US2418842A

US2418842A - Scanning oscillator

Info

Publication number: US2418842A
Application number: US477985A
Authority: US
Inventors: Boris J Kinsburg
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1943-03-04
Filing date: 1943-03-04
Publication date: 1947-04-15
Anticipated expiration: 1964-04-15

Description

S CANNING- OSCILLATOR /A/l/ENTOR B. J. /f//VSBURG A T`TO RNEV April l5, 1947- B. J. KINSBU'R 2,418,842

SCNNING OSCILLATOR l Filed March 4, 1943 2 Sheets-5116?, 2

30 THERM/STOR 38 www5- THERM/S TOR /Nl/ENTOR By B. J. K//VSBURG' ATTORNEY Patented Apr. 15, 1947 SQANNING OSCILLATOR Boris J. Kinsburg, New York, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 4, 1943, Serial No. 477,985

(Cl. Z50-36) 6 Claims.

This invention relates to oscillation circuits and particularly to vacuum tube oscillators of the so-called RC type in which the frequency determining circuit consists solely of resistance and capacity elements.

RC oscillators are commonly used as scanning oscillators for testing purposes particularly Where the sweep frequency range is such that oscillators having tuned circuits are too expensive or impracticable. In such prior scanning oscillators the output frequency is caused to vary over a desired frequency band by cyclically varying the impedance of one of the elements in the frequency determining network. This cyclic variation involves the use of some mechanical means, such as a motor-driven condenser or potentiometer which gives rise to objectionable noise coinponents in the output of the oscillator. Although some of these noise components, such as those due to eccentrizities of the condenser plates, are synchronous with the sweep frequency they produce a displacement in thei return .traceso that it is not directly superimposed on the forward trace and therefore must be suppressed or blanked out. This blanking out is usually accomplished by convtacts on the condenser or potentiometer shaft but l such contacts introduce peaks of noise which are Aalso very objectionable.

Other noise components, such as those due to deformations on the condenser plates duringrotation, often are not synchronous with the sweep frequency and therefore produce on the oscilloscope screen moving patterns which make accurate observations very difficult or impossible.`

For these and similar reasons all known forms of mechanical drive are often found unsatisfac- 'tory for scanning oscillators in testing systems.

The obj ect of this invention is an improved RC type oscillator inwhich the frequency is varied without the use of any form of mechanical drive.

According to the invention one or more of the resistance elements in the frequency determinin circuit of such an oscillator are of the ther- -mally sensitive type and the resistance of these elements is periodically varied over the'range `necessary to produce the required variation in frequency by heating current applied from an external source of current of the desired sweep frequency. Thermally sensitive resistors suitable for this purpose are -now called thermistors and arewell known as evidenced by U. S. Patents 2,184,847, 2,276,864 and 2,280,257 to G. L. Pearson.

As shown by these patents, the resistance of .thesedevices may be vvaried directly by the heating effect of the current in the thermistor or indirectly by means of a heating element in an electrically separate circuit. The indirectly heated type is preferred for the purposes of this invention particularly for oscillators of very low scanning frequency. In cases where the scanning period is so short that the resistance of indirectly heated thermistors does not vary sulficiently to cause the oscillatorto sweep 4over the required frequency range, the heating current circuit may be connected directly to the thermistor. However, when two or more thermistors in a frequency determining network are to' be controlled simultaneously they must be decoupled from each other by suitable impedances in the heating circuit.

In the drawing,

1 is a single tube scanning oscillator according to the invention in which the thermistors are indirectly heated;

Fig. 2 is a similar oscillator with directly heated thermistors; Fig. 3 is a two-stageoscllator; and Fig. 4 shows the connections for directly heating the thermistors in the cir :uit of Fig. 3.

In Fig. 1 the vacuumtubel is an RC oscillator of the general type disclosed in such patents as 1,442,781 to Nichols and 2,236,985 to Bartelink in which an iterative network comprising series condensers and shunt resistors is connected between the plate and grid circuits. As pointed out in the Bartelink patent, the frequency of such an oscillator can be varied over a wide range by varying one or more of the shunt resistors. In the circuit of this invention the network comprises four condensers 2,13, l and li and four resisters ii, l, 8 and 8 of which one or more such as 8 and 'l are thermistors with heating elements lll and Il connected in parallel to a pulse generator Il which may comprise a diode rectier I2 which passes to the elements pulses of heating current at the frequency of the alternating source I3. While the rectifier circuit shown is preferred as a simple and inexpensive device for obtaining a suitable scanning frequency, it will be understood that it may b e replaced by any other known device such as a relaxation oscillator, for producing the required low frequency heating pulses.

The rectier may be biased, as by a battery lil, to limit the duration ofthe rectified pulse to a desirable fraction' of" the positive half cycle at the plate of the diode. In this way the temperature range of the thermistor between successive pulses may be varied to produce the desired variatonin the frequency range of the oscillator. The

thermal inertia of the thermistors Il] and Il will ordinarily produce a sufficiently gradual change in their resistance but if necessary the Wave form of the heater current pulses may be modied by one or more auxiliary thermistors such as i5 in series or parallel with the heater circuit and the eiect of these auxiliary thermistors may be varied as required by the addition of series or shunt connected resistors.

While the thermal inertia of thermistors is sucient to produce a gradual change in resistance over the heating period they are nevertheless so sensitive as to undergo large variations in resistance between successive pulses of relatively low frequency and hence the output voltage of the oscillator across the resistor I3 will vary over a wide band of frequencies at the frequency of thesource i3.

In some cases particularly where the scanning .frequency is relatively high, that is, 60 cycles per second or higher, and the oscillator must scan a wide frequency band, the indirectly heated thermistors may not be suiiiciently sensitive. In such cases the thermistors G and 'l may be replaced by thermistors 6 and l' of the directly .heated type as shown in Fig. 2. If the frequency `of the oscillator is varied by means of two or more of such thermistors, suitable resistors or retard coils i8, I8v must be used in the heating circuits to prevent the heating circuit from forming a low impedance by-pass in shunt with the thermistors. If resistors are used they should be of a value greater than the maximum resistance of the thermistor and if retard coils are used their impedance at the lowest frequency of the oscillation should be much greater than .the maximum resistance of theltherrnistor in or- .der'to obtain the maximum range of frequencies from the oscillators. Y

InY Fig. 3 the principles of the invention are applied to a two-stage RC type oscillator cornprising two conventionally coupled tubes fil, 22 with a, coil 23 for conducting the output to the work circuit 24. Due. to the addition of the second tube very little phase shift in the feedback path is required. The signal voltage across the `resistor 725Y in the plate circuit of the output tube is impressed on the grid 96 of the-tube 2l through a small blocking condenser 2'! and an indirectly heated thermistor 28. The grid circuit of this `series with, a small condenser 32 across the re- The sister SB in the plate circuit of the tube 2l. variations in the resistance of this thermistor with the. variations in the, output of the tube 2t will -tend to keepthe alternating potentials applied to the. grid of tube 22, and hence the output of the oscillator, constant over the scanning frequency range.

As in the circuit of Fig. l, the indirectly heated thermistors 28, 2S may if required be replaced byv directly heated thermistors 33, 34 as shown in. Fig. 4. When the thermistors are directly heated neither heating lead can be at ground potential but both must be at the saine potential with respect to ground to prevent the heating current from affecting `the bias on the grid 26. Regardless of the type of thermistor used the heating circuit should be balanced in order to stabilize the oscillator against uctuations in the voltage of the source I3 and other variations such as those due to aging of the tubes in the pulse generating circuit.

To obtain this balanced relation the pulse generating diode l2 is connected to the heater windings 35, 36 of Fig. 3 or to the directly heated thermistors' 33, 34 of Fig. 4 through a direct current amplifying stage comprising two tubes 3l and 3B. The tube 31 is a conventional amplifier with one heating lead 39 connected to the cathode end of the cathode load resistor till. The tube Sil is of the same type as 31 but has its grid connected to ground through a high resistor 4| and does not function as an ampliiier but merely as a means of bringing the lead 42 to the same potential as the lead 3S by adjusting the cathode resistor d3 to balance the quiescent plate currents of the tubes 3l' and 38. Under this condition the pulses from the diode l2 will be amplified in tube 3l and will vary the resistance of the thermistors as described above. The amplitude of the current pulses in the heating circuit will be relatively small compared with the steady plate currents of the tubes 37 and 38 so that the potential drop in resistor 43 will vary with the amplitude of the current pulses and keep the conductors 39 and 42 at substantially the same potential with respect to ground.

Obviously the required balance can be obtained in a very simple manner by using a full wave rectifier in place of the diode (2 but this would double the scanning frequency. Very often, however, a frequency of about 6B cycles is desired and the circuit just described gives this frequency directly from the usual commercial supply line.

What is claimed is:

1. In combination a vacuum tube oscillator having a frequency determining network consisting of resistance and capacity elements at least one of the resistance elements being a thermistor and means for automatically varying the resistance of the thermistor comprising a source of alternating current, a circuit including a rectier between the source and the thermistor and means for biasing the rectifier to control the variation in the resistance of the thermistor between successive alternations of the source.

2. The combination of claim 1 in which the circuit includes an auxiliary thermistor for controlling the rate of change of the resistance of the thermistor in the frequency determining network.

3. The combination of claim 1 in which at least two of the resistance elements are thermistors and the circuit is connected to each thermistor through an isolating impedance.

4. A vacuum tube oscillator comprising two vacuum tube stages having input and output circuits connected in tandem, a thermistor connected in shunt to the input of the iirst stage, a feedback connection including a second thermistor from the output of the second stage to the input of the first stage anda heating circuit balanced with respect to ground for cyclically Varying the resistance of both thermistors.

5. An oscillator according to claim 4 havingl a resistor in the output circuit of one of the tubes and a condenser and a third thermistor Vcon- BORIS J. KINSBURG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Osbon Nov. '7, 1933 Artzt June 8,1943 Black Oct. 7, 1941 Wise May 25, 1943 Pooh Nov. 3, 1942