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Hysteresis in vacuum tube oscillators
HYSTERESIS
IN VACUUM
TUBE
OSCILLATORS.*
BY
LAURISTON S. TAYLOR, A.B. D e p a r t m e n t of Physics, Cornell Univers'~ty.
IN AN earlier investigation, 1 it was shown how, when a blocking condenser and resistance were placed in the grid circuit of a vacuum tube oscillator, the oscillations were broken up into groups which for convenience are called " zules." It was pointed out that such a circuit was capable of operation in two states, namely, one of continuous and one of intermittent oscillation. The actual state of oscillation is determined chiefly by the constants of the oscillatory circuit in which oscillation is maintained by the vacuum tube. Thus for any given value of the grid condenser and by-pass, the two states can be produced by changing the inductance or capacity in the oscillatory circuit. An empirical relation was obtained for the zule frequency F in terms of these constants. Thus L2 -
F=A~
kC2
2L,
where A and k are constants and L2, L1, and C2 are in the oscillatory circuit. The constants of that part of the circuit determine the rate of accumulation of charge on the grid and consequently the state of depression of the grid potential. The grid depression was then shown to be one of the two major factors in determining the state of oscillation, the other being a type of oscillation hysteresis. At the start of a zule, the mean grid potential was found to be above the dynamic grid potential cut-off. In this study a more precise location of the mean Eg was made. At the start of the zule, Eg was slightly higher than the static cut-off and at the end of the zule was slightly lower than the dynamic cut-off. Thus the actual range of variation of the mean E~ is determined by these two cut-offs, the first of which remains fixed for any given tube. * Submitted /or publication May 2, 1927. 1L. S. Taylor, JouR. FRANK.INST., 203, p. 35I, Mar,, I927. 227
228
L A U R I S T O N S. TAYLOR.
lJ. F. I.
There was subsequently made available a synchronized cathode-ray oscilloscope with a~ linear time axis, 2 by means of which the theory could be checked. Two assumptions had been made: ( I ) That the circuit when oscillating in the intermittent state did not suffer any appreciable wave distortion within the zule. (2) That the oscillation frequency within the zule remained constant. When the wave was observed by means of the oscilloscope, both these assumptions were found to be true. Fig. t is a copy of the oscilloscope pattern showing the wave form in the oscillatory circuit. Synchronization was adjusted for the zule frequency and not the oscillation frequency, and so the pattern shows directly the envelope of the oscillations. Intermediate potential variations were seen faintly in the background. T w o points in particular will be noted. First, the zule builds up quickly to a maximum at the beginnin 9 of oscillation and then drops off more gradually as the grid depression proceeds. The amplitude falls off continuously to zero. Secondly, between zules there is no oscillation nor irregularity of any kind. In a paper by Appleton, Watt and Herd, :~ similar to the one mentioned above -~ which was submitted at about the same time, the investigators stated that the oscillations were " suddenly quenched." Fig. I shows the actual fact to be quite the contrary as pointed out, since in the particular case the oscillations die off to zero in roughly three times the time necessary to build up. A similar misstatement was made by the author 4 regarding a Colpitts circuit and while this circuit was not actually checked by the oscilloscope, it seems reasonable to suppose that the zule dies off slowly and not suddenly. Fig. 2 shows the envelope of the grid potential variations over a zule. The hysteresis action taking place as caused by the grid depression is shown strikingly in this figure. The envelope is no longer symmetrical, but varies about a mean which decreases in some manner with time. Oscillation ceases at a certain value N of the grid potential, as determined by the dynamic cut-off. From this point the mean increases exponentially to a higher value M which is determined by the static cut-off. During the interval 2 Bedell and Reich, Pittsfield Meeting, A.I.E.E., May 25, 1927. "A.ppleton, Watt and Herd, Proc. Roy. Soc., A, 3, P- 615 (I926). 4L. S. Taylor, J.O.S..d.I?.S.I.. r2, p. 149 (Feb., I926),
Aug., i927. ]
229
HYSTERESIS.
N M the circuit is again seen to be in a state of non-oscillation. Thus the theory given before was found to be true. There was previously some doubt as to whether the transition from one state to another was perfectly continuous. Without further,.adjustment of the circuit constants, it is possible to effect this transition by means of an external biasing potential applied to the grid. When observed by means of the oscilloscope during this process, it was found that the transition proceeded continuously, and that there was no sudden jump between the two states. Distortion of the zule form was found to occur under certain conditions. By application of a strong bias to the grid, such that the zule was of comparatively long duration (several seconds), it
@ FIG. I.
FIG. 2.
was found that the oscillations built up slowly to the maxima and then died off very rapidly. This is seen to be just the reverse of the form for the more rapid zules as shown in Fig. ~. The length of the zule was found to vary slightly with the application of an external grid bias when the zule frequency was slow (i.e., less than about 20 per minute). For very low zule frequencies the change was quite marked. Another point of uncertainty was checked. For extremely high zule frequencies it was previously thought that an entire zule occurred during a single impressed e.m.f, cycle so that the zule effect disappeared and continuous oscillation alone resulted. Actually it was found that the zules " overlapped " and the complete zule was never formed. Fig. 3a shows the action taking place within the oscillatory circuit. Oscillation reaches a maximum at A and starts to die out in the usual manner. However, before the grid potential is depressed to the cut-off point, the leakage of the trapped charge begins to exceed the rate of recharging and at B the next zule begins. Thus there is seen to be no
230
LAURISTON S. TAYLOR.
[J. F. I.
longer an intervening period of non-oscillation. Fig. 3 b shows the form (to a different time scale) as the zule frequency is still further increased. Eventually the form is found to be but a ripple in the successive amplitudes of oscillation, and finally even the ripple disappears and all hysteresis action is absent. From the latter two patterns it is clear that there is no state PIG. 3.
8
a
"-
•
b
of equilibrium at which the grid potential remains fixed at a negative value such as reported by Appleton. 3 As long as the grid condenser and by-pass resistance play any part in the hysteresis action, no such point of equilibrium can exist. Fig. 4 shows the oscillation of the grid potential at high zule frequencies. In general it is the same as discussed above, except FIG. 4-
lv
between the zule maxima. Here it will be noted that the oscillations occur about a rising mean potential similar to that shown in Fig. z. I am indebted to Mr. H. J. Reich, of this laboratory, for his assistance in the use of the oscilloscope which was developed by Prof. F. Bedell and himself.
IN VACUUM
TUBE
OSCILLATORS.*
BY
LAURISTON S. TAYLOR, A.B. D e p a r t m e n t of Physics, Cornell Univers'~ty.
IN AN earlier investigation, 1 it was shown how, when a blocking condenser and resistance were placed in the grid circuit of a vacuum tube oscillator, the oscillations were broken up into groups which for convenience are called " zules." It was pointed out that such a circuit was capable of operation in two states, namely, one of continuous and one of intermittent oscillation. The actual state of oscillation is determined chiefly by the constants of the oscillatory circuit in which oscillation is maintained by the vacuum tube. Thus for any given value of the grid condenser and by-pass, the two states can be produced by changing the inductance or capacity in the oscillatory circuit. An empirical relation was obtained for the zule frequency F in terms of these constants. Thus L2 -
F=A~
kC2
2L,
where A and k are constants and L2, L1, and C2 are in the oscillatory circuit. The constants of that part of the circuit determine the rate of accumulation of charge on the grid and consequently the state of depression of the grid potential. The grid depression was then shown to be one of the two major factors in determining the state of oscillation, the other being a type of oscillation hysteresis. At the start of a zule, the mean grid potential was found to be above the dynamic grid potential cut-off. In this study a more precise location of the mean Eg was made. At the start of the zule, Eg was slightly higher than the static cut-off and at the end of the zule was slightly lower than the dynamic cut-off. Thus the actual range of variation of the mean E~ is determined by these two cut-offs, the first of which remains fixed for any given tube. * Submitted /or publication May 2, 1927. 1L. S. Taylor, JouR. FRANK.INST., 203, p. 35I, Mar,, I927. 227
228
L A U R I S T O N S. TAYLOR.
lJ. F. I.
There was subsequently made available a synchronized cathode-ray oscilloscope with a~ linear time axis, 2 by means of which the theory could be checked. Two assumptions had been made: ( I ) That the circuit when oscillating in the intermittent state did not suffer any appreciable wave distortion within the zule. (2) That the oscillation frequency within the zule remained constant. When the wave was observed by means of the oscilloscope, both these assumptions were found to be true. Fig. t is a copy of the oscilloscope pattern showing the wave form in the oscillatory circuit. Synchronization was adjusted for the zule frequency and not the oscillation frequency, and so the pattern shows directly the envelope of the oscillations. Intermediate potential variations were seen faintly in the background. T w o points in particular will be noted. First, the zule builds up quickly to a maximum at the beginnin 9 of oscillation and then drops off more gradually as the grid depression proceeds. The amplitude falls off continuously to zero. Secondly, between zules there is no oscillation nor irregularity of any kind. In a paper by Appleton, Watt and Herd, :~ similar to the one mentioned above -~ which was submitted at about the same time, the investigators stated that the oscillations were " suddenly quenched." Fig. I shows the actual fact to be quite the contrary as pointed out, since in the particular case the oscillations die off to zero in roughly three times the time necessary to build up. A similar misstatement was made by the author 4 regarding a Colpitts circuit and while this circuit was not actually checked by the oscilloscope, it seems reasonable to suppose that the zule dies off slowly and not suddenly. Fig. 2 shows the envelope of the grid potential variations over a zule. The hysteresis action taking place as caused by the grid depression is shown strikingly in this figure. The envelope is no longer symmetrical, but varies about a mean which decreases in some manner with time. Oscillation ceases at a certain value N of the grid potential, as determined by the dynamic cut-off. From this point the mean increases exponentially to a higher value M which is determined by the static cut-off. During the interval 2 Bedell and Reich, Pittsfield Meeting, A.I.E.E., May 25, 1927. "A.ppleton, Watt and Herd, Proc. Roy. Soc., A, 3, P- 615 (I926). 4L. S. Taylor, J.O.S..d.I?.S.I.. r2, p. 149 (Feb., I926),
Aug., i927. ]
229
HYSTERESIS.
N M the circuit is again seen to be in a state of non-oscillation. Thus the theory given before was found to be true. There was previously some doubt as to whether the transition from one state to another was perfectly continuous. Without further,.adjustment of the circuit constants, it is possible to effect this transition by means of an external biasing potential applied to the grid. When observed by means of the oscilloscope during this process, it was found that the transition proceeded continuously, and that there was no sudden jump between the two states. Distortion of the zule form was found to occur under certain conditions. By application of a strong bias to the grid, such that the zule was of comparatively long duration (several seconds), it
@ FIG. I.
FIG. 2.
was found that the oscillations built up slowly to the maxima and then died off very rapidly. This is seen to be just the reverse of the form for the more rapid zules as shown in Fig. ~. The length of the zule was found to vary slightly with the application of an external grid bias when the zule frequency was slow (i.e., less than about 20 per minute). For very low zule frequencies the change was quite marked. Another point of uncertainty was checked. For extremely high zule frequencies it was previously thought that an entire zule occurred during a single impressed e.m.f, cycle so that the zule effect disappeared and continuous oscillation alone resulted. Actually it was found that the zules " overlapped " and the complete zule was never formed. Fig. 3a shows the action taking place within the oscillatory circuit. Oscillation reaches a maximum at A and starts to die out in the usual manner. However, before the grid potential is depressed to the cut-off point, the leakage of the trapped charge begins to exceed the rate of recharging and at B the next zule begins. Thus there is seen to be no
230
LAURISTON S. TAYLOR.
[J. F. I.
longer an intervening period of non-oscillation. Fig. 3 b shows the form (to a different time scale) as the zule frequency is still further increased. Eventually the form is found to be but a ripple in the successive amplitudes of oscillation, and finally even the ripple disappears and all hysteresis action is absent. From the latter two patterns it is clear that there is no state PIG. 3.
8
a
"-
•
b
of equilibrium at which the grid potential remains fixed at a negative value such as reported by Appleton. 3 As long as the grid condenser and by-pass resistance play any part in the hysteresis action, no such point of equilibrium can exist. Fig. 4 shows the oscillation of the grid potential at high zule frequencies. In general it is the same as discussed above, except FIG. 4-
lv
between the zule maxima. Here it will be noted that the oscillations occur about a rising mean potential similar to that shown in Fig. z. I am indebted to Mr. H. J. Reich, of this laboratory, for his assistance in the use of the oscilloscope which was developed by Prof. F. Bedell and himself.