US2219175A - Television intermediate frequency separation circuit - Google Patents

Description

R. B. DOME TELEVISION INTERMEDIATE FREQUENCY SEPARATION CIRCUIT Filed Nov. 18, I938 IYWVe Poberb 5p e, y k r,

His/Att Patented Oct. 22, 1940 UNITED STATES PATENT OFFICE 3 Robert B. Dome, Bridgeport, Conn, assignor to General Electric Company, a corporation of New York Application November 18, 1938, Serial No. 241,248

2 Claims My invention relates to an improved arrangement for separating in a television system voice modulated oscillations from picture modulated oscillations which appear in a circuit common to each 5 at some point in the system. While not limited thereto, my invention is particularly suited for use in a television receiver to separate the voice modulated intermediate frequencies from the picture modulated intermediate frequencies appearing in the output circuit of a converter tube common to each intermediate frequency.

Television transmissions consist of both audio and picture signals each on their own individual carrier waves. The carrier waves are separated in the frequency spectrum, according to present day practice, by several megacycles, as for example, 3.25 megacycles. Considerations of economy and convenience require that only one frequency converter be used in the television receiver to transform the audio and picture signals to audio and picture modulated intermediate frequency currents to be suitably amplified before being supplied to appropriate translating devices. The anode circuit of the frequency converter thus contains two signal components: audio modulated intermediate frequency oscillations and picture modulated intermediate frequency oscillations. Since these intermediate frequencies are amplified by separate amplifiers, it becomes necessary to separate each intermediate frequency from the other in order that each may be supplied to its own individual amplifier free from any interfering effects of the other.

An object of my invention is toprovide an im- 35 proved and simplified arrangement for separating in a television receiver of this type the voice modulated (audio) intermediate frequency oscillations from the picture modulated (video) intermediate frequency oscillations in order that each termediate frequencies before being supplied to appropriate translating apparatus.

A further object of my invention is to provide a separating arrangement for video and audio intermediate frequencies in which the tuned input circuit of the audio intermediate frequency am plifier is more sharply resonant than that heretofore attainable in receivers employing only a single frequency converter stage. To this end, my invention contemplates that the tuned input circuit of the audio intermediate frequency amplifier shall be free from the loading effect of the resistance which must be employed to load the input circuit of the video intermediate frequency amplifier for broad frequency response,

may be suitably amplified at their respective in- An additional object of my invention is to provide, in a television system having a single frequency converter stage for producing both audio and video intermediate frequencies, an arrangement for separating out at high gain the audio modulated intermediate frequency oscillations.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing which illustrates an embodiment of my invention.

Referring more particularly to the single figure of the drawing, my invention is illustrated as embodied in a television receiver having an input circuit connected to a dipole antenna Ill, H and ground l2. The signal energy received in the an-v tenna and ground system is supplied to radio frequency amplifiers, which may be included in the apparatus represented conventionally by the rectangle l3, where the received energy is suitably amplified before being supplied to a frequency converter M. The frequency converter is connected to mix the received signal 0501113,? tions with locally generated oscillations to produce in a manner well known in the art oscillations of intermediate frequency.

The audio carrier wave is spaced in the fre-- quency spectrum, according to present day prac-- tice, 3.25 mega/cycles from the picture carrier wave. The tuned circuits of the radio frequency amplifier and of the converter stage have a sufliciently broad response that both the audio and picture carrier waves are received on the antenna ground system, amplified, and supplied to the frequency converter. The generator of local oscillations, included With the apparatus l3, has a frequency such that (when mixed with both the audio and picture carrier waves) the resulting picture modulated intermediate frequency ap-, pearing in the output circuit I5 of the converter M has a frequency higher than that of the audio modulated intermediate frequency which likewise appears in the output circuit l5 of the converterl The audio and picture modulated intermediate frequencies which appear in the output circuit l5 of the frequency converter are separated by a separating impedance network It, presently to be the apparatus represented conventionally by the rectangle IT. After amplification, the audio modulated intermediate frequency oscillations are detected and the audio frequency components are amplified at audio frequency before being supplied to a suitable translating device which may, 'for example, be a loudspeaker [8.

The picture modulated intermediate frequency oscillations after their separation from the audio intermediate frequency oscillations by the network I6, are supplied to a picture intermediate frequency amplifier included with the apparatus represented conventionally by the rectangle It. The picture intermediate frequency oscillations are amplified, detected, and the modulation components of the intermediate frequency are suitably separated and amplified before being supplied to an appropriate translating device which may, for example, be a cathode ray picture tube 20.

The manner in which the audio and picture modulated intermediate frequencies are separated by the network IE will now be considered. The network is comprised of a transformer 21 and associated condensers 22, 23, by an inductor 24, resistors 25, 2E, and El, and by condensers 28, 29 and 31. The output circuit l5 of the frequency converter I4 is connected through a primary winding 3!) of the transformer 2! and through the resistors 25 and 21 to the positive terminal of a source of anode potential, not shown. The secondary winding 3! of the transformer 2| is connected to the input circuit 32, 33 of the audio intermediate frequency amplifier included with the apparatus H. The condensers 22 and 23 tune the respective transformer windings 30 and 3! to the frequency of the audio intermediate frequency oscillations. The transformer windings 3B and 3! are preferably loosely coupled to enable the tuned circuit 23, 3! to be highly selective. The high selectivity'of the tuned circuit thus provided effectively separates the audio modulated intermediate frequency oscillations from the picture modulated intermediate frequency oscillations and supplies the former without interfering effect of the latter through the input circuit 32, 33 to the: audio intermediate frequency amplifier.

The picture modulated intermediate frequency oscillations have a higher frequency than have the audio intermediate frequencies and therefore pass through the condenser 22 in preference to the higher impedance of the transformer winding 30. The condenser 28 has a relatively large capacity and thus maintains the upper terminal of the resistor 21 at ground potential for frequencies of the order of the audio and picture intermediate frequency oscillations. The condenser 31 has a relatively large capacity and thus maintains the upper terminal of the resistor 25 at the high frequency potential of the upper end of resistor 25. Condenser 3'! thus serves as a blocking capacitor to keep the anode direct potential off the grid of the first amplifier in apparatus IS. The condenser 29 tunes the inductor 24 as a series resonant circuit to the frequency of the audio modulated intermediate frequency oscillations. Since the series resonant circuit 24, 29 is con nected across the input circuit 34, 35 of the picture intermediate frequency amplifier. and has a low impedance to the audio modulated intermediate frequency oscillations, the latter oscillations are very effectively shunted around the input circuit 34, 35 of the pictur intermediate frequency amplifier.

The picture modulated intermediate frequency oscillations produce a potential across the resistor 25. This potential is supplied through the condensers 28 and 31 to the input circuit 34, 35 of the picture intermediate frequency amplifier. The input circuit 34, 35 has a certain amount of circuit stray capacity, amplifier tube shunt capacity and stray capacitance to ground due to the transformer primary winding 30, conductor l5, and tube l4, which may be represented by the lumped capacity 36. The Values of the inductor 24 and the condenser 29 are so chosen that the capacity 36 adds to the capacity of the condenser 29 (in series) to tune the inductor 24 as a shunt resonant circuit to the frequency of the picture modulated intermediate frequency oscillations. The frequency response of the shunt tuned circuit thus provided is broadened by the use of the resistors 25 and 26, the resistor 25 serving additionally to supply direct current from the source of anode potential, not shown, to the anode of the converter i4. Thus while the series resonant circuit 24, 29 operates with its low impedance to shunt from the input circuit 34, 35 the audio modulated intermediate frequency oscillations, the parallel tuned circuit 24, 29 and 33 has a high terminal impedance to the picture modulated intermediate frequency oscillations and operates at high efficiency to supply the latter oscillations to the input circuit 34, 35 substantially to the exclusion of the audio modulated intermediate frequency oscillations.

My invention has the important advantage that the tuned input circuit of the audio intermediate frequency amplifier is not loaded by the resistors 25 and 26 which must be used to give the input circuit of the picture intermediate frequency amplifier its necessarily broad frequency response. This enables the tuned input circuit of the audio intermediate frequency amplifier to be sharply resonant at the frequency of the audio modulated intermediate frequency oscillations. It should be noted in this respect that the resistor 25 does not load the audio intermediate frequency tuned circuits since the low impedance series resonant circuit 24, 29 is effectively in shunt to this resistor.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and in the instrumentalities employed, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a television receiver in which sound modulated oscillations and picture modulated oscillations occur in the same circuit and are supplied therefrom to separate amplifiers, said oscillations having different frequencies, a coupling between said circuit and said amplifiers comprising a tuned circuit and a resistance connected in series, said tuned circuit being tuned to the frequency of one of said oscillations, means to supply oscillations from said tuned circuit to a respective one of said amplifiers, and means to supply oscillations from across said resistance to the other of said amplifiers and to prevent said resistance from loading the input to said one amplifier at the frequency of oscillations supplied to said one amplifier, said means comprising a circuit havin at the frequency supplied to said other amplifier,

a capacitive branch and an inductive branch, said branches being connected in shunt to said resistance and to the input of said amplifier and resonating with each other at the frequency of oscillations supplied to said other amplifier, and one of said branches being itself resonant to the frequency of oscillations supplied to said one amplifier thereby to by-pass oscillations of said last frequency around said resistance and to prevent said resistance 'from loading the input to the means to supply oscillations from across said resistor to said picture amplifier, said last means comprising an inductance and a capacity in series across said resistor resonating at the frequency of the sound modulated oscillations, and said series combination having at the frequency of the picture modulated oscillations inductive reactance of such a value as to resonate with the shunt capacity of the input of the picture amplifier at that frequency, a resistance including the resistance of said resistor, in shunt to the circuit formed by said series combination and said shunt capacity of such value as to damp the resonance characteristic of said circuit to produce a broad band pass characteristic for picture modulated oscillations, said combination Operating simultaneously to prevent damping of said first-mentioned tuned circuit by said resistance whereby said first-mentioned tuned circuit may be sharply resonant to sound modulated oscillations.

ROBERT B. DOME.

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