US1921087A - Wave signaling system - Google Patents

Description

Aug. 8, 1933. w, A, MacDoNALD wAvE SIGNALING SYSTEM Original Filed Feb. 15, 1931 ATTORNEYS Patented Aug. 8, 1933 WAVE srGNALmG SYSTEM William A. MacDonald, Little Neck, N. Y., as-

signor to Hazcltine Corporation, a Corporation of Delaware Original application February 13, 1931, Serial No. 515,528, patented October 4, 1932, No. 1,881,235. Divided and this application March 14, 1932, Serial No. 598,640, and in Canada November 13,

a1 claims. (c1. 25o-2o) This invention relates to radio Signaling and more particularly to radio receivers of the superheterodyne type. This application is a division of my copending application Serial No. 515,528, filed February 13, 1931, and issued October 4, 1932, as Patent No. 1,881,235.

'I'he principal objects of this invention are to`v obtain in a superheterodyne type of radio receiver, high sensitivity, selectivity, uniformity of amplification, freedom` from cross-talk and interfering noises and whistles and negligible radiation of oscillations, over a wide range of frequency.

The usual superheterodyne receiver arrangement comprises an antenna and ground, or a loop antenna, a radio-frequency amplifier provided.

with signal selecting circuits, a modulator or first detector, a local oscillator for producing local oscillations which are combined with the incoming carrier signal frequency at the modulator to produce a new carrier signal wave of intermedi- VVVate frequency, an intermediate frequency amplifier, a second detector for producing the modulation component of the intermediate frequency waves, an audio-frequency amplier, and a sig-l nal translating or sound producing system.

In accordance with this invention there is employed an which utilizes a coupling system having a. pair of coupled windings, one of which is tuned to the intermediate frequency. and the other to a frequency above the intermediate frequency range but below the radio frequency range, thus affording high attenuation in the intermediate amplifler to signals of the radio frequency range.

It is to be preferred that the radio-frequency system be highly selective, by virtue of which extraneous signals are highly attenuated, so that the tendency toward undesired beats in the modulator-is further reduced.

An important feature of the invention is the provision of an intermediate frequency amplifler which selectively and uniformly transmits the intermediate carrier frequency and the associated sidebands. The intermediate frequency amplifier comprises o ne or more vacuum tube amplifier stages coupled by an arrangement of coupling .circuitsmhich act to provide the above noted desirable transmission characteristic. One` intermediate frequency amplifier which readily transmitsall the frequencies of sidebands Since this type of coupling system provides a somewhat decreased transmission over a range of frequencies between the resonances, other of the intel-stage coupling systems are proportioned to have a single resonance peak which lies between the resonance peaks 0f the first- -mentioned coupling system, whereby the cumulative effect upon the entire intermediate-fre.- quency amplier is to provide a substantially luniform transmission over the entire sideband range of frequency.

The interstage coupling systems of the intermediate-frequency amplifier which are characterized by a single resonance peak, preferably each comprise a transformer having a primary winding and a secondary winding coupled sumciently close so that a condenser connected across one of the windings tunes the system as a whole to the intermediate frequency. It is preferable,

although not essential, that the winding which is so tuned is the anode circuit, or primary winding, since the input impedance of the coupling system is thereby made relatively highand great attenuation of. undesired signals is obtained. The other winding of the transformer is preferably naturally resonant below the range of `radio-frequency signals to be received, that is,

it is capacitively reactive to broadcast range.

A feature of the intermediate-frequency transfrequencies in the former construction is the adjustment of the range firequ'ency, by furnishing the necessary effective capacity. To secure this required effectivel capacity a proper form factor should be chosen for the winding which will provide a distributed winding capacity of the desired value; if

necessary, the winding capacity may be, supplemented by an external or added capacity.

The combination ofa sharply selective radio'- frequency amplifier and the intermediate-frequency. amplifier having the broad, uniform, transmission characteristic is particularly advantageous, Since it permits a moderate amount of mistracking of thel received signal frequency and of the local oscillator frequency without seriously impairing the quality of transmission through the intermediate-frequency amplier.

The Vabove'and other features will more fully appear from the following detailed description the antenna circuit transformer 17, and is simi-- duce oscillations of a desired frequency.

when read in conjunction with the drawing, of which:

Fig. 1 illustrates a superheterodyne radio receiver the component electrical units of which are arranged in accordance with this invention;

Fig. 2 illustrates transmission characteristics of the intermediate-frequency amplifier and of portions thereof.

Fig. 1 illustrates a complete superheterodyne receiver embodying features of the present invention. The receiver includes an antenna circuit 10 coupled to a radio-frequency amplifier designated in general as 11. The radio-frequency amplifier 11 comprises a vacuum tube amplifier 12 of the four-electrode, or screengrid type, which comprises aY cathode 13, an anode 14, a control electrode, or grid .15, and a screen-grid 16 partially surrounding the anode. The amplifier tube 12 is coupled to the antenna through a radio-frequency transformer 17, the primary winding 18 of which is connected in the antenna circuit and the secondary winding 19 of which is connected to the grid 15 of the amplifier 12. the signal frequency by means of a variable condenser 20 shunted across the secondary wind ing 19.

a vacuum tube modulator tube 21, also of the four-electrode type, comprising a cathode 22, an anode 23, a control electrode, or grid, 24, and a screen-grid 25. The grid, or input circuit, of the modulator is coupled to the anode ofthe radiofrequency amplifier 12 by a radio-frequency transformer 26, the primary winding 27 of which is connected to the anode 14 and the secondary winding 28 of which is connected to the grid 24. The coupling transformer 26 may be similar to larly tuned by a variable condenser 29 shunted across the secondary winding 28.

The receiver is provided with a local oscillator system 30 comprising a three-electrode vacuum tube 31 and associated circuit elements which are so proportioned as to cause the'tube 31 to pro- The oscillator system is provided with a variable tuning condenser 94, and an output coil 32 which is connected in the input circuit of the modulator 21 by virtue of its connection between the modulator cathode 22 and ground.

The anode circuit of the modulator 21 is coupled to an amplifier designated generally as 40, which is adapted to transmit a frequency band which is lower in the frequency scale than the signal frequencies transmitted by the radio-frequency amplifier. Since the frequency band transmitted by the amplifier 40 lies between the audio-frequency range and the radio-frequency tuning range, this amplifier is called an intermediatefrequency amplifier. The intermediate-frequency amplier comprises two screen-grid vacuum tubes 41 and 42 which are the same type as the radiofrequency amplifying tube 12. The first intermediate-frequency amplifier tube 41 is coupled to the modulator 21 through a coupling system comprising two 'similar tuned circuits 43 and 44. One of these tuned circuits 43 which is connected in the anode circuit of the modulator, includes an inductance 45 and a capacity 46; and the other of the tuned circuits 44, which is connected to the grid of amplifier 41, includes an inductance 47 shunted by a capacity 48. The inductances 45 and 47 are coupled magnetically.

The second intermediate-frequency amplifier 'Ihe coupling transformer 17 is tuned to tube 42 is coupled to the first intermediate-frequency tube 41 by an intermediate-frequency transformer 49, the primary coil 50 of which is tuned by a fixed condenser 51, and the secondary winding 52 of which is shunted by al resistance 53. An adjustable tap 54 connects a point of the resistance to the grid of tube 42.l

The output of `tube 42 is coupled to the input of a detector tube 60 by means of a coupling transformer 55 which maybe similar to transformer 49. As the intermediate-frequency amplifier constitutes another feature of the invention it will be dealt with in greater detail subsequently.

The second detector is of the two-electrode type commonly known as a Fleming valve; allthough it is shown in form as a three-element tube having a cathode 61, a plate 62 and a gridy 63, the cathode and plate are connected together to constitute a single cathode, so'that the tube is in effect -a two-electrode detector having a cathode and an anode, the grid acting as an" anode in this case. The input of the detector is coupled to the output of the intermediate-frequency amplifier by a connection between the high potential end of the secondary of transformer 55 and the anode (in this case the grid) of the detector and by a connection from the low potential end of the said secondary winding, through a resistancef65, to the cathode. The resistance 65 is shunted by a condenser 64 for providing a low impedance path for the intermediate frequency signals around resistance 65.

An audio-frequency amplifier, designated generally as 70 is connected to the detector circuit in thefollowing manner: A resistance 66 is connected at one end to the point between resistance 65 and the secondary winding of transformer 55.

The other end of resistance 66 is connected through a clocking condenser 67 to one end of a potentiometer, or tapped resistance 68, the other end of which is connected to the cathode 61 of the detector. The input potential for the audio amplifier is the voltage between the low potential end of resistance 68'and the tap.

The audio-frequency amplifier 70 comprises 120 threestages of amplification. The first two stages include respectively amplifying tubes 71 and 72, resistance-coupled in tandem in a conventional manner by shunt resistances 76 and 77 and blocking condenser 78. Ther output of tube 72 is 125 coupled to the last audio amplifying stage, which comprises a pair of tubes 73 and 74 connected in the well-known push-pull relation. The output of the push-pull stage is coupled to a loud speaker 75.

The receiver is adapted to be tuned by a unicontrol arrangement; this isy effected by operating variable condensers 20, 29, and 94 from a single shaft; this operation is represented by the 135 dotted lines 56 and 57.

The sources of operating potentials such as the filament heating sources "and the grid, screengrid and anode potentials, are not shown in the drawing. These potentials may be supplied by any o the well-known methods. There are indicate in the drawing potentials which are well adapted for application to the various leads; these potentials aregiven with respect.` to ground potential.

For the purpose of automatically controlling the 4strength of the signal current delivered to the audio amplifier, there is provided a volume controlling system which automatically regulates the amplification of the receiver so that the detected, 'or audio, signals remain substantially 150 uniform. The volume controlling arrangement is of the type described i'n a paper presented before the Institute of Radio Engineers by H. A. Wheeler and published in pages 30-34 of the Proceedings of the Institute of Radio Engineers, January, 1928. The system comprises a connection extending from the lower end of resistance 66 to the control electrode 15 of radio-frequency amplifier 12 and to the control-electrode of intermediate-frequency amplifier 41. The connection 80 is led to the control electrodes of ampliiers 12 and 41 by connections to the low-potential ends of the grid circuit windings 19 and 47, respectively, of 'the associated coupling systems. There are includedin the connection 80 resistances 81 and 82, the function of which will be more fully explained later. For the purpose of keeping the grid potential from the cathodes, but still enabling the grid circuits to be cmpleted,

there are provided blocking condensers 83 and 84.

The above-described automatic volume control system is disclosed and claimed in the copending applications of Harold A. Wheeler, Serial No. 203,879, filed July 7, 1927 and Serial No. 495,386, led November 13, 1930, now U. S. Patent 1,879,863, issued September 27, 1932.

The receiver is provided with a number of resistors, some of which furnish biasing potentials for vacuum tube grids, and others of which are inserted in the leads supplying operating potentials to the electrodes of the tubes. There are also provided by-passing condensers at advantageous points. These elements contribute toward good operation; and since they are in general use and are well-known in the art, no further details are given here.

The following is a brief description of the operation of the receiver: A radio signal received by the antenna 10 is selectedin the Wellknown manner by theselective circuits of coupling transformers 17 and 26, which are tuned to the same frequency. The modulated carrier signal, after being amplified in the yradiofrequency amplifier is impressed upon the grid circuit of the modulator 21. The oscillator 30 is tuned in conjunction with the selective circuits 17 and 26 to apply to the' modulator a frequency which differs from the signal frequency by a desired amount. The oscillator frequency may be either greater than, or less than, the radio carrier frequency, but it is preferably greater than the radio frequency. By .virtue of the wellknown phenomenon of modulation, there is produced in the outputt of the modulator a new carrier frequency which is equal to the difference between the frequency of the received radio signal and the local oscillator frequency. This difference in frequency is commonly known as the intermediate carrier frequency, since it is lower than the radio-frequency of the received signal but is above the audible range. The intermediate carrier frequency has associated with it the side band frequencies with which the signal is modulated.

The selective coupling circuits of the intermediate-frequency amplifier are adjusted to freely transmit the intermediate carrier-frequency and the associated side bands and to effectively exclude all .other signals.

pressed upon the two-element detector 60 in the output of which there appears the modulation component, that is, the audio-frequency signals. The audio-frequency, signals are amplified in the audio-frequency amplifier in the well-known The amplied output of the intermediate frequency amplifier is immanner and are converted into sound by the loud speaker 75 connected to the output of the audio^ amplifier.

The following is a briefy outline of the operatio'n of the volume controlling circuit:

.jfWhen a modulated carrier signal is impressed upon the two-electrode detector, there appears across resistances 65 and 66 a voltage having two components, one an audio-frequency component and theother a direct current component. The direct current component is proportional to the strength o'f the received carrier signal. An in` crease ofthe received carrier -signal has the effect of increasing'thel 4Voltage across resistances 65 and 66, that is, of causing the potential of point 85 to become more negative with respect to ground. Since the potential at point 85 is impressed through the connection 80 upon the grids of amplifying tubes 12'y and 4l, the effect is to render these grids more negative when the signal strength increases. Likewise when the signal strength decreases, the grids of amplifiers 12 and 41 become less negative. Due to the variation of the potential of the amplifier grids in this manner, attendant upon the variation of l the received signal strength, the amplification increases when the signals are weak and decreases when the signals are strong, so that the net effect is to m'aintain the signals at the second detector substantially uniform in strength. The audiofrequency component of the detected signal is prevented from appearing at the grids of amplifiers 12 and 41 by-virtue of the filtering action of resistances 81 and 82 and condensers 84 and 83.

Referring now specifically to the oscillator system, it comprises. a three-electrode oscillating vacuum tube 31 having a cathode 90, an anode 91 and a control grid 92. The grid circuit includes an oscillatory circuit comprising an inductance 93 and a variable capacity 94. The inductance 115 93 is connected at one end to the grid 92 and at the other end through the parallel arranged capacity 95 and resistance 96 to ground. There is shunted across capacity 95, a small variable capacity 106 for enabling a proper adjustment 120 to be obtained. The cathode is grounded through a parallel arranged resistance 97 and capacity 98. The function of the resistance 97 is to .provide a biasing potential for the grid.

For the purpose of establishing the condition of oscillation, the anode 91 is connected through a coil 99 to the point between the grid circuit inductance 93 and capacity 95. The coil 99 is vso situated relative to coil 93 that a substantial degree of inductive coupling exists between the two coils. By virtue of this circuit arrangement of the oscillator there exist in common with both the grid and the anode circuits of the oscillator, the capacity 95 and the mutual inductance M of coils 93 and 99. These common, lor mutual impedances are made sufficiently large so thatI the tube 31 is set into oscillation; and the frequency of the oscillation is the frequency ,at which the circuit including inductance 93 and capacities 94 and 95 is resonant. 14

Since the voltage across the capacity 95 is least A at the highest frequencies of the range over which the oscillator is tuned, and is greatest at4 the lowest oscillator frequency, it follows` that the effect of this capacity in producing an oscillating voltage is greater at the lower frequencies than at the higher frequencies. It further follows that the effect of the mutual impedance M in producing an oscillatory voltage is greater at high frequencies than atlower frequencies. The use to is more fully disclosed and claimed in my' aforesaid application Serial No. 515,528, of which this is a division.

The intermediate-frequency amplifier is so designed that there is provided a fairly uniform transmission of all the frequencies of Jthe side bands of the intermediate-carrier frequency, and furthermore, means are provided for insuring that no frequencies other than those of the desired` signal are transmitted to any substantial degree.

f-The coupling system which couples the output ,f of the modulator to the input of the intermediate-frequency amplifier is of the double-tuned type, that is, it comprises a pair of syntonously tuned circuits coupled electromagnetically, one of the tuned circuits'43 being situated in the modulator anode circuit and the other tuned circuit 44 in the grid circuit of the first intermediate-frequency amplifier. The degree of magnetic coupling is preferably somewhat over-optimum so that the transmission band of this coupling system is somewhat broadened by virtue of the pair of slightly spaced resonant peaks Which it is well known are obtained by this arrangement. The frequency spread of the resonance peaks should preferably be about equal to or greater than the frequency range of the side bands with which the intermediate carrier-frequency is modulated.

The two succeeding intermediate- frequency transformers 49 and 55 are preferably identical, although this identity is not essential. These transformers `are so constructed that thereexists a close electromagnetic coupling between their primary and secondary windings, whereby the coupling system tunes as a whole at the resonant frequency of the winding which is shunted by the condenser. Each of the two transformers 49 and 55 is characterized by a single resonance, rather than a double resonance such-as is obtained from the first intermediate-frequency coupling sys-` p tem.

There is, an advantage from a commercial standpoint, as well as from a transmission standpoint, in providing one coupling system of the double-resonance type and the other -couplingsystem of the single-resonance type. It has been found difficult to adjust theV double-resonance system commercially so that the two resonance peaks are properly spaced and are of approximately the same height. Hence, it is desirable that there be only one of these double-resonance systems. The advantage from the transmission standpoint is that the single-resonance of the transformers 49 and 55 can be made to -lie midway between the peaks of the double-resonance coupling system. whereby the over-all transmission provided by the intermediate frequency amplifier is substantially uniform over the side band range of frequencies.

Curve a of Fig. 2 illustrates the transmission` frequency characteristic of the double-resonant system. The ordinates represent the ratio of the gain at resonance to the gain near resonance and the absciss represent frequencies in kilocycles per second. The frequency marked zero represents the intermediate carrier-frequency, and the absciss on either side of the carrierfrequency are the frequencies of the side bands.

Curvej'fb of Fig. 2 represents the transmissionfrequency characteristic of the transformers 49 and 55. The combination of the two types of coupling system cooperates to produce a transmission characteristic of the type illustrated in curve c of Fig. 2. The intermediate-frequency amplifier can be readily constructed so that the over-all response at a side-band frequency of four kilocycles fromthe intermediate carrierfrequency is as much as of the response at the carrier-frequency.

By providing a relatively fiat-topped response characteristic of the type of curve c, slight mistracking between the oscillator frequency and radio-frequency carrier frequency, that is, variations in the difference between the radio-frequency carrier and oscillator frequency, cause no serious change in the over-all sensitivity over the important side-band range. ln considering' an intermediate-frequency amplifier of the type under discussion including coupling systems utilizing double tuning between the output of one tube and the input of the second tube, together with coupling systems utilizing but a single resonant circuit tuned to the intermediate frequency, it has been found preferable, although not essential, to arrange the coupling systems utilizing but a single physically resonant circuit so that the resonant circuit is connected between the cathode and anode-of the preceding amplifying tube rather than between the cathode and control grid of the succeeding tube. Where the very highest order cf performance is desired,

it is usually desirable to connect the coupling systems of adjacent stages in this way, for such-confrequency amplifier, thus producing interfering beats by the second detector.

It is usualy desirable to tune the primary cir- A cuit of transformer 55 rather than the secondary circuit because this transformer operates into a two-electrode type of detector, namely, detector 0. This form of detector imposes agshunt load on thetransformer, so that if the secondarycircuit'were tuned instead of the primary circuit, theeffect would be to materially impair the resonance characteristic of. this transformer. By tuning the primary circuit and employing a stepdown transformation ratio, the impedance of the secondary circuit may be arranged to match. the impedance of the system into which it feeds,thus producing the most efflcient condition of operation.

If for some design reason it is necessary to tune the secondary rather than the primary circuit, then the detector may be connected across a portion of the tuned secondary, the impedance of which is approximately equal to the impedance of the load circuit.

It is important in the design of the intermediate-frequency coupling-transformers that the natural period of the winding which is not tuned by a physical capacity shall have such electrical constants that it will be naturally resonant at a frequency lower than. the 'lowest broadcast frequency to be received. The factors which determine the resonant frequency of.' this winding are: Its natural inductance, the distributed capacity of the winding and the capacity of the devices connected across its terminals. All of these elements must be considered in the selection of the proper resonant frequency such that the winding is always capacitively reactive to frequencies in the broadcast band.

If this precaution is not followed and the untuned winding is arbitrarily chosen so that its natural period falls within the broadcast band, then lt has been found that there is usually a high orderfof amplification to broadcast frequencies, and voltages of signal frequency which would nor- V.mally be of small magnitude produce undesirable heterodyne whistles in the receiver.

The following values have been found satisfac- \tory in the design of an intermediate-frequency coupling transformer, although all of the values may be modified Within wide limits and still be within the scope of the present invention:

Intermediate frequency: 1'1 5 kilocycles per sec- Coupling coefiicient between primary and secondary windings=37% I Capacity across primary winding=100 micromicrofarads approx. g The above inductance values are "obtainable by windingon a one-half inch core in bobbins onequarter inch wide, a primary coil of 800I turns of No. 38 double silk-covered copper wire' and a secondary winding of 600 turns of No. 38 double silk-covered wire. The bobbins containing.'v the primary and secondary coils are coaxially placed side by side and are enclosed by a shielding can of 1%-inch diameter. A

When the transformer is constructed in accordance with the above specification the resonant period of the secondary winding is about 350 kilocycles per second. This value of the resonant frequency is not necessarily the best value for all receivers; the best value for the resonant frequency will depend somewhat upon the particular design of the receiver. If it be desired to make the resonant period higher than the value of about 350 kilocycles per second, the number of turns on the secondary winding should be somewhat less than the value given in the above table; or, on the other hand, if it be desired to reduce the num-/ ber of secondary turns and still maintain the resonant frequency at about 350 kilocycles per second, the coil should be constructed to have a higher distributed capacity.

It has been found that greater sensitivity is required in certain localities than in` other localities. To enable the receiver to meet any of the required conditions of sensitivity, one of the intermediate-frequency transformers is provided with an impedance shunted across its secondary winding so that the sensitivity may be adjusted to any first portion of said amplifier being responsive to signal voltage corresponding to the frequency of a received signal, a second portion of said amplifler being responsive to signal voltage of an intermediate frequency which is lower than the received signal frequency but higher than the audio frequency range, said coupling means of said intermediate frequency system including a coil and capacity connected in shunt and tuned to said intermediate frequency, and a second coil coupled to said first mentioned coil and capacity, having a nautral yperiod the frequency of which is lower than that of the received signal but higher than the intermediate frequency, whereby signals of said intermediate frequency are freely transmitted through said second portion but sig- .nals of said received signal frequency are highly attenuated in said second portion.

2. A signal response system including a signalfrequency vacuum tube amplifier tunable over a range of frequencies, a modulator-oscillator system, a secondvacuum tube amplifier tuned to the difference frequencies between those of said oscillator and saidy signal frequency amplifier, said second mentioned amplifier including a coupling means coupling the output of one tube to the in-l put of a succeeding tube, said coupling means including a coil and capacity resonant to a frequency equal to the difference between said oscillator and signal frequencies and a second coil electromagnetically coupled to said first coil having a natural period resonant to a frequency lower "than said signal frequency, but higher than said difference frequencies, whereby said signal frequencies are highly attenuated while said difference frequencies are freely transmitted, in said second-mentioned amplifier.

3. A signal response system including a radiofrequency amplifier tunable over a range of frequencies, a modulator-oscillator system, a second amplifier tuned to the ldifference frequencies between said oscillator and signal frequency amplifiers, said second mentioned amplifier including coupling means coupling the outputA of one tube to the. input of a-succeeding tube, said coupling means including a coil and capacity resonant to a frequency equal to the difference between said oscillator and signal frequency, a second coil coupled to said first coil whose natural period is of l such value as' to be inductively reactive to said second mentioned frequency and capacitively reactive to said radio frequency.

4. A signal response system including tuning means adjustably operative over y a frequency range, a modulator-oscillator system and amplifying means coupling the output of said modulator to a detector, said amplifying means including a coupling system having a pair of syntonously tuned circuits' tuned to a second frequency range differing from said first mentioned range and a second coupling system supplying signal voltage to said detector, said detector being the vacuum tube type having a cathode, anode and control-electrode, said second coupling system including a coil and capacity connected in shunt and a second coil coupled lto said first mentioned coil and connected between the control-electrode and cathode system of said detector, said second coil having such value as to be capacitively reactive to frequencies of the rst mentioned rangev but inductively reactive to frequencies of said `second mentioned range, whereby the output impedance of Asaid second coupling system is of the same order of magnitude as the impedance of the detector connected thereto.

is naturally resonantat a frequency above said intermediate frequency but below said radio-frequency signal range.

6. A signal response system including tuning means adjustably operative over a frequency range, an oscillator-modulator system and an ampliiier coupled between the output of said modulator and the input of a detector, said amplifier being tuned to transmit signals lying within a second frequency range which diifers from said first-mentioned frequency range and including a coupling system which supplies signals of said second range to said detector, said detector having at least two electrodes, and said coupling system including a coil effectively connected between said two electrodes, said coil having` such value as to be capacitively reactive to frequencies of the first-mentioned range but inductively reactive to frequencies of said second frequency range, and a second coil, in shunt with capacity, coupled to said first-mentioned coil.

'7. A signal response system including tuning means adjustably operative over a frequency range, an oscillator-modulator system, and amplifying means coupling the output of said modulator to a detector, said amplifying means including a coupling system having a pair of syntonously tuned circuits tuned to a second frequency range 'which differs from said first-mentioned range and a second coupling system supplying signal voltage to said detector, said detector being the vacuum tube type having two electrodes, said coupling system including a coil effectively connected between said two electrodes, having such value as to be capacitively reactive to frequencies of the first-mentioned range but inductively reactive to frequencies of said second frequency range, and a second coil, in shunt with capacity, coupled to said first-mentioned coil, and tuned to said second frequency range, whereby signals in said first-mentioned range are highly attenuated in said amplifying means, 'and also the output impedance of said second coupling system is of the same order of magnitude as `the impedance of said detector.

8. A signal response system including tuning means adjustably operative over a frequency range, an oscillator-modulator system and amplifying means coupling the output of said modulator tothe input of a tube having a low input impedance as compared with the output impedance of a tube of said amplifying means, said amplifying means being tuned to transmit a second frequency range differing from said rstmentioned range and including a coupling system supplying signal voltage to said tube of low input impedance, said coupling system including a coil effectively connected to the input terminals of said low impedance tube, said coil having such a value as to be capacitively reactive to frequencies of the first-mentioned range but inductively reactive to frequencies of said second frequency range, whereby signals of said first-mem tioned range are attenuatedin said amplifying means, and also the output impedance of said coupling system is of the same order of magnitude as the input impedance of said low impedance tube, and a second coil, shunted by capacity, coupled to said first-mentioned coil, said second WILLIAM A. MACDONALD.

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