US2026759A - Superheterodyne receiver - Google Patents

Description

Jan. 7, 1936. A. H. TURNER SUPERHETERODYNE RECEIVER Filed March 31, 1934 Patented Jan. 7, 1936 q 7 V UNITED. STATES PATENT OFFICE SUPERHEIERODYNE RECEIVER Alfred H. Turner, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1934, Serial No. 718,363

9 Claims. (01. 250-20) This invention relates to radio receivers and is shown for accomplishing the objects of my in more particularly to means for anda method of vention; and

eliminating the effect of undesirable frequency Fig. 2 is a cross-sectional view of a special form modulation and distortion of signals when re of transformer which I prefer to use in obtainceived onavery selective superheterodyne. ing periodic variation in the frequency of the 5 In general, the higher the carrier frequency, local oscillator. the more difiicult and costly it becomes to main Referring now to Fig. l, I show in a more or tain frequency stability of the transmitter. less conventional manner an ordinary radio fre- There are instances in short-wave and ultra quency amplifier stage, a first detector, an inter- 10 short-wave transmission where crystal control of mediate frequency amplifier, and a local oscilla- 10 the transmitter frequency is undesirable for reator with output from which the incoming carrier sons of cost or weight, as in airplane and portable Wave is heterodyned. The incoming radio fretransmitters. In the simplest transmitter conquency wave is preferably amplified by the screen sisting of a modulated oscillator, frequency modugrid tube l I and is heterodyned with the oscilla- 1 lation is unavoidable and is greater the higher tions of a local generator l2. The screen grid the carrier frequency. When received on the tube l3 represents a first detector while another usual selective superheterodyne receiver intended screen grid tube 44 represents the first stage of for sound reception, speech from such a tran'sintermediate frequency amplification.

mitter is nearly or quite unintelligible because the The various coils of my special transformer, as intermediate frequency swings so far beyond the shown in Fig. 2, are represented in the diagram 20 reception band-width. of Fig. 1 by comprehending the same within the Accordingly, it is among the objects of my inbroken line rectangle I5. In this transformer I vention to provide a device in which the incoming provide three windings l6, I1, and [8, which are carrier waves are not only heterodyned by a local preferably embedded in a core offinely com- 25 oscillator, but the frequency of the local oscilminuted iron the particles of which are separated 25 lator is periodically varied at a super-audible from one another by a non-metallic material such frequency to as great an extent as the possible as bakelite. As is well known in the art, there range of frequencies occurring in the transmitter. are numerous compositions of iron with insulat- Another object of my invention is to provide a ing material such as would be suited to this purdevice as aforesaid in which the resulting interpose. The chief characteristics. of such core to mediate frequency current will vary continuously material are that its permeability may be caused in frequency and amplitude at a super-audible to have a value of approximately 10, and it does q cy i not introduce appreciable loss even at very high In carrying out my invention I find that the radio frequencies. It is, therefore, well suited to conversion efficiency, although not as great as the conditions under which my systemis intended when the oscillator frequency remains constant, to operate. is, nevertheless, quite satisfactory due to the in- Referring again to Fig. 1, it will be seen that crease in the amplification which is obtained by the tube 12 produces oscillations due to the inusing a sharply tuned intermediate frequency tercoupling of the grid coil l6 and the plate 40 amplifier instead of one that is broadly tuned. coil 11. A third coil I8 is inductively related to 40 The intermediate frequency amplifier does not the coils l6 and H, a core 25 common to all three accept all of the output of the first detector, but coils being provided. As previously stated, the this loss is partially or completely made up by the core 25 is composed of finely comminuted iron greater ratio of amplification. The use of a and. a binder. The coil I8 constitutes a pick-up sharply tuned intermediate frequency amplifier device for transferring some of the oscillator 45 has an additional advantage in that it is possible energy to the grid circuit of the first detector to take full advantage of a frequency stabilized tube I3 where it heterodynes with the incoming transmitter, if the receiver oscillator be returned carrier wave. to itsnormal unmodulated condition. In order that the frequency of my local oscil- Myinvention in its more detailed aspects may lator may be swung gradually over a range at best be understood from the following description least as great as the possible variation in frewhen read in connection with the accompanying quency of the incoming carrier wave, I prefer to d aw ng in wh 7 l use an auxiliary oscillator comprising an electron Figure 1 shows a circuit diagram of a portion tube l9 having an external grid circuit which .7

Of a Superheterodyne receiver in which apparatus includes the coil 20 and an external plate circuit 55 which includes the coil 2!. The coils 20, 2! and an output circuit coil 22 are mutually intercoupled by means of a conventional laminated iron transformer core. The frequency of the oscillator I 9 is preferably made super-audible, al though at a much lower frequency than that of the incoming carrier wave.

The oscillatory energy induced in the coil 22 is transferred to the coil 23 which surrounds the structure including the powdered iron core 25, and its'transformer coils l6, I! and I8. Energization of the coil 23 induces magnetism in a field occupied by the core 25 and by the laminated iron shell 24. The degree of saturation may be controlled by controlling the amplitude of the oscillations. One device which enables this adjustment to be made manually comprises an adjustable potentiometer 26 in the anode circuit of the oscillator IS.

The auxiliary oscillator l9 serves to wobble or vary the frequency of the local oscillator l2 by varying the flux density in the core 25 and hence the effective inductance of the coils l6 and il. Furthermore, the range of frequencies within which the oscillator l2 may be wobbled may be easily controlled according to the extent of the frequency excursions of the transmitter, this object being gained by manual adjustment of the potentiometer 26, which in turn controls the amplitude of oscillations derived from the oscillator l9. 7

As a modification of my invention, I find it possible to dispense with the auxiliary oscillator 59 and to produce frequency modulation of the local oscillator I2 by causing the latter to become selfquenching. One way to accomplish this is to choose a greater capacitance for the grid condenser 21. Another way is to increase the resistance of the grid leak 28. Either way, or the adoption of both ways is with the object of obtaining a time constant for the grid leak-condenser combination such that the tube l2 will generate oscillations intermittently. The interruption frequency should, of course, be superaudible. According to this modification of my invention less desirable results are obtainable than according to the preceding disclosure. The reason for this is that no satisfactory method has as yet been found for avoiding extreme am plitude-modulation of the locally generated oscillations along with their frequency modulation. Notwithstanding this disadvantage, however, it is possible in certain circumstances to obtain satisfactory reception of voice signals andwith a freedom from distortion that is quite remarkable, considering the difficulties usually encountered when frequency control of the transmitted carrier wave is not all that it should be.

In the operation of my improved radio receiver it will be understood, therefore, that it is possible to overcome the difiiculties heretofore encountered. Distortion of the received signals is avoided in spite of poor frequency control at the transmitter. The continuously varying frequency of my local oscillator is such that a response may be had during some portion of every frequency excursion thereof. The sharpness of tuning of the intermediate frequency amplifier is-such that the signals will be rejected when not properly heterodyned. I have found that my improved radio receiver and the improved method of operating it are such that very satisfactory results may be obtained even when receiving signals from a transmitter comprising a voice modulated oscil lator operating at 100 megacycles.

1. In a radio receiver, means including anoscillator for heterodyning an incoming amplitude modulated carrier wave, means including a second oscillator for cyclically varying the frequency of the first oscillator to provide a relatively wide frequency band heterodyned wave, and a sharply tuned intermediate frequency amplifier adapted to selectively accept a portion of the heterodyned wave which is in a substantially undistorted narrow frequency band including .the amplitude modulation.

2. In a radio receiver, a network containing a generator of local oscillations and means for hetcrodyning said local oscillations with an incoming amplitude modulated carrier wave, means including a magnetic core structure of finely com minuted iron embedded in insulating material for inductively relating difierent portions of said network, means including a second oscillator having a utilization circuit which includes a coil surrounding said last named means and core structure for cyclically varying the fiux'density of said core structure thereby to vary the frequency of the first mentioned generator and the width of the heterodyned wave band, between predetermined wide limits, and an intermediate frequency amplifier tuned to accept a portion of the last named band between said limits.

3. A device in accordance with claim 2 comprising means for manually adjusting the band width of the frequency excursions made by the first mentioned oscillator.

4. In a radio receiver, a network comprising radio frequency stages, intermediate frequency stages and a heterodyne oscillator having a plurality of inductively related coils embedded in a core of high magnetic permeability, means including a second oscillator for periodically varying the flux density of said core and for wobbling the frequency of the heterodyne oscillator, and means for sharply tuning said intermediate frequency stages whereby they are caused to accept a portion only of the heterodyned wave thereby to discriminate in favor of undistorted signals and against signals which are distorted by undue frequency modulation accompanying amplitude modulation of a received carrier wave.

5. The method of reducing the distortion of intermediate frequency signals in a receiver the intermediate amplifier stages of which are relatively sharply tuned, which includes generating local oscillations at a super-audible frequency, causing said oscillations to vary the flux density of a magnetic field, superimposing within said field oscillations which are locally generated and oscillations of a received carrier wave, and causing the variations in flux density of said magnetic field to frequency modulate the locally generated oscillations over a band substantially commensurate with the frequency excursions of said received carrier wave, whereby a relatively wide band of intermediate frequency signals may b derived only a portion of which band is intermittently acceptable to said intermediate amplifier stages.

6. Radio receiving apparatus adapted for substantial elimination of distortion efiects comprising radio-frequency, first detector, and intermediate frequency stages, means including a heterodyne oscillator the output energy from which may be combined with that of a received carrier wave to produce a difference frequency, a transformer having a plurality of windings embedded in a substantially closed structure composed of iron dust and a binder, two of said windings being in circuit with said heterodyne oscillator and another of said windings being in the input circuit for said first detector stage, means including a supersonic frequency oscillator for varying the flux density in said iron dust, thereby to frequency-modulate the energy of said heterodyne oscillator, manually adjustable means for controlling the frequency band width of the heterodyne oscillator, and means for sharply tuning said intermediate frequency stages so that only substantially undistorted amplitude modulations of a predetermined relatively narrow band of intermediate frequency signals are accepted.

'7. In a. superheterodyne receiver for anamplitude modulated carrier wave, the combination with a signal detector of a tunable local oscillator, means for cyclically varying the frequency of said local oscillator about a predetermined mean frequency adjustment to provide wide frequency excursions proportional to variations in the frequency of a received carrier wave and a relatively wide band of intermediate frequency signals, and an intermediate frequency amplifier tuned to accept a relatively narrow amplitude modulated and undistorted portion of said intermediate frequency band.

8. The combination with a superheterodyne receiver for amplitude modulated carrier wave signals comprising a. tunable local oscillator, a first detector and an intermediate frequency amplifler, of means for preventing variations in the frequency of a received carrier wave from causing signal distortion, said means comprising inductive coupling elements for said oscillator and detector, a common core member for said elements, a coupling winding associated with said core, a second oscillator having an output circuit connected with said winding to cyclically vary the permeability of the core member and the frequency of the first named oscillator about a predetermined mean frequency to provide a relatively wide band intermediate frequency signal, and at least one tuned circuit in said intermediate frequency amplifier for accepting only a'minor portion of said wide band intermediate frequency signal.

9. In a superheterodyne radio receiving system for amplitude modulated carrier wave signals, the combinationof a first detector, an oscillator tunable within a predetermined frequency range and including inductive windings, a coupling winding for said detector associated with said last named windings, a common core for said windings, means for cyclically varying the permeability of said core at a superaudible frequency comprising a second oscillator having a coupling winding associated with said core, means for adjusting said last named oscillator to vary the permeability of said core and the frequency of the first named oscillator about a mean frequency in a band of frequencies proportional to the variation in frequency of a received carrier wave, and a tuned intermediate frequency amplifier connected with said detector sharply responsive to intermediate frequency signals in a relatively narrow band with respect to the band of intermediate frequencies produced by interaction of said first named oscillator and a received carrier wave, thereby to amplify a selected amplitude modulated portion of the intermediate frequency signals.

ALFRED H. TURNER.

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