US1881284A - Wave signaling system - Google Patents

Description

Oct. 4, 932.

w. A. M DONALD WAVE smmnme SYSTEM Filed Dec. 1, 1950 2 Sheets-Sheet 1 My m 1A A m WWW W,

4, 1932. w. A. M DoNALD 4 WAVE SIGNALING SYSTEM Filed Dec. 1, 1930 2 Sheets-Sheet 2 500 vo'oo I550 FREQUENCY-lf/LQCYCLEJ FE/FJECO/VO I INVENTOR ATTORNEY Patented Oct. 4, 1932 UNITED STATES PATENT OFFICE WILLIAM A. MACDONALD, 01' LITTLE NECK, NEW YORK, ASSIGNOR TO HAZELTINE CORPORATION, OF JERSEY CITY, NEW JERSEY, CORPORATION OF DELAWARE WAVE SIGNALING SYSTEM Application filed December 1, 1930. Serial No. 499,229.

This invention relates to high frequency signaling, and more particularly to systems especially adapted to the selection and amphfication of radio-frequency waves.

The principal object of the invention is to obtain in a radio-frequency amplifing system, a high degree of sensitivity, selectivity and fidelity and at the same time to reduce the tendency toward oscillation of the repeating elements of the amplifier.

Broadly stated, the system by which the above object is accomplished comprises an untuned radio-frequency amplifier preceded by a tuning, or band-frequency selective, system comprising a plurality of tuned circuits coupled by the proper amount of coupling to provide substantially the maximum possible transfer of energy from one tuned circuit to the next, consistent with a band transmission characteristic suited to freely transmit all the important frequencies of the sidebands and to discriminate against all other frequencies. Such a band selective system is disclosed in my copending application Se rial No. 469,487, filed July 21, 1930.

A feature of the invention is the radio-frequency amplifier, which is of the untuned, that is, untunable, type and has interstage coupling means so designed that each stage is non-regenerative. The amplifier repeating elements are preferably coupled by impedance coupling arrangements which effectively place capacitive loads upon the output circuits of each repeater over the tuning frequency range. These capacitive reactances are preferably provided by a choke coil or transformer connected between the anode and cathode of each repeater. The chokes or transformers are so constructed that their distributed capacity resonates with their inductance at a frequency slightly below the lowest frequency of the tuning range; there.- i by causing the chokes or transformers to be effectively capacitive throughout the tuning range. It is understood, of course, that the distributed capacity includes that of the associated amplifying tubes and any associated circuit elements. I

There are certain advantages inherent in an untuned radio frequency amplifier, partieularly when the output impedance connected to a repeating element is capacitive. One advantage resides in the fact that the coupling impedances of the amplifier can be made resonant somewhat below the tuning range, thereby causing the voltage transfer to the succeeding stage to be a maximum at the low frequency range. At the high frequency range the voltage decreases with increased frequency. That this is an advantage follows from the fact that most tuning or selective systems which must, of necessity, be associated with a radio-frequency amplifier, are characterized by an energy transfer which increases with increasing frequency. By properly proportioning the coupling elements of the amplifier, then, compensation of the characteristic of the tuning system may be efi'ected, and the overall response characteristic of the amplifier and selective system may be made uniform or at least regulated within wide limi A second important advantage in the use of an untuned radio-frequency amplifier is the ease with which oscillation can be prevented. It is well known that when the impedance connected to the output of a vacuum tube amplifier is capacitive, any voltage fed back from the output to the input of the amplifier is in the wrong phase to aid the input voltage, and hence the condition required for oscillation is non-existent. In consequence, when the output impedance is made to resonate below the tuning range, it is capacitive throughout that range and not conducive to oscillation.

Another feature .of the invention is the manner of coupling the several tuned circuits of the tuning, or preselecting, system so that 1ts response characteristic compensates for that of the amplifier, or gives to the receiver a desired over-all characteristic. It is contemplated to regulate the characteristic of the tuner by the proper combination of mag netic and capacitive coupling between the tuned circuits.

The combination of an untuned amplifier with a preselecting system possesses advantages not found in an ordinary tuned radiofrequency amplifier. In the first place, the

presence of the several tuned circuits, ahead of the first repeating element efiectively suppresses currents of undesired frequencies and substantially prevents harmful modulations which produce cross-talk. Furthermore there is attained a high de ree of selectivity together with high amplification and freedom from oscillation.

A further and more com lete understanding1 of the invention may e had from the fo owing detailed description and the accompanymg drawings, of which:

Fig. 1 illustrates an untuned radio-frequency amplifier connected to the output'of a three-circult tuning, or pre-selecting, stem of Iiwhich the circuits are coupled inductively;

1g. amplifier and a preselecting system, which is similar to that of Fig. 1 except that the.

circuits of the pre-selecting system are coupled .capa'citively instead of inductively;

Fig. 3 illustrates an arrangement similar tothose of Figs. 1 and 2, in which the circuits of the preselector are coupled both inductively and capacitively;

Fig. 4 illustrates the'combination with a preselecting system of an untuned amplifier comprising screen-grid tubes;

Fig. 5 shows the variation over the tuning and C5,. The tuned circuit 10 is coupled to, tuned circuit 11 by a small inductance L in series with L L being magnetically associated with coil L of circuit 11. Likewise circuit 11 is coupled to circuit 12 by a small inductance L in series with inductance L The small coupling inductances L and L preferably are constituted by one or two turns of wire and are located at the low potential ends of inductances L and L ,'respectively, so that substantially optimum coupling is obtained. Optimum coupling as used herein, means the degree of coupling which results in the maximum transfer of energy from one tuned circuit to the next while maintaining but a single resonant peak; the order of this optimum coupling isabout one or two per cent.

To minimize undesired electro-static cou-" pling between the several tuned circuits of the pre-selector, metal shields 13, 14 and 15, shown by dotted lines, are arranged to eifecshows a combination of a untuned inductances, it should be understood that the coupling may be obtained without these additional coils slmply by so situating inductances L L and L that the necessary co-efiicient of coupling between them is obtained.

' )The three circuits 10, 11 and 12 are tuned by means of the variable condensers C C2,

and C respectively and the circuits are simultaneously tuned to the same frequency by gauging the condensers in a uni-control arng between the successive circuits-is have been shown as rangement as indicated by the dotted line 40. The radio-frequency amplifier, which is connected to the output of the pre-selector, is

of the untunable type and comprises three thermionic repeaters, 16, 17 and 18, joined in tandem.- In practice, the output terminals 1 and 2 of the amplifier are connected to a detector and voice frequency amplifying system, but for convenience the detector and voice frequency amplifier are omitted as are also the power supply gources for thethermionic repeaters, inasmuch as these are wellknown and constitute no part of the inven-- tion.

The untunedcoupling arrangements be tween the amplifier tubes are of the impedance type. Considering the coupling between the repeating tubes 16 and 17, the arrangement comprises a radio-frequency choke coil 19 connected between the anode'of tube 16 and the positive terminal of the'B-battery. A blocking condenser 21 is connected between the anode of tube 16 and the control electrode of tube 17. The usual high gridleak resistance 22 is connected between the control electrode and the cathode of tube 17. The untuned coupling arrangement between tubes 17 and 18 is very similar to that betweentubes 16 and 17 except that. the anode lead 'of tube 17 is tapped into the coupling coil 20 b means of a variable contact, thus effective y making coil 20 an auto-transformer.

' The couplin coils 19 and 20 are preferably wound with fi ne wire and supported upon cylindrical forms although this construction is not essential. lSy winding the coils in this manner, however, the distributed capacity of the coils is reduced to an extremely low value. The coils, however, may be constructed by any of the well known methodsfor securin low distributed capacity. The coupling coi s 19 and 20 are characterized by being capacitively reactive over the entire tuning rangeof frequency that is, the inductance is sufiicient- 1y large so that it resonates with the distributed capacity of the winding, the output capacity of the associated tube and the input capacity of the next succeeding tube at a frequency lower, but not greatly lower, than the lowest frequency of the tuning range. As a pling arrangement is higher for the lower frepling coil L quenci'es of the tuning range than for the higher frequencies; hence the ratio of output to input voltage of a tube and its associated coupling arrangement is higher for the low frequency range than for the high frequency range. At the same time the capacitive load of the plate circuit shifts the phase of the output currents sufliciently so that the internal plate to grid coupling of the tube does not cause positive re generation.

In the chart of Fig. 5, the ordinates indicate voltage ratios and the abscissas represent frequencies of the ordinary broadcasting range, that is,- from 500 to 1500 kilocycles per second. Curve a illustrates a typical volt age ratio or response characteristic of one of the untuned amplifier coupling systems. The curve shows that the highest voltage is developed at the lower frequencies, that is, near the resonant frequency of the coupling coil. At the high frequency end'the voltage ratio is only about one-half of that at the low frequency end.

The signals are impressed upon the first tuned circuit ofthe preselector by an antenna A, the lead from which is preferably tapped into coil L a few turns from the low potential end. Although in the schematic circuit diagram, coil L is shown as an autotransformer, a two-winding transformer may be employed with equal facility.

The first tuned circuit composed of inductance coil L and variable condenser C is tuned to resonance and the-resonant current is transferred to the second tuned circuit composed of inductance coil L and variable capacity C 'by means of the small cou- The second tuned circuit is likewise tuned to resonance and the energy conveyed therefrom to the third tuned circuit composed of inductance coil L and variable capacity C by means of the coupling coil L The third tuned circuit is similarly tuned to resonance and the desired signal voltage applied to the input of the amplifier. When the tuning condensers are ganged together, as illustrated, the three circuits are simultaneously tuned by a uni-control device. .The high potential terminal 3 of the preselector is adjustable so that any desired signal voltage may be impres'sed upon the untuned amplifier. This variable connection, although desirable under some conditions, is not necessarily essential to the invention. Tapping in across a portion of the third tuned circuit in the manner described has been found to be particularly efiective when the negative regeneration of the untuned amplifier is high, for withthis-connection a smaller loading effect occurs in the third tuned circuit; hence, the selectivity is better than it might otherwise be. I

In addition to the loading effect resulting from the negative regeneration of-an untuned amplifier, the adjustable connection of the system from the preselecting system to the amplifier furnishes a very satisfactory method of sec-urin volume control, for as the high potential Tead is moved toward the grounded end of inductance L the voltage across the output terminals of the preselector diminishes, thus effecting control of the signal voltage.

It is not essential that the signal voltage to the amplifier be supplied by an auto-trans-' former connection such as terminal 3 of coil L but it may be suppliedto the amplifier with equal facility by means of a two-winding transformer of which the mutual inductance between the primary and secondary is made variable to accomplish the desired volume control.

A tuning system composed of coupled tuned circuits alone, such as that of Fig. 1, in which the tuned circuits are coupled magnetically, is characterized by a response whichrises with increasing frequency over the tuning range. When the antenna is naturally resonant above the tuning range, as is usual y the case, its impedance is effectively capacitive; which causes the voltage delivered by the antenna to the selector system to increase with increasing frequency. Since the voltage characteristics of the antenna and of the magnetically coupled tuner both slope upward with frequency, the over-all responsecharacteristic of the antenna and preselecting system of Fig. 1 rises with frequency as shown by curve b of Fig. 5.

Fig. 2 illustrates a radio-frequency recelving system which is in general slmilar to that of Fig. 1. The preselector, however, instead of utilizing magnetic coupling between successive tuned circuits, employs capacltlve coupling. The preselector comprises three tuned circuits 25, 26 and 27, each including an inductance tuned by a variable capacityas in the preselector of Fig. 1. The tuned circuits are coupled only by coupling con-- densers C and C 1 In the drawings each coupling condenser is shown as being connected between a tap in one tuning coil and the high potential end of the next succeeding tuned circuit. It 1s not essential that the coupling condensers be tapped into the coils in this manner, but it is often convenient to follow this expedient for with this connection the coupling capacities 1L may be larger and are not required to have such precise values, as would be the case if the high potential ends of all succeeding tuned circuits were directly coupled by the capacities.

The pre-selector systemof the receiver shown in Fig. 2 is characterized by a voltage ratio or response which increases toward the higher end of the tuning range. This increasing response results from the fact that coupled pre-selector system, includin 1 ly and capacitively. selector, it comprises three tuned circuits, 30', I 31 and 32. Circuit 30 is magnetically coupled the coupling capacities convey more current at higher than at lower frequencies. A typical response characteristic of the capacitivellly t e antenna, is illustrated by curve of i 5.

Due to the effect of the capacitive couphng,

to circuit 31 by a small coil L and circuit 31is magnetically coupled to circuit 32 by a similar coil L Coils L 'and L correspond-to' coils L and L respectively of Fig. 1 and may be constructed and arran d in the same manner. In addition to being magnetically coupled the tuned circuits are also coupled. capacitively by means of condensers C and C14 n the manner of the capacitive coupling of the re-selector ofFig. 2. Condensers C and may be tapped respectively into coils L and L in the man- Q ner explained in connection with the pre select r of Fig. 2, if desired.

'Th dualistic coupling between the seve'rhl tuned circuits of the pre-selector of Fig. 3

may be made use of to provide the receiving 35 system with desired response characteristics.

The capacitive and magnetic couplings may be arranged to mutually aid or to mutually oppose each other dependhig upon the result desired. The addition or the subtraction. of the coupling effects makes possible an alteration of the volta ratio, or. response characteristics of t e pre-selective system within wide limits. For example, the magnetically cou'pled pro-selectors have a substantially uniform response over the tuning range, while the effect of the capacitive coupling is to transfer the reatwt energy at the higher frequencies. If 1t is desired, therefore, to obtain an extremely stee gain characteristic between the input an the output of the tunin system, favoring the higher frequencies, t e capacity coupling shouldfibe arranged to aid the magnetic cou ling. If, on the other hand, a more uni orm gain characteristic between the input and the output of the tuning system is desired, the capacity coupling should be arranged-to oppose the i maignetic coupling.

ig. 4 illustrates a radio-fre uency receiving system similar to that of that the untuned amplifier utilizes four-element amplifying tubes of the screen-grid type. instead of. three-element tubes shown in Fig. 3. The useof screen grid tubes is often advantageous because of the higher system ig. 3 except amplifications that can be secured as com paged with the conventional three-element tu es.

Fig. 6 shows the over-all gain characteristic of a receiver employing an'antenna, a

selector system composed of three tuned circuits and an untuned am'plifiercomposed of a several stages of repeating elements having capacitively reactive loads in the output circuits. The res onse characteristic shown in this figure is o tained by designing the antenna and preselector system to have a voltage ratiocharacteristic which is complementary to that'of the'unttmed amplifier; hence,

the overall characteristic is substantially uniform overthe tuning range. The importance of being able to alter the slope of the respgnse characteristic of the selectin comes apparent when it is consi ered that the slope of the characteristic of the untuned am lifier may not lie whollywithin the'contro of the designer. For exam 1e, it. may be necessary to employ ampli ing tubes having either extremely small or extremely large input and/or output capacities; and the amphfication characteristic will depend upon the tubes employed. The slo ofthe res onse characteristic of a m ti-stage ampl' er will vary in accordance over-all response, or voltage ratio, of the amplifier will be the product of that bf the several stages. For. example, if the characteristic of a single stage varies from an amplification factor three at the high frequency end to six at the low frequency end, then if four stagesof amplification be emplo ed, the slope of the over-all amplification c aracteristic at the frequency limits will be in the ratio of' 16 to 1. In order to provide a substantially uniform gain characteristic for the completed. radio-frequencyreceiver, it is necessary to slope the again characteristic of the antenna and preselector system in the opposite manner to that of the amplifier and to provide a slope having an amplification .at the ends of thetuning range in the ratio of 1 to 16.

What is claimed is:

1. In a radio receiving system, in com-- bination, an untunable radio-frequency amplifier and a frequency-selecting arrangement comprising a plurality of coupled tunable circuits, said amplifier comprising a thermionic repeater having an output load which is capacitive over/the receiving frequency range whereby said'amplifier operates in a stable condition, said selecting arrangement being connected in tandem, ahead of said amplifier whereby said system is selective and free from cross-talk. 2. A radio receiving system in accordance with claim 1 in which said tunable circuits are coupled electro-magnetically.

3. A system in accordance with claim 1 in which said tunable circuits are coupled 1 both electrmmagnetically and electro-statically. I i

- 4. A system in accordance with claim 1 in which said tunable circuits are coupled both electro-magnetically and electro-statically in phase opposition, whereby the voltage ratio between the input and output of said plurality of tunable circuits is approximately uniform over the tuning range.

5. A system in accordance with claim 1 in fier, and means for variably adjustin the impedance of said inductive coupling, w ereby thestrength of signal voltages supplied to said amphfier may be re lated.

In testimony whereof I a my signature.

WILLIAM A. MACDONALD.

which said tunable circuits are coupled elec- I tro-magnetically and electro-stati'cally, said electro-static and electro-magnetic coupling being arranged to mutually aid each other,

whereby the ratio between the input and out- I put voltages of said plurality of tunable circuits varies oversaid frequency range at a rate more rapid than would if said plurality of tunable circuits were only electromagnetically coupled.

' comprising a plurality of tunable circuits 6. In aradio receiver a preselector comprising a plurality of variably tuned cir-. cuits arranged in tandem and coupled to- 7 In a radio receiver, including a series.

of resonant circuits tunable over a certain frequency range, can untunable amplifier comprising at least one stage of amplification, a coil connected to the output circuit of said amplifier stage having such valueof inductance that it is resonant at a frequency fixed lower, but not greatly lower, than the lowest frequency of said range, whereby the ratio of output to input voltage of said amplifying stage decreases with increased frequency, and grounded metallic shields interposed between the series of tunable resonant circuits whereby undesired coupling between said series of tunable circuits is reduced.

8. In a radio receiver a tuning system arranged to be syntonously tuned over a range in frequency, an untunable amplifierstage comprlsing a vacuum tube having an anode circuit and a load in the anode circuit,

which is capacitively reactive throughout the tuning range of frequencies, a mutually inductive coupling between the output of said tuning system and the input of said ampli-

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