US2440622A - Modulation - Google Patents

Description

April 27, 1948; e. 1.. USSELMAN 2,440,622

' MODULATION Filed Aug. '2, 1945 2 Sheets-Sheet 1 A 4400001 7/0 Farm mu:

' INVENTOR George L. flue/man BY .Aov-M,

ATTORN EY April 27, 1948. e. 1.. USSELMAN MODULATION Filed Aug. 2, 1945 2 Sheets-Sheet 2 fi/P/W/VG 0/2 a mar/ma MM:

INVENTOR V George [1. arfelmalz ATTORNEY In both embodiments a point intermediate the ends of the tank circuit is coupled to the cathodes of the tubes and the control grids of the respective tubes are cross coupled to the tank circuit to operate in the oscillation generating circuits approximately 180 out of phase with respect to each other and to the radio frequency voltage on the anodes of the said respective tubes.

In the embodiment illustrated in Fig. 1 a keyed tripping circuit and additional modulating circuits are arranged to be coupled to the tube elec-' may be used if desired. A point centrally located between the ends of inductance LI is connected to the positive terminal of a source of direct current potential and to ground and the cathodes of tubes VI and V2 by a capacitor CID which bypasses voltages of the generated frequency. This point is also connected to a point intermediate the sections of capacitor CI by a resistance R that uncouples the capacitive and inductive branches of the tank circuit so that split tuning will not take place. In other words, resistor R prevents the upper and lower halves of the tank circuit from being resonant at different frequencies in case their constants are not similar. The use of R is desirable in order that all parts of the tuning condenser be kept at the same direct current potential and thereby prevents arcing or condenser breakdown. It also increases the rating of the condenser since it has only to be insulated for the A. C. voltage on it.

A phase shifting network comprising series capacitors C2 and C3 and shunting resistances R3 and R4 with R5 connect a point on the inductance LI to the screen grid 8 of. tube VI and the screen grid II] of tubeVZ. These connections include radio frequency bypassing and coupling and direct current and audio frequency blocking condensers C4 and C5. As illustrated the tap on LI and the lead to anode 2 are on the same side of the grounded point on LI. The tap may be moved to the other side of the grounded point without-altering the principle of operation. Ra-

' dio frequency bypass condensers C6 and C1 couple the low radio frequency voltage ends of resistances R4 and R5 to'the cathodes of the tubes. Capacitors C6 and C1 are large' and are used to complete the radio frequency circuits of the phase shifting networks but are small enough to block currents of the frequency of the modulating potentials applied from any desired'means, such as a tripping circuit or transformer, to-leads I and 3, and thence differentially to the screen grids 8and I'll through resistors R4 and R5. The control grids I4 and I6 are cross-coupled by coupling condensers II and I3, to points on LI at opposite sides of the point thereon put at radio frequency ground potential by condenser CH3. A common cathode biasing resistance RI is shunted by a radio frequency bypassing capacitor An output value. A modulation source A for telephone may be coupled to a transformer TI primary winding. The secondary winding of this transformer is coupled to points on the resistances R8 and R9. A switch S may be included in this coupling. The resistors R8 and R9 are grid leak resistances. The secondary winding of transformer TI is tapped up on these resistors to points at which the resistance matches the impedance of transformer secondary Winding.

the

The common cathode resistance Ri maintains substantially constant bias on the control grids I4 and I5 of tubes VI and V2 since the sum of the currents through both tubes is substantially constant if the'modulating potentials are properly balanced. 'Inmy system they are balanced by proper selection of resistances R4 and R5 and condensers C8 and C9 and application of proper modulating potentials to leads I and 3.

Consider the system as described as an oscillation generator andassume the electrodes are energized and tank circuit C ILI tuned to the desired operating frequency. Oscillations are generated due to regenerative feedback between the-anode and control grid in each tube. Note the R; F. voltages are opposed on the anodesreversed and opposed on the control grids so that the set up is right for regenerative operation. Each tube as connected is a regenerative generator. The-two tubes are entrained by the tank circuit LICI to operate as a single generator operating at a common frequency.

The screen grids 8 and Ill of tubes VI and V2 are coupled by phase shifting networks C2C3, R3R4 with R5, to a voltage point on vLI whereat the radio frequency voltage phase is the same as the phase of the voltage on the anode 2. The phase of this voltage as applied to the grid 8, however, is relatively advanced about 90 by the phase shifting network. Thepotential on the control grid I4 however is 180 out of phase with respect to the potential on the anode 2. As a consequence the screen grid 8 and control gridI4 are excited substantially in phase quadrature relation. The sum or resultant of theexcitation on the control grid I4 and the screen grid 8 of the tube VI is lagging something less than 90, say 45 displaced relative to the voltage in the circuit C ILI, so that the phaseof the anode current output of tube VI is lagging the phase of-the oscillating current in tank circuit C ILI by about 453 grid-I8 of tube'V2 is the same as that'on the screen grid IQ of tube V2 except for the phase advance due to the phase shifting network. The

'sum of the resultant A. C. voltage on the control grid I6 and the screen grid I0 is leading, this time something less than 90, say 45, relative to the phase of the current in the tank circuit CILI so that the anode current output of tube V2 is say 45 leading this time the phase of the oscillations in the tank circuit CILI The frequency of operation of the individual tubes and of the tubes as entrained by the circuits depends on the phase relation of the radio frequency voltages on the tube electrodes and on the state of conductivity of the respective tubes.

" in my U. SpPatent $2 2,326,314.

"Hill-k6 direct currentipotentialsare appliedtoithe .1 wscreen gridsandthe .phase of the electrode volt- 'tages are asndescribedabove the system will scillate at a trequency depending primarily upon the tuning of circuit CILI.

Theiexcitation energy from theanode 2 end of the tuned circuit to the screen. grid 18 is relatively advanced 90 by the phase shifting network. 'The -excitation energy to the screen grid ill -of' V2 is also relatively advanced by its phase Y shifting network but is initially 180 out of phase 'with respect to the anode 4 voltage so that the exc'ita'tion on the screen grid it is lagging the anode-4 Voltage by about-90.

The phase of theoscillations generatedin the tank circuit CILI is the resultant :of the anode -powerdeliveredto it from the tubes 'VI and V2. Similarly the phase of the power delivered by the tubes Vl and V2 is the resultant ofthe phase'of the control grid excitation volt- 4 ages byway of condensers H and I 3, and the excitation voltages on the screen grids8 and It.

Since the resultant grid excitation in tube V! is -lagging it delivers power of lagging phase angle tothe tank circuit ClLI. 'In contrast since the. resultant grid excitation in tube V2 is leading it delivers power of leading phase angle'to the tankcircuit C lLl. As long as the tubes VIV2 deliver equal power-as is the case when no modulation ispresent, then the oscillator operates at. "the normal frequency to which CILI are tuned. However, if one tube delivers more power than the other *the'oscillator frequency will change by pulling the frequency. one way or the other. *For'example, if during the modulation cycle the of modulatingz..potentialssuch as those reprewsentingwolce. signals andvthe like. -"When the switch ison contacts- 12 the leads 1';- andBemay 1 bezconnected toltheresistors l and or :a tri-pping keying circuit:- such as I disclosed: more in idetail cin my. Uzi S. application: 'Serial" #521;907,

, filed Eebruaryi 11; 1944. "This keyedrtrippingor switching circuit including tubes viarid V4 supplies-at the-leaded!) and 32 directcurrentpotentials which are appliedby way of leads I and 3 to-the screening electrodes-8 and I0; The direct current potentials are made of a value sufchtas to alternately key the tubes V I and *V2 from a l more conductive condition to a less conductive or a non-conductive condition. "Preferably thepo- 'ten'tials swing from a positive value at which the the anode of the said one tube td the grid of the other tube. In this manner currentis caused screen grid of tube Vi is biased more positive and the screen grid of tube V2 is less positive then since Vl delivers phase retarded power and more power than tube V2 the frequency of the oscillatorwillbe decreased. On the otherhand, if the power output of the tube V2 is increased by-making its screen grid Ill more positive and thepotential of screen-grid 8 of tube Vl is made less positive, since the phase of the power 'from gtubeVZ is-leading-the' frequency of the oscillator will be increased. 'Modulation potentialapplied differentially to the screen grid electrodes by leads land 3 results in frequency modula- .tion of "the, oscillations generated in accordance with the modulation potentials. Amplitude variation "of the modulated radio frequency by the signalsor by power supply variations are corrected'due to thedifferential action. The modulationpotentials may representtelegraphy code .orlsimilar-signals or music or voice. The lat-- ter maybe applied by a transformer secondary winding tapped to supply the direct current potentials'ior the screen grids or telegraph signals ,may be applied by control circuits as disclosed Modulation mayalso or. alternativelybe applied from source Ajdifferentially to. the control grids l4 and I6, when switch S is closed.

In Fig. II have shown somewhat completely modulating and keying means which may be used .with theoscillation generators of Figs. 1 and 2. Switches SI and S2 are used to switch the modu- .lator to the ffphone source, contacts a, through ,transformerTz or to thetripping circuit tubes V3 and iVdcontacts. b. A double pole triple throw Qswitch may be used for this purpose. The leads to flow through tube-V3 while tube V4 iscut off or vice versa.

A point intermediate the resistances 43 and i4 is connected to the positive terminal of a source of direct current potential. A point. intermediate the terminals of resistances 45 and 6 is connected to a point-on a-potentiometer resistance' la shunting a source of potenti'alB2 the positive terminal of which is grounded. "This source of potential is shunted by a condenser for potentials of the keying frequency. *The resistances "45 and 46 whichare potentiometers have points thereon connected to theleads fill and 32and thence by leads land-3 to the screening electrodes of tubesVl and V2 for mod-ulation purposes. The outer ends of the resistances 45 and Marc connected to an'alterna'ting varyingpositive voltage source, i. e., to the anodes of tubes V3 and V4 which, as described' hereinaft'er,. swing alternately positive and less positive as the control gri'ds of the tubes-or-0ne thereofis 'controlledby signal potentials. "The potential-on the leads 39 and'32 therefore is a resultant-of the potentials on the anodesof tubes V3 andVd andof the potentialfrom the main direct current source DC and the potential de- *rived from source B2 by way of the potentiometer 49 and also of and V4.

The tripping tubes V3 and V4 have their control grids coupled'respectively by resistances 58 the conductivityof the tubes V3 and filr'and' 59 and'63, to ground; andbyreon resistance 6| connected by switchS3 either tothe positive terminal of a source B3 or to ground. A' point on potentiometer resistance-:63 is connected to a source of signals, forexample, keyed pulse energy, or keyed direct current or keyed alternating current energy. The-ot=her trol grid oftube V4"wit-h-respect to its cathode.

' on contact KI.

7 The source B3 is sometimes necessary to apply a fixed negative bias to the grid of tube V3 in order to get positive tripping action. The ar-' rangement is suchthat the tube V3 is biased to outofl in the absence of marking and spacing signals either by the drop in potential in resistances 58 and 6| or if necessary by negative potential from the source B3 with switch S3 on contact KI instead of on K2. The current then is switched through tube V4 and the resistances 43, 44, 45 and 46, the direct current source and the tap on potentiometer 49, etc., are such that the potential at the anode of V4 drops the required amount. This falling potential is fed to the screen grid I by leads 32 and 3 and is suflicient to swing the screen grid I0 negative or less positive enough to block or bias tube V2 and cut off or reduce current therethrough. At the same time the anode of tube V3 swings more positive because current through tube V3 is cut oil or reduced, and its anode is at or near the direct current source potential. The elements are dimensioned and adjusted to values such that the screen grid for .tube VI is made positive by this positive potential supplied by leads 30 and I, and tube VI draws current so that oscillations of a lagging phase are supplied by tube VI to the tank circuit and the frequency of operation of the system goes down and oscillations of one frequency are transmitted. They might well represent no signal or space signal.

Now when keying signals are applied an alternating current or direct current potential fed at the input and applied to the grid of tube V4 switches the current through tube V3 by blocking or biasing tube V4 and by virtue of the cross-couplings 42 and II. With the current switched through tube V3 and cut off in tube V4 a more negative potential is fed by leads 30 and I to the screen grid 8 of tube VI and a more positive potential is applied from the anode of tube V4 by leads 32 and 3 to the screen grid I0 of tube V2 and this tube V2 becomes conductive to supply oscillations in the tank circuit LICI of leading phase so that the frequency of the oscillations becomes higher and oscillations of this frequency may represent the marking condition.

' Alternating current or direct current may be used for keying purposes. If alternating current pulses are applied to the control grid of tube V4, through which the current is switched in the absence of signals, the positive excursions of the alternating current have no effect because the tube V4 is already drawing maximum current. The negative cycles of the keyed alternating current however, make the grid of tube V4 negative to cut oif current therein and switch the same through tube V3 as described hereinbefore. Adjustment of the point on potentiometer BI permits operation such that the positive bias on the tube V4 is overcome by the applied'signals to insure the current switching action. If necessary more negative potential may be applied to the grid of V3 from the source B3 by putting the switch S3 1 have found that when telephony signals or tone modulation is applied to the screen grids of a modulator, distortion in the modulated outputmay result. In Fig. 1 I show a circuit arrangement by which the signal modulation may be applied to the control grids of the modulator tubes as well as to the screen grids. In this embodiment phone signals or tone signals may be applied to the control grids and telegraphy or phone or tone signals maybe applied to the potentials are then superposed on the positive bias and applied from source B differentially by way of leads I and 3 to the screening electrodes 8 and Ill. The modulating potentials may represent tone or phone signals. During this operation and when the switches SI and S2 are on the b contacts the switch S is open to disconnect the source A from the control grids of the tubes VI and V2.

When telephony signals or tone modulation is applied to the screen grids 8 and III of my system some distortion in the modulated output may 7 result.

In Fig. 1 I show a circuit arrangement by which the signal modulation may be applied to the control grids of the modulator tubes instead of to the screen grids. In this embodiment phone signals or tone signals may be applied to the control grids from source A by way of transformer TI and resistances R8 and R9 when the switch S is closed. Application of the telephony or tone modulation to the control grids produces substantially undistorted voice or sine wave modulation of the frequency of the output in accordance with the signals. Then the switches SI and S2 may be placed on the 0 contacts to supply an adjustable D. C. potential to the screen grids 8 and I0.

If desired tone or phone modulation from source A may take place simultaneously with telegraphy modulation by signals applied to the leads 49 and 5 I. To do this the switch S is closed as stated above and the switches SI and S2 are moved to the 1) contacts so that the output of the tone keyer is applied to the screen grids while the output of the phone or signal source A is applied to the control grids.

I have shown an improved signalling system of this type in Fig. 2. In Fig. 2 I have used reference numeral corresponding to those used in Fig. 1. The arrangement in general'is as shown in Fig. 1. The tripping circuit is shown by a rectangle but the details there may be substantially as in Fig. 1. I

The source B2 is omitted in the practical arrangement illustrated in Fig. 2 and the potentiometer resistance 49 shunted by condenser CIII now also is in shunt to the main direct current source.

The radio frequency circuits are substantially similar to those in Fig. 1. However, in the arrangernent of Fig. 2 I have made the oscillation generator more stable so that its average or mean frequency is more constant in spite of voltage changes and ambient temperature changes. To

do this I include resistors R.I3 and RI 4 in the anode leads ,to the tank circuit. Moreover, to further enhance stability the oscillator tank circuit CILI is built with a relatively large capacitive reactance CI and low inductive reactance LI. The condenser CI and the coil LI are both constructed of materials and methods resulting in low coefiicients of expansion. Moreover, the tank circuit LICI is enclosed in a temperature control boxv TCB, the interior of which isheldzat a.substantiallyiconstant temperature byimeansiknown inxthe. art-but not shown here. Thea'ends of:;th,e tank. circuits-are as stated connecteditotheanodes sand 4 by-the: damping resistors. Hi3 and Bit. :To further improveqthe operating characteristics. the tank circuit =-LI is coupled to the. control gridlfi of an electron discharge device V through resistor 16 and coupling condenser 78. 82 is the grid leak resistance and St is the cathode return resistance shunted by condenser 85. Anode potential is supplied from the main source through resistance 81. The output of the system is then taken from the anode and cathode of the stage V5 by way of ground and condenser 89.

The constant temperature at which the tank circuit GIL! is held holds the capacity inductance thereof substantially constant. The resistances RI3 and RM and 16 act to minimize any effect of the tubes VI, V2 and V5 on the average frequency of the oscillations generated in the tank circuit due to temperature changes in the tubes and to changes in tube characteristics when tubes are changed. It has been found that connecting a resistance across a parallel tuned circuit does not change its frequency response. How ever, if a reactance, inductive or capacitive, is connected across a parallel tuned circuit then its frequency response is changed. The anodes of tubes VI and V2 present a considerable amount of capacitive reactance when connected across the tank circuit as shown in Fig. 1. In this case a change of tubes or a change of tube temperature will change the anode to suppressor grid capacity and consequently it will change the oscillator tank circuit tuning. This, of course, changes the oscillator frequency. When resistors Rl3 and RM are connected between the tank circuit and the anodes of VI and V2 as shown in Fig. 2, a variation of tube anode capacity will not change the tank circuit tuning and the oscillator frequency to such an extent. The larger resistors R43 and Rid are made the less the tube capacity will aifect the oscillator frequency. It would be advisable to make Rt 3 and RM as large as possible and still maintain oscillations in the oscillator tank circuit. The tube grid connections are usually made quite far down on the coil toward the center point and therefore do not materially affect the oscillator frequency. The anodes are usually connected to the ends or near the ends of the tank circuit and consequently they have a much greater effect on the oscillator frequency. Also, the capacities of the grids are usually much less than that of the anodes. It can also be said that the greater the impedance (resistance and reactance) that is connected across a parallel tuned circuit the less effect it will have on the tunin of the tank circuit.

The resistances RI 3 and El 4 also will cut down the amount of frequency deviation possible. However, this deviation is considered ample anyway so that it is desirable to sacrifice some of it in the interest of an average constant frequency. It may also be noted that the resistor 16 is tapped on inductance Ll at a point equally opposite the tapping point of condenser C2 (in the phase shift network) so that the two approximately balance their effect on LI. The output tube V5 has been added to lighten the load on the tank circuit CILI to thereby aid in holding a more constant aver- 10 and RI 4 should ba made; as: large as. practical andiistillmavesatisfactory operation, ofv thefrequency modulator circuit: .o'luring:- modulation.

The source A andthe tripping circuit; are con- 1; nested and, operate substantially :as: described above. The; modulation source Bahowever: is omitted. Withthe switches .Sl andrsii' inthe position shown, i. e., on contacts 0 direct current biasing and operating potentials only are applied to the screening electrodes. The switch S may then be closed and differential modulation of the control grid potentials takes place to modulate the frequency of the oscillations generated as described above by making the tube VI say with the phase lagging excitation on the control grid and screen grid predominate when the modulation swings in one direction, and to make the other tube V2 with the sum of the voltage on the screen grid and control grid of leading phase predominate when the modulation swings in the other direction.

With switches SI and S2 moved on the b contacts the tripping circuit is connected to the leads 3 and i and thence to screen grids l0 and 8 to modulate the generated oscillations in accordance with telegraphy signals as described hereinbefore. Then the output comprises telegraphy modulated oscillations which may also be modulated by tone signals from source A. With the switches S open telegraphy signals only are developed by the keying and tripping circuit.

What I claim is:

1. In apparatus for generating and modulating oscillatory energy, two electron tubes each having an anode, a cathode, a control grid and an auxiliary grid, a tank circuit parallel tuned substantially to the desired operating frequency, means coupling each of said tubes and tank circuit in a regenerative oscillation generator including said tank circuit coupling the anodes and cathodes of said tubes in pushpull relation and coupling the control grids and cathodes of said tubes in pushpull relation, the arrangement being such that the phases of the voltages of the regenerated frequency on the anode of each tube are displaced in phase about 180 degrees with respect to the phase of the generated voltages on the grid of the same tube, additional couplings between said tank circuit and said auxiliary electrodes such that when oscillatory energy of the desired operating frequency is developed in said tubes and tank circuit the phases of the voltages of the generated frequency on the control grid and auxiliary grid of each tube are displaced by substantially and means for differentially modulating the gains of said tubes in accordance with signals.

2. Apparatus as recited in claim 1 wherein impedances are interposed in the coupling between the tank circuit and the anode of each tube.

3. In apparatus for generating and frequency modulating oscillatory energy, two electron tubes each having an anode, a cathode, a control grid and an auxiliary grid, a tank circuit parallel tuned substantially to the desired operating frequency, means including said tank circuit coupling the anodes, cathodes and control grids in push pull relation, said .push pull couplings to the control grids being crossed relative to said push pull couplings to said anodes so that when oscillatory energy of the desired operating frequency is developed in said tubes and tank circuit the phase of the voltages of the generated frequency on the anode of each tube is substantially out of phase relative to the phase of 11 the voltage on the grid of each tube to provide regeneration, a phase shifting network coupling said tank circuit to said auxiliary electrodes so that when oscillatory energy of the desired operating frequency'is developed in said tubes and tank circuit the phases of the voltages of the generated frequency on the control grid and auxiliarygrid of each tube are displaced by-substantially 90", an output circuit coupled to said

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