US2373223A - Crystal-controlled relaxation oscillator - Google Patents

Description

April l0, 1945.,I A.l M. BRAATEN 4-CRYSTL CONTROLLED RELAXATION OSCILLATOR Filed Jan. 27, 143

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:Ni/wrok ARTHUR M. B19/1A TEN Y A TTOR NE Y Patented Apr. 10, 1945l cRYsrrAL-CONTROLLED RELAXATION oscILLA'roR VArthur M. Beraten, Riverhead, N. assigner to Radio Corporation of America, avcorporation of Delaware A l ApplicationJanuaryfZZ, 1943, Serial No.y 473,671 1 claim. (c1. 25o-36)V This invention relates to crystal-'controlledrelaxation oscillators.

I am well aware that piezo-electric elements have been used in the past for controlling the l frequency of various forms of oscillation genertion to provide an improved crystal-controlled relaxation' oscillator having circuit arrangements such that the yfrequency shallbe stabilized in a j high degree. u

It is another object of my invention to provide a single tube generator which may be used under certain conditions as a substitute for the well-known doubletube multivibrator circuit.

In my Patent No. 2,070,647, a multivibrator cir-v cuitwas sho-wn which accomplished results similar to those for which the instant invention is intended. As will be developed in the description to follow, it will be seen that the present invention offers many advantages over the system of the well-known multivibrator."

In some respects, furthermore, the present invention is an improvement upon relaxation oscil- Alaters ,of the tetrode. type such as illustratedby my Patent No.r 2,058,559. p 'The basic principle involved in the present circuit arrangements is one Where oscillations are produced by operating the tube in such mane ner that it exhibits a negative transconductance characteristic. This is done without the aid of secondary emission. An oscillation generator is, therefore, produced having more reliable operating characteristics than was heretofore possible. 4 l i 'My invention will lnowbe described in more detail, reference being made to the accompanying drawing, in which:

Fig. 1 shows diagrammatically a. circuit arrangement which may be operated in either of two `or more different ways depending upon the setting of double-throw switches as shown; Fig. 2 shows a modication of the circuit ar rangement of Fig. 1, a feature being the coupling arrangements for the output circuit; and

Fig. 3 showsstill another modification of the invention inv whichthe crystal control is applied between the cathode and the grid nearest the anode, and the 'output circuit is inductively` coupled to the 'anode circuit.

Referring first to Fig. 1, I show an electron discharge tube V having a cathode K, an anode A,.and three grids I, 2. and 3. A potentiometer disconnected across the terminals of the direct current source 5. The rst grid I is negatively place making grid 3 still less positive.

biased with respect to the cathode K by means of the battery t. The grid 2 is maintained at the most positive potential, being connected to the positive terminal ofthe source 5 through the resistor 8. The anode A is maintained at a less positive potential, connection being made to a suitable point on the potentiometer 4. The grid 3 is connected to the cathode K through a resistor ii and the battery 'I which maintains this grid at a negative potential.

A piezo-electric element I0 is arranged to con# nect in shunt with resistor 9, or, alternatively. between grids 2 and 3. These alternative con-l `nectionsare rendered possible by the use of a capacitors I3.' The single-pole double-throw switch i4, has its .movable contact member grounded and is provided for 'grounding' either the cathode or the second grid 2 according to the external circuit requirements. AThe operation `of the generator as shown in Fig. 1 is as follows: A Assume for the moment that the piezo-electric element I0 is omitted from the circuit. Grid 3 is used as the control electrode. Ii it is made negative, part of the electron stream between cathode and anode will be repelled, causing a decrease in the anode current.` The repelledA electrons are attached to the positively charged grid 2, and this results in an increase in the current drawn by grid 2. The resulting eect isa negative transconductance characteristic between the grids and 2, and the circuit of grid 2 exhibitsa negative resistance. t f Assume now that, due to some disturbance, there is a minute decrease in the voltage of grid 3. This causes an increase in the current in the circuit .of grid 2 with a resulting increase in poten.A tialdrop across resistor 8. The. potential of grid 2 is thus decreased somewhat. This change of potential acting through con-denser I2 causes a decrease in the potential of grid 43. A corresponding decrease of potential of grid 2 then takes This cycle of events continues until the potential of the grid 3 becomes so low that the cut-olf point is reached. Thus all the electrons from the cathode are absorbed by grid 2 and no current reaches the anode A. Any further decrease of potential of grid 3 has no effect on the electron stream within the tube. Condenser` I2 now begins to discharge through resistors 8 and 9. During the discharge period, the potential on Vgrid 3 increases. The cycle of events described is now repeated but in the opposite direction. Grid 3 becomes more and more positive until saturation is reached and no further increase in anode current canv take place. The current drawn by grid 2 is then reduced to a minimum.` The condenser I2 will now discharge in the opposite Adirection and the cycle of events will be repeated. The alternate discharges of the condenser will continue as long as energy is supplied from the sources 5, 5, and 1. A series of aperiodic pulses of potential occurring in cyclic order will thus be obtained, the frequency of recurrence of the relaxations being determined by the values of the capacitor I 2v harmonic relation may be maintained between the control frequency and the frequency of the generator. In any event the system can be locked in step with the applied voltage and the generated frequency will be dependably controlled thereby.

The function of the piezo-electric crystal I0 is, therefore, to apply the necessary stabilizing force for controlling the frequency of the generator. When the crystal I0 is connected across theresistor 9, an alternating voltage corresponding to the potential of grid 2 is applied to the crystal electrodes. If the circuit is adjusted so that its frequency is the same, or very nearly'the same, as the resonant frequency of the crystal, then the crystal will be set in vibration, and, due to the piezo-electric action, a voltage will be applied to grid 3 which is very nearly in phase with the potential of grid 3, and of lsufficient amplitude to pull the oscillator in step, after which it will remain locked in step. The circuit may, ofcourse. be adjusted to a sub-harmonic or a harmonic of the crystal frequency and will be controlled thereby.

Referring now to Fig. 2, I show thereina modification in which the same tube V is used. Furthermore, this tube preferably possesses the same electrodes as those of the tube V in Fig. 1. Accordingly, like reference characters are applied to like parts in different figures of the drawing.

In Fig. 2 a resistor I5 is shown connected between the source 5 and the grid I. The capacitor I 2 is at all times connected betweengrids 2 and 3. Thev piezof-electric elementvl is arranged tobe connected either inV shunt with'resistor I5 orin shunt'with resistor 8 depending upon the position in Awhich the double-pole double-throw switch II is set. When theswitch I'I is set in the position connecting ihe'piezo-electric element inV shunt with resistor 8 the switch :I9 may be used to short the resistor vI5.l

The output circuitis preferably coupled across resistor 9 by means of capacitors I6. A ground connection may be made to the negative terminal of the source 1 as shown, or, if desired, the cathode K may be grounded.

When the piezo-electric element is in shunt with resistor 8, the circuit arrangement of Fig. 2 operates in very much the same manner as that of Fig. 1, and nofurther description of operation is necessary,

When the piezo-electric device is in shunt with resistor I5, the alternating voltage impulses appearing across resistor I5 excite the crystal I0, which then controls the frequency of oscillation.

In Fig. 3 I again show a discharge tube V, the electrodes of which may be the same as shown in the other figures. These electrodes are, therefore, given like reference characters. In Fig. 3 theanodecircuit includes a primary winding of a transformer I8. The secondary of this transformer may be used to develop an output potential for serving any desired utilization device.

In Fig. 3 the piezo-electric element IIJ is shown connected permanently in shunt with resistor 9. The capacitor I2 is also permanently connected between grids 2 and 3. In other respects, the similarities between the circuit arrangement of Fig. 3 and of Figs. l or 2 will Ibe recognized by those skilled in the art, so that the operation of this cir-cuit will be well understood in view of the foregoing description.

It should be noted that the use of grid I is more or less optional since it is possible'to construct and operate a relaxation ocsillator in ac'-y cordance with my invention, while eliminating the grid l altogether. The amplitude of the oscilf lations generated is, however. subject to control by the bias applied to grid I and. for this reason (iii the use of a pentode rather than a tetrode tube is preferred.

The constants of the oscillating system described herein may be made such that oscillations are obtained which are not strictly of the relaxation type. The device then functions somewhat differently. However, since the circuit remains the same in appearance, such an arrangement will be described here. Referring to Fig. 3, consider capacitor I2 to be made small enough so that the system will not oscillate of itself. When the crystal III is connected in the cir-cuit, the system will oscillate at the frequency of the crystal. The capacitor I2 in this case functions merely as a coupling capacity to feed energy from grid 2 back to grid 3 in the proper phase to sustain the vibrations of the crystal. An alternative use for the circuit arrangement of Fig. 3 is thus provided which extends beyond the scope of relaxation type oscillators.

In the conventional showing of the cathode in each of the figures, it would appear that tubes of the indirectly heated cathode type are preferable. It will be understood, however, that the invention is not limited to the use of such tubes. A tube with a directly heated filament type cathode can be used j ust as well.

It is not necessary to ground the system exactly as shown in the different circuit arrangements, nor is grounding necessary for proper operation. It should be further understood that when grounding is desired or necessary for certain applications of the device. the ground connection may be made at any point in the circuit.

Features of one gu're may be combined with those of another figure without departing from the spirit o f my invention. Any satisfactory method of coupling to an external circuit may be used including that form known as electron cousecond and third grids, means including indel pendent circuits for maintaining a negative bias on the rst and third grids with respect to the cathode, each of said independent circuits having a separate biasing source connected therein, and the circuit connection between the cathode and first grid being arranged and adapted-to automatically control the amplitude of the oscilla` tions generated, a direct current source connected between the cathode and the second grid, also between the cathode and anode, said source connections being substantially non-reactive and of such impedance values that the second grid is more positive than the anode, and piezo-electric means interconnecting two of the electrodes of said tube in such manner as to x the generated frequency.

ARTHUR M. BRAATEN.

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