US2259596A - Band receiving system - Google Patents

Description

Oct. 21, 1941. F. K. VREELAND BAND RECEIVING SYSTEM Original Filed Aug. 1, 1927 2 Sheets-Sheet l 0a. 21, 1941-. r-. K. VREELAND 2,259,596

BAND'RECEIVING SYSTEM 0rigina1 F i led Aug. 1, 1927 2 Sheets-Sheet 2 INVENTOR Patented Oct. 21, 1941 STATES TENT arms 2,259,595 f F BAND RECEIVING SYSTEM Frederick K. Vreeland, Montclair, N. .L, assignor to Vreeland Corporation, New York; N; Y., a corporation of New Jersey 1 Application July 11, 1929, SerialINc. 377,409, now

Patent No. 1,850,973, dated March 22,1932, which is a division of application Serial No. 209,650, August 1, 1927, now Patent No. 1,725,433, dated August 20, 1929, Divided and this application March.,18,-' 1932,1SerialNo.

This. application is a division of application Serial No. 377,409, filed July 11, 1929, on which Patent 1,850,973, issued March 22, 1932, which is itself a division of application Serial No. 209,650, filed August 1, 1927, on which Letters Patent No.

"1,725,433, issued August 20, 1929.

7 i The invention herein described relates to a system of receiving alternating currents including a band of frequencies, particularly such a band of frequencies as comprise the transmission band of ajmodulated signal wave, The general purpose of the invention is to receive the component frequencies of such a band With such uniformity as to avoid material distortion of the modulated wave, and to exclude frequencies outside of the band which the system is designed to receive. vention is to provide means for shifting the position of the band in the frequency scale at will, by a simple adjustment, so that the system may be Another purpose of the inreadily adapted to receive modulated waves of any desired carrier frequency, including the side bands of such modulated waves; A particular object of the present invention is to secure the bodiment of the invention the band selector unit is combined with an antenna or other collector,

andna compensating reactance is employed to compensate the indeterminate reactance introduced by the collector and preserve the necessary symmetry of the system. Other features of the invention relate to the combination of a plurality of such units, each having a band characteristic, in a receiving and amplifying system, giving a high degree of amplification over a band of frequencies 'with a' high selectivity or power of exeluding frequencies outside the desired band. Other desirable features of the invention are explained at length. The original applicationon which were issued LettersPatent No. 1,7 25,433 has been restricted to the band selector as a unit and specifically to that form of band selector unit in which the common reactance bridges the common terminals of the two reactive couples while the claims of the divisional Patent 1,850,973 are directed to the combination of such a band selector with other elements of a receiving system. The'claims of the present application are directed 5 Claims; (01.250 20) particularly to other-combinations of the elementsof a receivingsystem embodying diiferent specie s of the invention, having as one particular object increasing the selectivity of the system without introducing distortion, andincluding as specific means to this end a plurality of systems of balanced reactances in specified relations; also other fcbmbinations including specific relations between the selective means'anda collector, a detector and an amplifier, and other features as willappear.

Whe n selectivity, the power of separating a signal wave o'fpne carrier frequency from undesired waves-er different carrier frequencies, is

accomplishedby the usual method employing a tuned circuitor circuits, thefrequency characteristic oi the receiver is essentially peaked, since thereisonly one frequencyatwhich the capacity and inductance r'eactanc'es of the circuits are balam n, At anyother frequency there will be an unbe en edr cee ens e' nlit System Whieh cuts downithe response to such'frequency. In receiving a modulated wave, comprising a band of frequencies, such a system will receive one frequency crime band effectively, and the other frequencies of the' band'less eff ectivelyor not ,at all, with resulting signal distortion.

In thecase where a plurality of synchronously tuned circuits are employed cascade, in the usual w ay se1ectivity is increased since the am- 'plificationat "peak frequency is increased .in geometric -'ratio and the amplification at any other frequency is increased in a much smaller ratio,

but this. selectivityis necessarily secured at the expensejof tone quality, since the side bands are relativelyreduced according'to the same law. It has beenf-propose'd to improve the reception of sidelban'ds by introducingdamping into the synchronously tuned circuits, but this only results in partial mitigation of. thedistortion and this mitigation is gained at theexpense of selectivity.

In my Patents Nos. 1,666,518, April 17, 1928,

1,632, 74, SeptemberA, 192a, and 1,730,987, October'3, 1929, I have described means whereby substantially uniform reception is obtained at all frequencies included in the band of a modulated wave, the means'specifically claimed in these patcntsbeing the uses]: successive stages of amplification'having different frequency characteristics, and in combination producing a band character'isne;

By'means of the invention common to the Patent 1,725,433, the Patent 1,850,973 and the present application I am able to secure a similar unifor'm'ban'd characteristic in a single selector unit all the frequencies within its characteristic band,

and is non-responsive to frequencies outside this band. When the system is suitably constructed, as hereinafter described, the cut-off at the limits of the band is exceedinglysharp. By the use of such a selector unit I am able to secure distor tionless reception of the entire'band of'frequencies included in a modulated wave, and effectively eliminate the frequencies of interfering waves.

Because of the sharp cut-off this uniform band reception is accomplished without any loss of selectivity. Comparing the frequency characteristic of my selector unit with that of a-pair of selective circuits tuned by resonance in the usual way it is found that the broadening of the band over the effective frequencyran'ge is accomplished without any increase of the width of the curve at its base, which determines the selectivity of the system.

Any number of my band selector units may be employed in cascade. In one arrangement that is especially effective they may be used for' example as coupling units in a multistage ampli- These features are illustrated and the apparatus employed is fully explained in the accompanying drawings and in the following description. In the drawings: Figure 1 represents schematically one of my band selector'units, in generalized form. Figure2 is avector diagram showing the relation of the currents in the various parts of the system of Figure '1. 5 i Figure 3 is a typical curve representing the frequency characteristic of one of my band selector units. It shows also for comparison a frequency characteristic of an ordinary tuned circuit. Figure 4 shows'a' radio receiving system embodying one of my band selector units associated with an antenna or collecting circuit on the one hand and an aperiodic amplifying and detecting system on the other. v Figure 5 shows a band selector unit employed as a preliminary selector or pre-selec'tor with a band amplifier.

Figure 6 shows a radio receiving system embodying a plurality of my band selector units, one being associated with a collector as in Figure 4 and the others being employed as coupling units in a multi-stage radio frequency amplifier.

Figure 1 shows one of my band selectorunits in generalized form. It employs two reactive couples X1 X2,each comprising capacity and inductance reactances 01.111 and C2 L2 which are preferably balanced at the same frequency and partially balanced at all frequencies included in the band, combined with a third reactance Xa, which is shared in common by the two reactive couples and completes the balance of the reactances. This third reactance is small in relation to the reactances of the two reactivecouples. It serves as a band forming reactance tending to balance the unbalanced portions of the two reactive units and renders the system responsive with substantial equality to all frequencies within a band whose width depends upon the relative values of the band forming reactance and the other reactances. For frequencies outside of this band, whether higher or lower than the frequencies included within the band, the unbalanced portions of the reactances of the two reactive couples become greater or less than the effective reactance of X3, which is hence unable to balance them so that, the system as a whole has an over-all reactance which prevents its transmission of currents of such frequencies outside the band. The reactance X3- may be untuned and either in inductance; a capacitance, or a mutual inductance, Figure 1 showing the reactance in generalized symbolic form. r v T In using my band selector unit as a frequency selector the impressed electro-motive force may be applied in any suitable way, shown schematically by the electro-motive element Ein the diagram, and the output of the unit may be taken off in any suitable way, as,-for example, by means of a pick-up coil S coupled to the inductance L2 as shown. Other specific means of applying and taking off the signal energy are shown in Figures 4, 5 and 6.

The operation of the band selector unit'may be more readily understood by'reference to the vector diagram Figure 2. Let the currents set up bythe impressed electro-motive force E in the three branches X1, X2 and X3 be I1, I2 and I3 respectively. These three currents are considered positive when they flow in the direction from the common point a of the branches to the common point i). Since the total current flowing into or out of points a. and b must be zero, the current I2 in the common reactance X2 must be equal and opposite to the vector sum of currents I1 and I2 in the other two branches. This relation is shown by the vector diagram Figure 2, 0 being the phase angle between the currents I1 and I2.

This phase angle varies from zero to 180 degrees in the following manner, depending upon the frequency of the impressed electro-motive force E. i

For any given band selector there is a critical frequency F1, at which the inductance and capacity reactances L1, C1 and L2, C2 of the branches X1 and X2 are balanced in themselves. The overall reactance of the circuit C1, L1, L2 and C2 will then be zero, the current will be in phase with the electro-motive force and its magnitude will depend upon the effective resistance of the system. The currents I1 and I 2 will then be in substantially opposite phase relation, considered from the junction points a and b, the angle will be approximately 180 degrees, and the current I3 will be approximately zero, the resistance of the system being considered small.

There is another critical frequency, F2, at which the unbalanced reactance of the branches X1, X2 in parallel is equal and opposite to the reactance of the branch X3. The reactances of the system as a whole are thus balanced if the currents I1 and I2 are in phase, the phase angle c being zero, in which case I3 will be approximately equal capacity reactance of C1 'or 02.

to the arithmetical sum of I1 and I2, the effect of resistance being considered small.

At "any frequency between these limits F1 and F2 the unbalanced reactance of the branches X1 and X2 will have a value intermediate between zero and X3, the phase angle g0 will lie between the limits 180 degrees and zero, and the current I3 will adjust itself between the limits zero and 2I1. If the resistance of the system is low and the value of X3 is sufficiently small in relation to "the other reactances, the current I2 will be substantially constant at all frequencies between F1 and F2, and effectively excludes all fr'equencies outside this band.

'If the resistance and other losses of the system are low, as they are preferably, the cut-off at the limiting frequencies is very sharp, and the frequency characteristic of the band selector unit has the form shown in Figure 3. V

The width of the band depends upon the relation .or the reactance X3 to the other reactances of the system. Thus if X3 is an inductance, as shown in Figures 4, 5 and 6, the band width depends upon the relation of this inductance to the inductances L1 and L2. If the reactance X3 is a capacity, the band width is determined by the relation of the capacity reactance of X3 to the In the case where thet'common reactance is a mutual inductance, the relation is similar to that existing in the case :of a simple inductance.

In general the width of the band expressed as a fraction of the mean or carrier frequency, is

equal to the ratio of the reactance X3 to the balanced reactances of the branches X1 and X2 very approximately. Thus when X1 and X2 are equal tion at a carrier frequency of 1,000 kilocycles with aband width of 20 kilocycles, the limitingfrequencies are 1,010 and 990 kilocycles and theratio of L3 to L1 (or C1 to C3, as the case may be) becomes 2 to 100. That is, L3 is equal to 2% of L1. It will be understood that this example is merely illustrative, and that the quantities employed may be varied over wide limits to suit the particular case in hand.

The band Width may be determined within reasonable limits by choice of the relation of the common reactance X: to the other reactances. If X3 is made too large the band loses some of its uniformity, and shows a depression or valley at the middle. In practice, however, the band is substantially uniform when designed for the frequency range represented by a modulated radio signal wave, for example if the system is designated to transmit a band 20 kilocycles wide, which includes substantially all the side band frequencies of a modulated wave.

When X3 is a By making X3 7 -X3 may remain constant,

ances, constant and similar.

variable the band width may be adjusted at will, to be broad or narrow as conditions or the convenience "or pleasure of the operator may require.

It is of interest to note the relation of the band "characteristic of the band selector unit to the characteristic of a tuned selective circuit. Thus if the mutual reactance X3 is omitted the two branches X1 and X2 together constitute a resonant circuit tuned to a certain frequency F1, this being one of the limiting frequencies of the band of the selector unit. The resonance characteristic curve of such a tuned circuit is shown by the dotted lines in Figure 3 in its characteristic sharply peaked form.

When the-common reactance IQ is added to the system the curve takes the band form shown in full lines, the limiting frequency F1 corresponding to the natural frequency of the tuned circuit, and the limiting frequency F2 being below or above this frequency, depending upon whether the reactance X3 is inductive or capacitive.

When the reactance X3 has a suitable small value in reference to the other reactances, the widths of the two curves at the base are sub- .stantially the same, showing that the uniform band reception is achieved without any loss of selectivity, but rather with a noteworthy gain.

It will be noted that the gradient of the cutoff in the band characteristic is much sharper than the slope of the resonance curve, since at any frequency outside the band IQ becomes a shunt or by-pass of sma'l-lreactance across the then large unbalanced reactance of X1 and X2, and so effectively prevents transfer of energy from one to the other. This sharp cut-off is a noteworthy feature of the selectivity of the band selector. I

The curves shown in Figure 3 are reproduced from records made by an oscillograph of the performance of an actual apparatus at a frequency of 600 kilocycles.

The band of reception may be readily adjusted in the frequency scale by varying the capacities C1 C2 orthe inductances L1 L2 or both. Usually For example the capacities C1 C2 may be variable condensers of the usual type, preferably equal, and operated by a single or common control. The band frequency of the system may thus be adjusted to any point in the frequency scale within the limits deter- 'mined by the ratio of the maximum and minimum capacities of the condensers. In this case if the reactance X3 is an inductance of constant value, the band width, considered as a fraction of the mean frequency, is constant, being deterr'nined by the ratio of the constant inductanoes.

Similarly if the frequency is adjusted by varying the inductances, as it may readily be, for example, by inserting similar short circuiting rings or tubes in the inductance coils, the frequency of the band may be adjusted at any point within the'limits determined by the greatest and least value of these inductances. In such case if the reactance X3 is a capacity, the band width, expressed as a fraction, will be constant, whatever the position of the band in the frequency scale.

While the inductances and capacities may both be made variable it is usually preferable to make one pair of reactances, for example the induct- The other pair of reactances which are of opposite sign, e. g. capacitive, in the case assumed, are also preferably made similar and similarly variable. It is usually desirable to make the band forming reactance X3 of the same sign as the fixed reactances, thus if the fixed reactances are inductive, X3 will be an inductance; if the fixed reactances .are capacitances, X3 will be a capacitance. In

this case, if X3 is constant, the band width, ex-

pressed as a fraction, would be constant as above explained. 'In general the reactive coupling, due

'to the reactance X3, between the component resonant circuits or reactive couples should be of sufficient magnitude to make the frequency response of the system broader than that of the individual circuits, as illustrated in Figure 3, though the variation of the coupling may be used to narrow the response to any desired degree permitted by the other constants of the system.

By making X3 variable as heretofore noted any desired relation of band width to the frequency may be secured.

In Figure 4 I have shown one of my band selector units employed as a frequency selector in a radio receiving system. The reactive couples X1 and X2 and the common reactance X3 are indicated by the same symbols as in the generalized schematic diagram Figure 1. The band selector unit is associated with the antenna or collectorA by a primary coil P coupled with the inductance L1 of the reactive couple X1. The bandselector unit may be associated with an aperiodic amplifying and detecting system, such as the detector D and audio frequency amplifier ljustment is not necessary. It is sufficient to have the magnetic circuits of the two coils interlinked. By varying the degree of interlinkage, the electro-motive force applied to the detector may be varied from zero to a maximum. The maximum occurs when the coils are closely coupled, and

ithe minimum when their fields are not interlinked at all.

The antenna coil P is preferably closely coupled to'the inductance L1. Usually I prefer a step-up ratio of turns, i. e. the number of turns of the antenna coil P is less than the number of turns of theinductance L1. In the case of such a stepupratio the effective capacity introduced into the reactive element X1 by the antenna is less than the antenna capacity, in proportion to the ratio ofturns. For this reason, and for other reasons that will be understood, this inductive coupling is usually preferable to connecting the antenna and ground directly across the capacity C1.

- The effective capacitance introduced by the antenna, or in general the effective reactance introduced by the collector, into the reactive element X1 is an indeterminate factor which if not compensated, would unbalance the symmetry of the system, and, if large enough, would distort the band characteristic. A feature of the pres- .ent invention which avoids such unbalance and distortion is the introduction of a compensating reactancein one of the reactive couples corresponding to the indeterminate reactance introduced into the other reactive couple. ample, in the case where the element that intro- For ex- "is capacitive, as shown in Figure 4, symmetry :compensate for any desired value of the capacity of the collector, but I prefer to make it a fixed capacity larger than the largest value of the effective capacity that will be introduced into the element X1 by the collector. I then employ an adjusting capacity Cx in parallel with the capacity C1, to make up the difference between'the compensating capacity Co and the effective capacity introduced into the systemby the 'collector.

It will .be readily understood that any equivalent devices for producing similarity in two circuit elements will be applicable to the specific case of the two circuit elements, one of which includes a collector, inthe band selector unit of the present invention.

In the arrangement shown in Figure 4, the position of the band of reception in the frequencyscale is determined by adjusting the capacitiesor condensers C1 C2 simultaneously by a-common control movement, whereby the frequency of the band of reception may be changed at will without altering its uniform band character.

In Figure 5 I show one of my band selector units employed as a preliminary selector, or preselector, with a coupled collector, in conjunction with a band amplifier of the type set forth. in Patents Nos. 1,666,518, April 17, 1928, 1,682,874, September 4, 1928, and 1,730,987, October 8, 1929. This is a very desirable improvement over the combination including a tuner, set forth in my Patent No. 1,730,987;

By a suitable choice of the inductances and capacities of the band selector unit and the band amplifier, the frequency characteristics may be made to coincide so that they may be adjusted in the frequency scale by a single or common control means, as shown and fully explained in my former application.

' A very important characteristic of the described band selector system is that any number of band selector units may be employed in cascade, thus greatly increasing the selectivity of the a system, without narrowing the response curve. When the conventional system of tuning by resonance is employed the use of synchronously tuned circuits in cascade inevitably sharpens the response curve, thus trimming the side bands and destroying the fidelity. When band selector units are used in cascade any desired degree of selectivity may be obtained without narrowing the effective band of response. There is no diminution of signal strength at any part of the useful reception band, but the use of successive units serves to steepen greatly the gradient of the cut-off, thus improving selectivity. The use of a selector unit of the type described as a pre-selector in advance of the first amplifier tube, is also important. Whenv a single tuned circuit is-used, as is customary at the present day, a powerful signal of foreign frequency introduces forced oscillations which are impressed on the grid of the first amplifier tube and modulate the desired signal oscillations, producing cross modulation or cross talk, so that when the desired signal is tuned in the interfering signal is heard superimposed upon it.

Such cross modulation is prevented by the use of a pre-selector unit of the type described, which is double tuned by means of the two reactive to the arithmetical sum of I1 and I2, the effect of resistance being considered small.

at any frequency between these limits F1 and F2 the unbalanced reactance of the branches X1 and X2 will have a value intermediate between zero and X3, the phase angle (p will lie between the limits 180 degrees and zero, and the current I3 will adjust itself between the limits zero and 2I1. If the resistance of the system low and the value of X3 is sufficiently small in relation to the other reactances, the current I2 will be substantially constant at all frequencies between these limits.

At frequencies above or below these limits, the combined reactance of the branches X1 and X2 will be greater than X3 or of opposite sign to X3, as the case may be, so that 2% cannot balance the unbalanced reactances of X1 and X2 and the over-all reactance of the system as a whole is large, and this unbalanced reactance will reduce the current in I2 to a small value. The band selector thus is responsive to and transmits with substantial equality all frequencies included in the band lying between the limiting frequencies F1 and F2, and effectively excludes all frequencies outside this band.

If the resistance and other losses of the system are low, as they are preferably, the cut-off at the limiting frequencies is very sharp, and the frequency characteristic of the band selector unit has the form shown in Figure 3.

The width of the band depends upon the relation of the reactance X3 to the other reactances of the system. Thus if X3 is an inductance, as shown "in Figures 4, 5 and 6, the band width depends upon the relation of this inductance to the inductances L1 and L2. If the reactance X3 is a capacity, the band width is determined by the relation of the capacity reactance of IE to the capacity reactance of C1 or C2. In the case where the common reactance is a mutual inductance, the relation is similar to that existing in the case of a simple inductance.

In general the width of the band expressed as a fraction of the mean or carrier frequency, is equal to the ratio of the reactance X3 to the balanced reactances of the branches X1 and X2 very approximately. Thus when X1 and X2 are equal and X3 is an inductance having the value L3, the band width is equal to L3/L1. When X3 is a capacity having the value C3, the band width is C1/C'3. When X3 is a mutual inductance having the value M3, the band width is M3/L1. To cite a specific example in the case of broadcast reception at a carrier frequency of 1,000 kilocycles with a band width of 20 kilocycles, the limiting frequencies are 1,010 and 990 kilocycles and the ratio of L3 to L1 (or C1 to C3, as the case may be) becomes 2 to 100. That is, L3 is equal to 2% of L1. It will be understood that this example is merely illustrative, and that the quantities employed may be varied over wide limits to suit the particular case in hand.

The band width may be determined within reasonable limits by choice of the relation of the 1 common reactance X3 to the other reactances. If X3 is made too large the band loses some of its uniformity, and shows a depression or valley at the middle. In practice, however, the band is substantially uniform when designed for the frequency range represented by a modulated radio signal wave, for example if the system is designated to transmit a band 20 kilocycles wide, which includes substantially all the side band fre quencies of a modulated wave. By making X3 variable the band width maybe adjusted at will, to be broad or narrow as conditions or the 'con venienc'eor pleasure of the operator may require.

It is of interest to note the relation of the band characteristic of the band selector unit to the characteristic of a tuned selective circuit. Thus if the mutual reactance X3 is omitted the two branches X1 and X2 together constitute a resonant circuit tuned to a certain frequency F1, this being one of the limiting frequencies of the band of the selector unit. The resonance characteristic curve of such a tuned circuit is shown by the dotted lines in Figure 3 in its characteristic sharply peaked form. 7

When the common reactance X3 is added to the system the curve takes the band form shown in full lines, the limiting frequency F1 corresponding to the natural frequency of the tuned circuit, and the limiting frequency F2 being below or above this frequency, depending upon whether the reactance X3 is inductive or capacitive. L

When the reactance X3 has a suitable small value in reference to the other reactances, the widths of the two curves at the base are substantially the same, showing that the uniform band reception is achieved without any loss of selectivity, but rather with a noteworthy gain.

It will be noted that the gradient of the cutoff in the band characteristic is much sharper than the slope of the resonance curve, since at any frequency outside the band X3 becomes a shunt or by-pass of small reactance across the then large unbalanced reactance of X1 and X2, and so effectively prevents transfer of energy from one to the other. This sharp cut-off is a noteworthy feature of the selectivity of the band selector.

The curves shown in Figure 3 are reproduced from records made by an oscillograph of the performance of an actual apparatus at a frequency of 000 kilocycles.

The band of reception may be readily adjusted in the frequency scale by varying the capacities C1 02 or the inductances L1 L2 or both. Usually X3 may remain constant. For example the capacities C1 C2 may be variable condensers of the usual type, preferably equal, and operated by a single or common control. The band frequency of the system may thus be adjusted to any point inthe frequency scale within the limits determined by the ratio of the maximum and minimum capacities of the condensers. In this case if the reactance X3 is an inductance of constant value, the band width, considered as a fraction of the mean frequency, is constant, being determined by the ratio of the constant inductances.

Similarly if the frequency is adjusted by varying the inductances, as it may readily be, for example, by' inserting similar short circuiting rings or tubes in the inductance coils, the frequency of the band may be adjusted at any point within the limits determined by the greatest and least value of these inductances. In such case if the reactance X3 is a capacity, the band width, expressed as a fraction, will be constant, whatever the position of the band in the frequency scale.

While the inductances and capacities may both be made variable it is usually preferable to make one pair of reactances, for example the inductances, constant and similar. The other pair of r'e'aotances which are of opposite sign, e. g. capacitive, in the case assumed, are also preferably made similar and similarly variable. It is usually desirable to make the band forming reactance X3 of the same sign as the fixed reactances, thus if the fixed reactances are inductive,

X3 will be an inductance; if the fixed reactances are capacitances, X3 will be a capacitance. In this case, if X3 is constant, the band width, expressed as a fraction, would be constant as above explained. In general the reactive coupling, due

to, the reactance X3, between the component :resonant circuits or reactive couples should be of sufficient magnitude to make the frequency re- ;sponse' of the system broader than that of the individual circuits, as illustrated in Figure 3, though the variation of the coupling may be used to narrow the response to any desired degree permitted by the other constants of the system.

By makingXs variable as heretofore noted any desired relation of band width to the frequency may be secured.

In Figure 4 I have shown one of my band selector units employed as a frequency selector in a radio receiving system. The reactive couples X1 and X2 and the common reactance X3 are indicated by the same symbols as in the generalized schematic diagram Figure 1. The band selector unit is associated with the antenna or collector A by a primary coil P coupled with the inductance L1 of the reactive couple X1. The band selector unit may be associated with an aperiodic amplifying and detecting system, such as the detector D and audio frequency amplifier A1 in any suitable way. I prefer to form this association by an adjustable aperiodic coupling which will give control of the strength of signal impulses applied to the system. A convenient arrangement for this purpose is an aperiodic pick-up coil S which is in variable inductive relation with the inductance L2 of the band selector. Since the purpose of this coil is to derive from the current in L2 an electro-motive force which is applied to the detector, tuning or frequency adjustment is not necessary. It is sufficient to have the'magnetic circuits of the two coils interlinked. By varying the degree of interlinkage, the electro-motive force applied to the detector may be varied from zero to a maximum. The maximum occurs when the coils are closely coupled, and

the minimum when their fields are not interlinked at all.

The antenna coil P is preferably closely coupled to the inductance L1. Usually I prefer a step-up ratio of turns, i. e. the number of turns of the antenna coil P is less than the number of turns of theinductance L1. In the case of such a step- "upratio the effective capacity introduced into the reactive element X1 by the antenna is less than the antenna capacity, in proportion to the ratio of turns. For this reason, and for other reasons that will be understood, this inductive coupling is usually preferable to connecting the antenna .and ground directly across the capacity C1.

The effective capacitance introduced by the antenna, or in general the effective reactance introduced by the collector, into the reactive element X1'is an indeterminate factor which if not compensated, would unbalance the symmetry of the system, and, if large enough, would distort the band characteristic. A feature of the present invention which avoids such unbalance and. distortion is the introduction of a compensating reactance in one of the reactive couples correspending to the indeterminate reactance introduced into the other reactive couple. For example, in the case where the element that introduces the indeterminate reactance is a collector and the'reactance introduced by the collector element X1 by the collector.

is capacitive, as shown in Figure 4, symmetry may be restored byintroducing a compensating capacity Cc, which is shown in parallel with the capacity C2. This capacity may be adjusted to compensate for any desired value of the capacity of the collector, but I prefer to make it a fixed capacity larger than the largest value of the effective capacity that will be introduced into'the I then employ an adjusting capacity 0.. in parallel with the capacity C1, to make up the difference between the compensating capacity Cc and the effective capacity introduced into the system by the collector.

It will be readily understood that any equivalent devices for producing similarity in two circuit elements will be applicable to the specific case of the two circuit elements,'one of which includes a collector, in the band selector unit of the present invention.

In the arrangement shown in Figure 4, the position of the band of reception in the frequency scale is determined by adjusting the capacities or condensers C1 C2 simultaneously by a common control movement, whereby the frequency of the band of reception may be changed at will without altering its uniform band character.

In Figure 5 I show one of my band selector units employed as a preliminary selector, or preselector, with a coupled collector, in conjunction with a band amplifier of the type set forth in Patents Nos. 1,666,518, April 17, 1928, 1,682,874, September 4, 1928, and 1,730,987, October 8, 1929. This is a very desirable improvement over the combination including a tuner, set forth in my Patent No. 1,730,987.

By a suitable choice of the inductances and capacities of the band selector unit and the band amplifier, the frequency characteristics may be made to coincide so that they may be adjusted in the frequency scale by a single or common control means, as shown and fully explained in my former application.

A very important characteristic of the described band selector system is that any number of band selector units may be employed in cascade, thus greatly increasing the selectivity of the system, without narrowing the response curve. When the conventional system of tuning by resonance is employed the use of synchronously tuned circuits in cascade inevitably sharpenstheresponse curve, thus trimming the side bands and destroying the fidelity. When band selectorunits are used in cascade any desired degree of selectivity may be obtained without narrowing the effective band of response. There is no diminution of signal strength at any part of the useful reception band, but the use of successive units serves to steepen greatly the gradient of the cut-off, thus improving selectivity.

The use of a selector unit of the type described as a pre-selector in advance of the first amplifier tube, is also important. When a single tuned circuit is used, as is customary at the present day, a powerful signal of foreign frequency introduces forced oscillations which are impressed on the grid of the first amplifier tube and modulate the desired signal oscillations, producing cross modulation or cross talk, so that when the desired signal is tuned in the interfering signal is heard superimposed upon it.

Such cross modulation is prevented by the use of a pre-selector unit of the type described, which is double tuned by means of the two reactive couples X1 X2 and reduces the grid swing of the first tube due to forced oscillations to a point that is not sufficient to modulate perceptibly the desired signal wave.

Both of these features are of great practical value and both are embodied in the arrangement shown in Figure 6 which includes a plurality of double tuned selector units, one being employed as a pre-selector coupling the collector with the amplifier, the others being employed as interstage coupling elements of a radio frequency amplifier.

This arrangement includes a plurality of amplifier tubes A1, A2, D, which are coupled in cascade, but the interstage coupling means in this case is not a single tuned transformer unit as in Figure 5, but a double tuned selector unit. The coupling means in each case comprises two reactive couples X1 and X2, each of which includes an inductance and a capacitance. The inductances or the capacitances or both are variable for the purpose of frequency selection. In the preferred arrangement shown the inductances are fixed and the capacitances C1 and C2- are variable. These two reactive couples are associated with each other by means which permits the transfer of oscillatory energy between them, which means, in the arrangement shown, comprises the reactive element X2 Whose reactance is common to both couples. Preferably the reactance X3 is so related to the other reactances in the system as to balance the reactances of the complete selector unit at a plurality of frequencies so that the system is responsive to all the frequencies included in the transmission band, as above explained.

The output of the first tube, such as A1, is impressed on a selector unit X1, X2, X3, which serves as the coupling means connecting the tubes in cascade and the selector unit is also operatively connected to the second tube such as A2. The means shown for impressing the output of the first tube on the selector unit is the connection between the anode of the first tube and the reactive couple X1 and the connection at serves to connect the second couple X2 to the grid or input circuit of the second tube, impressing signal oscillations thereon.

In the arrangement of this figure three of these selector units are shown, operating in cascade and coupled by the amplifier tubes A1 and A2, the first being employed as a pre-selector in advance of the first tube A1, and being associated with collecting means, here shown as a loop collector L1, the second being employed as an interstage coupling unit, coupling the tubes A1 and A2 in cascade, and the third coupling the amplifier tube A2 to the detector D.

It will be noted that each of the selector units is double tuned by means of the variable condensers C1 and C2.

It has been stated above that the means shown for impressing the output of the first tube on the selector unit is the connection 0 and that the connection d serves to connect the second couple to the input circuit of the second tube, but it will be obvious that instead of the direct electrical connection shown, a transformer or other suitable coupling may be employed, as is well known in the art, as shown, for example, in Figs. 4 and 5. It is characteristic of the selector unit described that the impedance between the points 0 and b and the points 01 and b is high at all frequencies within the band of effective response of the system and relatively low at frequencies outside of this band.

The two or more band selector units are preferably made alike, for convenience in mechanical construction. The inductance L of the loop is made approximately equal to that of the inductor L2, and a small compensating inductor La added, so that all the units are made symmetrical and all may be similarly adjusted by a single control means.

It will be understood however that complete symmetry is not essential, provided there is such similarity as will give the various band selector units similar frequency characteristics, the variable reactances being similarly variable so that they may be all operated by single control means as shown, which serves to adjust the pre-selector and the interstage coupling unit simultaneously by a single operation, so that the frequency response of the several selectors 'issimultaneously and similarly adjusted and the system as a whole is made responsive to any desired signal frequency or band of frequencies.

It will be understood that other modifications and applications of the system may be made without departing from the essential principles of the invention.

I claim as my invention: 7

1. In a system for receiving the transmission band of a modulated signal wave, a collector, an amplifier receiving signal energy from the collector, a double tuned band pre-selector connected between the collector and the amplifier and comprising two variable reactive couples, each having reactances that are partly balanced at the frequencies included in the transmission band, an untuned reactance common to both couples whose value is so related to the reactances of the couples that the unbalanced portion of these reactances is balanced and the system is made responsive to all frequencies within a definite band which is broader than the effective response of the component couples, said preselector having means for compensating for the reactance of the collector, and single control means for simultaneously varying said variable reactive couples to shift the band of response of the system in the frequency scale.

2. In a system for receiving the transmission band of a modulated signal wave, a collector, a double tuned band pre-selector comprising two variable reactive couples each having reactances that are partly balanced at the frequencies included in the transmission band, an untuned reactance common to both couples whose value is so related to the reactances of the couples that the unbalanced portion of these reactances is balanced and the system is made responsive to all frequencies within a definite band which is broader than the effective response of the component couples, an amplifying and detecting system receiving signal energy from the collector through the band pre-selector and responsive with substantial equality to all frequencies included in the reception band passed by the band pre-selector, said pre-selector having means for compensating for the reactance of the collector, and single control means for simultaneously varying the variable reactive couples of the band pre-selector to shift the band of response of the system in the frequency scale.

3. In a system for selectively receiving any desired transmission band of modulated signal waves, the combination with means for collecting and means for detecting the said Waves of a system of reactances, operatively connected with the collecting means to receive the collected energy therefrom andoperatively connected with the detecting means to impressthe selected signal band on the detecting means, said system comprising two reactive couples each having reactances that are partly balanced at the frequencies included in the transmission band, an untuned reactance common to both couples whose value is so related to the reactances of the couples that the unbalanced portion of these reactances is balanced 'andlthe'system is made responsive to all frequencies within a definite band, said system having means, for compensating for the reactance of the collecting means and means for varying said reactances for selecting at will any desired transmission band and cooperating with said system of reactancesto eliminate distortion simultaneously irrespective of the degree of increase in selectivity.

4-. In-a system for selecting any desired trans mission band of modulated signal waves, the combination with means 'for'collecting and means for detecting said Waves of a plurality of systems responsive to all frequencies within a definite band, the first of said systems of reactances having means for compensating for the reactance of the collecting means, means for varying said reactances for selecting at will any desired transmission band, and means for cooperatively associating said systems of reactances to increase Q the selectivity of the receiving system without imparting distortion thereto.

5. In a system for selectively receiving any desired transmission band of modulated signal reactances is balanced and the system is made responsive to all frequencies Within a definite band, said system of reactances including means for compensating for the reactance of the collecting means, means for varying said reactances for selecting at will any desired transmission band without materially modifying the relative intensities of the frequencies included in the selected band, and means for amplifying the entire transmission band of the selected signal to a substantially constant degree.

FREDERICK K. VREELAND.

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