US2037756A - Modulation system - Google Patents

Description

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Apfifi MODULATION SYSTEM Filed Sept. 27, 1929 2 Sheets-Sheet 1 m frequency EOWCE 2 9' v 5 I I I 11 ATTORNEY BGHM ET MODULAT I ON SYS TEM Filed Sept. 27, 1929 2 Sheets-Sheet 2 m m m m g g ,WWILJ Z v 0 b J QQELET Patented Apr. 21, 1936 EJNITED STATES PATENT OFFICE assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application September 27, 1929, Serial No. 395,505 In Germany October 1, 1928 7 Claims.

The present invention is concerned with a method adapted to divide or split in a transmitter station a certain output radio frequency oscillation into two or more other oscillations (os- 5 cillation bands) of different frequencies, though substantially like amplitude, with the original carrier frequency being incidentally omitted.

The ensuing or final frequencies thus obtained, if they differ but comparatively little from each other or from one another, inter alia, may be employed also to advantage for the simultaneous transmission of one and the same communication, and this is particularly advisable in short-wave work for the purpose of overcoming the 15 phenomena of fading.

For a better understanding of the idea underlying this invention and the operation of the circuits incorporating it reference may be had to the drawings, in which,

20 Figure 1 illustrates diagrammatically one embodiment of our invention;

Figure 2 illustrates by curves certain conditions existent in the circuits as employed:

Figure 3 illustrates our invention more completely;

Figure 4 illustrates amodification of our invention in which certain type of chokes are utilized;

Figure 5 illustrates a Wheatstone bridge arrangement for accomplishing the purpose of the 30 invention;

Figure 6 illustrates a modification of the circuit shown in Figure 5; and,

Figures 7 to 10 inclusive illustrate the invention as applied to circuits without the need of direct current being superimposed.

In Figure 1, it is assumed that the terminals pp from some suitable radio frequency generator are fed with radio frequency current i1=J1 sin cult whose amplitude is constant by the adoption of 40 means and ways later to be explained, i. e., independent of the impedances connected at the points ea or of the current there derived. Now, as shall hereinafter be demonstrated, it is possible to produce in the branch 2 two radio fre quency oscillations of like amplitudes, though different frequencies if current in the parallel branch or shunt LC (which will hereinafter be referred to as a coupler branch) is modulated in a certain way. It shall be assumed that the inductance contamed in the coupler branch is due to an ironcored choke-coil whose reactance [01L active in respect of the comparatively small radio frequency currents is acted upon both by a direct current vi derived from a direct current source b and a modulating current im=Jm sin wmt produced by an alternating current generator 02. Hence, through the winding e of the said ironcored choke-coil there flows a resultant pulsating current ir=i +im. The resultant reactance an; of the coupler branch for the radio frequency wl varies with 'Zr and im, respectively. Now, the parts of the said branch are chosen in such a way that its resultant reactance an active for the radio frequency wl disappears simultaneously with the modulator current im, indeed, all that is necessary for that purpose is that at the instant when the winding e of the iron-cored choke-coil is only traversed by the direct current i the corresponding inductive reactance (.0114 is equal to the capacitive reactance The shape of the coupling reactance $1; in dependence upon the modulating current im is shown in Figure 2, where graph A shows the dependence of the variable reactance 01111 of the iron-cored choke-coil active for the radio frequency and therefore proportional to the differential quotient of the inductance upon the resultant current ix.

For the point P of the abscissa which corresponds to the direct current i the corresponding ordinate w1L of graph A equals in amount to $k=im tan B=Jm sin wmt. tan [3 (1) For the voltage vector Ea at the terminals aa there holds generally this equation:

If impedance vector 2 in the numerator is considerably larger than am, this latter quantity may be disregarded, and the fraction Z-l-x may be taken to be equal to unity, and then there is obtained with sufficient closeness and approximation:

It may be noted parenthetically that the approximation will be particularly close if Z, as is recommendable, is substantially an active resistance.

It will be seen that the amplitude of this potential, if also J1 is kept at a constant value by suitable means, will not remain constant but is modulated in dependence on an; at the rhythm of the frequency wm. If the coupler branch experiences no appreciable ohmic losses, then its impedance we will be practically pure reactance, and the voltage Ea leads the current J1 by an angle of degrees. The instantaneous value as of the voltage will then be:

e,,=] sin (w t+ ;)x cos w tx (4) or in the light of Equation (1) a=JlJm tan {3. cos mt. sin wmt (5) If by the adoption of special means care be taken so that J1 remains constant regardless of the changes in the coupling reactance m then the factors J1 Jm tan B in the last equation form a constant product 270, so that there holds good this formula:

6a=2K sin wmt. COS wit (6) or else,

ea=K Sin(w1+wm) tK. sin (col-arm) t (7) The current J2 set up by the potential Ea in branch Z is:

Hence, also the amplitude J2, in the presence of constant J1 and X is not constant, indeed, it varies conjointly with the factor (Bk at the rhythm of the modulating frequency wm, in other words, it splits again into two currents having the frequencies (601+(dm) and (w1wm) and equal amplitudes.

It will be noted that it is, as matter of fact, possible, by the aid of modulation as described of a radio frequency current i1, to obtain two different radio frequency oscillations of the same amplitudes, without obtaining any of the original carrier frequency. In a similar way it would also be feasible, by the superposition (or heterodyning) of several modulating currents, to obtain several pairs of radio frequency in the branch 2, as will be readily understood by introducing in Equation (1) for im an expression 2 Jm sin wmt with different values of m.

Hence, if modulation is effected simultaneously, say, at 100, 200, 300, 400 cycles, the result will be several radio frequency currents, which differ from one another by like amounts.

It will be noted from what precedes that in a way as disclosed it is possible to split a given radio frequency current into two or more radio frequency oscillations of different frequencies by inserting between the terminals pp and the load branch .2 a coupling variable as to size and sense, which, in the course of a period or cycle of the modulating current, in accordance with the variations of the coupling factor occurring in Equation (8) will pass through zero twice and incidentally change or reverse its sense.

In the above derivation the presupposition has been made that it is possible to maintain the amplitude value of 1 at a constant value independently of variations of the coupling .Bk. A simple scheme adapted to insure this is shown in Fig. 3. The current i1 whose amplitude J1 is to be kept constant, is taken from an oscillation circuit L"C" which is tuned to resonance with the radio frequency o1, said oscillation circuit in turn being fed with energy from a source qq in loose coupling relationship therewith by way of a coupling coil L. Owing to such loose coupling, the coil L' suffers practically no reaction at all from the circuit with which it is thus associated, with the result that its current remains constant and that a constant A. C. voltage is induced in the coil L. Since the oscillation circuit L"C" is tuned to the frequency of this constant induced potential, the current J1, as is well known, will be maintained at a constant value automatically irrespective of all changes of impedance of other connected circuits. The load impedance 2 in Fig. 3 consists of the impedance of circuit I including the transferred resistance of amplifier V coupled therewith through circuit II.

What is important is that the ohmic resistance of the coupler branch should be negligibly low, for in that case the coupler circuit, each time that the reactance wk disappears, represents a perfect short-circuit for impedance 2 so that the modulated amplitude of i2 then passes through the zero value each time and contains no modulated component of the carrier wave 401. Since the losses, when radio frequency currents are dealt with, are primarily occasioned by the iron-cored choke-coil L, it is reduction of the iron losses that should be aimed at in the first place. It is a good plan to employ in lieu of ordinary iron-cored choke-coils, stretched or straight conductors coated with a thin layer of iron of the kind used in radio frequency work for other purposes see,

for instance, German patent application No.

A suitable circuit scheme for this purpose is illustrated in Fig. 4.

As there shown, the coupling branch is divided into two like parallel branches LC, comprising inductances L consisting of straight iron-coated conductors, say, conductors wrapped with a capillary iron wire or covered by electro-plating with a thin film of iron. The direct current ig and the modulating current im are fed at the corners ee, whereas the connections for the radio frequency current i1 and the modulated radio frequency current i2 are at points aa. A small variable inductance La serves for accurate adjustment.

However, also in this arrangement it is rather diflicult to eliminate entirely the influence of the ohmic losses.

In what follows a number of circuit arrangements shall be described wherein the undesirable influence of the ohm or loss resistances is practically precluded entirely or in which these are even put to a practical use.

In the scheme illustrated in Fig. 5, the circuit of the constant radio frequency currents i1 is coupled with the circuit carrying the modulated radio froquency current i2 by the aid of a Wheatstone bridge (11111 are the terminals for the radio frequency current 21 and for the modulating current azaz are the terminals for the direct current z' and the modulated radio frequency current i2. The variable inductances L1L2 may also in this instance consist of ordinary choke-coils or, better still, of the said iron-coated or iron-wire wrapped straight conductors. It is recommendable to choose the condensers C sufficiently large in order that they may constitute no unduly high reactance for the modulating current im, and to tune the inductances LO together with the condensers C contained in their branches with the modulating frequency wm. However, this latter instruction need not be observed absolutely.

It is furthermore not absolutely necessary that the currents im and i1 should be passed through one and the same winding of the choke-coil L as shown in Figure 1, on the contrary, it may be advisable or preferable, inter alia, to provide distinct windings therefor being connected in series in relation to each other, though diiferentially in relation to the direct current excitation, for the purpose of insuring absolute phase equality of the currents im in both limbs.

The operation of the circuit scheme shown in Fig. 5 is as follows:

At the instants when the modulating current "5m is of zero value the two inductances L1 and L2 are perfectly equal, and the corner points can are equipotential as regards the radio frequency on so that i2 will also disappear. During a half cycle of the modulating frequency aim the modulating current im in one of the variable inductances, for instance, in inductance L1 is unidirectional with the direct current, and in limb L2 opposite in direction to the direct current so that the inductance of the first limb is lower than in the second. During the next half cycle this relationship of the inductances is inversed. Hence, if at a definite instantaneous value and direction of 2'1 and im the modulated radio frequency current 2'2 has a corresponding instantaneous value and a corresponding sense, then after a half cycle of the modulating frequency (if current i1 has the same size and direction as before, and current im the same instantaneous value, though contrary sense) for the current i2 the same instantaneous value as before, though of opposite sense. Currents i2 and 11 are associated by the relation i2=i1 f(im) where the function flim) can be regarded and designated as coupling factor. This coupling factor passes simultaneously with im through zero and it changes its sign together with im, hence, it is a periodical quantity substantially proportional to im having a frequency of variation (01m), whence the inference that the current i2 is a purely modulated radio frequency current, in which there occur no unmodulated remnants of the carrier frequency.

Calculating for a Wheatstone bridge comprising arm impedances a1, (12, as, (14 (Fig. 6) the coupling factor, 1. e., the ratio between the diagonal currents J2:J1, there is found this relation:

appears, assume both the value a, otherwise the amplitude:

is independent of the modulating current, hence. the denominator Equation (9) remains constant. The numerator, however, becomes equal to a3(a1az) =2a3Aa=2aKim so that the Equation (9) assumes this form J2=J1.Kim (11) where K a constant. What follows therefrom is the fact that J2 is modulated at the rhythm of current im perfectly, in the absence of any trace of the original carrier frequency.

In the scheme shown in Fig. 5, both arms have constant reactances, it being understood,

variable reactances controllable by the modulating current. In this instance, opposite reactances must always change in the same sense, i. re-

actances a2 and a4, see Fig. 6, in the presence of a certain direction of the modulating current, must both grow, and the reactances a1 and both decrease, and vice versa.

In all of the embodiments hereinbefore described, recourse has been had to the superposition of a direct current and a modulating current. However, this is not absolutely necessary, indeed, the schemes hereinafter to be described by reference to Figs. '7 to 10 are not in need of direct current.

In the arrangement shown in Fig. '7, the inductance of arm L2 is subject to marked varie as a function of the modulating current im. nowever, the inductance of arm L1 is subject to but slight change, indeed, it should most preferably i be entirely constant for instance, when L1 is an air-core choke-coil. To compensate for the load resistance, a small variable ohm resistance connected in series with the choke-coil L. The

two choke-coils L2 and L1 are so chosen that, in

the presence of a certain mean absolute value of the modulating current im the inductance of the choke-coil L2 for radio frequency current i1 is equal to the inductance L1. But if the absolute value falls below the average, then L2 becomes smaller than L1. Hence, also in this arrangement the coupling factor, similarly as in the preceding embodiment, is a periodic function. How ever, in this instance it passes through zero four times during a cycle of the modulating current and incidentally changes in sense or sign, with the result that current i2 is here modulated by the frequency of 2mm.

In the scheme shown in Fig. 8 the two induc tances L1 L2 consist of two transformers having their secondaries connected in opposition. The mutual inducance of the iron-cored transformer L2, in the presence of a definite mean absolute value of 111 is here equal to the mutual inductance of course, that all of the four arms could consist of of transformer L1, the latter again being but little variable or most preferably constant.

If the transformer L1 is entirely free from iron, it is advisable to provide in parallel thereto a variable resistance radio frequency for the purpose of compensating the iron losses of the other transformer.

Also in this case the effects of the two transformers upon the secondary circuit compensated each other at the instant when the modulating current z'm passes through the above-mentioned mean value. In the presence of larger absolute values of the modulating current, the action of L1 predominates, and for lower values of the modulating current, the action of L2 will prevail, so that the modulating effect is here the same as in the previous scheme.

Another means and way of modulation resides in the use of condensers of well-known type comprising one or two movable or vibratory electrodes.

The arrangement in Fig. 9 shows the use of such a condenser. In this instance, the coupling branch consists of a constant inductance and a condenser furnished with one vibratory electrode. This scheme resembles the one illustrated in Fig. l. The distinctive feature between the two resides in that the resultant coupling reactance (El: is controlled not by the co-action of a direct current and the modulating current, but rather by the vibratory electrode of condenser C. The two parts of the coupling branch are here so chosen that in case of a median position of the vibratory electrode the inductive reactance of the choke-coil and the capacitive reactance of condenser C neutralize each other so that the coupling reactance :L'k disappears. In the presence of smaller inter-electrodes distances an; becomes inductive and at larger distances capacitive. Hence, also in this instance the coupling reactance reverses in sign, and this results in modulation of current 2 at the rhythm of the oscillations of the vibratory electrode.

Another similar arrangement is shown in Fig. 10. In this case the coupling between circuits i1 and i2 is provided by a Wheatstone bridge, here comprising four condensers C1, C2, C3, C4. One among the latter, e. g., C1 is furnished with a vibratory electrode, and when the latter occupies a median position, the bridge is in a balanced condition and current i2 disappears. Larger intor-electrode distance result in a definite sense of coupling, while smaller distances cause the opposite condition. The operation of this scheme will be readily understood in the light of what has been explained above.

In case of a bridge arrangement, as will be understood, choke-coils could be employed instead of condensers in two neighboring limbs or bridge arms. Also condensers furnished with vibratory electrodes could be employed in two arms. For instance, if arms C1 and C2 were so provided, then the vibratory electrodes would be compelled to oscillate in phase opposition, while they would oscillate in phase if they were contained, for instance, in the arms C1 and C3.

It may also be mentioned that in so far as the operation of the bridge schemes hereinbefore de-- scribed is concerned, all that is important is that at least in one arm a resistance be modulated, and that the other arms should be so compensated or balanced that the bridge is balanced when the said modulated resistance has a median value.

Hence, the desired result can also be obtained if instead of the vibratory-electrode condensers, tube resistances of well-known type responsive to or influenced by modulation are used, and if the resistances of the other arms are chosen correspondingly.

It may be noted in conclusion that in explaining the operation of several bridge schemes (Figs. 5, a, '7 and 10) the assumption has been made for the sake of simplicity that the variable reactance when being of median value, becomes equal to another reactance, though this is not absolutely necessary. Indeed, it sufiices that the bridge arrangement should be so proportioned that, in the presence of mean values of the resistances contained therein, proportional relationship between the four arms of the bridge should be present as required for a balanced state of the bridge. However, the sensitivity of the scheme will be the highest when the relation between the arm resistances is equal.

We claim:

1. In an arrangement for producing a plurality of radio frequency waves for transmission from a source of high frequency oscillations which are to be suppressed, a transmission line adapted to be connected to said source, a circuit connected across said line, said circuit directly including at least two reactances of opposite sign, one of which is adapted to be varied by modulating currents about a mean value, said reactances being normally resonant at the carrier frequency, means for impressing modulating currents across only one of said reactances, output terminals for said circuit, and utilizing means connected to said output terminals.

2. In an arrangement for producing a plurality of radio frequency waves from a source of high frequency oscillations which are to be simultaneously suppressed, a circuit connected in parallel with said source, said circuit including a capacitive reactance-and an inductive reactance which are normally resonant at the carrier frequency, a source of biasing current and a source of low frequency modulating current coupled with one of said reactances, said reactance being varied by the modulating currents about a mean value twice per cycle of said low frequency modulating current, and a load circuit connected with said circuit, said one reactance having an inductive reactance equal to the capacitive reactance in the absence of modulating current.

3. In an arrangement for producing a plurality of high frequency waves by means of a carrier wave which is to be suppressed, a circuit including series reactances of opposite sign, one of said reactances being an iron core inductance, means for applying said carrier wave to the terminals of said circuit, a magnetizing winding on said core, a source of direct current connected with said winding, said circuit being normally resonant at the frequency of said carrier wave, and means for impressing modulating currents across said iron core inductance.

4. Means for producing a plurality of signal modulated radio frequency waves from a source of carrier frequency which is simultaneously suppressed, including a bridge circuit the arms of which are composed entirely of reactances, one of said reactances being arranged to vary in value about a mean value, means for impressing carrier frequency oscillations across a pair of conjugate nodal points on said bridge, a source of direct current, means for connecting said source of direct current across another pair of conjugate nodal points on said bridge circuit, a source of modulating current, means for connecting said source of modulating current across said first named pair of nodal points, said modulating current being connected differentially with respect to said direct current, and a load circuit connected to said bridge circuit.

5. In an arrangement for producing substantially equal side band frequencies from a source of carrier frequency and simultaneously suppressing said carrier frequency, a plurality of reactances connected in series with said source, two of said reactances being of opposite sign, said reactances being normally resonant at signal frequency, means for applying a modulating current across only one of said reactances such that the value of said reactance varies about a mean point, and a load circuit connected with said reactances.

6. Apparatus for producing a plurality of signal modulated radio frequency waves from a source of carrier frequency which is to be suppressed comprising a bridge circuit the four arms of which are composed entirely of reactances, one of said reactances being arranged to vary in value about a mean value, a source of carrier frequency oscillations across a diagonal of said bridge, a source of direct current serially connected with an inductance across the other diagonal of said bridge, a source of modulating current also connected across said first named diagonal, and a load circuit connected across said second named diagonal.

7. Apparatus for producing a plurality of signal modulated radio frequency Waves from a source of carrier frequency which is to be suppressed comprising a bridge circuit, the four arms of which are composed entirely of reactances, a source of carrier frequency oscillations connected across one diagonal of said bridge, an additional reactance in series in said connection, a source of direct current and a source of modulating current connected in parallel across the other diagonal of said bridge, and a load circuit connected in series with said additional reactance across said first diagonal.

OTTO BoHM. MENDEL OSNOS.

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