US2275287A - Carrier controlled modulator - Google Patents

Description

I H V I ,5 23) CARR/ER E {5 4 POWER $5555 E l6 nMPL/F/ER March 3, 1942. M. G. CROSBY 7 2,275,287

' CARRIER CONTROLLED MODULATOR Filed June 1, 1959 3 Sheets-Sheet 1 4 xz 38 .43 44 43 3 6/ 4, \o as y g W EIU g I /C Y E Enl- $60 W J- C/ X 90 80 I g S, a 3 k Q E ;60- 3/0250 1 g;- B: 240- 5 a w 3) .1520. Q .0 0 l l I l l I I l 0 I0 40 6'0 /00 APPLIED PERCENT MODULATION INVEN TOR.

ATTORNEY.

MUfAY G. CROSBY March 3, 1942.

M. G. CROSBY N 2,275,287

CARR I ER CONTROLLED MODULATOR Filed June 1, lss 3 Sheets-Sheet 2 /20) 04/22/52 2 2 WAVE E 70 sou/ c5 3 7' ANTENNA INVENTOR.

MURRA Y. 6. C7305 B Y ATTORNEY.

March 3, 1942. M. G. CROSBY v CARRIER CONTROLLED MODULATOR Filed Jdne l, 1939 3 Sheets-Sheet 3 mmtzmi mmkmm INVENTOR. MURRA Y 6. CROSB Y ATTORNEY.

Patented Mar. 3, 1942 2,275,287 CARRIER CONTROLLED MODULATOR Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application June 1, 1939, Serial No. 276,772 17 Claims. (Cl. 179l71.5)

This application concerns a type of amplitude modulation system in which the carrier is maintained at peak amplitude during periods of no modulation and is biased down in proportion to the average modulation level. For example, in the case of 100% modulation, the carrier is biased to one half its unmodulated value and modulated 100% in a normal manner. For lesser amounts of modulation, the carrier is biased to more'than half its normal value and a lower percent modulation is applied. Thus the modulation capability of the transmitter may be made almost zero at the time of maximum carrier and no modulation, but is shifted to a capability 100% and one-half carrier as the modulation is fully applied.

In the prior art of carrier-controlled amplitude modulation, the conventional practice is to maintain a constant modulation percentage of substantially 100% and vary the strength of the carrier in accordance with the strength of the modulating wave. In this way the carrier power is made very low during idle periods and power is only required as the modulation is applied. Due

to the intermittent character of most modulation material, and due to the fact that a wave with 100% modulation may be amplified more efllciently than one without modulation, this type of carrier-control effects a more eillcient transmitter. It has the disadvantage, however, that no carrier is present during idle periods so that automatic volume control may not be used eflectivelyat the receiver. The fact that the carrier is low during idle periods also allows interfering signals to enter the receiver with an intensity which is high compared to the desired signal and thereby makes the system susceptible to interference.

In the system of the present invention, the control applied to thecarrier is opposite to that applied by the prior system described above. The carrier is maintained at its peak amplitude during the idle periods so that it may be used for automatic volume control purposes at the receiver and is present with a maximum strength to combat interference. This system also has advantages with regard to transmission emciency. At the transmitter the reduced modulation capability requirement during the idle periods allows the use of more efliclent amplification during these periods. That is, the efficlency of the amplifier is highest in the absence of modulation and is decreased as the mean amplitude of modulation increases. This operation requires less plate dissipation in the power amplifier tube than is required in the conventional uncontrolled amplitude modulation systems. Higher power outputs and higher efficiencies are, therefore, ob-

tainable with the same tubes as used in the conventional systems.

At the receiver an average noise gain of about one and one-half to one in voltage is obtained in addition to the gain obtained at the transmitter. This is due to' the fact that during the idle periods a carrier of twice normal amplitude is present so that during these intervals the signal-to-noise ratio is twice what it would normally be. That is, a two-to-one gain has been attained. During the condition of full modulation, the noise gain is one-to-one since the carrier is then at the normal value of one-half amplitude with the modulation swinging it up to peak amplitude. .The average noise gain is then something between one-to-one and two-to-one or about one and one-half to one. This average noise gain depends upon the average depth of modulation. In the case of program/modulation where the o average depth of modulation is about the average gain would be considerably more than one and one-half to one.

In describing my invention, reference will be made to the attached drawings wherein Figures 1 and 5 illustrate modifications of my modulation system wherein the carrier amplitude, which is maximum in the absence of modulating potentials, is reduced in the presence of modulation. Figures 3, 4, 5 and 6 illustrate'modiflcation of the carrier level controlling means of Fig. 1, while Fig. 2 comprises curves showing the percent modulation in terms of applied percent modulation and carrier amplitude for different amounts of modulation.

In applying this type of modulation, I have found that the modulation is accompanied by an inherent volume expansion at the transmitter. This is due to the fact that during the periods of low modulation, there is a larger amount of carrier to be modulated than'during the periods of high modulation. Referring to Fig. 2, curve A shows how the carrier amplitude varies as the modulation is applied. With the modulation input level adjusted so that 100% modulation is produced when the carrier is reduced to one-half peak amplitude, at the input which would normally correspond to modulation the carrier will have an amplitude of 1.5, but only enough voltage will be present to modulate a carrier with an amplitude of 1.0. Consequently, the resulting percentage of modulation will be 50/15 or 33.3%. Atan applied modula- Fig. 2 which gives the resulting percentage ofmodulation in relation to the modulation that would exist without this inverse carrier control. Curve C shows the percentage of modulation that would be produced with the normal type of uncontrolled carrier transmission under ideal conditions. A comparison of curves B and C shows the type of volume expansion that takes place. At 100% modulation, the resulting percentage of modulation is normal, but for lower values of applied modulation the resulting modulation is lower than normal. This results in a decreased intensity or contraction of the portions of the modulating wave which have a low intensity, or stating it conversely, results in an increased intensity or expansion of the portions of the modulating wave which have a high intensity. In the invention about to be described, this volume expansion is compensated for by applying a volume compression which reduces the intensity of the portions of the modulating wave having a high intensity. This will result in a characteristic similar to C but lower on thescale. Added amplification raises this new characteristic on the scale. Such a volume compression may be used where the circuit quality demands high fidelity, but for many purposes the compression circuit may be. eliminated.

Figure 1 shows a specific embodiment of the invention. Carrier source I has an output winding Ii coupled to a tuned winding I! in turn coupled to the input electrodes i5 and it of modulated tube which is of the pentode type using suppressor-grid modulation. Power amplii'ler 23 has it input coupled by a winding to the tuned output inductance 25 coupled to the anode 28 of tube 20. The amplifier 23- amplifies the output of the modulator and applies it to antenna 34, but the output of the modulator may also be connected directly to the antenna.

'Modulating potentials are applied to Jack 35 and fed through transformer 36 to the control grid 31 of modulation amplifier tube 40. Amplifier tube 4. is resistance coupled by coupling 38 to the control grid ll of amplifier 43. The anode 44 of amplifier l3 feeds modulating potentials through transformer 48 to the suppressor-grid I! of modulated tube 20 and through transformer 50 to diode rectifier 52 and 53. The diodes 52 and 53 rectify the modulating potentials and produce a rectified voltage across resistor 51 which is by-passed by condenser 88 for the audio frequencies, but not for the slow changes below about 50 cycles. This rectified voltage is used as part of the bias for the suppressor-grid I 9 of modulated tube 20 so thatas the strength of the modulating poterilals increases, the suppressor-grid is biased more negative and the strength of the carrier is reduced in accordance with curve A of Fig. 2. Additional negative bias for the suppressor-grid is supplied from source 59. The suppressor-grid circuit in-- cludes atime constant circuit 60.

The same rectified voltage is also fed through I time constant circuit 60' to the injector-grid 6| of amplifier 40 so that the amplification of 40 is reduced in accordance with the strength of the modulating wave. The reduction in amplification produces the volume-compression which compensates forthe inherent volume expansion transmission, I have found that there is a tend- -ency towards over-modulation when large wave proper bias to reduce theamplitude of the carrier amplified in 20 and thereby. increase the modulation capability. of the modulator. Consequently, at the beginning of such large wave fronts, there will-be a slight interval of overmodulation. .This interval is small so that the distortion is almost unnoticeable, but where real high fidelity is desired, one of the methods shown in Figs. 3, 4 and 6 or a combination of these methods, may be used to remove the distortion.

In Figure 3, the resistance of a diode III is shunted across the time constant resistor 60" .which represents the resistance in either time constant circuit ll or I in Fig. 1. When a sudden wave front of audio frequency is applied to leads III which correspondto the leads at points X and Y of Fig. 1, the rectified voltage appears across resistor I1 and condenser 58 and is applied to the series combination of condenser C and resistor 00'. Until the condenser C has time to charge, all of this voltage will appear across resistor ll". When this happens, the diode 1. draws current so that its resistance becomes low and shunts 6 to form a time constant circuit with a lower RC product, and, therefore, a lower time constant. This momentarily lowered time constant passes the rectified voltage on to the tubes 20 and 40 to be controlled with a greater speed and thereby applies the carrier bias to electrode ll of tube 20 or the volume compressing bias to electrode ii of tube 40 with a speed great enough to either shift the carrier to the operating point quick enough or reduce the volume so that the first part of the wave front will not come through to cause over-modulation. A battery :2 is inserted in circuit with the diode II to delay the operation of the shunting eflect of the, diode resistance and thereby ad- Just the amount and rapidity of the shunting efi'ect;

As indicated above, a similar compensating means may be connected with the resistance and condenser 6. of Fig. 1. This is shown also in Fig. 3. That is, resistance 60 is shunted by a rectifier II. in series. with a source 82' for the purpose specified above.

igure 4 makes use of somewhat the same principle as Fig. 3 except that condensers and OI are used in place of the diodes Ill and 10' of Fig. 3. This condenser has the eifect of shunting down the resistor 6! when the rate of change of the voltage is high. Consequently, a suddenly applied voltage will be passed by condensers 90 and 90' but the slower changes will be passed by Gil.

Figure 6 shows a separate volume compressing amplifier which may be used with the system of Fig. 1 with gain controlled amplifier tube 0 replaced by an ordinary amplifier tube 40 coupled to tube II to modulate therein the wave energy and with the modulation potentials fed to terminals IM and out of terminals I03 to the input of tube 40'. Variable gain amplifier I05, which may consist of a tube like 40 of Fig. 1 has its gain reduced as the modulating potentials are increased. This reduction in gain of the amplifier I is in accordance with rectified modulating potentials derived from the output of rectifier I06 having its input coupled to amplifier I00.-

The time delay in rectifier I06 and network RC is compensated for by a time delay circuit I01 which imparts a delay to the modulatingpotencircuit of Fig. 6 is different from that incorporated in Fig. 1 in that ordinary automatic volume control is used in Fig. 1 and in Fig. 6 a separate path is set up for the rectifier so that thepotentials fed to it are not volume controlled. The type. of control of Fig. 6 may be adjusted to reduce the volume in exact proportion to the strength of the modulation potentials and, therefore, may be made to exactly compensate for the inherent volume expansion introduced in the carrier-controlled system. However, the simpler auderiving modulated wave energy from said-tube, means for controlling said tube for maximum carrier wave amplitude output from said tube, means for modulating said carrier wave in accordance with modulating voltages, and means for lowering the carrier wave amplitude output of said tube as a iunctionof the mean amplitude of the modulating voltages.

2. A system as recited in claim 1 wherein said last named means includes a time delay circuit.

3. In a modulation system, a carrier wave am plifier tube, means for causing wave energy to be modulated to flow? in said tube, means for deriving modulated wave energy from said tube, means for controlling said tube for maximum carrier wave amplitudeoutput from said tube, means for modulating said carrier wave in accordance tomatic volume control type of compression of I Fig. 1 may be made sufiiciently accurate for many purposes. a

A further means of removing the inherent volume expansion caused by this type of modulation is by the use of inverse feed-back. The inverse feed-back may be applied by the known method in which a portion of the modulated wave is rectified and fed back into the amplifier for the modulating potentials with a phase which reduces the gain of the amplifier. Such feed-back tends to remove variation in gain like those of this volume expansion. The feed-back circuit has blocking condensers which do not allow the slow variations of the carrier amplitude to be fed back, but do allow the fast variations corresponding to the modulating frequencies to be passed.

Figure 5 shows a modification of my inverse carrier-controlled transmission to control-grid modulation. Carrier source I has its output coupled by inductance's I22 and I23 to the grid I26 and cathode I25 of tube- I26 which is neutralized by condenser I21. Modulating potentials are applied at Jack In through transformer I30. Battery I32 furnishe permanent grid bias for rid I24. This bias is such that in the absence of modulating potentials at I29 maximum carrier output is supplied by-tube I26. The drop in resistor R furnishes the variable bias which shifts the amplitude of the carrier down as the modulation is applied. When no modulation is being applied, part of the bias for the control grid I24 of tube I28 is furnished irom the drop in R due to the grid current flowing through it. when modulation is applied, the grid current increases and the bias consequently, increases so that a reduced carrier is obtained from I26. The amount of the reduction may be controlled by varying the proportion of the bias being furnished by the battery I32 and resistor R. Condenser I36 smooths the variations in the bias so that only the slow variations corresponding to an increase in level of the modulation will be passed and the audio fre- I ance of the supply serves as the resistance R.

What is claimed is: 1. In a modulation system, a carrier wave amplifier tube, means for causing carrier wave energ? to be modulated ot flow in said tube, means for with modulating voltages, means for lowering the wave amplitude output of said tube as a function of the mean amplitudeof the modulatin voltages, and means for reducing the amplitude of the modulating voltages as a function of the mean amplitude thereof.

4. A system as recited in claim 3 wherein said two last named means include a time delay circuit.

5. In a modulation system, a carrier wave amplifier tube having input electrodes and output electrodes, meansfor impressing wave energy to be modulated on said input electrodes, means for deriving modulated wave energy from said output electrodes, biasing means for biasing the electrodes of said tube for maximum carrier wave amplitude output from said tube, means energized by modulating potentials for impressing modulating voltages on an electrode of said tube for modulating said wave energy, and rectifier means excited-by voltages characteristic of the modulations on said wave energy .for applying to an electrode of said tube a potential which is a function of the mean amplitude of the modulations on said wave energy.

6. A system as recited in claim 5 includin means for controlling the amplitude of said modul ting potentials as a function of the mean am pntude of the modulations on said wave energy.

'7. A system as recited in claim 5 wherein said last rectifier means inherently introduces a time delay,' and wherein means is connected with said rectifier for compensating the effect of said time delay.

8. In a modulation system, an electron discharge tube having input and output electrodes, said tube having an additional electrode. a circuit connected between said input electrode and a circuit connected between said output electrodes, means for causing wave energy to be modulated to flow in said first circuit and modulated wave energy to flow in said second circuit, means for I adjusting said tube'to supply carrier wave energy of maximum'amplitude to said output circuit, a source of modulation potentials, mean for impressing modulating potentials on electrodes of said tube, a rectifier, means for impressing said' modulating potentials on said rectifier, means coupling the output of said rectifier to, said additionai electrodein said tube to control the potential thereon in accordance with the mean amplitude of the modulating potentials and means for controlling the amplitude of the modulating potentials in accordance with their mean amplitude.

0. In a modulation system, an electron dischargetube having input and output electrodes,

said tube having an additional control electrode,

a circuit connected between said input electrodes and a circuit connected between said output electrodes, means for causing wave energy to be modulated to flow in said first circuit and modulated wave energy to flow in said second circuit, means for adjusting said tube to supply carrier wave energy of maximum amplitude to said output circuit, a source of modulating potentials, amplifying means coupled with said source of modulating potentials and with electrodes of said tube, a rectifier, means for impressing said modulating potentials on said rectifier, and a time delay circuit coupling the output of said rectifier to said additional control electrode in said tube to control the potential thereon in accordance with the mean amplitude of the modulating potentials and with said amplifying means for controlling the amplitude of the modulating potentials in accordance with their mean amplitude.

10. A system as recited in claim 9 including time delay compensating means and a circuit coupling said time delay compensating means to said time delay circuit.

11. In a modulation system, an electron discharge tube having input and output electrodes, said tube having an additional electrode, a circuit connected between said input electrodes and a circuit connected between said output electrodes, means for causing wave energy to be modulated to flow in said first circuit and modulated wave energy to flow in said second circuit, means for adjusting said tube to supply carrier wave energy of maximum amplitude to said output circuit, a source of modulating potentials, means for impressing modulating potentials on electrodes of said tube, a rectifier, means for impressing said modulating potentials on said rectifier, and means coupling the output of-said rectifier to said additional electrode in said tube to control the potential thereon in accordance with the mean amplitude of the modulating potentials.

12. In a modulation system, an electron discharge tube having input and output electrodes coupled in high frequency circuits, said tube having a control grid, means for normally biasing said control grid to a value such that said tube has maximum high frequency output, a circuit for applying modulating potentials to said control grid and a resistance in said circuit wherein a potential drop is produced to lower the rier wave energy as modulated from said maximum value an amount proportional to the mean amplitude of said modulating voltages.

14. The method of modulating wave energy of carrier wave frequency which includes the steps of, producing wave energy of carrier wave frequency of a desired maximum amplitude, producing voltages representative of signals, modulating said carrier wave in accordance with said voltages, reducing the amplitude of said carrier wave energy as modulated from said maximum value an amount proportional to the mean ampliture of said voltages, and reducing the amplitude of the produced voltages representative of signals an amount proportional to the amplitude of the said voltages.

15. In a modulation system, a carrier wave amplifier tube, means for causing carrier wave energy to be modulated to flow in said tube, means for deriving modulated wave energy from said tube, means for controlling said tube for maximum carrier wave output from said tube, means coupled with said tube for modulating said carrier wave in accordance with modulating voltages, and means for lowering the carrier wave amplitude output of said tube substantially directly in proportion to the mean amplitude of the modulating voltages.

16. In a modulation system, a carrier wave amplifier tube, means for causing wave energy to be modulated to flow in said tube, means for deriving modulated wave energy from said tube, means for controlling said tube for maximum carrier wave amplitude output from said tube, means coupled with said tube for modulating said carrier wave in accordance with modulating voltages, means for lowering the wave amplitude output of said tube substantially in proportion to increases in mean amplitude of the modulating voltages, and means for reducing the amplitude of the modulating voltages as a function of the mean amplitude thereof. v

17. In an amplitude modulation system, an electron discharge device having input and output electrodes coupled in-alternating current circuits, means for causing wave energy of carrier wave frequency to be amplitude modulated to fiow in said circuits, means for deriving modulated wave energy from said tube, means for controlling said tube for maximum carrier wave energy amplitude output fromsaid tube, means for controlling said tube output in accordance with modulating voltages to thereby amplitude modulate said carrier wave energy output, and

MURRAY G. CROSBY.

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