US2346499A - Television apparatus - Google Patents

Description

April 11, 1944.

' H. .1. -DE N. MccoLLUM l TELEVISIQN APPARATUS Filed Aug. 5, 1942 yym, M v x Patented Apr. 11, 1944 UNITED y STATES PATENT I OFFICE e f p 2,346,499

i TELEVISION APPARATUS Y Henry J'. De N. McCollum, Chicago, lll.

Application August 5, 1942, Serial No. 453,658

(Cl. Pis-7.5)

3 Claims."

My invention relates generally to television apparatus, and more particularly to improvements in the radio frequency ampliiler for apparatus of this type.

In television apparatus, particularly receivers, the cost of the radio frequency amplifier and detector represents a substantial portion of the cost of the complete apparatus, since a number of stages of amplification are required.

It is an object of my invention to provide an improved radio frequency amplifier and detector winch may be produced at a low cost, which is simple, and which' is capable of transmitting ,an adequately wide band of frequencies.

A further object is to provide an improved superregenerative ampliiier detector for television receivers in which the quench potential is applied during the return time of the saw-tooth wave provided by the horizontal defiecting circuit of the cathode ray tube.

Other objects will appear from the following description, reference being had to the accompanying drawing,r in which: l

Figure 1 is a schematic wiring diagram'of a television receiver incorporating the invention; and

Figure 2 shows the wave form of the horizontal deecting circuit.

Superregenerative oscillator detector ampliiler circuits are known to provide high gain and to be extremely sensitive to weak signals. Such circuits are, however, not generally used in radio receivers because of the difficulty of tuning them sharply. However, the very high carrier frequencies used in the transmission of television signals, and th'e breadth of the band, necessarily make it desirable thatthe tuning be rather broadso that there `will be a minimum of distortion of the video signal. v

It has not been ioundpossible, in the past, to use a superregenerative oscillator-detector-ampliiler in a television Areceiver in the manner it may be used in radio receivers, r.because the time during which the oscillation is quenched may coincide with the reception of a video or synchronizing signal and may thus result in the blanking out or distortion of a portion of the image on the screen of the cathode ray tube. This difficulty has been overcome by my invention, in which means are provided to synchronize the application of the quenching potential (upon the regenerative oscillator grid) with the return sweep of the horizontal deflection impulse. Th'uS, though the superregenerative circuit is ineiiective to transmit the received lsignal during a substantial portion of the time, such interruption does not ailect the image appearing upon the screen oi' the cathode ray tube or in any other way aect the operation of the receiver.

The invention is shown in Fig. 1 as forming part of a television receiver, which, in the main, may be of conventional construction and is diagrammatically illustrated as including a cathode ray tube i0, the vertical deecting coils i2 of which are coupled to a vertical deilecting circuit, indicated as a block il, while the horizontal deilecting coils i8 of which are indicated as coupled to a horizontal detlecting circuit, shown as a block I8. Both the horizontal deilecting circuit and the vertical deilectlng circuit are coupled to a synchronizing signal separator circuit 2li, the latter being supplied with' its signal from a video amplifier 22, the video ampliiler supplying the necessary signal to the control grid of the cathode ray tube I0.

The video amplier receives its signal through a broadly tuned input circuit 2t, which is inductively coupled with the tuned output circuit 28 lof a superregenerative detector, including an electron discharge device 28. The oscillator is illustrated as having the device 28 in the form of a triode having its plate 28 connected to a suitable terminal of a plate' current supply system through the broadly tuned mesh 28. The input circuit for the tube 28 comprises a tuned inductance capacitance mesh 80, which is inductively coupled to the antenna coil 32 'connected between an antenna 84 and ground.

One terminal of the tuned mesh 30 is connected to the cathode 38 of the triode 28, and thus connected to a negative terminal of a suit- -able power supply system, while the oth'er terminal of the mesh 30 is connected through a condenser 38 to the grid 48 of the triode 28. A grid resistor l2 is connected'between the grid 40 and cathode 38, and is in the fonn of a potentiometer having a sliding contactor 4! connected toaI conductor 46. The other end of the conductor 46 is connected to any suitable element oi the horizontal deiiecting circuit at which' the potential goes negative during the return portion of the sweep cycle.

As indicated in Fig. 2, the horizontal deiiecting circuit generates a saw-tooth wave 4l shown in this ligure. having .a short return portion during which the potential rapidly drops from its maximum positive value to zero or its lowest negative value. During the return portion of the wave,

the horizontal deflecting circuit is arranged to impress a negative potential upon the conductor 46. If the horizontal deflecting` circuit is of such a nature that this negative potential is impressed upon the conductor 46 only during the initial major portion of the return portion of the wave, that is, the portion indicated by the bracket 50 on curve 48, the results will be improved, and, if des'ired, a suitable phase shifting network may be coupled between the horizontal deflecting circuit andconductor. 46 so as to provide a negative impulse only during that portion of the wave included within the bracket 50. This, however, is

not essential, since the negative potential may be impressed upon the conductor 46 throughout that portion of the wave 48 during which the potential is decreasing or going negative.

It will be understood that the superregenerativ'e detector will commence oscillation when the most minute signal is impressed upon lts grid, and may, in fact, commence oscillation due to thermal asitation. Such oscillation rapidly builds up because of the great amount of feedback through the interelectrode capacitance in the tube 28, which continues until the grid drops to a suliiciently negative value (beyond cuto) to block oscillation. The circuit elements associated with the tube 28, particularly the values of -the condenser 38 and resistor 42, are so chosen that this blocking or quenching of the oscillator will take place at a frequency of approximately, but preferably slightly less than, 15,'750 cps., assuming that the receiver is operating at the standard number of frames per second and the standard number of lines per inch, and that, therefore,

the horizontal deil'ecting circuit generates a sawtooth Wave at the frequency of 15,750 cps. Since the superregenerative detector thus tends naturally to be quenched at a frequency close to 15.75

' kc., the impulse impressed across a portion of the grid resistor42, and provided by the horizontal deflecting circuit I8, will be effective to stabilize the quenching or blocking frequency at this value.

As a result, the detector will be blocked only during the intervals that the cathode ray beam is not traversing the screen, so that the quenching or blocking of the superregenerative detector cannot have any effect upon the image produced upon the screen of the cathode ray tube.

In utilizing the invention, the means 24, 26,

vand are tuned broadly to the carrier fresignals) will be transmitted to the video amplifier The single electron discharge device 28, with its associated circuits, thus performs the functionsv of a number of stages of radio frequency ampliilcation and detection. Due to its great simplicity, its use results in a considerable reduction in cost of the receiver, as compared with the present systems using a. number of stages for amplification and detection, usually with an undesirably high frequency intermediate stage.

While I have shown and described a particular embodiment of my invention, itwill be apparent to those skilled in the art that numerous modifications and variations may be made without departing from the underlying principles of the invention. I therefore desire, by the following claims, to include within the scope of my invention all such modifications and variations by which substantially the results thereof may be obtained by the use of substantially the same or equivalent means.

I claim:

l. In a television receiver having a cathode ray tube for producing the image, a horizontal sweep impulse producing means for said tube, and a video amplifier, the combination of a superregenerativeoscillator operating asan amplifier and detector, said oscillator having input and output circuits, means coupling said input circuit to a modulated carrier signal'source, means coupling said output circuit to the video amplier, and a circuit deriving an electrical impulse from the horizontal sweep producing means and impressing it upon said oscillator to quench the oscillation thereof in synchronism with the horizontal sweep impulse producing means during the return portion of the sweep impulse produced thereby. 4

2. In a television receiver having 'a cathode ray tube for producing the image, a horizontal sweep impulse producing means for said tube. and a video amplifier, the combination of a superregenerative oscillator-detector, said oscilla-- tor-detector having input and output circuits. means coupling said input circuit to a modulated carrier signal source, means coupling said output circuit to the video amplifier, and a circuit deriving an electrical impulse from the horizontal sweep producing means and impressing it upon said oscillator-detector to quench the oscillation thereof in synchronism with the horizontal sweep impulse producing means during the initial return portion of the sweep impulse pro` duced thereby.

3. In acathode ray type television receiver having means providing a saw-tooth wave for horizontal deflection of the cathode ray beam, the.

combination of a superregenerative` oscillator coupled in the receiver to operate as a modulated carrier wave amplier and detector, and means coupled to the horizontal deflection wave producing meansto stabilize the quench frequency of said oscillator `in synchronism with the sawtooth wave and` in phasey with the substantially vertical portion of said Wave.

J. N. MCCOLLUM-'

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