US2162988A - Radio receiver - Google Patents

Description

June 20, 1939. I e. M. WRIGHT RADIO RECEIVER Filed Jan. 15, 1937 BY 7 ATTRNEY Patented June 20, 1939 RADIO RECEIVER George Maurice Wright,

Woodham Ferrers,

England, assignor to Radio Corporation of America, a corporation of Delaware Application January 15, 1937, Serial No. 120,666

In Great Britain 2 Claims.

This invention relates to radio and like carrier wave receivers and more specifically to homodyne receivers of the locked oscillator type and to valve arrangements therefor, and the object of the main feature of the said invention is to provide an improved homodyne receiver which shall be stable in use and relatively simple to construct and operate.

In the drawing:

Fig. 1 shows a receiving circuit known in the prior art,

Fig. 2 is a circuit diagram of a system embcdying the present invention,

Fig. 3 shows a modified form of beam valve that may be used in the circuit of Fig. 2.

In order that the main feature of the invention may be the better understood consider first the case of an ordinary known self-oscillating detectcr receiving circuit as shown diagrammati- 20 cally in the accompanying Fig. 1 and the difiiculties which arise therein when such a circuit is sought to be employed for homodyne reception. The typical known self-oscillating detector receiving circuit shown in Fig. 1 comprises an aerial circuit 6 including a variable condenser and an inductance in series, the inductance being coupled to the coil in a parallel tuned circuit 2 included in the grid circuit of a first valve 3. The anode circuit of this first valve contains a further parallel tuned circuit 4, the coil of which is coupled to the grid coil 28 of a detector valve 5 whose plate circuit contains in series with the usual loudspeaker or other translating means 2| a reaction coil 6 adjustably back-coupled to the grid coil 213 and circuit 4. Now, if in such a circuit the reaction coil 6 be adjusted to produce self-oscillation and the whole arrangement be adjusted to operate as a homodyne receiver serious operating difficulties arise. Firstly, when the detector valve 5 is oscillating, any variation of the tuning in any of the circuits tends to cause more or less violent changes in the beat frequency (which is preferably zero) between the incoming carrier and the locally produced osciliations. Such disturbances become very manifest if the tuning condensers in the aerial circuit and in the grid circuit 2 of the first valve 3 are varied and these disturbances render accurate tuning of the circuits s0 difficult as to be almost impossible. The cause of these disturbances lies in the main in interaction between the input and output circuits 2 and 4 of the first valve via the self-capacity of the said valve and/or leakage any screening that may be provided between said input and output circuits.

Even if,

December 13, 1935 most careful precautions as to screening are resorted to with ordinary valves, and even if a screened grid valve be employed as the first valve 3, more or less serious operating and/0r constructional difiiculties remain. Secondly, owing to the residual carry through of the first valve 3 continuous oscillations are induced in the aerial circuit from the self-oscillating detector 5, and these induced oscillations cause radiation which may interfere with nearby receivers.

The present invention provides an improved homodyne receiver in which the above difficulties and operating disadvantages are avoided.

According to the main feature of this invention there is interposed in the signal channel prior to the stage at which homodyne detection is effected in a homodyne receiver, an isolating valve stage constituted by what is herein termed an electron beam valve, that is to say, a valve which resembles a cathode ray tube and wherein input signals are applied to control the cathode ray beam, output signals being taken from the circuit of the electrode towards which said beam is projected.

In carrying out the above feature of the invention it is preferred to employ as the isolating valve an electron beam valve as described in British Patent No. 453,137 or British Patent 403,973.

The accompanying Fig. 2 illustrates one way of carrying out the main feature of this invention. Here a homodyne receiver comprises the usual aerial circuit l comprising a variable condenser 22 in series with an inductance 23. This inductance is coupled to the coil of a parallel tuned circuit 2 which is included between the cathode 1 and the Wehnelt cylinder or other control electrode S of the electron gun of an electron beam valve as described in British specification No. 403,973 or in British Patent 453,137. Suitable positive voltage is applied to the accelerating anode 9 of the gun, and the collector electrode ill of the valve i. e. the electrode towards which the cathode ray beam is directed, is connected to the positive terminal of a source (not shown) of anode potential through a choke coil H. An electrostatic screen l2 having an aperture to admit the envelope of the electron beam valve is interposed in a plane between and substantially parallel to the accelerating anode 9 and the output electrode it of the electron beam valve this screen serving to reduce, as far as possible, any residual capacity between the input and output circuits of the electron beam valve. The screen 12 is preferably substantially co-planar with and electrically connected to a transverse apertured screen electrode I3 provided in the valve between the accelerating anode 9 and the plate electrode I0, connection between the screen electrode and the external screen being obtained by connecting both to the cathode, (which is earthed) the cathode-screen electrode connection 24 being preferably made inside the valve envelope, as shown. Thus the external screen I2 is in effect a continuation of the screen electrode l3. The output electrode It of the electron beam valve is coupled by means of a condenser I4 to one end of a parallel tuned output circuit i5 and also by the same condenser to the grid 25 of an ordinary oscillator valve 5 which is adjusted to operate over the substantially rectilinear portion of its characteristic. The plate-cathode circuit of this oscillator valve includes a resistance HS in series with a reaction coil 6, the reaction coil being coupled to the 'coil in the tuned circuit l5 and also to the grid coil 25 of a further valve I! which is biased to operate as a lower bend detector. This valve I? may be, as shown, an ordinary triode but is preferably a screen grid type of valve or a second electron beam valve operated as a detector. The valve 5'! feeds into a utilization device 2| through a suitable low pass filter !B.

It will be appreciated that, were there no external screen I2, a compromise would have to be effected in the design of -the electron beam valve shown between the conflicting requirements of good isolation and reasonably low operating hotentials mainly the anode potential. The former requirement involves the use of a long electron gun to anode space whereas the latter requirement involves that this said space should be made reasonably short. The screen and screen electrode combination I2, l3, improves the isola tion and therefore facilitates the obtaining of a better compromise in design. The above described combination of screen and screen elec-- trode has, however, the defect that the amount of isolation afforded is critically dependent upon the exact location of the aperture in the external screen relative to the internal apertured screen electrode-so much so, in fact that if, in a given construction the valve be replaced by another similar valve, the ordinary practically unavoidable dimensional differences between different valves manufactured to the same design may well be enough to cause the said construction, when fitted with the second or replacement valve, to greatly decrease the maximum degree of isolation. For this reason, and in accordance with another feature of this invention, it is preferred to use, instead of a simple apertured plate-like external screen as shown in Fig. 2, an arrangement as shown in Fig. 3. Here there is a screen l2 which is formed with a tubular projection sheath or hood 52a, the axis of which is substantially concentric with the axis of the isolation valve envelope when the said valve is inserted into the aperture in the external screen. The said sheath lZa projects from the screen at right angles to the main plane thereof and on one side thereof and is of such length that when the valve is inserted, that part of the envelope on the anode side of the screen electrode is inside and closely surrounded by the sheath as shown. The far end of the sheath iZa is reduced in diameter after the manner of a hood so that it follows the contour of the valve envelope and is provided with an aperture bushed at !2b with insulating material, the anode lead passing through the bushing. The reduction in diameter should not be carried so far that the effective self-capacity of the anode lead is materially increased. In Fig. 3 the screen electrode 52 is shown located close to the accelerating anode 9. This construction may be adapted for the arrangement of Fig. 2, the electrode l3 in any case being as nearly as possible in the plane of the screen I2.

This feature of the invention, namely the provision of an external screen with a hood-like extcntion may be applied with advantage whenever an electron beam. valve'is used as an isolation valve, and it is found that, by its use the effective capacity between input and output electrodes may be reduced to about ,5 of that obtaining with an ordinary known screen grid valve. By using an external screen without a hood and arranged in association with an internal screen electrode, the said effective capacity is only reduced to about of that obtaining with an ordinary screen grid valve.

Referring again to Fig. 2 it will be appreciated that by reason of the provision of a separate valve for the oscillator (the valve 5 of Fig. 2) it is possible to obtain a very close approximation to rectilinearity of reaction characteristic for this valve may receive a bias such as will cause it to operate over the substantially straight portion of its characteristic. Furthermore the obtaining of rectilinearity of reaction is assisted by the resistance i 6 in series with the reaction coil.

Preferably the inductance in the parallel tuned circuit i 5 which is capacity coupled to the output electrode of the electron beam valve is constituted by a primary of a transformer so designed that the secondary winding 26 does not unduly raise the high frequency resistance in the primary circuit. Such design may be accomplished by making the primary and secondary windings of the same length, and of the same number of turns, employing very fine wire for the secondary winding and constituting the primary winding by the usual stranded wire construction.

As stated, the detector valve I! may be a triode, but it is preferred to constitute it by a screened grid valve or better still by a second electron beam valve. The object of using a screened grid valve or a second electron beam valve for the detector is to reduce still further any lack of rectilinearity in reaction conditions resulting from the varying anode impedance with applied amplitude which occurs in a rectifying valve, for it will be appreciated that, if a triode be employed as the rectifying valve or detector, any such varying anode impedance will tend to transfer a corresponding varying impedance to the grid circuit via the inter-electrode capacity of the triode.

Having now particularly described and ascertained the nature of my invention and in what manner thesame is to be performed, what I claim as novel and desire to secure by Letters Patent is:

1. In a receiver, means for connecting the receiver to a source of signal modulated carrier energy, means for providing homodyne detection of received signal modulated carrier energy, means for transferring the intercepted energy from the first named means to said last named means comtection means including a detector having an input circuit and an output circuit and an oscillator tube having an anode circuit and a grid circuit, said grid circuit including said last named tuned circuit, said anode circuit including a resistance and a feedback coil in series, said feedback coil being coupled to said last named tuned circuit, and a utilizing device coupled to the detector output circuit.

2. In signalling apparatus, an electron beam tube having a cathode, a control electrode, an accelerating anode and a collector electrode, a resonant input circuit connected between said control electrode and said cathode, a resonant circuit to both said first named output circuit and 1 the detector tube input circuit, and a utilizing circuit connected to the detector output circuit;

GEORGE MAURICE WRIGHT;

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