US2369954A

US2369954A - Crystal oscillator circuit

Info

Publication number: US2369954A
Application number: US475732A
Authority: US
Inventors: Reginald L Downey
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1943-02-13
Filing date: 1943-02-13
Publication date: 1945-02-20
Anticipated expiration: 1962-02-20

Description

F eb.

- Fig.5.

1945- R. DOWNEY 2,369,954

CRYSTAL OSC ILLATOR C I RCUIT Filed Feb. 13, 1943 NoN-osc|LLAme OSCILLATING REGION REGION I G J 5 D a Zg s'mnrms U k REGION H d C e a L 5 a m I! a u w a O z Fig.2.

CONTROL ELECTRODQ CURRENT Ld (D E c g E 2, a FREQUENCY 5 s O i! a Inventory Regmald L.Downey,

Aral/ewe) HI Patented Feb. 20, 1945 CRYSTAL OSCILLATOR CIRCUIT Reginald. L. Downey, Scotia, N.: assiznorto General: Electric Company, a corporation of New York Application February 13, 1943, SerlalNo. 475,732.

3v Claims.

My invention relates to oscillator circuitsv employing piezoelectric devices.

In oscillator circuits employing piezoelectric devices and especially inv circuits employing an electron discharge device having a tuned anode circuit and having a crystal connected between the control electrode and cathode, it has been necessary to tune th anode circuit appreciably on the high frequency side of the operating frequency of the piezoelectric device in order .to obtain proper conditions for quick and stable starting of the oscillator. After oscillations have been started, the anode tuned circuit has been retuned for higher output. In such a case, however, if

oscillations are halted for any reasons whatso- 1 ever, oscillations may not start again unless the circuit is again tuned to. the high frequency side of the operating frequency. For high frequency crystals, such as those having a natural frequency in the range of 1,000 to 10,000 kilocycles, the optimum operation of the oscillator is usually obtained when the anode oscillatory circuit is tuned to about 102 per cent to 110' per cent of the crystal frequency, depending on the output, frequency stability and tube current conditions. For low frequency crystals, as for example, in the range ing of oscillations even though the anode tuned circuit is tuned to a frequency providing optimum operating conditions.

In accordance with my invention there is provided in combination with a piezoelectric or crystal oscillator having a tuned anode circuit, automatic means dependent upon the oscillator output for tuning the circuit between resonance at a frequency at which the crystal oscillates readily and at another frequency at which the desired output is obtained depending upon whether the oscillator is in non-oscillating or oscillating condition. In other words, the anode oscillatory circuit is chosen toibe resonant at a frequency different from and preferably higher than the frequency of the crystal and also higher than the frequency at which optimum operation is had after starting is effected but at which frequency the generation of oscillations readily starts. Means is also provided which is responsive to the starting of oscillations for automatically tuning the oscillatory circuit to a resonant, frequency which more closely approaches the frequency at which optimum operation is had. Thus in the absense of control electrode bias the anode circuit tuning is increased progressively farther on the high frequency side toward an optimum starting condition. This means may comprise a reactance device arranged to tune the anode circuit oi a crystal oscillator. The reactance device is controlled from the gridcurren-t ofthe crystal oscillator such a manner that anode tank circuit conditions are optimum for starting when the crystal is not oscillating and: optimum to. produce maximum output voltage when the crystal is oscillating.

The features of my invention which I believe to'be novel are set forth with particularity in'the appended claims. its organization and manner of operation, to: gether with further objects and advantages thereof may best be understood by reference to thefole lowing description taken in connection with the. accompanying drawing in which Fig. 1 .i'sla schematic. diagram of an oscillator circuit. embodying the principles of my invention, and. Figs. 2. and .3. illustrate operating characteristics or the circuit shown in Fig. 1.

In Fig. 1 there is shown an electron discharge oscillator circuit l0 comprising-an electron discharge device ll havin an anode, 11, a. control electrode 13, and a cathode Hand having a crystal connected between arid, itemdeathude 1 land a tuned circuit connected between the. anode l2 and the cathode M. The oscillator control electrode is biased by eans o r sistors 18: and '20. A n d n d ir it- 5 comprising. an oscillatory circuit including an inductance 16 and :a variable capacitor I1 is; connected between: the. anode, l2 and the cathode l4. through-a source of potential 40 and ground. oscillatory circuit 1515 completed from the anode. l2. to the. cathode: for radio frequency currents .by means of. .a. suitable kbypass condenser 2|. Radio frequency in the grid circuit is bypassed around the resistor ill by a suitable condenser 22. The cathode is grounded.

- In Fig. '2 there are illustrated typical characteristics of the. oscillatorcircuitshown-inFig. 1. In

Fig. 2a, variations of anode current of the elec- My invention itself, both. as to decreased from a maximum, as by varying the variable capacitor l1, oscillations begin at point a and the anode current follows the solid line to the point D where oscillations cease. The anode current rapidly rises to c where the current remains constant at lower frequencies as indicated by the line cd. If the condenser is then tuned from the low frequency end toward the high frequency end, oscillations will not begin again until the point e is reached, at which point the anode current will suddenly drop to J on the original curve. If the tuning is continued in the high frequency direction, the anode current will follow the curve toward a. If, however, the tuning is reversed so that the condenser is tuned in the direction of 15 lower frequency, the anode current goes back along the curve toward b. The area defined by the letters I), c, e, f, is a region of instability. In

thi range of frequencies, the crystal oscillator will not begin to oscillate if the resonant frequency of the tuned circuit is lower than the frequency represented by the letter f on the curve of I 2c although oscillations continue, if once started. Therefore, if it is desired to assurecertain starting, the oscillatory circuit must be tunable to a point between a and f on the anode current characteristic. Starting is progressively better as the frequency is increased so that the optimum starting point is well out toward the high frequency end of the curve, as at the point 9,

for example.

On the other hand, it is desired to operate the oscillator circuit at a frequency at which maximum output is obtained. In Fig. 2c there is illustrated the variation in oscillator output, plotted as ordinates, against frequency, plotted as abscissa. It is to be noted that the frequencyjs plotted on the same scale in each of Figs. 2a and 20, so that direct comparison can be made between these figures. shows that while starting conditions are optimum at a point such as g in Fig. 2a, at that frequency the output is very low. For maximum output it will be seen that a frequencycorresponding to the point It is desirable. It isto be noted that this 45 point is in the unstable zone where starting is uncertain.

-Means has been provided for automatically shifting thetuning of .the oscillatory circuit between-frequencies at which optimum starting conditions prevail and at which optimum operating conditions are had.

Connected across the anode tuned. circuit is a form of inductance type reactance tube circuit including a phase shifting circuit comprising the series connected inductance and the resistance 26. There is also shown a blocking capacitor 2! in the series circuit including inductance 21 having acapacity such that it has low impedance to the oscillations produced. Across the phase shifting circuit there is disposed an electron discharge device preferably of the pentode type comprising an anode 3|, a cathode 32,

control electrodes

33, 34 and 35. The anode 3i is connected to the pper end of the inductance 25, the control electrode 33. is .connected to a point between the capacitor 21 and the resistor 28, the electrode 34 is connected to the positiveterminal of a suitable source of potential 36, the electrode is connected to the cathode and the cathode 32 is connected to ground through the biasing. resistor 31 shunted by the capacitor 38. The lowerend of the resistor 28 is connected to ground for alternating. currents by means of a suitable condenserdl.

5 age of the tuned circuit l5.

Comparison of Figs. 2a and 2c The reactance of the winding 25 and the resistance of the resistor 26 are chosen so that the radio frequency voltage on the electrode 33 is approximately 90 out of phase with the output volt- Under such conditions, the discharge device 30 draws current from the oscillatory circuit which is 90 lagging, thereby giving the effect of an inductance of approximate value L 1 Rgm where L0 is the effective inductance of the reactance circuit, L is the inductance of the coil 25, R is the resistance of the resistor 26 and gm is the mutual conductance of the electron discharge device 30, i. e'., the rate of change of plate current with respect to changes in control electrode voltage.

With the circuit as shown in Fig. 1, if oscillations cease, as when the transmission period for the transmitter with which the oscillator is associated is ended, the oscillator output ceases, resulting in minimum bias of the control electrode 25 33 of the electron discharge device 30. Therefore,

the gm of the discharge device increases and L0 is decreased. Therefore, the effective reactance of the tuned oscillatory circuit i5 is decreased and the resonant frequency of the tuned circuit is increased under the assumed conditions. The variation of resonant frequency of the tuned circuit with the voltage on control electrode 33 is shown in Fig. 3 in which frequency is plotted along the abscissa (to the same scale as in Fig. 2) and the 35 control electrode voltage is plotted along the ordinates. Accordingly, the oscillator is automatically tuned to a frequency such as represented by the point gin Figs. 2 and 3.

After oscillations start, as when the oscillator is again switched on, the reverse action takes place. The eifective inductance is increased so that the reactance of the tuned circuit is decreased and accordingly the resonant frequency of the anode circuitis changed to correspond with the point It, for example. In other words, the current in discharge device 30 i phased to -simulate a reactance of sign and. magnitude to effect the desired operation.

The operating and starting points forthe oscillator circuit depend upon the bias of the electron discharge device 30 including

the'resistances

18 and 31. In order to provide control and enable the operator to select the operating and starting frequencies, one of these resistances is made 5 variable. Accordingly, the resistor l8'is of the conditions is relatively wide as seen in Fig. 2a.

In Fig. 2b there is plotted control electrode current for the tube I as ordinates, against frequency of the tuned anode circuit l5. Maximum current occurs near the operating frequency of the crystal l 9.

The inductance values of the windings l6 and 25 are made as nearly equal as practicable but large enough to prevent undue loadingof the output circuit by the resistanceii fi. .Use of the 7-5; resistor 31 and condenser 38,.in 'theLcathode' clr'-.

cuit of the device 30 stabilizes operation and prevents excessive current being drawn by the discharge device 30. The capacitor'M not only bypasses the radio frequency current to ground but ma also be proportioned to effect a time delay in the operation of the circuit if such is desired.

Tubes of relatively high gm are necessary for the reactance tube or electron discharge device 38. It has been found that four type 1852 tubes connected in parallel work satisfactorily and with such an arrangement it was possible to operate a crystal ground to oscillate at 100 kilocycles at the point of maximum grid current and even on the low frequency side of this point at approximately maximum output and still achieve instantaneous starting.

A. capacity-resistance type phase sp itting circuit may be employed, if desired, instead of the inductance-resistance type circuit.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention in its broader aspects, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. The combination, in a crystal oscillator, of an electron, discharge device having an anode, a cathode, and a control electrode, a piezoelectric crystal connected between said control electrode and cathode, an oscillatory circuit connected between said anode and cathode, said oscillatory circuit being resonant at a frequency higher than the frequency of said crystal and higher than the frequency at which optimum operation is had after starting is effected but at which frequency the generation of oscillations readily starts, and means responsive to oscillations in said oscillatory circuit for lowering the resonant frequency of said circuit to a frequency substantially more closely approaching said frequency at which optimum operation is had.

2. The combination, in a crystal oscillator, of an electron discharge device having an anode, a cathode, and a control electrode, a crystal connected between said control electrode and cathode, an oscillatory circuit connected between said anode and said cathode, said oscillatory circuit being tuned to a frequency at which the generation of oscillations under the control of said crystal readily starts but different from that at which optimum operation of said oscillator i had after the generation of oscillations starts, and means responsive to the generated oscillations to maintain said circuit-tuned. to a frequency substantially more closely approximating said frequency at which optimum operation is had and upon cessation of oscillation to retune said circuit to said frequency at which oscillations more readily start, whereby both ready starting and optimum operation thereafter is had.

3. The combination, in a crystal oscillator, of an electron discharge device having an anode, a cathode, and a control electrode, a crystal con,- nected between said control electrode and cathode, an oscillatory circuit connected between said anode andsaid cathode, said oscillatory circuit being tuned to a frequency at which the generation of oscillations under the control of said crystal readily starts but different from that at which optimum operation of said oscillator is had after the generation of oscillations starts, and means device to simulate a reactance of sign and magnitude to maintain said oscillatory circuit tuned to said last mentioned frequency.

4. In combination, an oscillator of the type having an electron discharge device having an anode, a cathode and a control electrode, a piezoelectric device connected between the control electrode and the cathode, and an oscillatory circuit connected between the anode and cathode, and means dependent upon the output of said oscillator for automatically tuning said circuit between resonance at a frequency at which said device readily begins to oscillate and at another frequency at which a desired output is obtained according to whether said piezoelectric device is in non-oscillating or oscillating condition, respectively. i

5. In combination, an oscillator of the type having an electron discharge device having an anode, a cathode and a control electrode, a piezo-' electric device connected between the control electrode and the cathode, and an oscillatory circuit connected between the anode and cathode, and means in shunt to said oscillatory circuit for varying the effective reactance of said circuit in accordance with the output of said oscillator thereby to vary the resonant frequency thereof between frequencies at which said device readily begins to oscillate and at which a desired output is obtained, according to whether said piezoelectric device is in non-oscillating or oscillating condition.

6. In combination, an oscillator of the type having an electron discharge device having an anode, a cathode and a control electrode, a piezoelectric device connected between the control electrode and the cathode, and an oscillatory circuit connected between the anode and cathode, and a phase shifting circuit in shunt with said oscillatory circuit and operative in accordance with the output of said oscillator to tune said oscillatory circuit between resonance at a frequency at which said device readily begins to oscillate and at another frequency at which a desired output is obtained according to whether said piezoelectric device is in a non-oscillating or an oscillating condition, respectively.

7. In combination, an oscillator of the type having an electron discharge .device having an anode, a cathode and a control electrode, a piezoelectric device connected between the control electrode and the cathode, and an oscillatory circuit connected between the anode and cathode,

and means dependent upon the output of said oscillator for automatically tuning said circuit between resonance at a frequency at which said device readily begins to oscillate and at another frequency at which a desired output is obtained according to whether said piezoelectric device is in non-oscillating or oscillating condition, respectively, and means for adjusting the. point of operation for the desired output of said oscillator.

8. In combination, an oscillator of the type having an electron discharge device including an anode, a cathode and a control electrode, a piezoelectric device connected between the control electrode and the cathode, resistance means in shunt with said piezoelectric device for providing bias for the'control electrode of said discharge device, an oscillatory circuit connected between the anode and cathode of said electron discharge device, and means dependent upon the output of said oscillator for automatically tuning said circuit between resonance at a frequency at which said device will readily begin to oscillate and another frequency at which substantially maximum output is obtained according to