US3153201A

US3153201A - Oscillator-limiter-detector apparatus and method

Info

Publication number: US3153201A
Application number: US157285A
Authority: US
Inventors: Walter D Knight
Original assignee: Individual
Current assignee: Individual
Priority date: 1961-12-05
Filing date: 1961-12-05
Publication date: 1964-10-13
Anticipated expiration: 1981-10-13

Description

Oct. 13, 1964 w. D. KNIGHT 3,153,201

OSCILLATOR-LIMITER-DETECTOR APPARATUS AND METHOD Filed Dec. 5, 1961 8 RFC 6 7| 24 T 5 u fi I INVENTOR. WALTER D. KNIGHT A T TOPNEV United States Patent Ofifice.

j 3,153,201 Patented Get-1 1964 oscuirnrou-rnnrunuurncron APPARATUS AND METHQD Waiter D. Knight, Berkeley, Calif., assignor, by mesne assignments, to the United States of America as repre-' sented by the Secretary of the Navy Filed Dec. 5, 1961, Ser. No. 157,285 2 (Ilairns. (Cl. 329 -150) Pound and W. D. Knight, Review of Scientific Instruments, volume 21, page 219 (1950), permits of sensitive operation with good signal-to-noise ratio only in the range of RF levels between 0.01 and 1 volt. Schemes have been contrived which extend the range to microvolt levels, using the technique of increasingv the gain of the RF amplifier. It has been shownthat reliable operation can be achieved at low levels by incorporating a limiter in the feedback network. This, along with suitably high RF amplification, facilitates stableoperation at low voltage levels,

An object of the present invention is to provide a modification of the cathode-coupled circuit which possesses someof the advantages of both the marginal and the limited modes of operation, and to provide a method for operating such a circuit.

Other objects and many of the attendant advantages of V this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein the single figure is a;

circuit diagram illustrating a preferred embodiment of the circuit'of this invention. 7

The illustrated preferred embodiment of the circuit of this invention comprises a pair of multielectrode electron tube sections. These can be separate vacuum tubes but preferably form sections of a multi-unit tube, shown as a twin triode but conceivably a triode-pentode or other combination. By Way of example the figure shows a 6BQ7A and the values of the circuit parameters are appropriate for this tube. The first half 2 of the tube is a cathode follower to which is connected the tank circuit including coil L and condenser C. The second half 4 of the tube operates both as a cathode-driven detector (which provides a small degree of automatic level control), and as a plate-limited RF amplifier. Regeneration is provided by the capacitor 6.

The amplifier plate circuit is returned to a variable direct voltage source, illustrated as a potentiometer 8. This plate supply voltage source is constructed so as to supply a plate voltage to the amplifier, between plate and cathode, which never exceeds about one half of the normal operating plate voltage for which the particular tube is designed. The desired limiting action of the apparatus is achieved by imposing this low plate voltage. The signal developed across the cathode resistor 10 of the first section of the triode is cathode-coupled to the section 4 of the triode through capacitor 12. The signal appearing at the cathode 14 is amplified and limited at the plate 16' and fed back to the input grid 18 via capacitor 6.

is the detector filter.

and capacitor 34 forms an RF filter to keep RF voltage I The plate load 20 for tube section 4 is shown as an RF choke, typically 1 millihenry, which'is preferable, although a resistance could be substituted for it" of say l to 10 kilo-ohms. It is across this load 20 that there is developed the regenerative voltage fed back through impedance 6 to grid 18. Impedance 6 should be at least as high and preferablyhigher than the LC impedance.

The cathode load 22101 tube section 4 is shown as an RF choke, typically 1 millihenry, but could be a resistance although this would not work as well.

At junction 24 appears, for example, +50 volts D.C.,

while at junction 26 appears ground which is 0 volts D.C.

Hence 6 must be a D.C. blocking device such'as a condenser as shown of small capacitance or aresistor plus a blocking condenser.

The network including resistor 28 and capacitor 30 The network including resistor 32 out of the output appearing at 36, which typically may be an audio voltage superimposed on the rectified RF voltage. The 33-oh-m resistors shown in the plate circuit and grid circuit of tube section 2 are parasitic suppressors.

Operation The operation of the apparatus in its especially useful function in nuclear magnetic resonance studies-is as follows. The coil L of the tank circuit surrounds'a sample of material whose nuclear magnetic resonance is to be studied. The oscillation of the device startsby perhaps random noise. A D.C. voltmeter or like instrument is placed to measure the output, namely the voltage between output 36 and ground. The potentiometer 8 is vacuum tube voltmeter is removed from the circuit.

Usuallythe sample of material to be examined is immersed in a magnetic field, which field is varied at an audio rate. As the magnetic field is Varied and a point is reached at which the sample exhibits nuclear magnetic resonance, the 'Q of L diminishes because of absorption of energy by the sample. As the Q goes down, the voltage across L goes down and hence the output at 36 decreases in proportion. It may be noted that when the magnetic field surrounding'the sample is varied continuously at an audio rate, there appears at the output terminal 36 an audio output superimposed on the direct voltage at 36. This output is what is measured in conducting the nuclear magnetic resonance studies and can be amplified with a tuned amplifier to reduce noise if desired.

This circuit is particularly suitable for application to problems in nuclear resonance which require RF levels between '1 and 20 volts R.M.S. Near one volt the operation is marginal; at higher levels the limiting action is a predominant. The arrangement shown in the drawing has been used between 1 and 10 me. andat levels between 1 and 10 volts R.M.S., with sensitivity equal to the best attainable with the usual marginal circuit.

Since there are no time constants in the audio-frequency range, very low modulation frequencies can be used, and motorboating does not occur. Additionally, it may be noted that this circuit easily works to frequencies of 50 me. or above, since a separate RF amplifier is not required, and since the feedback network is simple.

The method of the invention comprises operating the circuit of the invention by supplying a plate supply voltage to the amplifier tube section of such a magnitude that a the-potential difference between the plate and the cathode, is always maintained at less than one half of the normal operating plate voltage for the particular tube being used.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. :It is therefore to be understood that within j=the scope :of the appended claims the invention may :be

practiced otherwise than as specifically described.

What is claimed is:

1. An oscillator-limiter-detector comprising a first electron tube section serving'as a cathode follower; a tank circuit connected between ground and the grid of said cathode follower to provide the input to said cathode tolplifie-r remains always at less than one half the normal operating volt-age for said amplifier tube section; means connected between said input tank circuit and the plate of said amplifier tube section to feed back RF signal from said .plate to said input tank circuit; and means connected between ground and grid in said amplifier tube section to provide a filtered, detected out-put essentially free of RF content.

2. The apparatus of claim 1 wherein said 'two tube sections constitute sections of a rmulti-unit tube.

References Cited in the file of this patent UNITED STATES PATENTS Mackey Nov. 27, 1956 OTHER REFERENCES Ingram: Spectroscopy .at Radio "and Microwave Frequencies, Butterworths Scientific Publications, London, 1955, pages 102, 103, 235, 236 and 237-.

Blume: The Review ofScientific Instruments, vol. 29,

No. 7, July 1958, pages 574 through 1577.

Claims

1. AN OSCILLATOR-LIMITER-DETECTOR COMPRISING A FIRST ELECTRON TUBE SECTION SERVING AS A CATHODE FOLLOWER; A TANK CIRCUIT CONNECTED BETWEEN GROUND AND THE GRID OF SAID CATHODE FOLLOWER TO PROVIDE THE INPUT TO SAID CATHODE FOLLOWER; A SECOND ELECTRON TUBE SECTION OPERATING AS A CATHODE-DRIVEN DETECTOR, IN WHICH DETECTION TAKES PLACE BETWEEN ITS GRID AND ITS CATHODE, AND AS A PLATE-LIMITED RF AMPLIFIER; MEANS CATHODE-COUPLING THE SIGNAL FROM THE OUTPUT OF THE CATHODE FOLLOWER TO THE CATHODE OF SAID SECOND TUBE SECTION; MEANS SUPPLYING A VARIABLE DIRECT PLATE SUPPLY VOLTAGE TO THE PLATE CIRCUIT OF SAID AMPLIFIER OF SUCH A MAGNITUDE THAT THE PLATE VOLTAGE OF SAID AMPLIFIER REMAINS ALWAYS AT LESS THAN ONE HALF THE NORMAL OPERATING VOLTAGE FOR SAID AMPLIFIER TUBE SECTION; MEANS CONNECTED BETWEEN SAID INPUT TANK CIRCUIT AND THE PLATE OF SAID AMPLIFIER TUBE SECTION TO FEED BACK RF SIGNAL FROM SAID PLATE TO SAID INPUT TANK CIRCUIT; AND MEANS CONNECTED BETWEEN GROUND AND GRID IN SAID AMPLIFIER TUBE SECTION TO PROVIDE A FILTERED, DETECTED OUTPUT ESSENTIALLY FREE OF RF CONTENT.