US2114615A - Superheterodyne receiver - Google Patents

Description

Ap' 9, 193.. K. scHLEslNGER l MQ SUPERHETERODYNE RECEIVER Filed Jan. 6, 1957 Patented Apr. 19, 1938 PATENT OFFICE SUPERHETERODYNE RECEIVER Kurt Schlesinger, Berlin, Germany Application January 6,

1937, Serial No. 119,193

In Germany January 13, 1936 Claims.

The present invention relates to receivers employing locally generated oscillations and its main object is to prevent disturbing radiation of. local oscillations.

The radiation of local oscillations in superheterodyne receivers is interfering even in the case of comparatively long waves, but in the case of shorter waves it is considerably amplified. Specially designed tubes, so-called mixing tubes,

410 are known in practice, in which it is possible to eliminate capacitative couplings between the local oscillator and the receiving channel by internally fitted screening means between the oscillator system and the mixing system. The

shorter the wavelength, however, the greater are the capacitative currents which flow by way of the remaining coupling capacities still present, which latter, generally speaking, are already within the order of .1 cm. A further reduction of these capacity Values is hardly possible, and a capacity of this magnitude is already fully sufficient for interfering radiation. The circuits are using screen grid tubes, such as have alr-eady been proposed for these purposes. Such circuits must also necessarily fail more and more proceeding from a critical short wave length downwards. In addition there is the fact that the gain in amplification of the preliminary tubes also sinks with the wavelength, and the economic efficiency of the tube will be reduced more and more.

Remedy has already been obtained according to the invention in an earlier application (Schlesinger Ser. No. 67,343, filed March 5, 1936) r35 by the fact that the detrimental capacity is neutralized by a special neutralization. The details of this method, which is not entirely simple to adapt to high frequencies, were then set forth in a later application (Schlesinger Serial No.

o 72,314, April 2, 1937) In the method therein set forth the cathode of the receiving system is connected with earth only by way of a choke and accordingly participates in the received oscillations. It is certainly possible by means of the method therein set forth to obtain a very good compensation of the radiation, which also remains to a certain extent independent of the frequency, but at the same time the sensitivity of reception is reduced to a comparatively large 50 degree.

1 In the present application the two requirements, freedom from radiation and the maintenance of the full sensitivity of reception, are fullled.

55 Embodiments of the invention will now be delas scribed with reference to the accompanying drawing, Figs. 1-3.

Fig. 1 shows the mixing stage of a super- ,heterodyne receiver using the neutralizing circuit according to the invention.

Fig. 2 shows a schematic diagram of the neutralizing circuit of Fig. 1, and

Fig. 3 shows a modification of the circuit of Fig. 1.

vIn Fig. 1 there is shown a typical mixing tube circuit directly connected to an aerial. The mixing tube I, as Well known, is divided by an inner screening means 2 into two separate tube systems, the one of which acts as oscillator system and the other as mixing system. The received signals are conducted to th-e latter by way of a first grid 3. The aerial 4 or the feed line 5 to the aerial is in the present case inductively coupled by way of a transformer 6. In order that powerful oscillations will be produced the cathode, grid and anode must be connected with one another in the oscillator system without interfering phase shifting and by means of short leads. This is preferably accomplished by means of a twin condenser la, 1b, the middle electrode of which is connected with the cathode 8. The grid circuit of the receiying grid 3 is dynamically earthed by wayy of a condenser 9 (non-dissipative ceramic embodiment or non-dissipative air condenser). If the reception is to take place with full volume of sound, it is necessary for the cathode 8 also to be dynamically earthed by way of a resistance, which must be small in relation to the gridcathode capacity I0 of the tube between the receiving grid 3 and the cathode 8. If the cathode l 8 is earthed in direct fashion it will be found that the neutralization of the radiation greatly deteriorates or ceases, or in certain circumstances the radiation is even increased, at the same moment. If on the other hand the cathode 8 is applied to earth by way of a choke ll, there is obtained an admirable neutralization', but the reception is lost, as the resistance of I I is greater than the resistance of the grid-capacity I0, and a receiving current of such negligible kind flows from 6 to the grid path that a sufficient receiving potential is unable to build up between grid and cathode.

In Fig. 2 there is proved the reason for this remarkable relationship between cathode earthing and neutralization circuit which latter represents a bridge-circuit. The neutralization is performed as already described in the earlier applications by a compensating condenser I2, a small adjustable capacity of the order of the detrimental capacity with the assistance of the opposite-phase potential which is available at the anode of the oscillator system. In Fig. 2 the capacities having special bearing have been particularly indicated. The receiving grid 3 is on the one hand in capacitative coupling with the mixing grid :I4-between the two there exists the detrimental capacity I3 passing through the screening grid of the hexode,-and on the other hand there is applied to the receiving grid 3 the neutralization potential b-y condenser I2, which is adapted in its order of magnitude to the value ES.

rI'he operating potentials of this bridge arm are derived from the rotating plate condensers 1a, 'Ib of the local oscillator, which owing to their size ensure that the potentials at the points A and G are of opposite phase. Between the grid 3.and

the cathode 8 there is a tube capacity I0, which is fairly large in practice (approximately 20cm. including leading-through capacity). So long as the cathode 8 is earthed by way of a choke II it is a simple matter by the balancing of I2 to raise the receiving grid 3 and the cathode 8 to the same potential. There is, however, no reception, as the resistance of I l is greater than that of lil. Now there exist between the'terminals G and A of the oscillatory system earth capacities I5 and I6 of general, greatly varying value.v rThe bridge: la, 1b, I5, I6 is, therefore, generally speaking, not in equilibrium. It is accordingly impossible to connect the cathode point 8 to earth without influencing the values of 1b, 1a, and in this way the bridge first balanced is likewise varied and the compensation of the radiation is destroyed. Y

The arrangement according to Fig. 1, therefore, in accordance with Fig. 2, represents together with the part-capacities against eartha double bridge-circuit. According to the invention a balancing of the two bridge-circuits between tuning condenser and aerial grid on the'one hand and 1between tuning condenser and earth on the other hand is performed simultaneously, and in this way there is rendered possible an earthing of the cathode 8 and together therewith a full sensitivity in respect of the reception together with the compensation of the radiation. In accordance with the invention there is arranged a differential condenser having thecapacities Il and I8 which may be varied by the rotary electrode I9. This differential condenser is lying in parallel to the oscillatory circuit and the two indefinite inherent capacities I5 and I 6. The value of the two capacities is chosen in the order of magnitude of the capacity I0 between receiving grid and cathode of the mixing tube for example equal to or greater than said capacity I0 so that the impedance of the earth connection -via the differential condenser forming a capacitive parallel connection is small in comparison with the impedance of the grid-cathode path of the mixing electrode-system. Further this capacity I0 is made as small as possible, for example by using a tube having the receiving grid separately led out of the glass bulb. The junction point of the condensers Ia and 'Ib may be adjusted to have earth potential by corresponding adjustmentof the differential condenser with its capacities VI'I and I8. After this has been done, the cathode 8 in itself could be earthed in direct-fashion without the previously adjusted neutralization I2, -I3 being destroyed, and a full volume of reception would take place.

It has been found in practice that in the case of the eddy currents which flow in the metal of the chassis and in surrounding conductors and are very diiicult to control it is not desirable to connect the three poles of the oscillator direct to earth. A very good neutralization, however, may be immediately accomplished if the earthing of 8 is performed with resistances as highly ohmic as possible. By reason of the stated dimensioning of the condensers I'I and I8 and simultaneously using a choke I I, which is to be regarded as highly ohmic in the wave band employed, there is no loss in the volume of reception, as the resistance of the total of Il and I8 is smaller than the resistance of I0. The powerful currents of the oscillator, however, do not flow by way of the differential to earth and accordingly do not cause any interfering coupling.

The arrangement of Fig. 2 has the disadvantage that two bridges have to be equalized at the same time, viz., the neutralization bridge I3, I2 and the earthing bridge I'I, I8. The arrangement of Fig. 3 is free of such disadvantage, in that only a single equalizing operation is necessary. Condenser I2 is eliminated and its function is undertaken by the already existing condenser IIJ by so adjusting the differential condenser II, I8, I9, that cathode 8 is not accu- `rately at earth potential, but has a small oscillation amplitude in reverse phase to the mixing grid I4 and earth. If the. rotor I9 of the differential condensers Il and I8 is displaced in the manner shown in Fig. 3 in relation to its neutral position, so that the capacity on the grid side is somewhat greater than the capacity I8 on the anode side,the cathode 8 passes into the correct potential range useful for compensation purposes, in which it is capable, by way of the grid capacity I0, of balancing out the receiving grid 3 and accordingly the aerial 4 against radiation. There is no loss with respect to the sensitivity of reception, Yas the received currents, proceeding from 3, are able to find their way to` earth without resistance by way of the parallel connection of the series 1b, I1 and 1a, I8, since in accordance with the invention the parallel connection of I'I and I8 should be large in relation to I0.

It has been found that a circuit according to Fig. 3 may be adjusted very well by the turning of one single armature I9 of the differential condenser connected with earth. At the same timev it appears, however, that it is not immaterial at what point the earthing of the armature I9 is connected. The stated stray currents in the chassis are apparently the reasons for this indefinite nature of the earthing point I9. To render also,V these ineffective it is proposed to enclose the oscillatory circuit', including its coil 25 and the differential I'I and II8, in a screening casing 20, which may also include the receiving tube I. This casing should be in direct connection with the rotor armature I9 of the differential condenser II, I8. It should, however, be insulated itself against the earth of the chassis 2l and connected therewith only by means of a single lead 22. In,

this way it is accomplished that the comparatively! powerful induced currents flowing in the wall of the screening means 29, together with their appertaining drop in potential, do not enter the circuits of the compensation bridge according to Fig. 34 '1"' portion 1b. Now in the case of three-electrode tubes the inner resistance of the anode path is comparatively low, whilst the grid resistance of these tubes is very high. There accordingly results an interference with the freedom of phaseshifting of the condenser 1a, 1b. .The procedure of compensating in accordance with the invention may be supported by the following additional means:

l. Inclusion of a parallel resistance 23 between grid-condenser portion "ib (Fig. 3). This resistance would simulate, and accordingly render ineffective, the phase shifting in the grid circuit caused by the tube.

2. Inclusion of a series resistance 2 inrthe lead to the control grid (entered in Fig. 1). The effect is the same as under 1.

3. The use of a four-electrode or five-electrode tube as oscillator tube. Tubes of this nature have in themselves a sufficiently large anode impedance and do not interfere with the equilibrium of the bridge. This method is superior to the others mentioned as regards reliability, particularly when exchanging the tubes, and is also largely independent of the particular currents and biases flowing in the tube.

I claim:

1. In a superheterodyne receiver means for eliminating the interfering radiation of the locally generated oscillations, a mixing tube including a mixing electrode-system and an oscillatory electrode system, an oscillatory circuit connected to the oscillatory electrode system, a neutralizing circuit between said oscillatory system and said mixing electrode system, said oscillatory circuit consisting of a combination of two condensers and an oscillatory choke and being connected between the anode of said oscillatory electrode systern and the cathode common to both said electrode systems, said cathode being connected to the junction point of said two condensers, said oscillatory circuit being connected with its two poles having a potential of opposite phase to earth by means of a, differential condenser the rotary electrode of which being connected with earth and being adjustable so that the potential of the cathode is variable for balancing out the neutralizing circuit as regards the local oscillations, the capacities of the differential condenser lying in parallel connection to the oscillatory circuit having a value equal to or greater than the capacity between receiving grid and cathode, the impedances of said earth connections via the differential condenser being small in comparison with the impedance of the grid-cathode path of the mixing electrode-system in respect to the received frequency.

2. In a superheterodyne receiver means for eliminating the interfering radiation of the locally generated oscillations, a mixing tube including a mixing electrode-system and an oscillatory electrode system, an oscillatory circuit connected to the oscillatory electrode system, a neutralizing circuit between said oscillatory system and said mixing electrode system, said mixing electrodesystem containing at least one screening grid, said oscillatory circuit consisting of a combination of two condensers and an oscillatory choke and being connected between the anode of said oscillatory electrode system and the cathode common to both said electrode systems, said cathode being connected to the junction point of said two condensers, said oscillatory circuit being connected with its two poles having a potential of opposite phase to earth by means of a differential condenser the rotary electrode of which being connected with earth and being adjustable so that the potential of the cathode is variable for balancing out the neutralizing circuit as regards the local oscillations, the capacities of the differcircuit between said oscillatory system and said mixing electrode system, said oscillatory circuit consisting of a combination of two condensersand an oscillatory choke and being connected between the anode of said oscillatory electrode system and the cathode common to both said electrode systems, said cathode being connected to the junction point of said two condensers, said oscillatory circuit being connected with its two poles having a potential of opposite phase to earth by means of a differential condenser the rotary electrode of which being connected with earth and being adjustable so that the potential of the cathode is variable for balancing out the neutralizing circuit as regards the local oscillations the capacities of the differential condenser lying in parallel connection to the oscillatory circuit having a value equal to or greater than the capacity between receiving grid and cathode, the impedances of said earth connections via the differential condenser being small in comparison with the impedance of the grid-cathode path of the mixing electrode-system in respect to the received frequency, said neutralizing circuit containing a variable condenser for adjusting neutralization of radiation.

4. In a superheterodyne receiver means` for eliminating the interfering radiation of the locally generated oscillations, a mixing tube including a mixing electrode-system and an oscillatory electrode system, an oscillatory circuit connected to the oscillatory electrode system, said oscillatory circuit consisting of a combination of two condensers and an oscillatory choke and being connected between the anode of said oscillatory electro de system and the cathode common to both said electrode systems, said cathode being connected to the junction point of said two condensers, said oscillatory circuit being connected with its two poles having a potential of opposite phase to earth by means of a differential condenser the rotary electrode of which being connected with earth and being adjustable so that the potential of the cathode is variable for balancing out they neutralizing circuit as regards the local oscillations, the capacities of the differential condenser lying in parallel connection to the oscillatory circuit having a value equal to or greater than the capacity between receiving grid and cathode, the impedances of said earth connections via the diierential condenser being small in comparison with the impedance of the grid cathode path of themixing electrode-system in respect to the received frequency, the rotary electrode of said differential condenser being adjusted to such a setting that the cathode of the mixing tube has a reverse phase oscillation of small magnitude suicent to neutralize the radiation of local oscillations, the bias ofV said cathode being conducted thereto by Way of a choke which has a very high ohmic resistance with respect to the received frequency.

5. In a superheterodyne receiver means for eliminating the interfering radiation off the loically generated oscillations, a mixing tube including a mixing electrode-system and an oscillatory electrode system, an oscillatory circuit connected to the oscillatory electrode system, said mixing electrode-system containing at least one screening grid, said oscillatory circuit consisting of a combination of two condensers and an oscillatory choke and being connected between the anode of said oscillatory electrode system and the cathode common to both said electrode systems, said cathode being connected to the junction point of said two condensers, said oscillatory circuit being connected with its two poles having a potential of opposite phase to earth by means of a differential condenser the rotary electrode of which being connected with earth and being adjustable so that the potential of the cathode is Variable for balancing out the neutralizing circuit as regards the local oscillations, the capacities of the.

dierential condenser lying in parallel ,connec- -tion to the oscillatory circuit having a value equal to o-r greater than the capacity between receiving grid and cathode, the impedances of said earth connections via the differential condenserv being small in comparison With the impedance of the grid cathode path of the mixing electrodesystem in respect to the received frequency, the rotary electrode of said differential condenser be'- ing adjusted to such a setting that the cathode' choke which has a very high ohmic resistancel with respect to the received frequency.

KURT SCHLESINGER.

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