US2048900A - Modulation - Google Patents

Description

2 Sheets-Sheet l INVENTOR GEORGE L. USSELMAN BY 2 ATTORNEY G. L. USSELMAN MODULATION July 28, 1936.

Original Filed July 20, 1932 |||||||||I lllllllllllllllll a) m m i222;E EE EEE m m m u u m m m M m n m 989 Q59 m M u R. QM .0 m A n W U Q o w W n m m w m T k 3 QESE N u QQESSQ g Y m Q m$ h m Q n W n -L MODULATION 2 Sheets-Sheet 2 Original Filed July 20, 1932 SELMAN GEORGE L. U BY ATTORNEY Patented July 28, 1936 'FFN MODULATION poration of Delaware Application July 20, 1932, Serial No. 623,558 Renewed January 11, 1936 7 Claims.

This invention relates to signalling means and in particular to means whereby the characteristics of high frequency oscillations, other than the amplitude, are varied in' accordance with signals tobe transmitted. It has been found that ordinary amplitude modulated high frequency oscillations in transmission from the sending station to the receiving station are subject to what is known as fading effects. This is a decided disadvantage since it introduces drop-outs and errors in the signal. Even where diversity receivers are used to receive the amplitude modulated signals the effect of fading is a serious disadvantage.

I" I have found that if the high frequency oscillations are modulated in phase or in frequency in accordance with the signal'to be transmitted they are less subject to the effect of fading than am plitude modulated waves. The reason why phase or frequency modulated oscillations in telegraph transmission are subject to a less extent to fading effects than oscillations modulated otherwise is that a greater amount of the transmitter power is available for transmission. In other words a transmitter can operate at full output on phase or frequency modulation, while it is necessary to reduce the power output to about one quarter for amplitude modulation. Assuming equal power output, reduction of fading may be slightly greater in phase or frequency modulation.

'Obviou-sly, this method of signalling is of the utmost importance since one of the main drawbacks to signalling is the effect of fading of the transmitted oscillations. Especially is this method-desirable now in telegraph transmission since it -is adaptable where diversity reception methods are not in use. 1

In my United States application Serial No. 616,026, filed June 8, 1932, I disclose in detail a method of and means for producing high frequency .oscillations and phase or frequency modulating the same in a novel manner with modulating oscillations or voice frequency modulations obtained from a novel thermionic tube arrangement which,-by asingle switching operation, may be made to operate as a modulating frequency oscillation generator, or as an amplifier for the voice frequencies, with which the high frequency oscillationsare to be phase or frequency modulated. The modulating frequency oscillations generated by this novel tube arrangement and the'amplified voice frequency potentials, whichever is used for modulating the high frequency oscillations, are passed through a symmetrical thermioniczstage which affects-the modulation in ent invention; while,

phase or frequency of the high frequency oscillations generated.

The present invention relates to an improvement in modulators of the type disclosed in the above referred to application. 5

It has been found through practice that when an ordinary phase modulator, or a phase modulator as disclosed in said application, is used on the high frequencies there is usually an unbalance in the excitation voltage delivered by the phase shifting circuit to the grids of the modulator tubes. This unbalance is caused by the effect of the grid capacity to ground on the phase shifting circuit. In its broadest aspect the object of the present invention is to provide means to correct or compensate or neutralize this unbalanced effect so that equal excitation voltage is maintained on each grid.

The novel features of my invention have been p nted ut with. part cu ar ty in the claims. a tached hereto.

The nature of my invention and the mode of operation thereof will be better understood by the following detailed description thereof and therefrom when read in connection with the drawings, throughout which like reference characters indicate like parts and which:

Figure 1 illustrates a, phase modulator which includes the novel correction means of the pres- Figure 2 illustrates a modification of the arrangement of Figure l.

Referring to Figure 1, a pair of electron discharge repeater tubes l and 2 ,which may be of the thermionic screen grid type, have their control electrodes 6 and 1 connected through leads 8 and 9 to a blocking condenser H], which is in turn connected with any source of substantially constant high frequency oscillations, as, for example, the source D. Oscillations from the source I) are impressed by wayof condenser ID on to the electrodes 6 and 1 substantially cophasally. The relative phase of these oscillations is shifted, however, since the oscillations reaching the grid 7 pass through a phase advancing means in the form of a variable condenser ll, while the high frequency oscillations reaching the grid 6 pass through a phase retarding means in the form of an inductance i2.

The anode electrodes l3-andl4 are connected together and to the terminal of a tank circuit l5 which includes an inductance I ;and a variable capacity Hi. The tank circuit i5 is tuned to resonance at the frequency of the oscillations sup d om -.D- The h gh freque y qs l z tions, slightly shifted in phase, and impressed on to the control electrodes 6 and I, are repeated and amplified in the tubes I and 2, and the energy from I and 2 appears in the tank circuit I5 shifted in phase an amount determined by the phase shift impressed on the energy by the elements I2 and II. Anode potential for the tank circuit I 5 is supplied by Way of a lead I6 connected with the positive terminal of source 20.

Energy for the filaments of the tubes I and 2 is supplied by leads I9 from the source 20.

A low impedance path for the high frequency oscillations repeated in I and 2 is provided by a by-passing condenser C connected, as shown, between a terminal of tank circuit I 5 and the filament of the tubes, thereby shunting said frequency around the source 20. rent biasing potentials for the control electrodes 6 and I of tubes I and 2 are supplied by way of resistances R1 and R2 from lead 22 connecting a point on the secondary winding 23 of modulation transformer T to a movable point on potentiometer P1 connected in shunt with a portion of the battery 20, or other source. High frequency oscillations appearing in the input circuits of tubes I and 2 some of which passes through resistors R1 and R2 are provided with a low impedance path to the filament by way of by-passing condensers C1 connecting the terminals of resistances R1' and R2 to the filament lead I9, as shown.

Applicant has now described the manner in which the high frequency oscillations originating in D are applied shifted in phase, but in a sense cophasally, to the control electrodes of the tubes K and L, amplified and repeated therein, and transferred therefrom substantially in phase to the tank circuit I 5, from which they may be utilized.

The manner in which these oscillations are modulated in phase' or frequency at signal frequency will now be pointed out.

The primary winding 24 of modulation transformer T is connected with any source of modulating potentials B in such a manner that modulation potentials are supplied in phase opposition to the control electrodes 6 and 'I of tubes I and 2.

The effect ofsaid potentials being applied in phase opposition to the control electrodes 6 and I, and the manner in whichsaid signal oscillations are applied thereto in opposition, modulate in phase or frequency the high frequency oscillations repeated therein, will be set forth now.

Assume that no modulating frequencies are applied to the control grid electrodes of tubes I and 2. The same direct current biasing potential is supplied to the control electrodes, and the high frequency oscillations impressed on the control electrodes are substantially in phase, being shifted only by the equal but different sense of impedances of the inductance I2 and capacity II. The anode circuit of tube I will supply, therefore, to the tank circuit, energy equal in amount to the energy supplied to the tank circuit I5 from the anode of tube 2. The energies from these tubes will be of different phase and the energy supplied from the tank circuit to the frequencydoubler or next stage will have a resultant phase determined by the phase of the separate energies supplied from the tubes I and 2, so that in this case the carrier oscillations will have no change or shift in phase.

In the discussion of the operation of the phase modulating arrangement of Figure 1 which follows it will be assumed that signal frequency potentials are beingsupplied in phase opposition Normal direct cur- I to the control electrodes 6 and I of tubes I and 2 by way of transformer T from source B.

The energy supplied to the tank circuit I5, from the tube which has the highest positive modulating potential applied to the grid electrode, will be greater in amplitude than the energy supplied to the tank circuit I5 from the tube having a lesser positive modulating potential applied to its grid. This is due to the fact that the effective potential applied to the grid of a tube governs the internal impedance of the tube, the amplification thereof, and other characteristics. Each of these variables in turn effect the amount of energy supplied to the anode circuit of the tube. Therefore, varying in phase opposition the effective potentials of the grid electrodes of the tubes I and 2, by modulating potentials from the source B, varies differentially the amount of energy said tubes will supply to the tank circuit I5. This in similar fashion varies differentially the phase of the oscillations appearing in tank circuit I5.

The maximum possible phase deviation of the carrier is determined by the adjustment of impedances II and I2 of the grid phase shifting circuits. The phase shift, therefore, of the resultant energy in the tank circuit I5, is limited by the adjustments applied to the phase shifting circuits. The frequency of the phase deviation of the carrier is determined by the modulating frequency. The amount of phase deviation of the carrier is determined by the amplitude of the modulating frequency and is proportional to the amplitude of the modulating frequency. The amount of phase shift or deviation of the carrier or the tank circuit oscillations may, accordingly, be limited by the adjustments of the phase shifting circuits connected with the input of the tubes. The amounts of phase shift or deviation of the carrier or the tank circuit oscillations is determined by the amount or difference of power deliveredby one tube over the power delivered by the other tube, and by the phase difference of the energy delivered by both tubes. In other words, the phase shift of the oscillations in the tank circuit I5 is the result of theincrease of energy delivered by one tube while the energy delivered by the other tube decreases a like amount because of the phase difference in the alternating current energy delivered by each tube, and is proportional to said increase and decrease.

The phase shifts which take place, as indicated above, will be multiplied or increased by a frequency multiplier in case the oscillations from the tank circuit I5 are fed by way of a frequency multiplier or mutipliers to the utilization circuit.

The phase or frequency modulator of the pres-' ent invention, as described hereinbefore, is similar in many respects to the phase or frequency modulator disclosed in application Serial No. 616,026, filed June 8, 1932. In the said application, however, the modulating potentials are applied to the screen grid electrodes of the tubes I and 2, whereas in the present arrangement the modulating potentials are impressed, as set forth above, on the control electrodes 6 and I. The result obtained and the operation is, in general, the same in each case.

In the present invention the screen grid electrodes 24 and 25 are connected, as shown, to a movable point on a potentiometer P2 connected in shunt with the battery 20. This arrangement permits regulation of the amplification of the grid electrodes 24 and and the filament lead I 9, as shown.

In practice, and in particular where high frequencies are to be used, the grid to filament and screen grid capacity K2 of tube 2 has the effect of reducing or subtracting from the amount of excitation voltage reaching the grid I, but the grid to filament and screen grid capacity K1 of tube I has the effect of increasing or adding to the amount of excitation voltage reaching the grid 6. In other words, the grid to filament and screen grid capacity Krin tube I tunes out part of the inductive impedance of element I2 and thereby reduces the total impedance between the source D and'grid 6. This increases the ex- I citation to grid 6. However, the grid to filament and screen grid capacity & in tube 2 by-passes part of the excitation on grid I to ground and thereby has a subtracting 'or reducing effect. This causes an unbalanced condition which produces objectionable distortion in the resultant signal modulated carrier.

This distortion may be overcome in accordance with the present invention by balancing the excitation voltage reaching the grids. There are numerous ways of accomplishing this balance but most of them have disadvantages which make their use undesirable. One such method is to make the value of resistances R1, R2 small as compared with the value of the impedances of K1 and K2. This has the disadvantage of requiring more excitation power from the source D and this method of balancing can not produce but can only approach a controlled balanced condition.

Preferably, a balanced condition is obtained in accordance with the present invention by introducing a variable resistance Rs, or if desired a fixed resistance element Rs in series with inductance I2, as shown in Figure 1. By choosing the proper value of resistance Rs, (which is preferably non-inductive) the amplitude of the excitation voltage on the grid of tube I may be made the same as that on the grid of tube 2. By analyzing this new phase retarding circuit vectorally it will be seen that Rs reduces by a small amount the phase retarding angle, but this may be corrected by slightly increasing the inductance of L. In this manner distortion arising from unbalanced excitation may be overcome without the usual disadvantages.

In a preferred embodiment of the invention the resistance Rs of Figure 1 may be eliminated and the inductance coil L, used for retarding the phase of the energy supplied to the grid of tube I, may be constructed of resistance wire so that the coil L, in addition to providing the phase retarding effect, serves as the voltage reducing means so that potential of the oscillations appearing on the grid of tube I may be exactly equal to the potential of the oscillations appearing on the grid of tube 2. This results in that a balanced effect is obtained and no distortion is introduced into the signal energy. Such an arrangement has been shown in Figure 2. The arrangement shown in Figure 2 is otherwise the same as the arrangement shown in Figure 1. A detailed description thereof is thought unneces sary at this point.

In operation of the arrangement of Figure 2 the highfrequenoy carrier from D is modulated in frequency in accordance with the signals from B in the same manner in which modulation-of the carrier by the signal is accomplished in the arrangement of Figure 1.

Moreover, the unbalancing of the modulators I and 2, due to the grid to filament and screen grid capacities K2 and K1 of the tubes 2 and I respectively, which results in distortion of the modulated signal, is overcome here or compensated by the effect of the resistance of the in= ductance I2 on the potential of the oscillations applied from the source D to the control electrodes 6 and I respectively. In this manner the potential of the high frequency oscillations appearing on the grids of each of the tubes I and 2 is made equal and no distortion is introduced into the signal energy.

The frequency modulated carrier free of distorted signal components may be applied to any utilization circuit. Theutilization circuit may be an antenna, a transmission line, or a translat ing device. Before utilizing the frequency translated waves they may be passed through a frequency multiplier or multipliers 38 to increase the frequency of the frequency modulated signals. The unit 30 may include an amplitude limiter which either precedes or follows the frequency multipliers. The amplitude limiter takes care of any amplitude modulation inherently produced in the frequency modulator. The signals from the unit 30 may be utilized in any known manner or may be passed to a thermi-v onic amplifier 3! wherein the amplitude of the signal modulated carrier is increased before said modulated carrier is impressed upon the work circuit, which may be an antenna F.

Having thus described my invention and the operation thereof, what I claim is:

1. Signalling means comprising, a pair of electron discharge tubes each having an anode electrode and a control electrode and having their anode electrodes connected in parallel, a source of high frequency oscillations, phase shifting means connecting said source to the control electrode of one of said tubes, phase shifting means connecting said source to the control electrode of the other of said tubes, one of said phase shifting means comprising a turn or more of resistance wire, and means for applying signal oscillations in phase opposition to the impedances of said tubes.

2. In a modulating device, a pair of electron discharge tubes each having an anode, a cathode and a control grid electrode, a circuit connecting the anodes of said tubes in parallel, said circuit including an inductance and a capacity, a resistance connected between the control grid and cathode of each of said tubes, a circuit for applying modulating potentials in phase opposition to the control grids of said tubes by way of said resistances, and separate circuits for connecting the control grid electrode of each of said tubes to a source of carrier frequency oscillations, said circuits including phase advancing and phase retarding means respectively, the circuit including phase retarding means also including amplitude regulating means, whereby the carriers on the respective grids are of equal amplitude.

3. In a signalling system, a pair of electron discharge tubes, each having a control electrode,

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an inductive reactance having one terminal connected to the control electrode of one tube, a capacitative reactance having one terminal connected to the control electrode of the other tube, a circuit for applying oscillations of like frequency and phase to the free terminal of each of said reactances, and a device connected with the control electrode of one of said tubes for regulating the amplitude of the oscillations reaching said control electrode by way of the reactance connected therewith.

l. In a phase modulating system, a pair of electron discharge tubes each having a control grid, a, cathode and an anode, an output circuit connected to the anode and cathode of each of said tubes, an inductive reactance having one terminal connected to the control grid of one of said tubes, a capacitive reactance having one terminal connected to the control grid of the other of said tubes, a circuit for applying carrier waves to be modulated of like frequency and phase to the free terminal of each of said reactances, circuits for applying modulating potentials in phase opposition to the like electrodes of said tubes, and a device for regulating the amplitude of the carrier to be modulated on the control grid of one of said tubes.

5. In a phase modulating system, a, pair of electron discharge tubes each having a control grid electrode and a cathode, an inductive reactance having one terminal connected to the control grid electrode of one tube, a resistance in said connection, a capacitive reactance having one terminal connected to the control grid electrode of the other of said tubes, an impedance connected between the control grid electrode and cathode of each of said tubes, a circuit for applying oscillations of carrier frequency in phase to the free terminal of each of said reactances, and a circuit for applying modulating potentials in phase opposition to said impedances.

6. In a signalling system, a pair of electron discharge tubes each having a control electrode and an anode and having their anode electrodes connected in parallel, a source of high frequency oscillations, a coupling device connected to said source, a, circuit including inductive reactance connecting said coupling device to the control 19 electrode of one of said tubes, a circuit including a capacitive reactance connecting said coupling device to the control electrode of the other of said tubes, amplitude regulating means comprising a resistance in one of said connections, a source 15 of signalling potentials, and circuits applying signalling potentials from said source in phase opposition to like electrodes of said tubes.

'7. In a signalling system, a pair of electron discharge tubes each having a control grid, an 2 anode, and a screen grid electrode between said anode and control grid, a source of high frequency oscillations, a, coupling device connected to said source of high frequency oscillations, a circuit including phase retarding means connecting said coupling device to the control electrode of one of said tubes, a circuit including phase advancing means connecting said coupling device to the control electrode of the other of saidtubes, an

amplitude regulating resistance in one of said 3 connections, a, source of signal potentials, a aircuit applying signal potentials in phase opposition from said source of signal potentials to the control electrodes of said tubes, and means for applying positive potentials to the screen grid electrodes of said tubes.

GEORGE L. USSELMAN.

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