly adjusting the feedback coupling I obtain a resonance curve of the crystal itself and the condition where suflicient coupling to maintain crystal cannot prevent frequency variations over oscillations is obtained only for frequencies very a relatively broad band. near the resonance frequency. In this case the As I make, according to my present invention, crystal becomes a more controlling factor in the the grid to cathode impedance low so that the circuit and therefore, much greater frequency stacrystal supplies energy to arelatively low 'imbility is obtained. V pedance circuit, the frequency band in which I have found that crystals vary greatly in their 35 there issufficientfeed-backto the grid to susconstants not only at different frequencies but 9 4 No. 540,310, filed May 27, 1931.

' ,tain oscillations Will be greatly reduced and the also for crystals ground to operate at the same crystal will control the frequency .of oscillations frequency. A representative crystal ground for much more closely. Thisactionis more fully 10o kilocycle'sand'mountedin its holder may be explained in my patentapplication, application ,an effective series resistance on theorder of 100 I ohms and effective equivalent inductive and 'ca- Accordingly, to ma, e the system more immune ,pacitive reactances of several megol ms'., In such from spurious oscillationsandtomake the crystal a case the-effective impedance ,added'betweengrid more effective in controlling the frequency of the and filament 'of the tube should also be on the oscillator.aretheprincipal objects of my present order of magnitude of ohms. Ilowever, due invention. Toaccomplish this I place a relativeto the'great' variations ,b tween crystals -I prefer 100 ly .low alternating current impedance between to make .the impedance, as well as the feedback the grid and filament of the vacuum'tube. 'This coupling yariableflso that adjustments may be low impedance, which, is preferably a relatively madejto suit individual crystals. Accordingly a .sma'll'resistance with'fa series'connected direct further objectof my invention is to permit ad- 'current blockinghcondenser as indicated by 30a ju'stments for optimum operation by making the EandBZ in Fig. 1, discourages spuriousgoscillations impedance variable. l j which might take place by virtueof anyunde Myinvention is more fully described in convsired coupling from theanodje to grid'circuit's of 'nection with the accompanying drawing w-herein; .th'd'tube. "For example, there will be feedback .Fig. 1 illustrates apreferredjform of my pres- I 55 through ithe plateto grid dielectf'riccapacity of "exitinvention; and 4 Fig. 2 illustrates a somewhat modified form utilizing a screen grid tube;

Turning to Fig. 1, I have indicated an electron discharge device or vacuum tube 2 having a cathode 4 energized through transformer 6 with alternating heating currents. By means of the condenser network 7 and ground connection 8, symmetry of the heating circuit is insured, eliminating to a desired degree, possible modulation which would otherwise occur due to the alternating E. M. F. applied to the filament heating circuit. Preferably the tube should be one designed espe cially for alternating filament heating current. Also direct heating current may be used. Between the cathode 4 and grid or control electrode 10 there is connected a piezo-electric frequency controlling crystal 12 as well as a parallel tuned circuit 16.

As in the Cady circuit referred to, some feed back occurs through the inter-element capacity of the tube 2 between plate or anode 14 thereof and the grid 10 as well as through the feed back or tickler coil 18. It is preferable that electrostatic shielding 20 be provided between the coil 18 and the inductance coil 22 of tunable circuit 14 as well as providing for the grounding of the direct current end of the tickler coil 18 by means of condenser 24. This reduces undesired capacity coupling which might cause spurious oscillations. Grid bias, is provided by the grid leak resistor 26 eifectively in shunt with the capacity -formed by the electrodes of piezo-electric frequency controlling crystal 12.

As so far described, the system, in general, may be made to act as a reasonably good crystal controlled oscillator. However, at certain frequencies, there will be a tendency to generate spurious oscillations determined by the electrical constantsof the circuit, the crystal being considered simply as a di-electric rather than a piezo-electric crystal. To eliminate this tendency to oscillate at spurious frequencies I connect across the grid and cathode circuits at points A and B a low impedance which may be in the form of a resistor 30a. in series with a direct current blocking condenser 32.

Now, should there be any tendency for the circuit to oscillate at some parasitic or spurious frequency, those undesired frequencies will be bypassed by the low impedance between the points A and B, and no energy of spurious frequencies will occur in the output circuit so designated on the drawing. However, at the natural frequency of the crystal, the crystal acts as a series tuned circuit, thereby presenting a path of low impedance relative tothat between the points A and B and thereby causing relatively large feed back to the grid 10 from anode 14 through the coupling between coils 18 and 22. This feed back will cause building up of voltages of desired frequency across the grid and filament and thereby cause energy of desired frequency to appear in the output circuit.

The purpose of the condenser in the series combination of low resistor 30a and condenser 32 connected across points A and B given in Fig. 1 is to prevent a reduction in the direct current impedance of the grid leak resistor. The grid leak resistor may be replaced by the series combination of a source of potential and an impedance to prevent the flow, of alternating current therein the impedance being, for example, a choke coil or resistor. I

Fig. 2 illustrates a modification wherein the plate circuit coil 18 is tuned by means of a variable condenser 40 and wherein the crystal 12 is placed on the otherwise grounded side of the parallel tuned circuit 14. To prevent interelectrode feed back, tube 2 is chosen of the screen grid type having a screen grid 42. For better frequency stability and as an added refinement, the cathode 4 may be energized from a direct current potential circuit diagrammatically indicated at 44 though tubes are now available which equally well, for this purpose, allow of the use of alternating heating current. Condensers 24 and 25 of Fig. 2 are, of course, the usual by-pass radio frequency condensers. Also in Fig. 2, shielding 20 is preferably used to prevent capacitive feedback from anode to grid circuits and may be connected to the screen grid 42 through the medium of by-passing condenser 43. It will be found in the circuits shown in Figs. 1 and 2, that by suitable adjustment of the tuning of the circuits, feed back through the inductive coupling may be in either the normal direction or it maybe reversed so as to ordinarily be degenerative and still the circuit will operate under crystal control. This may be explained by virtue of the fact that despite the degenerative feed back, suitable tuning of the grid circuit, or of anode and grid circuits will alter the phase of the voltages fed to the control grid 10 of tube 2 of Fig. 2, such that they are sufiiciently near the correct phase to produce oscillations at a frequency near the resonant frequency of crystal 12. However the frequencies generated with the feed back in one direction or the other will ordinarily be slightly different and usually on opposite sides of the exact crystal resonant frequency.

Here also in Fig. 2, the impedance formed by resistor 30 and large by-pass condenser 32 is low for undesired frequencies. However, for energy of a frequency corresponding to that of the crystal 12, the crystal 12 also acts as a circuit of low impedance and passes current from the parallel tuned circuit 14 at the desired frequency. Consequently, suiiicient voltage of the desired frequency is built up on the grid 10 of tube 2 so as to produce in the output leads 50 oscillations only of a frequency corresponding closely to that of a crystal.

The condenser 52 is merely a blocking condenser which allows passage of the high frequency energy generated but prevents passage of the direct current voltage applied to the plate. or anode 14.

In Fig. 2 where the crystal 12 has been placed between ground and one terminal of parallel tuned circuit 14 there are obviously some advantages in the mechanical construction of the oscillating system described.

Resistor 50 is the usual screen grid biasing resistor, and, choke 52 and source 54 maintain suitable control grid bias on grid 10.

It is to be noted, adverting to Fig. 1, that resistor or impedance 30a has been made variable so that as already pointed out, it may serve for inc crystals having different effective series resistances.

Having thus described my invention, what I claim is:

1. A vacuum tube oscillation generator comprising a vacuum tube having a filament, a grid and a plate, a piezo-electriccrystalin series with only oscillations of a frequency corresponding to the natural frequency of said piezo-electric are generated by said electron discharge device.

2. A crystal controlled oscillation generator comprising an electron discharge device having an anode, a cathode and a control electrode, the series combination of an electro-mechanical vibrator of relatively constant frequency and a high impedance, connected between said control electrode and cathode, a relatively low resistance, of the order of magnitude of the series resistance of the crystal at resonance, connected effectively across said grid and cathode, and means for establishing feedback between the anode and control electrode of said device whereby oscillations corresponding to the frequency of said electromechanical vibrator are generated by said device.

3. In apparatus of the character described, a vacuum tube having an anode, a cathode and a control electrode, a parallel tuned circuit and a piezo-electric crystal connected in series, the said series combination being connected between said control electrode and cathode, biasing means connected across said control electrode and cathode, a relatively low resistance effectively connected across said cathode and control electrode, said relatively low resistance being of the order of magnitude of the series resistance of the crystal at resonance, and, means for establishing feedback between the control electrode cathode circuit and said anode cathode circuit of said elec- "tron discharge device whereby oscillations of a frequency corresponding to that of said piezoielectric crystal are generated by said electron discharge device.

4. Apparatus as claimed in claim 3 wherein means are provided to prevent inter-electrode feedback between the anode and control grid of said electron discharge device, and, wherein means are provided for establishing substantially pure inductive feedback between the anode circuit of said device and the control electrode circuit of said device.

5. A vacuum tube oscillation generator comprising a vacuum tube having a filament, a grid and a plate, a piezo-electric crystal in series with a parallel tunable circuit connected between said grid and said filament, and, a relatively low resistance, ofthe order of magnitude of 100 ohms, connected effectively across said grid and filament whereby only oscillations of a frequency corresponding to the natural frequency of said piezo-electric are generated by said electron discharge device.

CLARENCE W. HANSELL.

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