US2403245A - Modulation system - Google Patents

Description

'.Jlly 2 1946- F. H. SLAYMAKER MODULATION SYSTEM Filed Feb. 23, 1942 5 sheets-sheet 2 INVENTOR, FRHNK h'. 5L HYMH/(EE f la l ML 1M,

ATTORNEY July 2, 1946. F. H. SLAYMAKER MODULIXTION SYSTEM Filed Feb. v2:5, 1942v 3 Sheets-Sheet 5 .SRS

ATTORNEY. u

Patented July 2, 1946 Y MonULA'rIoN SYSTEM Frank H. Slaymaker, Rochester, N. Y., assigner to `The Stromberg-Carlson turing Company, Rochester,

tion of New Yorkv Telephone Manufacv e N.\Y., a `corpora- I Application February 2s, 1942, serial' 10,432,052

The present invention relates generallyto signal modulated high frequency transmission systems and more particularly to a modulation booster for boosting the degree of modulation. While my invention' is applicable to and usable in any system Where it is desirable to increase the modulation intensity, it finds par- 2 Claims.. (Cl. 250-36) ticular utility in the intra-tank communication system disclosed in pending application Serial No.'430,592, filed Feb. 12, 1942, and assigned to the same assignee as the present inventiom It is, therefore, one object of my invention to handle 4carrier currents which are very weakly modulatedv without overloading the amplifying equipment.

Another object of my invention is the increase in modulation intensity from a very low degree, say 3 per cent, to a very high degree, say 100 per cent.

Still another object of my invention is to increase the modulation of the radio frequency carrier by operating on the carrier envelope itself without requiring the use of audio frequency in securing the increase.

Various other objects and advantages of the invention will appear from the following description when read in conjunction with the appended drawings, in which:

Figure 1 is a block, diagram illustrating the above mentioned intra-tank communicating system, with the modulation booster connected therein;

Figure 2 illustrates, in diagrammatic form, a typical carrier frequency transmission system, having my modulation. booster incorporated therein;

My system may also be used as a yconventional grid modulated class C stage. j' e Figure 3 shows the fundamental, schematic circuit of the modulation booster, with the bypass filters, filament supply and other parts of the circuit, not necessary for illustrating the present invention, omitted; Y t Figures 4 and 'I show the control grid voltageplate current characteristic for a tube used as a modulation booster in accordance with my invention.

Figure '1A shows the potential acrossthe tuned plate load of the circuit-of Figure -3 and it will be noted that these waves are modulated to a greater degree than those of the input vcarrier of Figure'l. -It will beunderstood thatthe out.-y .put Yload may be .tuned-to any Vdesired-,multiple of the :input frequency to .provide-:any desired .hare

2 monic of the input frequency in the output circuit, as later explained.

Figures 5, 6 and 8 are curves which Will be referred to in pointing out various features and advantages of the present invention; v I

Referring now to Figure 1, LI and L3 represent two antennas strung around the inside Vof a tank or other mobile unit. L2 and L4 indicate two portable loop antennas which are carried in the clothing, such as being sewed in the users belt, of the members of the tank crew. The port.. able modulator comprises a telephone transmitter (not shown) which functions tomodulate a tuned circuit comprising a condenser (not shown) and loop L2. The portable receiver comprises a telephone headset' and a` detector (not shown) connected to a tuned circuit comprising a condenser (not shown) and loop L4.

Oscillator OSC may be an electronic oscillator which supplies radio frequency oscillations Flv to radio frequency amplifier R. F. AMP, the output of which is connected to loop LL A radiofrequency electromagnetic field is therefore setup in the space within loop Ll, which field is absorptively modulated in accordance with thevariation's provided by the portable modulator, all

as disclosed in the above mentioned pending application. Y i

In any system where the modulated radio frequencies are weakly modulatedfas for example in the system illustrated, these radio frequencies f may be connected vto the circuit 20, illustrated in Figure 3 and the per cent modulation boosted. The potential of battery C is such that grid G `is biased to an operating point such as 2| of Figure 4 or'Figure "7, whereby one edge. of the modulated radio frequency or carrier envelope will fall on the linear portion of the Eg-Ip curve. The plate current in the output of tube V ,will then consist of a series of uni-directionalpulses varying in amplitude with the modulation and occurring at the same frequency as the carrier. Furthermore,- thevariousharmonics of the car,- rier fundamental' frequency are also present in the plate current. r The plate load, comprising inductance L and capacity C, may be tuned to the input frequency FI, or any of its harmonicsfand the potential across the output terminals of tube V willconsist of a true modulated radio frequency wave, but modulated at a much higher degree `or percentage than the app ed radio frequency in the input. This boost is illustrated by the envelopes of therinput ,and output ,waves in ,'lgand mote'point with respect v3 7A. These boosted waves appearing across the output terminals of Figure 3 are applied to the input of the radio 'frequency amplifier of Fig- Vure 1 which has its output connected to loop L3.

By tuning the CL circuit of Figure 3 to the sec ond harmonic of the oscillatory frequency FI, a

the frequency of FI and are then received and In this case the modulation is boosted but falls far short of reaching Figure 6 shows the Vcut-oil set at a pointnear V i 100 per cent.

zero and a low degree of input modulation. In

Vthis case` the percent modulation is boostedV to approximately 100 per cent. 1

.Figure 8 shows an absolute amplitude increase in rthe input modulation by an increase in the carrier strength. Similar results can be accom- 'plished by anV increase in the per cent modulation. In this casethe modulation is boosted to detected by loop L4 and the portable receiver, lthe Y loop circuit being tuned to frequency F2..

From the above explanation and vwith jpart-"lcu-Lj lar reference to Figure 7, it will 'be noted that a' series of pulses, of the same frequency as the ln-` put carrier, appear in the -platewcurrent of tubev around 100 per cent. Y It has already been explained that, in addition rto vthe Ifundamental radio frequencyrcarrier in V in Figure 3. Furthermore, since thepul'seswin the plate circuit are not sinusoidal, harmonicsn are present and by tuning the output 'circuit to the fundamental orany desired l'harmomc thereof, it is possible to extendrtne 'modulated funda.,- mental,v `or harmonic, to `a Work circu1t, such as the R F. V'amplifier 22 of Figure 1. It is an observed fact that the harmonics in the .output circuit are modulated to Substantially the same degree or per cent boost as the fundamental.

It Vwill be seen from Figurel that, while the i modulation hasbeenconsiderably increased, it is -not near A100 per cent. Modulation of ,the order of 100'per Vcent may be obtained Yby adjustmg `me grid bias -C so that the dips in the modthe cut-oir point kon the Eil-11D curve as illustratedin Figure This may also be obtained by adjusting the amplitude of the input potentials. f so that the dipsin the modulated port1onof-the carrier FI fall exactly ,at the cut-off pointfon the Eg-'Ip curve as illustrated in Figure :8. Of course Wise, fother'types ofrbia's can be used. .If the ayerage carrier amplitude is likely to change spontaneously, fixed biagmay not be desirable `since over-'modulation of the Aoutput can .occur 1f the carrier strengthdrops enough to place :the `modulation dips back'of cut-'olf `The use 'of Va self biasing scheme, such .as a grid leak bias, with f the grid leak and condenser having arsufliciently `long time constant to 'prevent Ythe bias Vfrom :following the modulatiom'will' eliminate manyef the difficulties arising 'from a shifting jinputg-mtensi-ty.

With a ygrid leak bias, ,the peaks in the monulation come just on Ithe edge tof -zeito .bias regard- Y less of thelinputintensity. When the'output. 1s

to be modulated `100 per cent1-,either the zabsolute amplitude ofthe input .modulation should be such thatrthe dips fall at cut-oif Vor .the fposition of vthe cut-olf looint yshould be 4.moved until it coincides with the dips in the .mouuiatnmY The position of the .cut-'off `point can be Vcontrolled jby using a pentode Afor 'the tube V aof Figure 3 :and adjusting the screen or suppressor grid potentials.

As an example of .the above described control, Figures 6, and 8 are illustrative. Figure 14 .shows the cut-'off set at a value such that .it is at a reto' the :modulation dips, which dips illustrate' a 10W degree of modulation.

l ulated -portion .of the :carrier FI fall exactly at y roV input Aof Figure 3 and applied to the grid circuit, Y

primarily with operation `intra-tank communication system, in which a the output, .fthe plate circuit also contains har- Y monies of 'the fundamental.- The output circuit may, therefore, be tuned to any of these harmonicsand the tube then serves as both a modulation yloopster and a frequency multiplier.y For example, the fundamental frequency Fl (say 1100 kc.) supplied by oscillator OSC in Figure l may be modulated very weakly in the above .mentioned intra-tank communication system, this frequency is then multiplied .t0 2200 kc. and the modulation `boosted by the modulation booster, so that the signal may be satisfactorily receivedby the receivertuned to ,2200 kc. I

As long as the modulated portion of theradio frequency envelope falls on the linear portion of the yEg-Ip .characteristic curve 'there is no troublesome Adistortionintroduced into the modulation of the various harmonics and `the modulation percentage is substantially the `same at the harmonics asatthe fundamental.

I am aware that frequency multipliers are .old n the art, but as far as I know they .have been used only on unmodulated carriers ybecause .it was felt that such multipliers would introduce .serious modulation distortion if an attempt .was

made to multiply a modulated carrier. I-I lam cognizant of the fact that, `in ,my system, distortion will appear if the modulated .portionnf the carrier does not fall on the linear portion of the characteristic curve of the tube, .but with an input of a very low percentage of modulation it can very readily be made to ,fall on vthis portion of the curve.

Another application of mymodulation booster system .is that it may serve asa .conventional grid modulated class C amplifier, inV `addition to its function as .a modulation booster and frequencyrmultiplier. To accomplish this, an audio frequency kcan `be added to the radio frequency as shown by the lower ,curveof Figure 5. 'This audio frequency will, in effect, swing the-openat1ng point of the tube and the .output .circuit will contain a radio frequency, modulated VwithV ghe audiofrequency signal, as illustrated 'in'.Figure 'In order to simplify the drawings, only `one edge of the input carrier is illustrated in Figures 6 and 8. Since the omitted portion of ,the carf river falls back of cut-olf, it is believed unnecessary'to show it. This explanation of my invention has dealt in connection with an very low degree of modulation is primarily produced and then -steppedup or boosted -to a very high degree, without the introduction Vof distortion, and operating on the high radio frequency currents. Application .of this invention may, however, .be found .mother classes .of service, :such asin the. radio vart .or in carrier systemftelepli' the modulated stage, distortionless modulationY is secured and a weakly modulated carrier can be fed into my modulation booster to secure 100 19erl cent modulation. Such an expedient would require a comparatively small amount of audio power and the desired radio frequency power could be secured by either class A or class B amplifiers following the modulation booster,

While I have gone into considerable detail in pointing out the various features of my invention, it will be apparent that many modifications thereof might suggest themselves to those skilled in the art and I accordingly do not desire to limit my invention to the specic details disclosed except as may be necessitated by the prior art and the appended claims.

What I claim is:

1. In an arrangement for multiplying the frequency and for increasing the percentage of modulation of a modulated signal, `a source of incompletely modulated frequency, an electron tube comprising a cathode, a grid, and an anode, an input circuit connected to said source and including said grid and said cathode, an output circuit including said cathode and said anode,

said output circuit being tuned to a harmonic of the frequency of said modulated signal, and a utilization network coupled to said output circuit, said grid being biased more negatively thanv that just suiiicient to prevent the ilow of space current in said output circuit in the absence of an input signal but of such a Value that in response to all of the momentary' peak values of said'modulated frequency space current is per-A mitted to flow in the output circuit.

2. In an arrangement for multiplying the frequency and for increasing the percentage of vmodulation of a modulated signal, a source of ncompletely modulated frequency, any electron tube comprising a cathode, a grid, and an anode, an input circuit connected to said source and including said grid and said cathode, an output circuit including said cathode and said anode, said output circuit being tuned to a harmonic of the frequency of said modulated signal, and a utilization network coupled to said output circuit, saidA grid being biased, the absolute value of the grid bias Abeing greater than that necessary to cut .oi the anode current in said output circuit in the absence of any input signal but of such a value that. in response to all of the momentary peak values of said modulated frequency space current is permitted to flow in the output circuit. i

FRANK H. SLAYMAKER.

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