US2144777A - Carrier frequency modulator - Google Patents

Description

Jan. 24, 1939. Y K. SCHLESINGER 2,144,777

CARRIER FREQUENCY MODULATOR Filed Jan. 7, 1936 Patented Jan. 24, 1939 UNITED STATES PATENT OFFICE 2,144,777 CARRIER FREQUENCY MODULATOR Steglltz, -Germany Application January 7, 1936, Serial No. 57,869 In Germany January 18, 1935 '1 Claims. (01. 179-171) The present invention relates to a carrier frequency amplifier arrangement foruse with photo-electric cells. Photo-electric cells are commonly used in television or like systems for pro- 5 ducing variable electric currents depending on the intensity of the light falling upon the photoelectric cell. p

The carrier frequency amplifier is employed with great advantage with a television transmit- 10 ter, as in the case thereof there is no dependency of the output potential on the duration of the exposure conditions of the photo-cell. Forms of embodiment are known in which the carrier frequency is produced electrically and is introduced 15 as anode potential into the photo-cell1.11 the amplification input. In these known systems an oscillator accordingly acts on the carrier frequency wave AT and the amplifier is tuned exactly to the same carrier frequency. v

20 These methods of operation, however, involve considerable dimculties. 4 I

Firstly, the oscillations applied to the photocell should have as large an amplitude as possible, thusintroducing the problem of serial cou- 25 pling between the input oscillator and the amplifier which can only be solved by shielding the oscillator and its output conductors from the amplifier in a very effective manner because it operates on the same frequency as the sensitive 30 amplifier. Secondly, means should be provided for compensating for the strong currents which fiow through the photo-cell, even when it is not being illuminated so that these currents flowing inthe dark state of illumination (which may be as referred to as dark currents) shall have no effect in the output circuit of the amplifier. In its non-illuminated state the photo-cell acts as a pure capacity whereas in its illuminated state it also acts as arectifier. The dark currents 40 are therefore out of phase with the light currents (i. e. the currents produced when the photo-cell is in its illuminated state). In order to compensate for these dark currents it is consequently necessary to provide .a phase com- 45 pensating bridge which requires great care.

The subject matter of the present invention is a method of operation, in whichthere is required neither a bridge connection for separating the exposure currents from the dark current 50 nor a screening means of any kind of particular quality between the oscillators and the amplifier.

The drawing explains the method according to the invention of obtaining the intermediate frequency. In the same I and 2 are oscillators,

5 which act on a wave which is even shorter than the desired intermediate wave. If, for example, an intermediate wave of two mega-cycles is desired, the oscillators I and 2 act on waves of approximately mega-cycles. The two local oscillators l and 2 differ in frequency by the de- 5 sired intermediate frequency; for example, I acts on 10 mega-cycles and 2 on 12 mega-cycles. By means of the differential condenser 3 the two local oscillator amplitudes are conducted tothe photo-cell 4 in equal amount. From a light 10 source I2 a beam of light I 4 is projected upon the photo-cell and this beam of light varies in accordance with the light and shade value of an image which for example appears on the film l3. This carrier oscillations of a desired fre- 15 quency are obtained which vary in accordance with the intensity of the beam light. This cell is biased by a battery 5, which may also be dispensed with, there being employed a lead choke 6. The resistance of 6 is large in respect of the local oscillation frequencies I and 2, but on the other hand is very small in respect of the frequency difference, i. e., the intermediate frequency, as compared with the inner resistances of the cell 4. Since the latter is very high (approximately 10' ohms), there may also be employed in place of 6 a resistance of approximately 10 ohms. The working resistance of the photo-cell 4 is shown in the drawing in the form of an intermediate frequency transformer 1, 8. This transformer offers a high resistance only in respect of the desired intermediate "frequency range, and may either be made, as shown, in the form of a pair of coils without appreciable parallel capacity and with parallel damping resistances or, particularly in the case of intermediate frequencies shorter than those stated, as regular band filter with damping and with parallel'condensers for tuning. In the case of a particular form of embodiment of the input 40 tube 9 with an extremely small grid cathode capacity the band filter coupling may also be replaced by a resistance coupling.

The method of operation of the connection systems in the drawing is as follows: If the cell 4 is not acted upon by light, comparatively pow erful currents of the frequencies f1 and f2 pass by way of the cell and the band filter I to earth.

So long as the potentials of the local oscillator frequencies reaching the grid of 9 are not sufficiently high to modulate each other in the tube 9, these dark currents remain wholly ineffective. In practice this is always the case, as the capacity of the cell 9 represents a much greater resistance than the impedance of the band filter l, 8 for the local oscillation frequency. The balancing out is improved by a screening means H], which keeps the building-up capacities small between the leads of the local oscillators and the input circuits of the amplifier. Immediately light impinges on the cell 4 an electronic emission occurs therein. The cell varies its character from that of a pure capacity to that of a rectifier. By reason of the valve effect of the lighted cell there occurs in the circuit 6, 5, 4, 1 the difierence in frequency f1-f2 with an intensity of current proportional to the light. This is selectively amplified by the intermediate frequency amplifier. By reason. of the frequency selection of the amplifier alone there is accordingly performed in the method according to the invention a separation between dark currents and lighting currents. It is merely necessary for this purpose that the ratio of the current amplitudes traversing the circuit of the photo-cell is adapted to both kinds of frequency selection of the amplifier. The weakening of the high frequencies f1 and f2 in the amplifier must be so good that no transverse modulation is able to occur. This transverse modulation, since the same supplies a constant amplitude, would moreover also have no detrimental effect, as the same in accordance with the invention could be compensated in the amplifier output behind the rec-- tifier there, as it provides the delusion of a constant preliminary lighting of the cell. The elimination of the local oscillation frequency is all the more successful the higher these frequencies have been selected in relation to the intermediate frequency. The absorption is assisted by small series resistances II, the resistance of which is large as compared with the capacitative resistance of the following intermediate frequency transformer.

I claim:

1. In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in ,amplitude with respect to the modulating ourrent, comprising in combination two local frequency oscillation generators, a light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image to be transmitted, each of said generators being tuned to frequencies, the'wave lengths of which being shorter than the wave length of said intermediate ca-rrier current frequency to be obtained, means for combining said local oscillation frequencies, thus obtainin' the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated With'said varying light intensity values and an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators in parallel with said combining means, means for connecting said parallel connection in series with said light responsive cell and said input of said amplifier, and connection means for connecting all said elements to form a cooperation unit.

2. In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in amplitude with respect to the modulating current, comprising in combination two local frequency oscillation generators, a light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image to be transmitted, each of said generators being tuned to frequencies, the wave lengths of which being shorter than the wave length of said intermediate carrier current frequency to be obtained, means including a differential condenser for combining said. local oscillation frequencies, thus obtaining the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated with said. varying intensity values, an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators each to a stator of said differential condenser, the rotor of said differential condenser being connected in series with said light responsive cell and said input of said amplifier, and connection means for connecting all said elements to form a cooperation unit, a suction potential source and an impedance'in series connected between said connection means of said generators and said light responsive cell in parallel connection to said generators, said impedance being constructed to have a great drop in potential with respect to said local oscillation frequencies and to have a very small drop in potential with respect to said differential intermediate carrier current frequency in comparison with the inner resistance of said cell for the said differential frequency.

3.In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in amplitude with respect to the modulating current, comprising in combination two local frequency oscillation generators, a light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image'to be transmitted, each of said generators being tuned to frequencies, the wave lengths of which being shorter than the wave length of said intermediate carrier current frequency to be obtained, means including a differential condenser for combining said local oscillation frequencies, thus obtaining the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated with said varying intensity values, an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators each to a stator of said differential condenser, the rotor of said differential condenser being connectedin series with said light responsive cell and said input of said amplifier; and connection means for connecting all said elements to form a cooperation unit, said amplifying means comprising a band filter tuned to said intermediate carrier current frequency range under avoiding special parallel capacities, said band filter being damped by parallel damping resistances.

7 4. In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in amplitude with respect to the modulating current, comp-rising in combination two local frequency oscillation generators, a light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image to be transmitted, each of said generators being tuned to frequencies, the wave lengths of which being shorter than the wave length of said intermediate carrier current frequency to be obtained, means including a differential condenser for combining said local oscillation frequencies, thus obtaining the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated with said varying intensity values, an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators each to a stator of said differential condenser, the rotor of said difierential condenser being connected in series with said light responsive cell and said input of said amplifier, and connection means for connecting all said elements to form a cooperation unit, said amplifying means comprising a band filter tuned to said intermediate carrier current frequency range under avoiding special parallel capacities, said band filter being damped by parallel damping resistances, said selective band filter coupling means being constructed to cause weakening of said local oscillation frequencies.

5. In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in amplitude with respect to the modulating current, comprising in combination two local frequency oscillation generators, a light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image to be transmitted, each of said generators being tuned to frequencies, the wave lengths of which being shorter than the wave length of said intermediate carrier current frequency to be obtained, means including a differential condenser for combining said local oscillation frequencies, thus obtaining the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated with said varying intensity values, an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators each to a stator of said differential condenser, the rotor of said differential condenser being connected in series with said light responsive cell and said input of said amplifier, and connection means for connecting all said elements to form a cooperation unit, said cell having cathode and anode, means for connecting said cathode to said local oscillation generators, means for connecting said anode to said selecting means and said amplifying means, said anode having a smaller operating earth capacity than said cathode.

6. In a television transmission system an arrangement for generating and modulating a carrier current frequency exactly proportional in amplitude with respect to the modulating current, comprising in combination two local frequency oscillation generators, at light responsive cell adapted to change its inner resistance proportional to a varying light beam in accordance with the light and shade values of the image to be transmitted, each of said generators being tuned to frequencies, the wave lengths of which being shorter than the wave length of said intermediate carrier current frequency to be obtained, means including a differential condenser for combining said local oscillation frequencies, thus obtaining the intermediate carrier current frequency resulting as a differential frequency of both said local oscillation frequency, means for selecting said intermediate carrier current frequency modulated with said varying intensity values, an amplifier, the input of which being selectively tuned to said intermediate carrier current frequency, means for connecting said generators each to a stator of said differential condenser, the rotor of said differential condenser being connected in series with said light responsive cell and said input of said amplifier, and connection means for connecting all said elements to form a cooperation unit, means for compensating rest potentials of said local oscillation frequencies occurring in the output of said selective amplifying means.

'7. In a television transmission system a carrier frequency amplifier in combination with a photo-electric cell for obtaining carrier oscillations, two generators for producing two basic oscillations, said both basic oscillations being of constant amplitude and frequency and differing in frequency by the desired carrier frequency and being higher in frequency value than said desired carrier frequency, means for combining said basic oscillations and including means for coupling said photo cell to said generators and to said carrier frequency amplifier, said coupling means including a differential condenser having stator plates connected, respectively, to said generators and the movable plate connected to said photo-cell.

KURT SCI-ILESINGER.

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