US2267732A - Radio receiving system - Google Patents

Description

Dec. 30, 1941.

c. w. HANSELL RADIO RECEIVING SYSTEM File'd Nov. 24, 1939 3 Sheets-Sheet l T0 ANTE/YNA n TRANSMISSION L/IVE Fig. 2

UTPUT 7'0 A/EXTSTAGE r0 ANTENNA PULS/NG 0sc/4AT0R OUTPUT z E AMPL/F/ERS SECOND 0575070? AND AUDIO AMPLIFIERS I HETERODYNE ZI I POTEA/T/OMETER INVENTOR CLARENCE W. F4IV$ELL NAM ATTORNEY I BEA T/IVG OSCILLA 70R PULSl/VG OSCILLATOR 12 Dec. 30, 1941. c. w. HANSELL 2,267,732

RADIO RECEIVING SYSTEM Filed Nov; 24, 19:59 5 Sheets-Sheet 2 F/L TER r0 ANTENNA I j 010,000.; 400/0 AMPLIFIER POWER I V Y V V V r 5 0 U R C E PULS/A/G OSCILLA r02 #7 70 A uo/b UTILIZATION DEV/CE AUDIO 3/ OUTPUT INVENTOR CLARENCE W HANSELL ATTQRNEY Dec. 30, 1941. I c. w. HANSELL 2,267,732

RADIO RECEIVING SYSTEM Filed Nov. 24, 1939 3 Sheets-Sheet 3 r0 ANTENNA 44 0 AN ENNA {12E 5 jmnnn I 36 INVENTOR CLARENCE W. HANSELL BY g ATTORNEY Patented Dec. 30, 1941 RADIO RECEIVING SYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Gorporationof America, a corporation of Delaware Application November 24, 1939, Serial No. 305,800

17 Claims. 1C1. 250-20) The present invention relates to improvements in radio receivers.

In the reception of very high frequency signals, the input impedance of the first vacuum tube is a large factor in loading the input circuit. This loading reduces appreciably the amount of oscillatory energy applied to the tube and is often responsible for a low ratio of signal to noise, particularly a low ratio of signal to tube noise and thermal agitation noise.

One of the objects of the present invention is to provide a receiver which possesses an improved signal to noise ratio, and particularly an improved ratio of signal to noise in the input circuit for the first stage.

Another object of the invention is to obtain greater selectivity in the first circuits of a receiver.

A further object is to simplify receivers by eliminating the requirement for heterodyning to a lower frequency in order to obtain a desired degree of selectivity.

The foregoing objects and others, which will appear later from a reading of the description of the invention, are achieved in one particular emi bodiment by storing a relatively large amount of signal energy in the form of high frequency oscillations in a low power factor resonant circuit and periodically effectively connecting the storage circuit suddenly to the signal grid of the first vacuum tube of the receiver, or by rendering the vacuum tube periodically active in a manner to accomplish an equivalent result. For this purpose, the vacuum tube input electron current loading is first effectively removed from its input 9 circuit and the input circuit correctly coupled to the source of signal oscillations, usually an antenna or transmission line. In this way a much larger current and voltage is built up in the low power factor input circuit than can be obtained by having the vacuum tube effectively connected, or maintained continuously active, in the input circuit. After the large amount of energy has been stored in the low power factor input circuit, it is quickly used to momentarily control electron currents by means of power input to the control grid of the first tube in the receiver, thus providing momentarily a greatly improved signal to noise ratio. If this is repeated periodically, it should be apparent that the power available for the input of the tube is momentarily much higher than the maximum obtainable continuous or average value. The receiver circuits are designed to be responsive only during the periods of high signal to noise ratio.

In practice, it is proposed to increase the input impedance of the first vacuum tube during receiver off periods by applying a high negative bias to the control grid, or to a space charge 2 grid between the cathode and the control grid.

As an illustration of the benefits to be derived from the present invention, let us assume that the first vacuum tube of the receiver, under one set of operating conditions at very high frequencies, has an effective input resistance of 2000 ohms, and that the receiving antenna is connected with first circuit of the receiver over a transmission line which has its characteristic impedance matched by the input impedance of th first circuit. If this first circuit is a concentric line resonator, its power factor may be as low as one part in ten thousand. Assume that, when used in the ordinary way, 10% of the received power is to be lost in the first circuit of the receiver (i. e., tuned grid circuit) and 90% applied to the input of the first tube. This would result in a final overall frequency selectivity for the input circuits to the first tube equivalent to a tuned circuit with a power factor of substantially one part in 500 (taking into account antenna, transmission line, circuit and tube loading). In the assumed case, if we now remove the tube loading and rematch the antenna line to the circuit we can build up in the circuit an oscillatory stored energy much larger than before. The power factor can then be reduced to only one part in five thousand (taking into account antenna loading and circuit losses), and with a given strength of signal to be received We can build u p ten times greateroscillatory energy and V10 times greater voltage and current in the input circuit. Furthermore, the circuit selectivity is now ten times as good as before.

If now the bias on the vacuum tube is momentarily reduced so as to throw the tube input electron loading on the circuit suddenly, it is possible to deliver momentarily to the vacuum tube ten times as much signal power with at least ten times greater signal to noise power ratio. If the vacuum tube is made to be effective repeatedly for short pulses, with relatively long off periods between them, we thus have a receiving system capable of delivering approximately ten times better signal to noise power ratio than in the original assumed ordinary case. If the intervals between pulses are made substantially equivalent to the time constant of the input circuit and are then made of shorter and shorter duration with correspondingly greater and greater momentary circuit loading by the mission line TL is connected across a correct portube, there should be obtained greater and tion of the tuned circuit so that the charactergreater signal to noise power ratio. pulse period is 1% of the total time and 99% of the time of the ordinary signal pulse is used to build up or store energy, then the power input to the vacuum tube during active periods may be about one hundred times greater than when the tube is active continuously, assuming that there is no limit on the power factor of the input tuned circuit or that thefrequency of pulses is great enough to neglect power factor. The noise power built up in the input circuit at, or just before, the beginning of each pulse period should be only as high as in the ordinary case, because of increased circuit selectivity, and the noise introduced by shot effect, and thermal agitation, upon the first grid, after the tube beistic impedance of the line TL is matched by the resonant input resistance of the circuit.

Under these conditions maximum power is delivered to the circuit. Also, the transmission line TL and the resistances R and R across the circuit have equal effect in determining the circuit frequency selectivity,

If now, the 2000 ohm load R is removed from the circuit 5|, 52, R and the connection of the comes active, should be only 1% as great as in.

the ordinary case. This is the theoretical optimum for the conditions assumed. In practice, however, all of the theoretically possible gain cannot be realized. In the case just given, in order to make practical use of the principles described, there should be one or more activepulses for the vacuum tube for each cycle or pulse of useful modulation, even though the active pulses are separated from one another by one hundred pulse time intervals.

Thus, by storing up the received power in a flywheel circuit, and then using it at a high rate for a small percentage of time, we may make the signal much higher in proportion to tube and circuit noise during the used periods.

Although the principles of the invention are described herein in connection with the first vacuum tube of a receiver, it. should be distinctly understood that they are equally applicable at any point in areceiver or signalling system where it is desired to overcome the effects of an ob-.

jectionable source of noise. For example, it may be desirable to overcome hum or other local noises in communication circuits in one or more stages following the first stage. The invention findsapplication in receivers capable of receiving signals from continuous wave telegraph and similar transmitters as well as in receivers capable of receiving speech waves.

The following is a detailed description of the invention in conjunction with the drawings, wherein Figs, 1 to 9, inclusive, show receiver circuits illustrating different embodiments of the invention. Throughout the figures of the drawings the same reference numerals are intended to designate the same or equivalent parts.

Referring more particularly to Fig. 1, there is shown an arrangement of circuits for illustrating the principles of the invention. These circuits might be capable of being used at relatively low frequencies. In Fi l I have shown a transmission lineTLfor delivering power to a circuit 5|, E52 tuned to the power frequency. This tuned circuit is constituted by an inductance coil 5| in parallel relation to a condenser 52. We may assume that the reactances of the inductance 5| and the capacity 52 are each 2 ohms at resonance and that losses in the circuit are equivalent to those which would be produced in a parallel resistance R of 20,000 ohms connected across the tuned circuit 5|, 52. The useful load, here indicated by R, is shown connected across the tuned circuit, or removed, by means of a switch or relay S. Let us assume this load R to be 2000 ohms.

To correspond with the usual case, we may assume that the 2000 ohm load R is connected across the tuned circuit 5|, 52 and that the transtransmission line TL to the circuit is readjusted to restore an impedance match between the characteristic impedance of the line TL and the input impedance of the circuit 5|, 52, R, then the oscillatory energy which can be built up in the circuit 5|, 52 by a given available power input will be increased'in the ratio of 11 to 1. Therefore, by connecting the 2000 ohm load R across the circuit momentarily, we can deliver power to the load R at a rate which begins at eleven times higher than could be obtained if the load were continuously applied. The mean or average increase in power delivered to the load, for the whole duration of the load connection, may be say 10 to 1. Therefore, by connecting the 2000 ohm load R to the circuit 5|, 52 repeatedly, with only momentary contact and sufiicient time between contacts, we may obtain an increase of about 10 to 1 in power delivered to the load, during active periods. If the load were a part of a signalling system, and contained a source of noise, we might therefore obtain a 10 to 1 increase in signal to noise power ratio during active periods.

In Fig. 2 there is shown an arrangement for employing the principles described in Fig. l in improving the signal to noise ratio in a very high frequency receiver circuit. This receiving circuit comprises a vacuum tube I0 used as the first tube in the receiver, a low power factor concentric resonant line input circuit M connected to the control grid of tube l0, a transmission line |3 extending from concentric line M to a receiving antenna, not shown, a pulsing oscillator l2 here shown conventionally in box form, and a pulsing control vacuum tube coupled between the oscillator and the first tube. The power received on the antenna is brought to the low power factor tuned circuit l4 over the transmission line I3, from which energy is fed to the control grid of the first tube I0. (In some cases a multi-grid tube may be used instead of the three electrode tube shown.)

Concentric line I4 should be considered as being typical of any desired type of low loss, low power factor tuned circuit which can be used, including resonant circuits made up of concentrated inductance and capacity as well as resonant cavities in conducting enclosures. In the form illustrated, it is electrically one-quarter of a wave long and consists of an outer conductor and a coaxial inner conductor both connected together at one end by a metallic plate which is connected to ground and to the cathode of tube I. This type of resonator is now well known in the art and is described in some detail in the article by Clarence W, Hansell entitled Resonant lines for frequency control, published in Electrical Engineering, August, 1935, pages 852 to 857, to which reference is made for a more detailed description.

During most of the time the first receiver tube W has a strong negative direct current biasing potential on the control grid which effectively keeps the space inside the tube clear of free electrons. Under this condition the tube input impedance, most of the time, is almost as high as if the cathode were not heated. During these periods of high negative bias the tube loads the low power factor concentric line l4 circuit relatively little and, by proper impedance matching of circuit M to transmission line I3, a relatively high signal current may be built up in the circuit M if this circuit is tuned to the signal. Also, the frequency selectivity of the resonant input circuit I4 is relatively high because the only substantial loading on it is its own losses and effective loading of the antenna line 13. I

By means of a pulsing control tube ll, normal operating bias may be applied to the first receiver tube Ill momentarily, at frequent intervals. This happens whenever thepulsing control tube I I has its negative grid potential swung below cut-off, for the anode circuit, by the oscillator input [2. A biasing power source, through resistance l5, supplies normal grid bias for tube l0 whenever the anode current of tube H is cut off, that is, when the tube l l is non-conducting. When ourrent flows in tube II, the potential drop through I resistance I5 is increased to such an extent that the first tube I0 is made to be non-conducting.

To obtain pulsing circuits, or pulsing oscillators, one skilled in the art may draw upon the radio television art in which pulsing oscillators and circuits are used.

During the moments of normal bias, the first receiver tube I0 fills up with moving electrons and is capable of being used as an amplifier or detector according to the design of the receiver. Of course, the presence of the electrons gives the tube a relatively low resistive component of input impedance. However, this low input impedance is thrown as a load upon the input circuit [4 only momentarily instead of continuously as in the,

ordinary receiver.

Due to the momentary character of the loading, the value of loading, while it lasts, is relatively very high and the tube is strongly excited by the relatively large energy stored in the input circuit Hi. If the active periods are say of the total time, then power may be taken from the input circuit at approximately ten times the rate which could be permitted in an ordinary receiver and will be at a rate correspondingly higher than the rate at which noise energy is generated in the tube.

If, as an example, the receiver is used to receive signals from a transmitter with an ultra short wave carrier frequency of 200,000,000 cycles per second, then the pulsing frequency might be on the order of 50,000 cycles with 5% on periods. Then each pulse would have the benefit of 4000 cycles of stored energy utilized during a period of 200 cycles.

If the effective power factor of the receiver input circuit is made to be as low as one part in 20,000, it alone can be relied upon for nearly all the high frequency selectivity of a radio telephone receiver. The first tube might then be used as a detector and followed only by a low pass filter to pass 0 to 10,000 cycles and an audio amplifier. Such an arrangement is shown in Fig. 4.

Fig. 3 is a modification of the receiver of Fig. 2 and shows how the principles of the invention can be applied to a superheterodyne receiver. In Fig. 3, the first heterodyne detector is supplied with energy both by the receiving antenna 21 and by the first beating oscillator 22. The low power factor energy storage circuit 23 is in the output of the first detector and. comprises the input circuit feeding the control grid of the first intermediate frequency amplifier vacuum tube 24, here shown as a screen grid device. The local oscillator 22 produces oscillations which are strong enough to eliminate from the output of the first detector 20 everything except the beat frequencies with the oscillator carrier. The pulsing oscillator I2 is here shown as feeding a push-pull pulsing tube circuit II. This pushpull pulsing tube circuit allows the first intermediate frequency tube 24 to be active only when the pulsing tubes have nearly zero instantaneous grid voltage from the pulsing oscillator I2. The output from vacuum tube 24 is, if desired, fed to one or more suitable intermediate frequency amplifier stages and to the second detector and audio amplifier stages here shown labeled in box form as 25, from which the signal can be heard in a loudspeaker or headphones. Any power at the pulsing frequency, or its harmonics, which does not balance out in the pulsing tube circuit II, is eliminated by frequency selectivity of the later circuits. The arrangement of Fig. 3 provides greater selectivity in the intermediate frequency circuits.

Fig. 5 illustrates a receiver wherein the first stage, supplied with energy from the antenna, is a detector having a push-pull input. In this figure the antenna feeds a low power factor resonant line system 26, here shown as a half wavelength coaxial line, via a pair of coaxial transmission lines 27. The first stage comprises a pair of vacuum tubes 28 whose grids are coupled to the resonant system 26 on opposite sides of the voltage nodal point. It should be noted that both the feeder lines 21 as well as the grids of the tubes 23 are symmetrically coupled to the resonant line 26 on opposite sides of its center. The outputs from the vacuum tubes 28 are in parallel and feed the audio frequency energy to a suitable audio frequency amplifier and/or loudspeaker. In this arrangement, the lead 29 in the grid circuits of the vacuum tubes 28 extends to a pulsing tube and pulsing oscillator arrangement of a type similar to that shown in Fig. 1 for supplying periodic bias pulses to render the tubes 28 conductive and non-conductive.

Fig. 6 shows a receiver arrangement which is a modification of the circuit of Fig. 5. In Fig. 6 the push-pull vacuum tubes 30 constituting the first stage function as amplifier tubes whose outputs feed a detector 3|, in turn providing audio frequency output signals. One feature of this circuit is that the pulses in anode direct current of tubes 30, at the pulsing frequency rate, are eliminated by both frequency selectivity and balancing.

If desired, we can use an extremely low power factor circuit in the output of the first receiver tube or stage of Figs. 2, 3, 4, 5 and 6 which is capable of smoothing out pulse modulation of the received high frequency energy but not the useful modulation, in which case we would have aradio frequency amplifier for amplifying the incoming modulated carrier, thus giving a much lower inherent receiver noise level.

Fig. 7 shows a receiver arrangement particularly suitable for low frequency operation. The lower power factor tuned circuit comprising an inductance coil and a parallel condenser, the combination of which is here labeled 32, is excited by energy from feeder 33 extending to a power source, which may be an antenna, not

shown. By means of a vibrator 34, the terminals of the tuned circuit 32 are periodically shunted by a condenser 35 which is of sufiicient value to change the normal resonant period of 32 from a radio frequency to an audible frequency which can be heard in headphones 36. This shunting of the tuned circuit 32 causes the oscillations therein to take place as a damped audio frequency wave which is audible in the headphones. In the circuit of this figure, as well as in the circuits of the previously described figures, oscillatory energy of relatively large value is built up in the unloaded low power factor circuit. This energy is intermittently utilized, by momentarily loading the circuit, in this case by means of vibrator 34, and the headphones 36.

Fig. 8 shows an arrangement also suitable for the reception of low frequencies in which coils 3'! are saturated iron core inductances arranged in series to each other but in parallel to a condenser 38 to form a tuned circuit for radio frequency energy. By means of a rotating commutator 39 periodically supplying direct current to coils 40 coupled to coils 3'1, the saturating current for inductances 31 is interrupted, thus changing the tuning of the circuit 31, 38 to an audible frequency. Connected across the low power factor tuned circuit is a pair of headphones M in series with a radio frequency choke coil 32. Condenser 33 is a radio frequency bypass for the headphones. It will be apparent that when coils 4b are properly poled then the radio frequency induced therein will balance out. In Fig. 8 the incoming signals received on the antenna are applied to the tuned circuit 31, 38 by means of transmission line 44 and coils 45, the latter being coupled to coils 31.

Fig. 9 is an alternative arrangement which can be used in place of the circuits of Figs. 7 and 8. In this figure, the low power factor tuned circuit 45 in which the oscillation energy is stored has an inductance 41 placed in series with the radio frequency inductance of the tuned circuit. This inductance 41 is short circuited by vibrator 34, while radio frequency energy is being stored in tuned circuit 46, and then thrown into the circuit of 46 by the opening of the contacts of the vibrator to bring about the frequency conversion to an audible frequency which can be heard in headphones 36.

It should be noted that the input currents and the pulses of useful current passed on by the pulsing tubes of Figs. 2, 3, 6 may be either amplitude modulated, frequency modulated or phase modulated by a useful signal modulation, and that the final demodulator should be chosen according to the type of modulation applied to the input currents at the transmitter.

What is claimed is:

1. A radio receiver having a vacuum tube, a low power factor radio frequency tuned circuit connected to the input of said vacuum tube, and means for periodically increasing appreciably the input impedance of said vacuum tube, for a relatively large percentage of the time compared to the time between said periods and at least once for each pulse of useful modulation received on said receiver whereby the loading of said vacuum tube on said tuned circuit is materially reduced during said periods of increased input impedance.

2. In a radio receiver having a vacuum tube and a low power factor tuned circuit associated with the input of said vacuum tube, the method of operation which comprises receiving energy in said receiver, effectively dissociating said vacuum tube from said tuned circuit, storing said energy in said tuned circuit, and then periodically operatively associating said tuned circuit suddenly with said input for relatively very short periods of time compared to the time between said periods during which said input is effectively dissociated from said input circuit.

3. In a radio receiver having a vacuum tube, a low power factor tuned circuit associated with the input of said vacuum tube, and an antenna coupled to said tuned circuit, the method of operation which includes the step of periodically rendering said vacuum tube inactive for a period of time much longer than the time during which said tube is active and at least once for each pulse of useful modulation.

4. In a radio receiver having a vacuum tube and a low power factor tuned circuit associated with the input of said vacuum tube, the method of operation which comprises receiving energy in said receiver, effectively dissociating said vacuum tube from said tuned circuit, storing said energy in said tuned circuit, and then periodically operatively associating said tuned circuit suddenly with said input for a relatively very short period of time and at least once for each pulse of useful modulation received on said receiver.

5. In a radio receiving system, an antenna, a low power factor radio frequency tuned circuit coupled to said antenna, a utilization circuit, and means for periodically operatively associating said utilization circuit, suddenly, with said tuned circuit for a relatively very short period of time compared to the time during which said utilization circuit is disassociated from said tuned circuit.

6. In a radio receiving system, a source of signals, a low power factor radio frequency tuned circuit coupled to said source, a vacuum tube having an input electrode connected to said tuned circuit, and means for periodically rendering said vacuum tube inactive for a period of time much longer than the time during which said tube is active, whereby the loading of said vacuum tube on said tuned circuit is effectively removed therefrom during said inactive periods.

'7. In a radio receiving system, a source of signals, a low power factor radio frequency tuned circuit coupled to said source, a vacuum tube having its control grid connected to said tuned circuit, means for applying bias to said grid, and a circuit including an oscillator for periodically increasing the bias on said grid to render said vacuum tube inactive and for suddenly decreasing the said bias on said grid to render said tube active, the inactive periods of said tube being appreciably longer than the active periods.

8. A system in accordance with claim 7, characterized in this that said system includes another vacuum tube whose input is coupled to said oscclillator and whose output is in circuit with said gri 9. In a radio receiving system, a source of signals, a low power factor radio frequency tuned circuit coupled to said source, a vacuum tube having an input electrode connected to said tuned circuit, means for periodically rendering said vacuum tube inactive for a period of time much longer than the time during which said tube is active, whereby the loading of said vacuum tube on said tuned circuit is effectively removed therefrom during said inactive periods, a filter coupled to the output of said vacuum tube,

and an audio frequency utilization circuit cou pled to said filter.

10. An ultra high frequency radio receiving system comprising an antenna, a low power factor tuned circuit coupled to said antenna, a detector and amplifier vacuum tuve having its input circuit connected to said tuned circuit, a filter circuit coupled to the output of said tube, an audio frequency utilization device coupled to said filter, and means for effectively periodically disassociating the input of said tube from said tuned circuit and for suddenly operatively associating said input with said tuned circuit, the periods during which said input circuit is disassociated from said tuned circuit being longer than the periods during which said input circuit is operatively. associated with said tuned circuit.

11. In a heterodyne radio receiving system, an antenna, a vacuum tube heterodyne detector coupled to said antenna, a local oscillator also coupled to said detector to beat with the waves received on said antenna for producing an intermediate frequency in the output of said detector, a low power factor oscillatory circuit in the output of said detector, an intermediate frequency amplifier vacuum tube having an input electrode connected to said low power factor circuit, a

utilization circuit including a second detector coupled to said amplifier, and means for periodically rendering said amplifier .vacuiun tube inactive, whereby the loading of said amplifier on said low power factor circuit is effectively removed therefrom during said inactive periods, the periods during which said amplifier is inactive being at least ten times longer than the periods during which it is active, whereby the value of the energy stored in said low power factor circuit during the inactive periods which is avail able for said amplifier is much higher than the steady state value obtainable if the amplifier were continuously active.

12. In a heterodyne radio receiving system, an antenna, a vacuum tube heterodyne detector coupled to said antenna, a local oscillator also coupled to said detector to beat with the waves received on said antenna for producing an intermediate frequency in the output of said d tector, a low power factor oscillatory circuit in the output of said detector, an intermediate frequency amplifier vacuum tube having a grid and cathode connected to said low power factor oscillatory circuit, an impedance in circuit with said grid for applying a negative bias to said grid relative to said cathode, a vacuum tube having an output electrode in series with said impedance relative to a source of polarizing potential, and a pulse oscillator coupled to the input of said last vacuum tube for periodically causing said last vacuum tube to become inactive at the frequency of said pulses, whereby the negative bias to the grid of said amplifier is increased solely during the active periods of said last vacuum tube to such an extent that said amplifier is made to be non-conductive during said active periods, the conductive periods of said amplifier being very short compared to the nonconductive periods thereof and at least one for each cycle or pulse of useful modulation.

13. In a radio receiving system, an antenna, a low power factor radio frequency tuned circuit coupled to said antenna, a utilization circuit also coupled to said tuned circuit, and means for periodically effectively disassociating said utilization circuit from said tuned circuit for a period of time at least twenty times longer than the time during which said utilization circuitis operatively associated with said tuned circuit.

14. Means for increasing the ratio of magnitudes of desired currents to undesired currents during periods of utilization of thedesired currents, comprising means for accumulating and storing energy of the desired currents, and means for periodically utilizing the stored energy for very short time periods compared to the time during which said energy is stored, the periodicity of utilization being at least once for each cycle or pulse of useful modulation.

15. In combination, a non-oscillating amplifier tube, a source of alternating current signal energy coupled to the input of said tube, and means for periodically effectively disassociating said amplifier tube from said source for a period of time at least ten times longer than the time during which said tube is operatively associated with said source, said amplifier being operatively associated with said source at least once for each cycle or pulse of useful modulation.

16. In a high frequency system, a source of high frequency signal energy, a utilization circuit coupled to said source, and means for periodically effectively disassociating said utilization circuit from said source for a period of time at least ten times longer than the time during which said utilization circuit is operatively associated with said source, said utilization cir'cuit being operatively associated with said source at least once for each cycle or pulse of useful modulation.

17. In a high frequency signal system having a tuned circuit coupled to the input of a vacuum tube, the method of increasing the ratio of magnitudes of desired currents to undesired currents during periods of utilization of the desired currents which includes the step of periodically rendering said vacuum tube inactive for a period of time much longer than the time during which said tube is active and at least once for each pulse of useful modulation.

CLARENCE W. HANSELL.

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