June 13, 1957 M. GREENSPAN ErAL 2,796,522 CRYSTAL-CONTROLLED Filed Aug. 21, 1953

INVENTOR vMari‘z'n Greenspan M v .r I Carroll E. Ybc/u'eyy

- - - Y .BY ’ AGENT , , 2,796,522 United States a atent {C Nice Patented June 18, 1957

1 2 1It is another object of the present invention to provide 2,796,522 a crystal controlled relaxation oscillator, the output of which is relatively insensitive to variations in circuit CRYSTAL-CONTROLLED RELAXATION parameters even at high division ratios. OSCELLATOR *It is another object of the present invention to pro Martin Greenspan and Carroll E. Tschiegg, Silver Spring, vide a relaxation oscillator in which the voltages along Md, assignors to the United States of America as rep the ?at portion of the capacitor discharge curve are made resented by the Secretary of Commerce to vary considerably from instant to instant. Application August 21, 1953, Serial No. 375,841 1It is another object of the present invention to pro 10 vide a relaxation oscillator in which an oscillatory volt 4 Claims. (Cl. 250—36) age with a constantly increasing amplitude is superim (Granted under Title 35, U. S. Code (-1952), see. 266) posed on the grid voltage of the oscillator tube or tubes. Further provision is also made to crystal stabilize the frequency of this oscillatory voltage. The invention described herein may be manufactured 15 and used by or for the Government of the United States 'It is a'further object of the present invention to in for governmental purposes without the payment to us of crease the frequency stability of relaxation oscillators any royalty thereon in accordance with the provisions of when working along the steep portion of the timing wave 35 United States Code (1952) section 266. form. It is another object of the present invention to pro The present invention relates to relaxation oscillators 20 and in particular to a method and apparatus for stabiliz vide a in which the various phases of an ing the frequency of relaxation oscillators ‘by means of asymmetrical output wave are locked in by a piezoelec a piezoelectric crystal. trical crystal. ‘In the prior art numerous attempts have been made In accordance with the preferred embodiment of the to stabilize the frequency of various types of relaxation 25 present invention there is provided a multivibrator, the oscillators. in the case of the ‘, Mar’ output frequency of which is synchronized with a har rison, Patent No. 1,919,795, provides a crystal-stabiliz monic or subharmonic of a piezoelectric crystal. The ing circuit. However, since the division ratios obtain piezoelectric crystal is coupled into the grid circuit of able with this circuit were rather small and the range the multivibrator, the crystal and the tube acting as a of frequencies limited, the circuit was not widely used. 30 immediately prior to the ?ring of the In the case of , which are inherently fre tube in whose grid circuit the crystal is coupled. In addi quency ‘unstable, stability has been provided by syn tion to the crystal oscillator the circuit resonates as a chronizing the frequency by means of an oscillatory volt low-Q amplitude-unstable oscillator, whose frequency is age coupled into the circuit from an external oscillator. locked on some harmonic or subharmon‘ic of the crystal The use of an external oscillator of necessity increased 35 frequency. Because the low-Q oscillator is amplitude the complexity of the circuits. unstable, successive of this circuit will pro The primary cause of frequency instability of relaxa duce output voltages of increasing amplitude, the ?ring of tion oscillators arises from the fact that the time of ?r the tubes of the multivibrator being controlled by one of ing of the oscillator tube is controlled by the rate of these voltage peaks. Since the amplitude of these peaks discharge of a capacitor in the grid circuit of the tube. is increasing rapidly, particularly during the period just As long as the ?ring of the tube takes place at a voltage before ?ring, the circuit can easily differentiate between which lies along the steep portion of the discharge curve, successive voltage peaks. the frequency stability is acceptable. However, if the Other uses ‘and advantages of the invention will be frequency is varied so that ?ring of the oscillator tube come apparent upon reference to the speci?cation and is to occur at a voltage which lies along the ?at portion 45 drawings. of the discharge curve, the frequency stability becomes {Figure 1 is a circuit diagram of a crystal-controlled very poor. ,This is due to the fact that along the ?at ‘blocking oscillator. portion of the discharge curve the variation in voltage Figure 2 is a series'of reproductions of pictures taken from instant to instant is almost in?nitesimal. In order from the face of a cathode-ray-oscilloscope, showing the ‘for a tube to ?re at exactly the same potential during 50 various wave forms obtained on the grid of the oscillator each cycle and therefore at the same time the tube tube. characteristics would have to remain just about constant Figure 3 is a circuit diagram of a multivibrator which which, of course, is impossible. is crystal controlled. -It is the primary object of the present invention to Figure 4 is a reproduction of pictures taken from the provide a simple method of and apparatus for crystal 55 face of a cathode-ray-oscillo'scope, showing the wave stabilizing the frequency of various types of relaxation form at the output of the multivibrator. oscillators. %It should be noted that the use of a crystal to stabilize It is another object of the present invention to provide rblocking oscillators is not speci?cally claimed in this appli for crystal stabilization of relaxation oscillators having cation, but the explanation of that circuit is included for high division ratios. the purpose of developing the overall concepts involved. it is another object of the present invention to pro The crystal~controlled .blocking oscillator is speci?cally vide crystal-controlled relaxation oscillators in which the claimed in co-pending application No. 376,770, ?led on frequency of the oscillator is synchronized with the fre August 26, 1953, by Moody C. Thompson, Jr., now quency of the crystal. U. S. Patent 2,761,971. ‘It is another object of the present invention to provide 65 Referring to Figure 1, the tube 11 has its cathode 12 crystal-controlled multivibrators in which the frequency grounded and its plate 13 connected through one wind of the oscillator is synchronized with the frequency of ing 14 of the pulse 16 to the B+ supply. the crystal. ' The grid 17 of the tube is connected through another Another object of the present invention is to provide winding 18 of the pulse transformer and through the crystal-controlled relaxation oscillator-s which require 70 variable capacitor 19 to ground. The capacitor 19 is only very minor modi?cations of the conventional oscil shunted by the ?xed capacitor 21 and the series combina lator circuit. ‘ ’ ' tion of the ?xed 22 and variable resistor 23. A 2,790,522 3 4 third winding '24 of the transformer 16 has one terminal 2. The overall shape of the wave is controlled by the dis connected to ground and the other terminal connected charge or exponential curve of the capacitor. Superposed through the crystal 26 to ground. on the capacitor-discharge Wave is the characteristic fre Assuming initially that the blocking oscillator has just quency voltage of the low-Q oscillator (curve 1‘) which, as ?red, the grid 17 is instantaneously driven very highly can be seen, builds up cycle by cycle. The curve g rep positive and then very sharply negative. The negative resents the grid potential at which the tube will ?re, there bias on the grid shuts oif the tube, and this negative charge by producing an output pulse from the blocking oscillator. is stored in the capacitors 19 and 21, thereby maintaining It will be noted that the magnitude of each positive peak, the tube in the biased-off condition. However, the such as h and 1', increases considerably over the prior posi capacitors gradually discharge through the 22 10 tive peak and therefore makes it possible to differentiate and 23, and the potential on the grid 17 rises along the between one pulse and the next succeeding or prior pulse exponential discharge curve of the capacitors. When even when operating on the ?at portion of the timing wave the grid has reached a potential at which the tube can form. again ?re, a large pulse is given out by the oscillator, The frequency of the blocking oscillator is’ controlled by the grid again being instantaneously driven very highly 15 varying the discharge time of the capacitor 19. Rough positive and then very highly negative. Owing to the adjustments may be made by varying the value of resistor very tight coupling of the pulse transformer the crystal 23. This oscillator was found to have a high degree of ‘and tube combine to form an oscillator which will oscil frequency stability at even large division ratios. late for a brief period before the ?ring of the blocking It will be noted in the above analysis that several volt oscillator. Initially the crystal is caused to ring as the ages are applied to the grid-to-cathoclecircuit of the tube. result of the large output pulse supplied to it but since These voltages include a crystal oscillator voltage and a the tube is biased almostto cutoff by the large negative low-Q oscillator voltage, both of which appear in the charge stored across the capacitors 19 and 21, thetube aforementioned grid-to-cathode circuit. cannot cooperate with the crystal to produce a true oscil Referring now to the preferred embodiment of the lator. However, since no tube is cut off 100 percent by ' present invention which is a crystal-controlled multivibra the normal biases which are applied to the grid, the tor, as shown in Figure 3, there is provided a tube 31 tube will begin to conduct to a very small extent as the which has its plate connected to B+ through resistor 32 bias on the grid rises. That is, before the tube actually and to the grid 33 of the tube 34 through the parallel ?res and causes the blocking oscillator to produce an combination of the resistor 36 and capacitor 35. The output pulse, there will be some conduction through the cathodes of the tubes are connected to ground through the tube, which conduction is su?‘icient to cause the crystal common resistor 37. The grid 33 is grounded through the and tube to act as an oscillator. These oscillations will resistor 38. The grid of the tube 31 is grounded through continue, until the tube again ?res, and are superposed the variable capacitor 39 which is shunted by the resistor on the grid voltage. In addition to the crystal oscillator 41 and a variable portion of the voltage divider made up the winding 18 in conjunction with the tube and feed 35 of resistors 42, 43, and 44. The other end of the resistor back circuit form a second oscillator, the winding 18 and 42 is connected to B+ as is the plate of the tube 34 its stray capacitances acting as a tank circuit for this through the resistor 46. The plate of the tube 31 is con oscillator. This low-Q oscillator produces what will here nected to the grid of the tube through the series combina inafter be called the characteristic of the block tion of capacitor 47 and piezoelectric crystal 48. This ing oscillator circuit. Since the Q of the tank circuit is circuit minus capacitor 47 and crystal 48 is a conventional very low, pulse necessarily having low-Q cathode-coupled multivibrator. It operates in the follow windings, this oscillator is amplitude-unstable, and the ing manner. If the left-hand tube has just started to con amplitude of the oscillations rise sharply as the capacitor duct, the plate voltage of this tube will fall thereby driving discharges. This is due to the steady increase of the the grid 33 of the tube 34 negative with respect to its cath transconductance of the tube in conjunction with the 45 ode and' this tube will be biased off. The fall in the low-Q of the tank circuit, the increase in transconductance plate voltage of tube 31 decreases the voltage impressed resulting from the decrease of bias on the grid of the across the capacitor 35 which will now begin to discharge tube as the capacitor discharges. This circuit oscillates because of leakage through resistor 38. After a predeter at a characteristic frequency which is determined by the mined time,'controlled by the time constant of the R-C resonant'frequency of the low-Q tank and breaks into circuit, the voltage on the grid 33 will rise su?iciently to oscillation prior to actual ?ring of the tube, this voltage allow the tube 34 to start to conduct; However, during also being superposed on the grid voltage. 'These oscil the period of conduction of vthe tube 31, grid current was lations of the low-Q circuit are synchronized with the ?owing through the capacitor 39, which current charged, crystal oscillations, thereby providing for frequency sta the capacitor negatively. Since the relative values of the bility of this oscillator. However, as pointed out above, plate resistors of the tubes 31 and 34 are chosen to be dif the low-Q oscillator is amplitude-unstable, the amplitude ferent, that of tube 34 being smaller, the tube 34 will of the oscillatory voltage increasing by as much as several draw a larger current than the tube 31. Therefore when volts per cycle. the tube 34 conducts, the cathode voltages of the two As a result of the above, the grid 17 of the tube has circuits rise sharply and because of the negative charge impressed upon it three distinct voltages, the gradual in 60 accumulated on the capacitor 39, the tube 31 will be biased crease of voltage as the capacitor discharges, the oscilla off. This will raise the voltage of the plate of the tube 31, tory voltage of the crystal oscillator, and the oscillatory thereby increasing the positive voltage on the grid 33 voltage of the low-Q oscillator. In other words, there to cause the tube 34 to fully conduct. Since the current is a regenerative build-up of an oscillatory voltage at drawn by the tube 34 is larger than that drawn by the a frequency which is characteristic of the particular cir tube 31, as already pointed out, the cathode voltage of the cuit, which voltage is superposed on a timing wave form tube 31 remains su?iciently high with respect to the of the'grid and serves to trigger the transition. At the initially charged condition of the capacitor 39 to maintain same time the characteristic oscillation is synchronized the tube nonconducting. However, the negative voltage with the oscillations of the crystal, which in conjunction across the capacitor 39 will gradually leak off along the with the tube acts brie?y as a crystal oscillator once each 70 exponential curve and will eventually reach a point where cycle of. relaxation. ' the tube 31 will again begin to conduct, starting the cycle The high degree of stabilityof the frequency of this agaln. blocking oscillator even at high division ratios is due to ‘ vThe frequency and stability of this type of circuit is, the increase in amplitude of the characteristic frequency like that of the prior-art blocking oscillators, inherently cycle by cycle. This can be shown by referring to Figure poor. The reason, as previously explained, is because the 2,796,522 5 6 grid voltage of the tube, or tubes, follows the discharge will be varying along a very ?at portion of the condenser’s curve of a condenser. When the ?at portion of the curve exponential curve. However, because of the presence of is reached, the change in voltage from instant to instant the crystal-stabilized amplitude-varying voltage on the is exceedingly small. If the frequency of the‘bscillator, grid of the tube each portion of the wave will be ‘locked in. however, is to remain constant, the tubes must start con It has been found possible to obtain division ratios of duction at precisely the same instant each cycle, which several hundred by means of the present invention land means that the exponential timing curve must remain with high frequency crystals of even ‘as high as 2000, identical from cycle to cycle, and the tube must respond with this circuit. At the higher division ratios, this cir to exactly the same voltage each time. This requires a cuit is not as insensitive to changes in circuit and tube degree of stability of the circuit parameters that is nearly 10 parameters as the blocking oscillator described above. impossible to obtain. However, at the lower division ratios the two circuits are The present invention overcomes this di?iculty in grid comparable. The use of the crystal in the multivibnator controlled relaxation oscillators in general by superim is not restricted to connections between the grid and plate posing on the exponential grid voltage an oscillatory volt of one of the tubes. The crystal may be inserted in other age which constantly increases in amplitude and the fre places for instance, between grid to ground. quency of which is stabilized by a piezoelectric crystal. This system for stabilizing grid-controlled relaxation The circuit of Figure 3 with the crystal 48 connected oscillators has also been found to be applicable to screen as shown operates in a manner which is very similar to coupled astable phantastrons. The crystal may be con the operation of the blocking oscillator already described. nected from the plate to the suppressor or from the screen It will be noted that both in the case of the blocking to the control grid of the tube. Division ratios of up to oscillator and the multivibrator there is a circuit 100 can be obtained with this circuit. from the plate of a tube to its grid-to-cathode circuit. It will be apparent that the embodiments shown are In the case of the blocking oscillator this is accomplished ‘ only exemplary and that various modi?cations can be by means of the windings 14 and 18 of the pulse trans made in construction and arrangement within the scope former 16 (see Figure 1) and in the present case it is ac of our invention as de?ned in the appended claims. complished—taking the tube 31 as the tube under dis What is claimed is: cussion—by means of the coupling between the plate of l. A crystal-controlled relaxation oscillator comprising the tube 31 and the grid 33 of tube 34 and cathode re a grid-controlled electron tube having a cathode, a plate, sistor 37 which again controls the voltages in the grid and at least one grid, a direct-coupled low-Q regenerative to-cathode circuit. feedback circuit including the stray parameters of said In this circuit the crystal 48 and the m-u‘ltivi'brator cir tube connected between the plate and grid of said tube to cuit act in a manner similar to a 'Pierce crystal oscillator form an oscillator circuit therewith, said low-Q circuit having a natural frequency distinct from the relaxation over a narrow range of biases. Also it is found that the frequency, a timing capacitor connected to the grid of said multivibrator has a characteristic frequency much in the tube for applying a rising exponential voltage to said grid, same sense as that of the blocking oscillator. This fre a piezoelectric crystal having a natural frequency har quency is determined by the circuit constants, stray in monically related to the natural frequency of said low-Q ductances, and tube capacities, and is set for any particu circuit coupled to the grid of said tube to form a Pierce lar circuit. This oscillator again acts as a low-Q 'oscil oscillator circuit therewith, the and low-Q later which oscillates with a constantly increasing voltage oscillator voltages derived from said oscillator circuits amplitude for a brief period prior to conduction of either being superimposed on said exponential voltage as said tube; that is, this oscillatory voltage and also that of the crystal oscillator will appear in the grid-to-cathode cir extponential voltage nears the conducting value for said tu e. cuit of a tube immediately prior to conduction of that 2. An oscillator as de?ned in claim 1 in which said tube. This is clearly shown by the oscillograph which crystal is connected between the plate and grid of said is reproduced in Figure 4. Unlike the oscillogram shown tu e. in Figure 2, this oscillogram is of the output voltage of 3. A crystal-controlled relaxation oscillator comprising the multivibrator, which is taken at the plate of the ?rst and second electron tubes each having a cathode, a tube 34. It will be noted that the left-hand portion of plate, and at least one grid, means including, power sup the curve immediately preceding ?ring contains a con ply means having its positive terminal resistively coupled stantly increasing oscillatory voltage, which voltage, it to the plate of each tube, a common cathode resistor con has been determined, is synchronized with the frequency nected between said cathodes and the negative terminal of the crystal 48. The fact that this oscillatory voltage of said power supply means, a ?rst capacitor connected is constantly increasing allows the multivibrator to easily between the ‘grid of said ?rst tube and said negative ter detect different peaks of this voltage ‘and therefore the minal, a second capacitor connected between the plate tube will ?re on the same grid voltage peak and conse of said ?rst tube and the grid of said second tube, and quently at the same time each cycle. the stray parameters of said tubes, whereby a direct Another feature of this circuit is that even when the coupled low-Q regenerative feedback circuit is connected multivibrator is used to produce an asymmetrical output between the plate and grid of said ?rst tube to form an voltage wave form both periods will lock on some sub oscillator circuit therewith, and a piezoelectric crystal harmonic of the crystal frequency. The duration of the 60 coupled to the grid of said ?rst tube to form a Pierce positive and negative portions of the output wave are oscillator circuit therewith, said crystal having a natural controlled by the time constants of the circuits containing frequency harmonically related to the natural frequency the capacitors 35 and 39. The capacitor 35 deter-mines of said low-Q regenerative feedback oscillator circuit. how long after the ?ring of tube 31, the tube 34 will ?re 4. A relaxation oscillator as de?ned in claim 3 in which and therefore determines the period of conduction of 65 said crystal is connected between the late and 'd of tube 31. Varying either the capacity of capacitor 35 or said ?rst tube. p gm resistance of resistor 36 will then control the width of the vpositive portion of the output wave. Capacitor 39 References Cited in the ?le of this patent. similarly controls the period of conduction of tube 34 UNITED STATES PATENTS and varying the time constant of its circuit controls the 70 width of the negative portion of the output wave. In 2,070,647 Braaten ______.. Feb. 16, 1937 this way it is possible ‘to control the width of both por 2,553,165 Bliss ______May 15, 1951 tions of the wave and also the period of the oscillations. 2,560,576 Hoeppner ______.__ July 17, 1951 When the output wave is made highly asymmetrical the FOREIGN PATENTS voltage on the grid of one of the tubes just before ?ring 75 251,782 Switzerland ______- Sept. 1, 1948