US2332811A - Limiter circuit - Google Patents

Description

Oct. 26, 1943. A RANKN ETAL 2,332,811

LIMITER CIRCUIT Filed Oct. :51, 1940 INVEN QRS Joknflmmid'zm' M iii-(ball BY 7% m-t/L ATTORNEY Patented Oct. 26, 1943 UNITED STATES PATENT OFFICE LIMITER CIRCUIT John A. Rankin, Port Washington, and Charles N. Kimball, Jackson Heights, N. Y., assignors to Radio Corporation of America, a corporation of Delaware Application October 31, 1940, Serial No. 363,626

6 Claims. (01. 178-44) quency channel chosen from a range of 2 to 4 megacycles. Usually the intermediate frequency value is between 2 and 3 megacycles. Hence, in receiver development work it is desirable to have a signal channel capable of producing frequency modulated waves in the 2 to 4 megacycle range, and wherein a deviation of 100 kilocycles is employed. By deviation, as employed in this specification, is meant the frequency departure of the modulated carrier above or below its center, or unmodulated, frequency value. Just as frequency modulation should be kept at a minimum in signal generators designed for amplitude modulation, so should amplitude modulation be eliminated as far as possible in frequency modulated signal generators. In the case of frequency modulation generators amplitude modulation introduced in the modulation process is subsequently removed by some form of amplitude limiter. Because limiter action is not perfect it is desirable to keep the amplitude modulation low in the original modulation process, but because a limiter can be used the requirements are not as rigorous as for amplitude modulation.

Accordingly, it may be stated that it is one of the main objects of our present inveition to provide a signal generator of the frequency modulated type in which a limiter is employed to eliminate amplitude modulation from the generator output, and the limiter employing in its output circuit a filter network functioning to remove undesirable harmonics generated in the limiter.

Another important object of this invention is to provide a limiter circuit for a frequency modulation signal generator wherein the plate load of the limiter tube takesthe formof a low pass filter whose cut-off frequency is less than double the center frequency of the generated waves whereby discrimination is had against the harmonics of the center frequency generated by the clipping action in the limiter circuit.

Still other objects of the present invention are to improve generally the simplcity and efliciency of signal generators of the frequency modulated type, and more particularly to provide such signal generators, adapted to, generate waves in the 2 to 4 megacycle range, which are reliable in operation and are economical to manufacture and assemble.

The novel features which we'believe to be characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a tube l of the SAC? type. This tube isprovided with a grounded cathode, a sig-' nal input-electrode 2, a screen grid 3, a suppressor grid 4 at cathode potential and a plate electrode 5. The signal grid 2 is connected by condenser B to the, source of frequency modulated carrier oscillations, and the grid side of condenser 6 is connected to ground through a leak resistor I.

The aforesaid oscillations have a center frequency f, and the oscillation source may comprise any well known network which is capable of producing oscillations having a center frequency in the 2 to 4 megacycle range, and wherein'the frequency deviation is 100 kilocycles. For example, and merely by way of illustration, there may be utilized in the source an oscillator of the tuned grid circuit type, and wherein there is employed across the oscillator tank circuit a reactance simulation tube. It is not believed necessary to illustrate this arrangement since it is well known. Briefly, in the latter case there is developed across the tank circuit alternating voltage which is in phase quadrature with the alternating voltage existing across the tank circuit. The

quadrature voltage is applied to the grid of the reactance tube, and the plate to cathode impedance of the reactance tube is connected across the tank circuit. As is very well known by those, skilled in the art, such an arrangement produces a reactive effect across the tank circuit whose magnitude may be varied as a function of the control grid bias of the reactance tube. The modulating audio frequency voltage is fed to the control grid of the reactance tube, and causes a variation in the tank circuit frequency by virtue of the variation of the simulated reactive effect across the tank circuit which is due to the re- 50,000 ohms for resistor I, the grid circuit of the limiter tube I develops the bias for limiting operation from grid current flow in the grid circuit. The screen grid of the limiter tube is connected to a source of positive potential through a resistor 8 having a magnitude of about 70,000 ohms.

A second resistor 9, of approximately 25,000 ohms, v

connects the screen end of resistor 8 to'ground.

. Condenser l and condenser H are each connected in shunt with resistor 9, and condenser [0 may have a magnitude of 8 microfarads, while condenser Il may have a magnitude of about 5,000 micro-microfarads.

The plate of the limiter tube is connected to the source of positive potential through a path which includes the coil [2, the sectioned load resistor l3 and coil l4. Each end of the coil I2 is connected to ground through a condenser.

Condenser 20 may have a magnitude of approximately 48.2 mmf., while condenser 2| may have a value of approximately 24.1 mmf. The coil I2 itself may have a value of approximately 48 microhenries (mh.). The load resistor I3 is divided into a plurality of 100 ohm sections. These sections may consist of 100 ohm carbon resistors connected to a switch permitting selection seconds so that any amplitude modulation of the oscillator is removed on the positive excursions of the waves, as well as by plate current cut-ofl' on the negative excursions. In other words, the plate voltage and screen voltage of the limiter tube are so chosen with respect to thetime constant of the grid circuit that the input-output characteristic shown immediately above the limiter tube is secured.

The primary function of the tube l is to act as an amplifier which is very easily overloaded. That is. grid current is caused to flow and plate current saturation is produced on one 'half of the si nal cycle. while plate current-cut-ofl is created on the other half of the cycle. The primary urposeof the tube is to wipe out all amplitud variations present in the output of the oscillator. and to pass on to the output terminals a c ns ant am litude frequency signal. The methmi of creating the required operating state for e limiter is by a combination of conditions. There is employed in the limiter tube zero initial ontrol grid bias with comparatively low values of screen and plate voltages so that the tube will overload very easily and plate current cut-off will be quickly reached.

Under the proper conditions frequency modulated waves of comparatively smallamplitude will swine the grid voltage into that zone which will cause plate current cut-off and plate. current saturation thereby limiting the magnitude of signal plate current variations in-the plate circuit. Since the operating point is established at the center of the linear portion of the plate currentcurve both halves of the plate current cycle will be equal in amplitude, but the current peaks will be flattened .out by the limiting action. In

I passing to the output terminals.

effect, then, the positive and negative signal peaks are clipped ofi.

The clipping action of the limiter tube results in the production of harmonics of the center frequency of the waves. 20-|2--2l is designed as a low pass series peaking network. This network is designed for the load resistor [3 of 1,000 ohms, and has a fiat response characteristic for all frequencies up to approximately 3.3 megacycles. In other words, and as shown by the characteristic curve located above the low pass filter network, the cut-ofi frequency of the filter is less than double f. In this way the harmonics off are prevented from The output transformer 52 is of the step-down type, and has its primary winding 5| of high inductance. Since it would usually feed into the grid circuit of an intermediate frequency amplifier stage being tested, the response across the secondary will be high and the voltage will be essentially the open circuit secondary voltage. Hence, the output voltage will be practically independent of frequency. It may be pointed out that a maximum output voltage of 3.3 volts can be had, and because of the decade output potentiometer, the minimum is 0.33 volt. This range of output should prove satisfactory for intermediate frequency alignment purposes. In place of a low pass filter there may be employed a band pass filter with a section to provide band elimination at the second harmonic of the signal or at any other objectionable harmonic.

While we have indicated and described a system for carrying our invention into effect, it wil be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications maybe made without departing from the scope of our invention, as set forth in the appended claims.

What we claim is:

1. In combination with a source of frequency modulated signals having variable amplitude and having a center frequency chosen from a range of 2 to 4 megacycles, a limiter network comprising a tube provided with input electrodes and output electrodes, a load impedance connected across saidoutput electrodes for developing purely frequency modulated signal voltage, a relatively short time constant network consisting of a condenser and a resistor coupling the limiter tube input electrodes to said source, a network arranged in circuit with said output impedance for preventing the development across said impedance of voltage of harmonics of the center frequency of said frequency modulated signals, and said last named network consisting of a shunt low pass filter whose cut-off frequency is less than double said center frequency.

2. In combination with a source of frequency modulated signals having variable amplitude and having a center frequency chosen from a range of 2 to 4 megacycles, a limiter network comprising a tube provided with input electrodes and output electrodes, a load impedance connected across said output electrodes for developing purely frequency modulated signal voltage, a relatively short time constant network consisting of a condenser and a resistor coupling the limiter tube input electrodes to said source, and a network arranged in circuit with said output impedance forpreventing the development across said impedance of voltage of harmonics of he Hence, the network' center frequency of said frequency modulated signals, said last named network consisting of a low pass peaking network arranged in shunt impedance connected across said output electrodes for developing purely frequency modulated signal voltage, a relatively short time constant network consisting of a condenser and a resistor coupling the limiter tube input electrodes to. said source, and a network arranged in circuit with said output impedance for preventing the development across said impedance of voltage of harmonics of the center frequency of said frequency modulated signals and said load impedance comprising a resistor composed of a plurality of sections, a switch element associated therewith, and a step-down transformer provided with 'high primary inductance connected to said switch arm.

4. In a frequency modulated wave generator adapted to generate variable amplitude-frequency modulated wave having a center frequency of the order of 2 to 4 megacycles, a limiter circuit for eliminating amplitude variations in the generated frequency modulated waves which includes an electron discharge tube provided with at least a cathode, a signal input electrode, a positive output electrode and positive screen grid arranged between the output electrode and said input electrode, condenser-resistor means having a time constant of the order of 2.5 microseconds for impressing frequency modulated waves upon said signal input electrode, the screen grid and output electrode potentials-being so chosen that amplitude variations in said impressed oscillations are eliminated, an output circuit coupled to said output electrode for developing output voltage of purely frequency modulated waves, and a low pass filter network connected across said output circuit for attenuating voltage of harmonics of the center frequency of said frequency modulated waves.

5. In a frequency modulated wave generator adapted to generate variable amplitude-frequency modulated waves having a center frequency of the order of 2 to 4 megacycles, a limiter circuit for eliminating amplitude variations in the generated frequency modulated waves which includes an electron discharge tube provided with at least acathodeja signal input electrode, a positive output electrode and positive screen grid arranged between the output electrode and said,

input electrode, condenser-resistor means having a time constant of the orderof 2.5 microseconds for impressing frequency modulated waves upon said signal input electrode, the screen grid and output electrode potentials being so chosen that amplitude variations in said impressed oscillations are eliminated, an output circuit coupled to said output electrode for developing output voltage of purely frequency modulated waves, and a filter network connected across said output circuit for attenuating voltage of harmonics of the center frequency of said frequency modulated waves and said filter network consisting of a low pass circuit having a cut-off frequency below the second harmonicof said center frequency.

-6. In a frequency modulated wave generator, an amplitude limiter circuit which includes an electron discharge tube provided-with at least a cathode, a signal input electrode, a positive outputelectrode, a positive screen grid arranged between the output electrode and said input electrode, a series condenser-shunt resistor network having a time constant of relatively short value of the order of 2.5 microseconds for impressing frequency modulated waves upon said signal input electrode, the screen grid and output electrode potential being so chosen that amplitude variations in said impressed oscillations are eliminated, and a filter network connected to said output electrode for attenuating voltage of harmonics of the center frequency of said frequency modulated waves.

JOHN A. RANKIN.

CHARLES N. KEMIB

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