US2144779A - Relaxation oscillator with variable frequency - Google Patents

Description

Jan. 24, 1939. K. SCHLESINGER 2,144,779

RELAXATION OSCILLATOR WITH VARIABLE FREQUENCY Filed Jun e 11, 19:57

huenlfor Patented Jan. 24, 1939 UNITED STATES RELAXATION OSCILLATOR wrrn VARIABLE FREQUENCY Kurt Schlesinger, Berlin, Germany, assignor to Radioaktiengesellschaft D. S. Loewe, Berlin- Ste'glitz, Germany ApplicationJune 11, 1937, Serial No. 147,801

v p The need frequently arises, particularly in the case of oscillographs, for a relaxation oscillation apparatus, the frequency of which is able to be varied within wide limits, the relaxation poten- 5 tial, however, remaining constant. If the oscillator is provided with a grid-controlled gas-filled thermionic tube this problem could be solved simply by a variation of the relaxation condenser. In practice, however, a variation in the capacity is comparatively expensive and complicated, particularly if a desired range of ill-10,000 periods is concerned, whilst resistance tuning would be cheap and practical.

It is an object of the invention to vary the frequency of a relaxation oscillation by varying its charging current and to provide a special compensation in order to maintain the potential of the oscillations constant. Further objects of the invention may be best understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. l is a connection scheme for varying uniformly the relaxation frequency within moderate limits, whilst Fig. 2 is a connection scheme for varying the frequency within wide limits.

In Fig. 1, I is the potential source, 2 the charging resistance, 3 the relaxation condenser and 4 the grid-controlled gas-filled thermionic tube, which is connected with its anode and cathode to the condenser. A braking resistance 6 is inserted in order to limit the discharge current. As well known, oscillations of constant frequency which are independent of the potential of the mains can only be obtained if the discharge tube derives its grid bias from the charging current, so that the same is coupled with the current and accordingly with the mains potential and participates in all variations in the opposite sense. For this purpose there is employed the resistance 5. If now the charging resistance 2 were varied, the bias 5 would also vary. As a result there would hardly occur the desired variation in frequency, but merely a variation in amplitude. This behaviour, although possibly to smaller extent, would always take place also in all other methods of generating the grid bias upon variation of the charging currents, which represent a part of the current loading the mains apparatus. Simple resistance regulation of the relaxation frequency by varying the resistance 2 could only be performed, if resistance 5 would be replaced by a biassing battery.

According to the invention, therefore, the following way is adopted: The charging resistance 2 Germany June 20, '1936 (01. ate-s0) is connected with a potentiometer l. The charging E. M. R, therefore, is varied at the wiper thereof. An electric stop resistor 8 prevents the charglhgpotential to drop below the ignition po tential-of the discharge tube. The potentiometer current flows also. through 5. Since the same is substantially constant it furnishes the bias at 5 with a constant basic component and already decreases the interfering dependency. According to the invention, however, there is also caused by a resistance e an artificial increase of the charging current upon decrease of the potential at l, as if the wiper of l is furnishing the full potential the resistance Q is short-circuited and ineffective. The more the wiper at l is moved downwards, the more is l shunted by 9, thus increasing the total current which reaches the grid resistance 5 along one of the two paths, either by way of 2 or by way of 8. It is essential that the charging resistance 2 is large as compared with the potentiometer resistance 1 and that the resistance 9 is approximately equal to the resistance 2. With the stated connection system it is possible, by shifting the wiper at l, to produce a variation in frequency such as 1:7 with a practically constant amplitude. The igniting potential may be applied to the grid in the usual way.

For additional amplification the oscillator is coupled with an amplifier E3 of any kind by means of a condenser I2 and a potential drivider I5, It. The significance of this potential distributor is as follows: The more the same sets down the grid alternating potential, the greater are the alternating potentials which require to be generated at the anode of the discharge tube, the greater, therefore, are the charging currents which require to be passed to the condenser 3, and the smaller is the eiiect of insulation faults and leakage or initial currents at the condenser 3 or the tube 4 with respect to these charging currents to be maintained constant. It is found that the ascent of the condenser potential gains considerably in linearity if the charging currents are of the order of l m. a. instead of 0.1 m. a., provided a potential source i of a few hundred volts is used. This fact is caused by unavoidable insulation faults of the order of 10 ohms, corresponding already to the initial current of such tubes.

Whilst by reason of the connection system according to the invention as shown in Fig. 1 it is possible to obtain a fine adjustment of the frequency within ranges of approximately one order of magnitude, a coarse adjustment in powers of ten is obtained by switching over the condensers. The connection system necessary for this purpose is shown in Fig. 2, A multiple-way double-leverswitch I I is provided for changing-over the condensers 3 together with its different braking resistances 6 and the appertaining resistances 9.

I claim:

1. In a relaxation oscillator comprising direct current source, a gas-filled grid-controlled thermionic tube shunted by a condenser in series with a braking resistance, a grid biassing resistance between the cathode of said tube and the negative terminal of said source and a charging resistance between the anode of said tube and the positive terminal of said source, means for varying the frequency, means to keep constant the grid bias of said tube and consequently the discharge potential of said condenser, said means consisting in a connection for keeping constant the current flowing through said biassing resistance when the current charging the condenser is varied for frequency variation.

7 2. In a relaxation oscillator according to claim 1, the means to keep constant the current flowing through said biassing resistance in a connection guiding the whole current taken from said source through said biassing resistance and keeping constant said current.

3. In a relaxation oscillator according to claim 1, the means to keep constant the current taken from said source consisting of variation means in the charging resistance.

4. In a relaxation oscillator according to claim 1, means to vary the oscillator frequency within wide limits in coarse steps, said means consisting of a set of charging condensers of difierent capacity each in series with an adapted braking resistance, a set of difierent charging resistances and a multiple-way double-lever switch inserting at once one member of either of said sets.

5. In a relaxation oscillator according to claim 1, an electronic amplifier and means to couple the output of said oscillator with said amplifier, said means consisting of a condenser in series to a potential divider, the intermediate terminal of said divider connected to the grid of said amplifier, said divider reducing the output potential of said oscillator for at least one order of magnitude.

KURT SCHLESINGER. g5

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