US2440621A - Frequency modulation - Google Patents

Description

April 27, 1948. G. L. IIJSSELMAN 2,440,621

FREQUENCY MODULAT I ON Filed April 25, 1944 s Sheets-Sheet :1

INVENTOR ZW/PQF Z. Mira M4 ATTO RN EY April 7, 1948. G. L. USS'ELMAN 2,440,621

FREQUENCY MODULATION I Filed April 25, 1944 3 Sheets-Sheet 2 INVENTOR 420?;5 Z. Z/JJEzMA/V.

ATTORNEY April 27, 1948.

G. L. USSELMAN' FREQUENCY monum'nou Filed April 25, 1944 lC'.Ea I (5 6% Ala L/cd 3 Sheets-Sheet 5 INVENTOR 62%?65 A Una/11AM ATTO R N EY Patented Apr. 27, 1948 ENT OFFICE FREQUENCY MODULA'IIIQN George L. .Usselman, Port Jefferson, N. Y., as-

.signor to Radio Corporationiof America, a. C01:

,poration of Delaware Application April 25, 1944, Serial No. 532,592 '1 Claims. (01. 179- 1715) This application discloses an improved frequency modulation system. In the improved modulation system of this application, carrier wave energy isgenerated stabilized as to, mean frequency and deviated :in frequency in accordance with signals-01" control potentials. The signals or control potentials may represent voice, music,-code, etc. For example, oscillations generated may be shiftedirom a first frequency representing a marking condition to, asecond frequency representing a, spacingcondition, or deviated in frequency by voice si als.

The means 'for generating oscillations comprises two tubes each in an oscillationgenerating circuit comprising a tank circuit regeneratively coupled to the tube electrodes. The tank circuits-ofthe two tubes are tuned to diiierent frequencies separated in thefrequency spectrum by the desired amount, the separation being at least as great as-the maximum total deviation to be used.

a l ,o h embo me t -a e d a r r u is common to the separate tube generators so that the'tubes are. entrained to operate at the same mean frequency.

\ In two embodiments the common feedback path is by way of a crystal which acts as a filter to limit the extent of deviation as well as stabilizing the mean frequency at which the separate oscillators are entrained. Ina further embodiment the connections between thetank circuits and the common feedback path include band pass filters which serve to limit the extent of-the deviation.

An object of the present inventionis to produce frequency modulated oscillations the averr efrequency of which is substantiallyconstant.

;A further object of thepresent inventionis to p oduce "f q ency m du at sc llat o s i which theamount of frequency deviation is limited by the frequency controlling device (crystal) or filter.

An additional object of the present invention is to produce frequency modulated oscillations the deviation of which is balanced vabout the average; frequency so 7 that amplitude modulation entailedin producing the deviation is substantially eliminated.

-Anotherobject of the invention is to produce frequency modulated oscillations in which the amount of frequency deviation may be ,a il dted within the limits of the frequency controlling circuits.

The manner in which the; aboveoblects and others are attained, and the advantages derived from the attainment of such objects will now be odes ,of the. tubes.

described indetail. in this description reference will bemade to the attached drawings wherein .Fig. 11. illustrates. by wiring diagram the essential features of a frequency modulation system arranged in accordance with my invention, Known refinements may he addedto the circuit shown. The circuit shown,however, is substantially complete asto the essential features. Fig. 2,.is amodificationof the. arrangement of Fi 1( In Fig. 1, a two-electrode crystal is in a common path betweenthc Q'Ontrol grids and cath- In Fig; 2 a three-electrode crystal isused in ,af'eedba'ck connection between the anod an grid Qfw ach b with a comm crystalelectrode in the gridside of the circuit.

Fig. 3..is a modification of the embodiment of Fig. 2. InFig, 3 band pass filters replace the crysjt-al filter ofFig. 2. Band. pass filters satisfactory for use in-theembodiment of Fig. 3 are shown in Fig. 3d. v

Nowreierring to Fig. 1, VI and V2 are the oscillation generatorand modulator tubes and for the sake of simplicity are shown as triodes but may each have more. than three electrodes. The control gridsA andfi of thesetubes are coupled in parallel by coupling and blocking condensers C to one electrode of the crystal X, the other electrode .of which connects the cathodes of the, tubes in parallel and to -ground.

The, anode. Ill of tubeVl is .connectedto a tank circuitLI-Cl, the other end of which is grounded by a radio frequency bypassing condenser BP. The inductance of LI is also connectedto the positive terminal. of a direct current source, a negative terminal of, which is grounded. The anodeeleotrode 12 of tube V2 is similarly coupled to ground and theudirectf current source bytank circuit C2'--L2.

The control grids 4 and .6 are connected respectively by'resistances R1 and R2 to the terminals of the secondary winding of atransformer T, the primary winding of which is connected with thesignalsource A. The resistors RI and R2 s pply. negative grid biasing potentials by virtue pf grid rectification in the" tubes. If more bias is desired a sources maybe connectedbetween the midspoint on the secondary Winding of transformer T and ground. .Radio frequency bypass condensers BP .are connected between the lower terminals of resistances RI' and R2 and the oathodes and. ground. These condensers shunt the radio frequency potentials around the secondary "winding of the. transformer T and thesourceS but are of high impedance to modulating po. tentials. l A v posite sides of the frequency at which the crystal X normally operates.

frequency at which the system would operate inthe absence of control or signal potentials. Say, for example, the tank circuit CI-LI is tuned to operate at a frequency on one side of the crystal The crystal frequency is assumed in the system to be the meanor carrier.

frequency and that tank circuit C2-L2 is-tuned to operate at a frequency an equal amount on the other side of the crystal frequency. The amount of shift or spread in the tuning of these tank circuits is determined by the flexibility of the system and by the amount or degree of frequency modulation required. In operation, the tuning of these circuits should be separated in the frequency spectrum an amount at least equal to the total deviation desired.-

Since the grids 4 and 6 of the two tubes are connected together and to the crystal X the tubes must operate or oscillate at the same or a common frequency despite the unlike tuning of the tank circuits. Since the tank circuits have unlike tuning the oscillator frequency will swing toward 4 generated oscillations is shifted from a first frequency denoting say marking condition to a second frequency denoting say spacing frequency. The keying is generally done by applying variable amounts of direct current or pulses of A. C. to a tube electrode. In the present case the keying energy is applied differentially. For this purpose a keying-tripping circuit similar to that shown in Fig. 40f my United States application Serial No. 521,907, filed February 11, 1944, is preferably used and the control grids or the screen grids (when screen grid type tubes are used) of tubes.VI. and V2 may be ke'yed. That is, the

tubes V and V5 ,of Fig. 4 of said application and their-inputandoutput circuits may be used to key the tubes VI and V2, Figs. 1, 2 and 3 of this applicationfi'lo arrange for such use of tubes Fig. 4 of thefsaid application are connected to the screen grid electrodes oftubes VI and V2 of Figs. 2 and 3, and of Fig. 1, when the same use tubes VI and V2 of the screen grid type. Keying of the screen grid potentials takes place substantially as described in said application. In the said application the screen grid potentials are varied froma positive value to a'negative value that one of the tubes delivering the most power.

if we assume that the tank circuits are, as

stated above, tuned to frequencies symmetrically spaced above and below the crystal frequency.

Under the conditions shown, differential modulation of the power delivered by tubes VI and V2 under control of the signal causes the carrier to be frequency modulated or deviated about the crystal frequency in accordance with the signals. The

degree or amount of frequency deviation is proportional to the signal amplitude, and the sense of deviation follows the changes in the signal amplitude with respect to a base value whereat carrier frequency output is produced and the number of frequency variations over a period of time is proportional to the signal frequency.

The output of the frequency modulator may be used directly or may be supplied as illustrated to the input electrodes 20 and 30 of two amplifier tubes V3 and V4 having their anodes coupled in parallel by an output circuit including inductance L3 coupled to output inductance L4. The control grids 20 and 30 are variably tapped on the inductances LI and L2 so that the desired excitation may be supplied to the control grids of tubes V3and V4. Chokes CHI and CH2 block the radio frequency potentials from the direct current grid bias source which is also shunted by radio frequency bypass condensers BP.

The advantage in using a crystal in these modulator circuits is that much more accurate carrier frequency is obtained. That is, the average frequency remains substantially constant. The amount of frequency deviation is limited by the resonance characteristic of the crystal and these circuits are in particular useful for frequency hift keying.

In frequency shift keying the frequency of the atwhich the tubes v2 and v3 are blocked. In the present case the screen grids are modulated above and below a positive value to swing the frequency of the generatedoscillations the desired amount above and below thecarrier frequency. The potentials on the screen grids are'not, however,'variedin the, negative direction an amount such as to block oscillation generation. The modulation operation here is more like the operation described and illustrated (Fig. 3) in my U. S. Patent No. 2,326,314, dated Aug 10; 1943.

'To arrange for the keying of tubes VI and V2 when they are triodes, as in Fig. 1, the grid leads of tubes VI and V2 are disconnected from the transformer T at points 0-0. The two grid'leads are then connected to the two lines of the movable points on potentiometers 45 and '46 of Fig. 4 of my said application Serial #521,907, dated Feb. 11, 1944. When the control grids are to be keyed the adjustments of potentiometers 45-46 and 49 Olf Fig. 4 of saidpriorapplication should be such that these grids are always negative. Keying of the grid potentials take place as described in said applicationand as described in my U. 5. Patent; #2326314. The keying circuit or said prior application and patent are if desired applied to the generator and'modulator tubes VI and V2 of Figs. 2 and'3 in like manner.

The embodiment of Fig. 2 is in manyrespects similar 'to the embodiment of Fig. 1. In Fig. 2, however, oscillators 'of-a different type are used and the oscillators are entrained in a different manner. In Fig. 2, VI and V2 have theiranodes I9 and I2 coupled respectively each to one end of tank circuits CI-LI and C2- -L2. A point on each of the tank circuits is connected toground by a radio frequency coupling and direct current sourcebypass condenser Bl? being thereby' connected to the'cathodes of tubes VI and V2. 9

nected to one terminal D of a three-terminal ci stai x, another terminain of which is "tapped modulator can operate. .Whemno signali'mod'u and the third terrain 33 cf which is tapped through coupling coh'de'n erWll toinductahcem. Note that tube VI has its an'ode and gridcon heated to spaced points onLI, vsr-hile cathode is connected to a point intermediate 's"aid spaced points so thatthe tube VI i'sman oscillation generator of the inodifiedHaftley "type, including in thegr-id feedback excitation circuit thecrystal "X. The tube v2, and the described circuits, "con si'cl'red separately is an oscillationgenerator-oi a similar type. I I

The tubes -V'I and V2are of the screen grid type instead of t'riod'es as in Fig. -=1; and the screen grids are supplied with direct current operating potentials over bypass condensers BP. The moduiation circuit is substantially as shown in Fig.1, and will not be discussed herein. When voice or similar signals aiused'they are applied at A to transforin'er T. When freque cy shif t signalling is to'be carried out atrlp fig-keying circuit asdescribed abovemay be connected to the control gridsbr screen grids.

The three-electrode crystal X'which has a filtering action has itscolhmon electrodes D conne'cted to the 'gri'd's of tubes V nd V2, While the electrode A-i-s tapped on tank circuit coil- LI and electrode is tappedoh tank circuit coil L2, asnescr hed abovey -Neutral-i'zing condenser NI is connected betweenl'the' ahodeend of ta-nk circuit coil lil andthe crystal filter electrode-D, While neutralizing condenser N2 is "connected between the anode end of tank coil L2 and the crystal filter electrode B.

Herep'as in 'Fig. 1, the tank "circuits "G2'L2 and (II-13 are tun'edtojfopposite sides ofthe "normal frequency6f"crystal filter -X-and if symmetrical operatio'nis desired,'which'is preferable, they aretun'ed to frequencies which differ from the crystal *frequencyin opposite direction by equal amounts. The filter electrode-A *and neutralizing condenser l' aire tappeusymmetncauy on each side of the=point atwhich LI is grounded. This point may be the center point of the tank circuit. Neutralizing --'condenser N2 and electrode B are -also symmetrically tappedwith respect to the point on inductance LZ connected to ground for R. F. voltages by thecondenser BP. The neutralizing condensersNI and N2 are adjusted "to just balance the "capacity elfect of each half of the crystal filterelectrodes so' that when the crystal is-not oscillating tl'ie g'rids 4 and 6 arein elTect connected to' a neutral point and no unwanted oscillations'can reach the grids fromthe anode tank circuits. "It is also essential that for best operation both sides "of-the modulator be substantially identical except for the equal and opposite tuning adjustments of the tank circuits. When the electrodes of the tubes are charged oscillations are generated in a well known manner, the crystal oscillates and feedback takes place therethrough, as described hereinbefore.

Under these conditions differential modulation of the potentials on the control grids of tubes VI and V2 by the signals causes the frequency of the entrained oscillators to vary in accordance with signals. The tube delivering the most power will cause a carrier frequency shift in the direction of the frequency for which the tank circuit of that tube is tuned. This shift will be substantially proportional to the differences of power delivered by the tubes VI and V2.

The crystal filter limits the band in which the lation isipresent and atzinst'anceswhen the modulation potentials are equaler their differences are Ii-zero; then "there remains lion-1y the carrier lre- "quency "which is substantially-fin I the :middle of the filter passband'because .Ithefifilter characteristic is shaped *for this purpose.-'I his is a balanced type I of frequency modulator as is the embodimentof-Fi'gfll, and the ampltiude modulation is balanced out and does not appear in the "output. i

The. main .:purpose 'of usi-ngithe crystalkis to maintaina substantially constant carrier frequency. However, thisvlimits the band width of frequency modulation. This seems-tosbe a funda- 'mental fact. This type "of modulator (using a crystal for frequency control) is useful where frequency shift keying is usedbecause usually :in this case a f-requency'sh'ilt "of :only a. few mundred cycles is desired. :In order toc-use frequency shift keying the. control (grids or screen .1 grids may be keyed differentially means of the keying-tripping circuit such as -that shown in 'Fig; 4 of my U. S. {application :Serial #52'L90fl, lfiled February 11, 1944. This latter circuitcan supply either; positive or negative-keying bia'sas desired depending on the adjustments..

The embodiment of Fig. c isressenti'ally. as .dis-

I closedin Fig. 2, except for the following main differences. H

Bandpass filters designated-thy rectangles :FI "and F2 and are in this: embodiment: connected to the capacitive branch -'of' eachs'of theftank circuits. L! is now: 'shunted' byscondensersCI C3 and C5, while L2 is shunted by icondensersCZ,

"desirable type, such as asingle crystal filter-as shown in Fig. 1, or'th-ey may be a multiple type crystal filter as shown in Fig. 2, or theyimay consist of acom'bination'ofwcoils and condensers arranged to have aband "pass characteristic. In Fig. 3a, IJhave shown filters satisfactory for use in the 'embodiment of *Fig. -3. In -Fig. 311, Lid and C6 and 0'6 and LG are the filter elements. NI and N2 are the neutralizing condensers. The condensers Q are coupling condensers used for coupling the filters and neutralizing condensers to a common point and thence to the condenser C. Now NI and N2 control the amount of feedback to the grids. If adjustment for complete neutralization is made there would be no feedback. Under or over-neutralization permits feedback as desired. If crystal filters are used as in Figs. 2 and 3, neutralization stops feedback through the crystal holder capacity thereby allowing feedback only through the oscillating crystal.

I claim:

1. In a timing modulation system of the type comprising two tubes, each having a plurality of electrodes, a tank circuit tuned to a first fre quency regeneratively coupling the electrodes of one tube in an oscillation generating" circuit, said connections including a feedback excitation circuit between the anode and control grid of the tube, a secon'd'tank circuit tuned'to a second frequency coupling the electrodes of the other tube in a regenerative oscillation generating circuit, said last named oscillation generating circuit including a feedback excitation circuit between the anode and control grid of the second tube, a band pass filter which passes a frequency spectrum including said first frequency andhalf of the band between said first and second frequency in said first feedback circuit, a band pass filter which passes a band of frequencies including said second frequency and half of the band between said first and said second frequency in said second feedback connection, and means for differentially controlling the gains of said tubes in accordance with signals.

2. In an oscillation generator, two electron discharge devices each having an anode, a cathode and a control grid, two circuits each comprising capacity and inductance in parallel tuned one to a frequency above, the other to a frequency below the desired mean operating frequency, a coupling between the anode of one tube and one tuned circuit, a coupling between the anodeof the other tube and the other tuned circuit, means for providing regenerative feedback between the anode and grid of each tube whereby each tube is connected in an oscillation generating circuit, said means comprising series coupling condensersv substantially directly coupled between the control grids of said tubes, a feedback coupling between one parallel tuned circuit and the junction point between said series condensers and a feedback coupling between the other parallel tuned circuit and the junction point between said series condensers, said feedback couplings having a portion in common which common portion with said substantially direct coupling between the grids of the tubes entrains'said oscillation generating circuits so that they operate at a common frequency, and a wave filter in said generating circuits which passes a bandof frequencies centered on a frequency intermediate the two frequencies to which said parallel circuits are tuned.

3. An oscillation generating system as recited in claim 2 wherein the filter is a band pass filter in the connection between the anode and grid of each tube.

4. An oscillation generating system as recited in claim 2 wherein the wave filter is in the feedback couplings and comprises a three-electrode crystal with one electrode coupled to the grids of bothtubes, the'second electrode coupledto one parallel tuned circuit, and the third electrode coupled to the other parallel tuned circuit.

5. In a signalling, system, two electron discharge devices each having an anode, a cathode anda control grid, two circuits each comprising capacity and inductance in parallel, tuned one to a frequency above, the other to a frequency below thedesired mean operating frequency, a coupling between the anode of one tube and one tuned circuit, a coupling between the anode of the other tube and theother tuned circuit, means for providing regenerative feedback between the anode and grid of each tube whereby each tube is connected in an oscillation generating circuit, said means comprising series coupling condensers substantially directly coupled between the control grids of said tubes, a feedback coupling between one parallel tuned circuit and the function pointbetween said series condensers and a feedback coupling between the other parallel tuned circuit and the junction point between said series condensers, said feedback couplings having a portion in common, which common portion with said substantially direct coupling between the grids entrains said oscillation generating circuits so that they operate at a common frequency, a wave filter in said generating circuits which passes a-band of frequencies centered on a frequency intermediate the two frequencies to which said parallel circuits are, tuned, anda sourceof modulating potentials coupled differentially to corresponding electrodes in said tubes.

6. A signalling system as recited-in" claim 5 and wherein said wave filter comprises a band pass filter in each of said feedback couplings.

7. A system as recited in claim 5 wherein the wave filter is a three-electrode crystal with one electrode coupled to the grids of both tubes, the second electrode coupled to one parallel tuned circuit, and the third electrode coupled to the other parallel tuned circuit. 7 H

I V GEORGE L. US SELMAN.

REFE ENCES e The following references are of record in the file of this patent:' a

UNITED STATES PATENTS Number Name Date 1,945,547 Pray Feb. 6, 1934 2,027,975 Hansell Jan. 14, 1936 2,030,125 Usselman Feb. 11, 1936 2,304,388 Usselman Dec. 8, 1942 2,345,101 Crosby Mar. 28, 1944 2,375,527 Crosby May 8, 1945

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