US2097918A - Magnetron detector - Google Patents

Description

Nov. 2, 1937. K FRn-z i 2,097,918

MAC-NETRON DETECTOR Filed Dec. 23, 1935 in /e efe/een maj/z and 9 cross/Avd.: A my 7 Y ATTORNEY.

Patented Nov. 2, 1937 UNITED STATES PATENT OFFlCE MAGNETRON DETECTOR Germany Application December 23, 1935, Serial No. 55,726 In Germany December 20, 1934 3 Claims.

This invention relates to magnetron detectors and is, of course, useful in receiving circuits.

It is generally known that a magnetron discharge tube having split anodes may be used as 5 a detector. However, the detector eifect is very low when the tube is used according to the teachings of the prior art. The magnetron tube was, therefore, thought to be better suited for use as an oscillator and in transmitting circuits.

Since the trend of practice in the use of ultrahigh frequency circuits is toward high-power output and toward greater efliciency of reception, every effort has been made in the past in designing magnetron tubes to adapt them for use with very high plate potentials. Such adaptation of the tubes requires that they shall be of large proportions. The dimensions of the electrode elements were made greater in order to provide a suicient cooling effect. It was assumed, 20 somewhat erroneously perhaps, that it would be impossible to operate a magnetron at relatively low anode potentials and with small anode radii if the object to be obtained was to operate such a magnetron as a self-oscillating detector having damping reduction. On the other hand,`even when the tubes were operated with high voltages up to several thousand volts, the detected currents were found to be negligibly small in relation to the total plate current. It is, therefore, an object of myV invention to provide a magnetron detector system in which the detector action is noticeably improved.

My invention will be best understood from the following detailed description when considered in View of the accompanying drawing in which Figure l shows a conventional magnetron circuit diagram which is referred to in explaining the principles of my invention;

Figs. 2 to 6 inclusive show characteristic performance curves which are also referred to in explaining my invention; and

Fig. '7 shows a preferred magnetron circuit diagram of my invention.

Before entering into a detailed description of the invention itself, the requirements for a favorable receiver circuit and detector tube will rst be discussed. Such tube is required:

1. To carry out a damping reduction of an input circuit tuned to a receiving wave, or deviating 51'0 therefrom by the value of an intermediate frequency;

2. To provide a detector effect of suflicient amplitude for favorable reception of signals.

From past experience it has been found thaty 55: magnetron discharge tubes operate more eniciently in transmitters than in receivers. The diiiiculty encountered when operating the magnetron as a detector appears to be that where the direct current voltages are applied symmetrically at nodal points with respect to the high frequency alternating currents to be demodulated, then the resulting variations in electronic emission become negligible in value. This is due to the fact that as the discharge tube oscillates a cloud of electrons strikes first one anode segment and then another, but the total number of electrons in the cloud remains substantially constant. Thus it becomes-necessary to find some method by which the emission current can be substantially varied under control of the input signals to be de-modulated.

Figure 2 illustrates the matter under discussion. In this diagram the current drawn from one anode segment is represented by the curve I1, whereas the current drawn from an opposite anode segment is represented by the curve I2. The algebraic sum of these two curves is represented by the horizontal line Ie, which is the value of the current in the operating circuit inter-connecting the anode system with the cathode.

It is obvious that where symmetry exits in the operating conditions as shown in Figure 2 that the tube cannot be made to operate eiiiciently as a detector. The difficulty is still further increased when attempting to operate the magnetron tube under a condition of saturation of the emission current-plate potential characteristic as shown at A in Figure 3.

The idea underlying the present invention resides in causing the current sum Ie to fluctuate as regards time in the rhythm of the low frequency modulation. The receiver circuit according to the invention is characterized by thefact that the magnetron has a symmetrically disposed multi-partite anode and is operatedin the static region of the space charge current.

Favorable results have been obtained where the operating plate potential was decreased to 65% of the saturation potential measured in the presence of the operating magnetic eld. The increase of the saturation potential by means of the magnetic eld will be later referred to.

Cathodes without distinct saturation have been found to be especially suited for carrying out the idea of the invention.

Fig. fi shows how the demodulation of modulated high frequency oscillations may be obtained under more or less favorable adjustments of the direct current anode potential. The curve l theretotal current goes up as the alternating potential in the tuned circuit swings the potential on the i' one anode segment more positive, while on the other anode segment it is swung less positive. This condition occurs because the operating point of ach anode is adjusted to a position such as C on the curve characteristic shown in Fig. 3 where there is a disproportionate increase of current to one anode in relation to the decrease of current in the other anode. Curve 3 represents the current values on the anodes while operating the magnetron tube at the point B of Fig. 3 so that there is a greater decrease in one anode than the increase in the other. The high frequency potentialsj- LZU depend in general upon the Value of the direct current anode potential. Hence, it is important to choose a point on the curve characteristic as shown in Fig. 3 such that the voltage variations on one anode segment shall be disproportionate to the voltage variations in the opposite sense which occur simultaneously on the other anode segment.

For curves 2 and 3, the working point lies in the space charge region of the Ie/Ua characteristic in Fig. 3. It will be readily* seen that a positive or 1negative voltage fluctuation dU produces an emission current which varies proportionately when one working point is chosen, and disproportionately if other working points are chosen. The characteristic depends also upon the geometrical structure and size of the tube. A proper reception is possible with magnetrons whose emission current fluctuates with superposed small voltage fluctuations on the partial anodes, in accordance with curves 2 or 3. Curve 2 of Fig. 4 represents the emission current Ie in dependence upon the 4:5resulting potentials of the partial anodes when the magnetron Works in the region of space charge. If there is applied to one group of anode segments of the magnetron the potential Us-l-dU, and to the other group, the potential IIa-ZU this detector effect can be statically measured. The same relation will be obtained as is obvious, if alternating receiving potentials are superposed on the direct potentials of the part anodes and eventually a course in accordance with Fig. 3 can be obtained.

As the curves 2 and 3 indicate, a distinct double detector effect will be obtained. The highfrequency oscillations therefore will, as may be said, be detected in push-pull. The emission current Ie will therefore be correspondingly larger, in case of self-excitation, than in the non-oscillating state to which curve 2 applies.

The approximate course of the current of the partial anodes is shown in Fig. 5 in relation to the superposed receiving oscillations dU. v

It must be emphasized that the detector effect has no relation to the 'bend of the space charge curve Ie/Ua since a working point adopted on this 0 icurve remains stationary also in the case of production of oscillations. In order to obtain a high detector eifect, the working point is required to be held in the space charge region, i. e. below the upper bend of the emission current-plate 'potential characteristic. The bend as such simply indicates that at this point the space charge region begins.

Hitherto it was held impossible to obtain selfexcitation of a magnetron with small anode radius at a weak magnetic field and low plate potentials.

The present invention makes use of the fact that the plate potential Ua varies with the square of the anode radius ra, namely:

ru: 6.72 H

' wherein H=magnetic field intensity, (see Hulls formula, Barkhousen, Elektronroehren, volume 1, 1931, page 67). When decreasing the anode radius ra the plate potential Ua necessary for the setting up of oscillations will be reduced according to power 2 of Ua.

From the space charge formula:

Uds/2 or when transformed:

16:2 milliamperes (approximately) II=350 Gauss (approximately) ra=1 mm.

In view of magnetic effects or other reasons it is possible to obtain for each wave and especially for the shortest waves, suitable small anode radii.

It is a favorable circumstance that the magnetic field increases the space charge region, or, in other words, that the space charge region is expanded with an increase in the voltage values.

Physically this can be explained by the fact that I the electrons due to the curvature of their courses remain for a longer period of time in the vicinity of the cathode than would be the case without the magnetic field. The expansion of the space charge region towards higher potentials is in the above stated example approximately 100% as regards the voltage values. Magnetrons operated in the expanded space charge region are especially suited for stations with duplex communication in which with a single tube, transmission and reception is to be carried out, since the magnetrons operate with higher plate potentials, thus being accordingly better suited for producing oscillations.

The above named Hulls formula is subject to correction at operation in the space charge region, which correction depends upon the shape of the tube and upon the working conditions, namely:

exceeds the known detector effect of magnetrons by several orders can be further increased by a sort yof low frequency feed back. This can be carried out by means of a circuit in which the detecting current in the plate-,cathode circuit is utilized to increase the entire detector eiect so that an effect will be obtained such as in a low frequency amplification.

This circuit is not to be confused with the reflex circuits often used for ordinary receivers and in which the audio and high frequency potentials are superposed on the same electrodes. In the case of the magnetron, high frequency and audio frequency actions upon the current circuits can be readily separated from each other Without the use of special blocking elements.

The inventive idea resides in the provision of a system wherein the detecting current of the magnetron will be substantially localized in the plate circuit and wherein a further plate current variation is produced in the same sense. In the extreme case, therefore, the anode-cathode circuit undergoes self-excitation in the rhythm of the detector current. As regards audio frequency all anode segments are obviously connected in parallel and operate in synchronism.

In order to influence the current Ie it is necessary to choose such variables as will influence as little as possible and only in the most favorable manner, the detector effect necessary for reception. Most suited to this end is apparently a magnetic cross field. If signal reception were to be carried out in accordance with the method heretofore used, and as above described, the angle u between the constant principal magnetic field and the tube axis would generally be zero degrees. However, by introducing an appreciable angle a, an increase in the plate current results, as seen from Fig. 6 in which the emission current is represented as a function of the angle between the principal magnetic field and the sym metry axis of the electrode system. The increase in the plate current can be explained by the fact that the curved component of the magnetic field decreases so that the electrons `move in paths of smaller curvatures and consequently a larger portion of them arrives at the anodes.

At an angle a=0 the electrons move in planes extending at a right angle to the axis of the electrode system namely in accordance with a curvature corresponding to the respective magnetic eld. Where the angle u differs from zero, the electron paths on the one hand have a smaller curvature and on the other hand, the electrons move in certain cases along helical courses whereby the axis of the helix assumes the direction of the resulting oblique eld, thereby, for the lmost part, reaching the anode after making several complete turns.

A magnetic cross eld produced by the detector current would in the working point W and at yu=0" produce a doubling of the demodulation frequency. Hence it is necessary to choose an angle a that is slightly different from zero, as for instance by turning the tube or by means of an auxiliary direct magnetic field. In this way the working point will be shifted toward W at which place the angle a is different from zero. 'I'hen the demodulation current which, as will be remembered, varies the angle a by small values about the working point W, again controls the electron movement in the correct manner, provided of course that the polarization of the magnetic cross eld takes place in the proper sense. Such a circuit is shown in Fig. 7.

The total plate current Ie passes throughthecoil S which excites the magnetic cross field. Since the coil S is an air gap coil it has but a very low self-induction. It has, preferably, approximately 200 turns, and is about 10 mm. in diameter. Its load is substantially negligible and hence the amplifier action in the range of the modulation frequencies becomes entirely independent of the frequency. The two circuits, namely the high frequency tank circuit LC, and the audio frequency circuit (from the anodes to the cathode K) are practically independent of each other within the angle range to be considered herein.

In Fig. 7, I show a quenching frequency source P coupled to the anode circuit. T is a low pass filter for blocking the high frequency. In circuit with the filter T, I preferably include further ltering means such as the inductance D and the capacitor Q (in parallel therewith) for causing the field coil S of the magnetron to be traversed only by detecting current. A responsive device of any suitable type may be included in the utilization circuit which is shown coupled to the anode circuit through the transformer R. It will be understood, of course, that between the transformer R and the indicator or other responsive device (not shown) any suitable audio frequency amplifier may be provided.

I claim:

1. In a detector circuit for radio reception, an electron discharge tube of the magnetron type having a cathode and cylindrically formed anode segments symmetrically disposed about said cathode, a source of potential applied in circuit between the cathode and anodes and of such value that an unsaturated condition of space charge is produced, said anodes being disposed at a minimum operating radius about said cathode thereby to minimize the value of anode potential required for damping reduction, and a magnetic eld producing device for directing the electron stream in said tube, said device having a coil coupled to the anode-cathode circuit.

2. In a radio receiving circuit, an electron discharge rectifier tube of the magnetron type, a centrally disposed cathode and a split-anode structure within the envelope of said rectifier tube. means including a resonant circuit interconnecting the anode sections for producing oscillations in said tube, an operating circuit eX- tending from a point on said resonant circuit to the cathode, an input device and an output device coupled to said operating circuit, frequency selective impedance connected to said operating circuit between the points of coupling of said input and output devices to said operating circuit and a eld coil for said magnetron shunting an impedance which is disposed in said operating circuit.

3. In a radio receiving system a detector circuit having a magnetron discharge tube whose electrodes comprise a linear cathode surrounded by a plurality of cylindrically segmented anodes, a tuned tank circuit interconnecting oppositely disposed anode segments, an operating circuit connected from the cathode to the mid-point of an inductive impedance forming part of said tank circuit, a source of direct current potential in said operating circuit, and means coupled to said operating circuit for varying the magnetic iiux density within the discharge zone of said tube in response to modulating energy to be detected.

KARL FRITZ.

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