US2171679A - Heterodyne receiver - Google Patents

Description

Sept. 5, 1939. o. WOHLFARTH El AL HETERODYNE RECEIVER Filed April 21, 1937 INVENTORS Q WOl/ZFARTH AND A TTORNE Y AAlAlAA 'vvvvvv I l I I I l I I I l I I I I I I I All I IHIHIHHI Patented Sept. 5, 1939 UNITED STATES HETERODYNE RECEIVER Otto Wohlfarth and Willi Patzschke, Berlin, Germany, assignors to Allgemeine Elektricitatz Gesellschaft, Berlin, Germany Application April 21, 1937, Serial No. 138,110 In Germany May 16, 1936 2 Claims.

The fundamental operation of heterodyne receiver apparatus known in the prior art in a general way is approximately as follows:

The antenna input circuit consists of a tuned simple or multiple circuit to which the mixer tube is connected. The heterodyne frequency fed to this tube is generated with an oscillation circuit whose rotary condenser is actuated jointly with that of the first or input circuit. The beat frequencies resulting from the mixture are further fed to circuits tuned to a fixed intermediate frequency and are amplified.

After rectification the audio component is fed through additional amplifier stages to the loudspeaker. Now, inasmuch as the requisite synchronism between the incoming signal frequency and the oscillator frequency is securable only with the expenditure of certain amounts of money and means, and even then only approximately, the suggestion has been made to insure the tuning of heterodyne receiver apparatus only by the oscillator frequency, the input circuit then being aperiodic. The oscillator frequency is suitably so chosen that there results a comparatively high intermediate frequency. The selectance of such a receiver is comparatively poor, in short, it does not measure up to what is attainable with other circuit organizations and with a similar amount of technical and circuit means.

Another drawback of these circuit organizations is that in the input end of the receiver, filters or networks are provided of necessity, in order that whistling points, ambiguities, and the reception of short-wave transmitters may be avoided.

Now, in order to overcome all of these difficulties, it is suggested according to the present invention to leave the oscillator frequency unaltered and to tune the input circuit; since this results in a variable intermediate frequency, the intermediate frequency portion must be designed so as to accommodate in the amplifier a broader band. The advantages which results in a given instance is that an outfit thus built has at least the selectivity, with the same number of tubes, as a tuned radio frequency receiver, or nonheterodyne set.

The circuit organization distinguishes itself from those heretofore known in the art in that no rotary condenser is used for the tuning of a circuit which does not contribute to an improvement of the selectivity. As compared with circuit organizations of the kind before mentioned having an aperiodic input, there are in addition obtained the following merits:

In order that the apparatus known in theart may be balanced to a given scale or dial, tuning is required in at least two circuits, namely, that of the oscillator and that of one or more intermediate circuits. In fact, it is only when both have been balanced and set with the requisite amount of care the indication will be sufiiciently accurate and the sensitiveness be high enough. In the solution herein disclosed and above referred to, adjustment of only one circuit is necessary, and yet at least the same selectivity and the same accuracy of tuning will be insured. The receiver could also be furnished with two tuning circuits synchronized or inter-locked either by that both are designed and connected as input band-pass filters or that the receiver comprises a tuned input stage. Contradistinct to the two-circuit straightahead receiver sets known in the earlier art, the double advantage is here secured, namely, that manipulation is simplified owing to the fact that feedback is dispensed with, and that a satisfactorily functioning automatic volume control is obtainable without any trouble.

Three exemplified embodiments of the heterodyne receiver apparatus here disclosed are shown in the accompanying drawing.

In the circuit arrangement of Fig. 1, l denotes a combined detector and oscillator or mixing tube, the input circuit 2 being tuned by condenser l0 to the desired carrier frequency, the local oscillation frequency being determined by circuit 3 which is preferably tuned to a fixed frequency for a single wave band. The intermediate frequency currents appearing in the plate circuit of tube l are fed through the aperiodic coupling means shown to the signal control grid of tube 4. The amplified signals are impressed across the plate H and cathode l2 of a diode detector tube 5, the detected component being further amplified in the audio frequency amplifier tube l3 whose plate circuit I4 is coupled to the loudspeaker l5.

With a view to improving the selectivity and amplification, the circuit arrangement of Fig. 2 provides a tuned radio frequency stage l6 between the antenna and the resonant signal circuit l of the mixer or first detector tube I. The variable condensers which serve to tune grid circuit 6 and anode circuit 1 to the signal frequency may be operated by a single control means indicated at IT. The aperiodic coupling, which is designed to transmit the entire band of intermediate frequency currents pr oduced as circuit 6 is adjusted throughout its tuning range, includes an inductance l8 and a condenser l9 and a resistor 20 connected across the anode and cathode of rectifier tube 5.

The circuit arrangement shown in Fig. 3 is similar to that of Fig. 1 except that a tuned band pass filter comprising the resonant circuits 2 and 22 areinterposed between theantenna and control grid of tube l, the variable condensers of which are preferably controlled by a single operating means indicated at 23.

While in the examples shown the oscillator frequency remains fixed for receiving all frequencies within a given wave band, it is within the scope of the present invention to adjust it to a different value for each wave band while maintaining it constant for all frequencies within a single wave band. It is also within the scope of this invention to choose the frequency of the local oscillator so high that the variation of the intermediate frequency resulting for reception over a wave band amounts to only a Smallfraction of the oscillator frequency.

Having described our invention, what We claim as novel and desire to secure by Letters Patent is: l. A radio receiver comprising the combination of a tube having a cathode, a signal grid, an oscillator grid anda plate, an input circuit tunable over a range of signal frequencies connected between said signal grid and cathode, means for varying the potential of said oscillator grid at a fixed frequency, an intermediate frequency amplifier designed to amplify a wide band of frequencies and having a control grid, a cathode and an anode, a resistor connected to said plate and adapted to have one end connected to a source of positive potential, and a condenser having one side connected to said plate and its other side to the control grid of said amplifier,

sa'id condenser having a low impedance to all currents of the difference frequencies between the signal frequencies and the fixed oscillator frequency.

2. The Vmethod of receiving radio signals covering a band of frequencies which comprises the steps of -generating a fixed oscillator frequency, combining the same oscillator frequency successively with the frequencies of said band, aperiodically amplifying currents of the resulting intermediate frequencies and deriving a direct current voltage from the amplified currents and controlling the amplitudeof said resulting intermediate frequencies with said derived voltage.

WILLI PATZSCHKE. OTTO WOHLFARTH.

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