US2442138A - Stabilized modulated oscillator - Google Patents

Description

, E. R. MANN ETAL' STABILIZED MODULA'IED OSCILLATOR May 25, 1948.

Filed Aug. 5, 1944 H" ks 772mm INVENTbRS f TTOPNEY Patented May 25, 1948 STABILIZED MODULATED OSCILLATOR Estle Ray Mann, Upper Montclair, and Morris Spector, Clifton, N. J., assignors to Allen B. Du Mont Laboratories, Inc., Passaic, N. J., a

corporation of Delaware Application August 5, 1944, Serial No. 548,158

This invention relates to a modulated oscillator in which provision is made by means of which the oscillator is kept sensitive and is also prevented from becoming overloaded. The OS- cill'ator is thereby stabilized.

Heretofore vacuum tube oscillators have been provided for testing or examining metals by placing pieces of the metals in or passing them through an inductance coil that is located in the grid tank circuit of the oscillator and modulating the oscillator by means of a low frequency voltage, The modulated output from the oscillator may be examined, for example, by means of an oscillograph; and since the pattern on the face of the cathode-ray tube of the oscillograph varies with the sort and condition of the metal placed in or passed through the coil, information relating to the metal is thereby rendered available.

Means are also available for selecting-or sorting metal pieces by placing them in or passing themthrough such coils and utilizing the signals so produced for sorting or selecting the metal pieces. 7

It is desirable to make such oscillators both stable and sensitive. Stability can usually be se-- cured by using push-pull vacuum tube circuits in the oscillator and providing symmetry of components and by modulating the oscillator by means of a stable signal of low frequency'compared to that of the oscillator. In this way the oscillator can be made to oscillate and still be very stable even very near its quench point, The low frequency modulator supplies energy at all times regardless of the amplitude of the high frequency oscillations, thereby sustaining the high frequency oscillations, even when they are very close to their quench point, so that they do not become entirely quenched.

However, when the load is removed from the output of the oscillator the amplitude of the high frequency oscillations may increase to such an extent that the oscillator becomes so much overloaded that the output oscillations become objectionably distorted.

By the present invention, the oscillator is kept sensitive and is also preventedfrom becoming overloaded. That is, the oscillator is kept oscillating and the amplitude of the oscillations are prevented from becoming too large.

The invention may be understood from the description in connection with the accompanying drawing which is a diagram of connections illustrating the invention.

In the drawing, reference character I indicates a coil into which the metal to be tested is placed Claims. (Cl. 25036) or through which it is passed. It is tuned by capaoitor I. A source 2 of low frequency current, say l03-cycles per second, is connected to one end of resistor 3 the other end of this resistor being grounded. A sliding contact 3' on resistor 3 is connected to the center of coil l.

Theends oi;coil l are coupled by leadst and 5 and capacitors 6 and 1 to the grids of push-pull oscillator tubes 8 and 9 which are provided with grid leak resistors IE] and II., and with cathode bias resistors l2 and I3 connected to a common variable resistor M which is connected toground.

The ends of. coil I are also coupled by leads it and ll and capacitors [8' and IE to the grids of push-pull cathode biased amplifier tubes 20 and 2| which are provided with grid leak resistors 22 and 23 that are connected to intermediate points 24 and 25, on cathode bias resistorsZS and 27 of tubes 20 and 2 '1, respectively.

The platev of tube 8 iscoupled by lead 28, capacitor 29, and variable resistor 30 to ground. An adjustable contact on resistor 30 is connected by lead 3| to the lead 5. The plate of tube 9 is coupled by lead 32, capacitor 33, and variable resistor 34 to ground. An adjustable contact on resistor 34 is connected by lead 35t0 the lead 4. This provides a push-pull oscillator of the known sort. The plates of tubes 20 and 2| are connected by leads 36 and 31 to a source of positive potential while the plates of tubes 8 and 9 are connected through plate load resistors 38 and 39 to the samesource of potential.

The cathodes of. tubes 20 and 2! are coupled by capacitors 40 and 4| to potentiometer gain control resistors 42 and 43. The choke coils M and 45, which are in parallel with the resistors 42 and 43, are filters toremove the low frequency modulating componentsthat are introduced by the alternator 2. The sliding contacts and leads 45 and. 41 couple the gain controls. 42 and it to the grids of tetrodes 48. and49. The cathodes of these tubes are'biased by resistor 50 that is connected to ground as are the adjacent ends of resistors 42, 43- andinductance coils 44 and 45.- A resistor 5! is provided in the usual way between a source of positive potential for the plates. of tubes-48, 49 and the screen gridsof these tubes, and capacitors 52 and 53 are. provided between the screen grids of these tubes and the: cathodes thereof. and ground. The load'resistors oftubes 48 and 49 are shown at 54 and 55.

The leads 56and 5'! and coupling capacitors 58 and 59are for applying the amplitude modulated high frequency signals on the plates. of tubes 48 and 49 to an indicating device not shown,

such as a cathode-ray tube, a meter, a lamp, a trigger circuit or a relay that are well known for observing or utilizing the high frequency envelope.

necting them to points 61 and 68, respectively,

on voltage dividers 69, 10 and H, 12 that are connected between sources of positive potential coil I one after the other, one of them will more and ground. As long as the points 61 and 68 are at a potential level higher than the ampli tude of a signal applied to the plates of these diodes 65 and 66 no current will flow through them. The signal fed into the cathodes of tubes 65 and 66 from the amplifiers 48 and His the amplified amplitude modulated high frequency signal from coil l The filter network comprising resistors i3, 14 and 15 and capacitors 16 and 82 contains a series resonant circuit consisting of the capacitor 1! and an inductor 18 in series. This resonant circuit 11, I8 is tuned to the modulating frequency which is the frequency of the source 2. A lead 89 connects a point between the resistors I0 and ll to one end of this filter. A resistor 8| is connected between the plates of diodes 65 and 66 and said filter network. I

The operation is as follows:

Apositive potential is applied at the point shown on the drawing above tube 20, whereupon the push-pull oscillator 8-9 begins to oscillate. The amplitude of the oscillations is adjusted by varyingthe-resistors I4, 30 and 34, the latter two' beingvaried together. This amplitude can be observed by means of the indicating device mentioned above for, observing the envelopeof these high frequency oscillations. The alternatori. feeds a low frequency potential of say 103 cycles, per second tothe coil l.

The amplitude of this potential is adjusted by positioning the contactor 3 on resistor 3 to providethedesired' amount of modulating signal. This modulating signal aids in keeping the high frequency oscillations stable without causing the high frequency oscillator to lose sensitivity.

,The stabilizing effect is also aided by providing symmetry. of components in the duplicate portions of the oscillator. 7

Due to the core losses in the sample caused by the, oscillating magnetic. field of coil I, the amplitude of the high frequency oscillations is decreased,-thus,causing the peaks of the modulated envelope to decrease. Such distortion is obviated with the present invention.

The output signals from the plate circuits of tubes 48 and 49 are coupled by'leads 56 and 51 and condensers58 and 59, respectively, to'diodes and filter circuits which detect the envelope produced by the modulated oscillator. The signals having the amplitude and phase of this envelope taken from the leads 56 and 51 may be used in usual waysto produce useful indications on a cathode ray tube, a meter, or to operate a relay by means of atriggering device, for example.

.If. the amplitude of the oscillations were allowed to increase, without automatic volume control means, the output from the oscillator would be an output from which the envelope had been removed from the amplitude modulated signal due to, overloading of the oscillator or overloading the two amplifiers 48, which follow the oscillator, or overloading both. With this invention, the amplitude of the oscillations are 4 limited to a predetermined maximum so that overloading does not occur and therefore the envelope is not removed.

The sensitivity of the oscillator is greatest when it is perating near its quench point as is usual with oscillators of this sort. It is therefore desirable in using the. device for a metal specimen in the test coil l to bring the amplitude of the oscillations to near zero. When this 'is'done, if two metal specimens differing in-some metallurgical properties are inserted in the test nearly quench the oscillator than the other. The

amplitude of the oscillations in the one case will be smaller than itwill be in the other case. The amplitude of the envelope in the one case will also be smaller than it will be in the other case.

' If, however, the metal specimen is withdrawn from the test coil the amplitude of the oscillations will increase to such an extent that overloading will occur and the envelope will be removed unless provision is made for preventing this.

To prevent this overloading a negative voltage is inserted at the proper time, namely, as the core is being withdrawn, between resistors l0 and H so that the grids of the oscillator tubes 8 and 9 will be driven sufiiciently negative to keep the amplitude of the oscillations down below the overload point. However, it is desirable that this prevention of overloadingzshould not begin when the oscillator is near its quench point. If this were done the voltage fed back by means of the automatic volume control means would be equivalent to introducing degeneration in the oscillator and its attendant circuits. The intro-' duction of this degeneration would serve the purpose of preventing overloading, but at the same time would diminish the sensitivity of the instrument sothat it might not indicate the difference between core losses of two unlike metal specimens which differ only slightly.

Therefore, some delay is provided for inthe automatic volume control means. This desirable delay is provided by the predetermined voltage that is applied at points 61 and. 68 taken from the voltage dividers 69, 10 and H, 12, respectively, so that the amplitudes of the output signals from the plate circuits of tubes 48 and 49 at-which the diodes 65 and 66 are allowed to conduct is predetermined. Signal amplitudes below the voltages at 61 and 68 do not allow these diodes to conduct. By this means the automatic long as the amplitudes of the signals from plates of tubes 48 and 49 are lower than the potentials at 61 and 68, but is applied when these amplitudes exceed the potentials at 61 and 6B.

By this means all the sensitivity desired for the oscillator is made available without degeneration when it is' operating near its quench point. As the amplitudes increase when a specie men is removed from the test coil and when sensitivity is no longer desired the automatic volume control introduces degeneration and decreases the sensitivity when high sensitivity is no longer needed. I 1 f The signal rectified by diodesBS and 66 contains a high percentage of the modulating frequency envelope. This rectified signal is filtered by the filter consisting of condensers i6 and 82, and resistors 13, 14 andl5 and the series network consisting of capacitance I1 and inductance 18 which is tuned to the modulating frequency. If this modulating "frequency were not removed from the rectified signal it would be reinserted at the point between resistors l0 and II of the oscillator and would remodulate the oscillator. This remodulation could be the same or different in both phase and amplitude from that inserted by the source 2.

With the delayed automatic volume control circuits, as a metal specimen is inserted in the test coil I, the amplitude of the oscillator diminishes slowly at first until it drops below a voltage predetermined by the potentials at the points 61 and 68 because at voltages higher than this the oscillator is relatively insensitive. When the amplitude of the oscillations have dropped below this level further insertion of the specimen into the coil produces large changes in these amplitudes because without this automatic volume control the oscillator would be much more sensitive to changes in core losses that are produced by the presence of the metal that is inserted into the test coil I. With this invention the oscillator retains its high sensitivity to small diiferences between core losses that are produced by different metals being in the test coil and, at the same time, this oscillator becomes relatively insensitive to the large core loss difierences which are shown when the test coil has no metal core and when it has a metal core inserted into it.

What is claimed is:

1. A stabilized oscillator comprising a pair of vacuum tubes each having a control grid, a source of low frequency current connected to each one of said control grids to amplitude modulate the oscillations of said oscillator, push-pull cathode biased amplifiers having control grids to which the outputs of said pair of vacuum tubes are respectively coupled, a filter for said low frequency current located between the output terminals of said push-pull amplifiers and ground, a pair of diodes having cathodes to which said push-pull amplifiers are coupled respectively, and a filter for said low frequency current located between the control grids of said pair of vacuum tubes and the plates of said diodes.

2. The device of claim 1 in which push-pull amplifiers are located between said filter and said diodes and connected thereto.

3. A stabilized oscillator comprising a pair of vacuum tubes each having a control grid, a source of low frequency current connected to each one of said control grids to amplitude modulate the oscillations of said oscillator, push-pull cathode biased amplifiers having control grids to which the outputs of said vacuum tubes are respectively coupled, a filter for said low frequency current located between the output terminals of said push-pull amplifiers and ground, a pair of diodes having cathodes to which a source of positive potential and said push-pull amplifiers are coupled respectively, and a filter for said low frequency current located between the control grids of said oscillator and the plates of said diodes.

4. The device of claim 1 in which said filter is resonant at the frequency of said low frequency current.

5. The device of claim 1 in which resistors are located between a source of positive potential and the cathodes of said diodes.

ESTLE RAY MANN. MORRIS 'SPECTOR.

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

UNITED STATES PATENTS Number Name Date,

2,138,138 Bruckner Nov. 29, 1938 2,264,369 Golicke Dec. 2, 1941 2,343,539 Edson Mar. '7, 1944 2,352,219 Olesen June 27, 1944 FOREIGN PATENTS Number Country Date 426,396 Great Britain Mar. 28, 1935

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