US2210497A - Amplifying system - Google Patents

Description

Aug. 6, 1940.

W. s. PERCIVAL AMPLIFYING SYSTEM Filed Oct. 29, 1937 Mama O Q -0.-70 //1 7'0 SOIl/VD REC 771 7523 ETC.

- INVENTOR W. 5. PERC/VAL BY MW/ ATTO R N EY Patented Aug. 6, 1940 PATENT OFFICE 2.210.497 AM'PLIFYING SYSTEM William Spencer Percival, Ealing, London, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application October 29, 1937, Serial No. 171,678 In Great Britain November 2, 1936 1 Claim.

This invention relates to radio receivers and is mainly concerned'with a receiver for the reception for both vision and sound signals in television reception.

The usual form of receiver for this purpose is one employing several stages of radio frequency circuits, tuned either to the carrier frequency, for example, 45 megacycles, or to an intermediate frequency of, for example, 10 megacycles, but

such receivers are not altogether satisfactory since it is difficult to obtain the required passband and adequate stage gain. With television transmissionthe modulation frequency range of the signals extends up to above 2.5 megacycles and hence, if faithful reproduction of the signals is to be obtained, the receiver must be capable.

of dealing with such a wide range of frequencies. It is the main object of the present invention to provide an improved superheterodyne receiver which is specially suitable for use in the reception of both the sound and the vision signals.

According to one feature of the invention, a superheterodyne receiver for television reception is provided in which both the sound and the vision signals are heterodyned by local oscillationsand the resulting intermediate frequency outputs are amplified by a low pass amplifier. Preferably the amplifier is of the type comprising a plurality of thermionic valves, the intervalve coupling or couplings including a low pass network or low pass networks designed in such a manner and terminated by appropriate resistances as to have the required characteristics. The use of an intervalve coupling designed as a low pass network and not as a band-pass net work is advantageous since it affords adequate stage gain and band-pass with a smaller number of components compared with the band-pass case with much less critical adjustment of these components. Y

Preferably the sound signals are rejected or suppressed at a suitable point in the low pass amplifier by the provision of an additional rejector circuit or circuits and a band-pass output circuit is provided for feeding the vision signals to a suitable rectifier.

According to anotherfeature of the invention, a superheterodyne receiver for television reception is provided in which the intermediate frequencysignals, after amplification, are fed to a band-pass output circuit for feeding the signals to a suitable rectifier, the terminating resistance (01. its-5.8)

the use of a band-pass network which consists of a parallel tuned circuit provided by an inductance in the anode circuit of the thermionic valve and the anode-to-earth capacity of the said valve, coupled by a series resonant circuit, to a further parallel tuned circuit connected to an additional valve or rectifier the capacity of which forms the capacity of the further tunedcircuit, the series resonant circuit being tapped into one or both of said parallel tuned circuits to take account of the difference in the capacities of said valves.

Other features of the invention will hereinafter appear.

In order that the nature of my invention and the manner in which it may be carried into practice may be readily understood, the invention will now be more fully described with reference to the drawing, the single figure of which illustrates, by way of example, a circuit of a superheterodyne receiver embodying the features of the present invention.

As shown, the receiver includes an oscillator valve VI, first rectifier V2, four intermediate frequency amplifier valves, V3, V4, V5 and V6, and a second rectifier V1, from which the vision signals are obtained. The valve VI is associated with circuit elements as shown and is arranged to generate oscillations of about 40 megacycles. The operation of the oscillator circuitwill not be described in detail as it forms no part of the present invention. The valve V2 is arrangedto function as an anode bend rectifier and is biased I bya suitable resistance '8 between cathode and to pass the sound carrier frequency and the television sidebands. The anode of valve V2 is connected' to the positive terminal of the anode high tension sourcethrough a pair of coupled coils H and I2, a load resistance I3 and a decoupling resistance [4 associated with 'a decoupling condenser l5. The intermediate frequency outputs of the valve V2 are applied through a coupling condenser it to the control grid of valve V3, a leak resistance I! being provided as shown. The

valve V3 is coupled to the valve V4 and the latter valve is coupled to the valve V5 and valve V5 to valve V6 through similar inter-valve couplings consisting of coupled coils H and I2, resistances l3 and the anode-to-earth and grid-to-earth capacities of the associated valves. Decoupling resistances I4 and condensers are provided as shown. The valves V3, V4, and V5 and V6 are,

in the specific example, of the pentode type and the cathodes of the valves are associated with biasing resistances l8 shunted by by-pass condensers 19. The required voltages for the screens of the valves V3, V4, V5 and V6 are obtained through the medium of dropper resistances 20 in series with by-pass condensers 2| across the positive and negative terminals of the high tension source as shown.

The low pass intervalve couplings are designed in conjunction with the anode-to-cathode capacity of the first valve and the grid-to-earth capacity of the second valve of each stage in such a manner as suggested in the specification of British application No. 5335/36 as to provide the optimum product of the pass-band and stage gain. The bandof frequencies passed by the first stage of the amplifier comprises the sound carrier frequency of about 1.5 megacycles, and the television carrier of about 5 megacycles with sidebands of about 2.5 megacycles. The lowpass amplifier described is arranged to have a cut-off at 7 megacy'cles and in the specific ex ample shown, the intervalve couplings of the low-pass amplifier may have the following values: Coil ll, 44 microhenries; coil i2, 25 microhenries; resistance [3, 2000 ohms; resistance I4, 500 ohms and the condenser B5, 0.04 microfarad. The coupling condenser 16 may be of 0.0005 microfarad, the anode-to-cathode capacity of the valves may be 11 microfarads and the grid-toearth capacity of the valves may be 16 microfarads, while the leak resistance ll may be of 100,000 ohms.

It is desirable in order to eliminate or reduce noise and interference due to reception of signals at certain broadcast frequencies, to arrange that the low-pass amplifier has a cut-off at the lower end of the intermediate frequency spectrum for instance at about 1 megacycle. Such a cutoff is effected through the medium of the coupling condenser l6 and leak resistance [1. In addition, by suitable choice of the condensers I9, it may be arranged that the biasing resistance is not shunted by this capacity at the lower frequencies and degeneration at these frequencies results. In the example shown the condensers is each may have a capacity of 0.01 microfarad. If the cut-off is arranged to extend to frequencies above 1 megacycle, some attenuation of the lower side band frequencies of the vision signals may result. In order to compensate for such attenuation these lower side-band frequencies may be boosted by employing an additional resistance and condenser, not shown, in the anode circuit of one of the stages between the resistance l3 and the condenser I5 which in effect sharpens the cut-off.

Since the sound carrier frequency occupies only a relatively narrow band at the lower end of the intermediate frequency spectrum, the sound signals can be substantially rejected or suppressed at a suitable point in the low-pass amplifier. In the example shown, such suppression is effected at the valve V4; by degeneratiomfor which purpose between the cathode of valve V4 and earth there is inserted a rejector circuit comprising an inductance 22 shunted by a condenser 23, the inductance being tuned to the sound carrier frequency of 1.5 megacycles. The intermediate frequency voltage across this rejector circuit is supplied to a separate sound amplifier (not shown) which may be connected to the pair of terminals S. If desired, a similar rejector circuit may be included in the cathode of valve V5 in order to reject further the sound carrier. Volume control may be elfected by the provision of a variable resistance 24 associated with a by-pass condenser 25.

The output from the valve V6 is fed via a bandpass output circuit to a diode rectifier Vl from which vision signals are fed to a cathode ray tube in the manner hereinafter described. The anode of the valve V6 is connected to the positive terminal of the high tension source through an inductance 25 and a decoupling resistance 2'! associated with a decoupling condenser 28. The anode of valve V6 is coupled through a series resonant circuit comprising a; condenser 29 in series with an inductance 30, to a shunt circuit comprising an inductance 3| tuned by the input capacity of the rectifier V1 between the anode of the rectifier and earth.

In order to avoid attenuation of the higher frequency signals it is essential to obtain a low impedance output from the'valve V6 since the diode load must be of low impedance having regard to the'unavoidable self-capacities and the input capacity of the cathode ray tube.

The diode load comprises a relatively low resistance 32 shunted by a series tuned circuit comprising a condenser 35 and an inductance 35. Since the rectifier circuit is a low impedance at the intermediate frequencies the coupling between valves V6 and V1 is a terminated bandpass network of low characteristic impedance, the input impedance of the rectifier being incorporated as part of the terminating resistance of the network the termination being such that the network is substantially non-reflective at the terminated end. The band-pass network which aids in completing the rejection. of the sound and other unrequired signals comprises the coil 26, which may have an inductance of 85 microhenries, in parallel with the anode-to-earth capacity of the valve V6, and the series resonant circuit, consisting of condenser 29 and coil 30 is tapped into the coil 26 at a point to take account of the differences in the capacities of valves V6 and V1. The condenser 29 may have a capacity of 11.6 micromicrofarads, coil Swan inductance of '78 microhenries. have an inductance of microhenries and is tuned as aforesaid by the input capacity of the diode rectifier V! between the anode of such valve and earth. Theseries tuned circuit which shunts the diode load causes the input capacity of the diode to be substantially the anode-tocathode capacity thereof; the condenser 35 in the example shown may have a capacity of 21.5 micromicrofarads and the coil 35 may have an inductance of 40 microhenries.

If desired, in cases where the capacity of valve V1 is larger than valve V6, the Series resonant circuit may be tapped into the inductance 3|. The connection of the series resonant circuit to either inductances 26 or the inductance 3! in the manner aforesaid can be employed to take account of the value of the effective termination resistance which is fixed, not by consideration of the design of the band-pass network alone, but by the requirements for a low impedance out- The coil 3! may J put from the diode rectifier. The leaking inductance resulting from tapping down of the inductance is effectively included in the inductance of theseries resonant circuit. The inductance 3E! may be physically eliminated by tapping down on both inductances 26 and 3i if the coil 3!, which is the coil at the loaded end of the network, has very close coupling between its sections such as results from using a suitable iron core. The damping provided by the iron core may be employed in some cases to replace the resistance 34.

The band-pass network is terminated as aforesaid by the input impedance of the diode rectifier in parallel with the resistance 34, the resistance 32 in the example shown being 5,600 ohms and the resistance 34 being 4,300 ohms.

During rapid decreases of the amplitude of signals applied to valve Vl, this valve, as is usually the case, does not conductat the peaks of the applied high frequency oscillations. at Consequently, the damping provided by the rectifier is removed during these periods resulting in a distortion of rapid decreases of picture illumination.

It is not, therefore, advisable to rely on the rectifier to supply the whole of the damping, and hence it is preferred to employ a composite terminating impedance comprising resistances 32 and 34.

The non-reflective terminated band-pass network has, in addition to the selective property mentioned earlier, the advantage that for the given shuntcapacities of valves V8 and Vi, the load in the anode of valve V6 is a maximum, so that the gain of valve V6 for a given pass-band is a maximum and also the power output from valve V6 is the highest possible.

The output from the diode valve V7 is fed through a network comprising a coil 3? damped by a resistance 38, and a condenser 39, such network being designed in conjunction with the input capacity of the cathode ray tube to pass all frequencies up to 2 megacycles without appreciable attenuation. The signals to be fed to the cathode ray tube are established across the terminal 40 and earth while signals for application to the synchronizing circuits are taken from a tapping point in the resistance 4 l.

In some cases it may be desired to employ an anode bend rectifier inplace of the diode Vl. Since such a rectifier has a high input impedance, a low pass network similar to those coupling the amplifying valves can be used to couple the valve Vt thereto. In this case, for selectivity purposes, a band-pass network should be employed at a suitable stage in the amplifier, for example, in the anode circuit of valve V2.

In cases where it is required to remove the intermediate frequency carrier and sidebands from the output more effectively, it may be advantageous to replace the diode V? by a push-pull arrangement, using an ordinary band-pass output.

It is to be understood that the invention is not to be limited to the specific values of components referred to, since these can obviously be varied according to particular requirements.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

In a system for the reception of television signals, wherein the sound and picture signals are transmitted on separately modulated carrier frequencies, a local source of oscillations, means for heterodyning the local oscillator with the sound and picture carrier frequencies to produce separate sound and picture intermediate frequencies,

common means for amplifying both the intermediate frequencies comprising a plurality of 'iermionic discharge tubes, means including a low-pass network for coupling the tubes whereby WILLIAM SPENCER PERCIVAL.

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