US2027986A - Superheterodyne receiver - Google Patents

Description

Jan. 14, 1936. R, I, KINROSS ,SUPERHETERODYNE RECEIVER Filed Nov. lo, 193s lNvENToR Wm/5 5A/67H RUPERT 1. KxNRoss BY l WW ATTORNEY Patented Jan. 14, 1936 UNITED STATES PATENT OFFICE Electric & Musical Industries, Ltd.,

Hayes,

Middlesex, England, a corporation of Great Britain Application November 1o, 1933, seria1N0.69'1,393 In Great Britain December 19, 1932 8 Claims.

The present invention relates to wireless receivers of the supersonic heterodyne type.

In supersonic heterodyne wireless receivers, oscillations due to a wanted signal are hetero- 5 dyned by oscillations from a local source to produce supersonic beat-frequency oscillations. Usually, the frequency of the oscillations from the local source is made variable, and means such as a tunable input circuit are provided to 1,0 select the wanted signal oscillations while attenuating 4oscillations due to unwanted signals; an amplifier iixedly tuned tothe beat or intermediate frequency is also usually provided.

Receivers of this type are found to introduce IIS a form of signal interference not normally eX- perienoed with other types of receivers. This interference, which is known as image frequency interference, may occur for example when the signal-selecting circuit of a receiver 20 is tuned to the frequency of one of two transmissions which are separated from one another by approximately twice the intermediate frequency of the receiver, or by certain fractions of twice the intermediate frequency. In these 25 circumstances, one signal may be heard as a background to the other, or, if the frequency difference between the wanted and unwanted oscillations fed to the intermediate amplifier lies within the audible range, the interference may 30 manifest itself as a whistle at the difference frequency. The interference is particularly noticeable when the interfering signal is that due to a powerful local transmitting station.

The amount of image frequency interference 435 experienced depends in part on the selectivity of the signal selecting circuit. In broadcast receivers, it is customary to employ some form of band pass filters for this purpose, but since considerations of cost usually limit the number of filter sections employed to two or three, sufficient selectivity cannot normally, be attained to restrict image frequency interference to an unobjectionable amount.

It is an object of the present invention to pro- 45 vide a supersonic heter'odyne wireless receiver in which image frequency interference is eliminated or reduced.

According to the present invention, a supersonic heterodyne wireless receiver comprises an V50 input circuit, a second circuit and a coupling between these circuits such that oscillations are transferred from said input circuit to said second circuit, one or both of said circuits being tunable to the frequency of a wanted oscillation,

(Cl. Z50- 20) capacitive coupling and an inductive coupling for the purpose of feeding into said second circuit oscillations in antiphase to the oscillations transferred thereto by the first-mentioned coupling. Preferably, the magnitudes of both said 5 capacitive and inductive couplings are made variable over suitable ranges.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invenl() tion itself, however, both as vto its organization and method of operation will best be understood by reference to the following description, taken in connection with the drawing in. which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect. v

The invention will be described by way of example with reference to the accompanying drawing in which Fig. 1 is a circuit diagram illustrating a part `of a superheterodyne receiver circuit constructed according to the invention, and

Figs. 2, 3 and 4 are curve sheets used in illustrating the principles involved in the invention.

Referring to Fig. 1, a supersonic heterodyne wireless receiver comprises a screen grid valve I of the indirectly heated type adapted to function both as the local oscillator and the first detector, and has an input band pass filter constituted by two inductively coupled tunable circuits. The first tunable circuit comprises a suitable coil 2 connected in parallel with a variable condenser 3, the aerial being connected to a tapping point on a high resistance potentiometer II. One end of the potentiometer 4 is connected to earth, while the other end is connected to a tapping point in the coil 2 through a small fixed series condenser 5.

The second tuned circuit comprises a coil 6 40 inductively coupled to the coil 2 of the first tuned circuit, and connected in parallel with a variable condenser 'I which is preferably ganged, as indicated by a broken line, with the variable condenser 3 of the first tuned circuit and with 5 the tuning condenser S of the circuit 9 which' determines thev frequency of the local oscillations. The low potential ends ofthe two tunable circuits constituting the input band pass filter are earthed. The cathode of the detector-oscillator valve I is connected to earth through a coil I I] inductively coupled to the coil I I of the tunable circuit 9, and the high potential end of the tunable 55 there being also provided between said circuits a circuit 6, 'I is connected to the control grid I2 55 of the detector-oscillator valve I through a grid leak I3 and grid condenser I4 in parallel.

The anode of the valve I is connected to the positive terminal Il of a source of high tension current through a parallel tuned circuit I8 tuned to the intermediate frequency. The circuit I8 is inductively coupled to a similar circuit I9 and the two circuits form a band pass filter. The terminals and 2| are connected to the input circuit of a suitable intermediate frequency amplifying valve.

Connected between the high potential ends of the tunable circuits 2, 3 and 6, 'I is a small condenser I5, which is preferably variable over narrow limits, and may conveniently be formed by two pieces of stout wire insulated at their ends by rubber sleeving or the like, the insulated ends being secured together in such a manner that the wires are movable in the sleeving to vary the capacity between them. Alternatively, the condenser I5 may be constituted by two screws so arranged that the distance between their heads, which face one another, is adjustable. The effect of this small capacity is to produce in the tunable circuit 6, I impulses of the unwanted frequency in opposite phase to those transferred inductively from the tunable circuit 2, 3 to the tunable circuit 6, 1, .and the value of the capacity can be made such that the two sets of impulses substantially cancel one another. So long as the unwanted image frequency lies outside the b'and to which the filter is tuned, substantially complete cancellation can be obtained.

Since the impedance of the condenser increases with increase in wave-length, the antiphase feed will decrease with increase in wavelength. It is found, however, that the band width of the type of band-pass filter described above narrows with increase in wave-length. Less anti-phase feed will therefore be required for perfect neutralization as the wave-length increases. This condition is satisfied only approximately by the use of the condenser, since the capacity feed from the latter decreases more rapidly with increase in Wave-length than is required by the condition that the band Width is also narrowing. Fullest advantage can only be taken of the capacity feed by arranging its value to be such as to give maximum neutralization towards the lower end of the wave band to be covered by the kind of band-pass filter described above.

Fig. 2 illustrates this point graphically, the amount of interference obtained using capacity feed being plotted vertically, while wave length is plotted horizontally. The capacity of the condenser I5 is preferably made such that when the receiver is arranged for reception within the medium wave band (20G-600 metres) maximum suppression of interference is produced at a wave length of about 320 metres. As will be seen, the amount of suppression obtained decreases to- Wards the upper part of the wave band and there is therefore connected between the aerial series condenser 5 and the coil 2 of the tunable circuit 2, 3, a coil I6 of small inductance which is so coupled to the coil 6 of the tunable circuit 6, 'I as to produce in that circuit impulses of opposite phase to` those transferred inductively thereto from the tunable circuit 2, 3.

'Ihe coil I 6 may comprise two or three turns of wire upon a wooden former of about 11/2 inches in diameter. The coil I6 may be mounted at the side of the coil 6 by means of a screw passing through a hole drilled eccentrically in the former. Both coils are mounted side by side upon the same base with their longitudinal axes parallel. The magnitude of the inductive coupling may be adjusted by rotating the former of the coil I5, the screw being tightened when the correct value is attained.

The sense of the coupling between the coil I6 and the coil E of the second tunable circuit is made such that the two sets of impulses induced into the second tunable circuit (one through the coupling 2, 6 and the other through the coupling I6, 6) tend to cancel out, the effect being arranged to be most marked at the higher wavelengths.

Fig. 3 shows the amount of suppression due to the inductive coupling alone. The magnitude of the coupling is preferably made such that for medium Wave-band reception maximum suppression is obtained at about 500 metres. By a suitable choice of values of the reverse-phase feed capacity and mutual inductance, it is possible to eliminate or reduce substantially interference due to image frequency oscillations over the whole of the Wave band to which the receiver is tunable.

Fig. 4 is a curve showing the combined effect of the capacitive and inductive feed. As before, interference is plotted vertically, and wave length horizontally, and it will be seen that substantially complete suppression of interference is obtained over two ranges of frequencies within the desired frequency band. It'will be apparent, that impulses at the wanted frequency as well as at the unwanted frequency are induced in the tunable circuit 6, 'I by the coupling due to the elements I5 and I6. Over the band of frequencies to which the input band pass lter responds, the amount of anti-phase feed is substantially uniform, and it is permissible to represent its effect as being to lower the response curve of the filter a substantially uniform amount relatively to its original datum line. Thus, by effectively removing or reducing the skirts of the response curve of the input filter, the invention enables interference due to signals within the frequency range of the skirts to be eliminated or reduced.

When the receiver is tunable to more than one wave band, the values of the feed capacity and mutual inductance may be varied by connecting additional condensers and inductances in parallel with those employed for the lowest wave band. The switches for this purpose may be ganged with the usual wave-range changing switch of the receiver.

In supersonic heterodyne receivers such as that described above, in which no high frequency amplifier is provided, and in which the tunable input circuit is connected directly to the first detector, there may be a certain amount of radiation by the aerial of oscillations due to the local source. It will readily be seen that the amount of radiation will also be substantially reduced by the use of the reverse-phase feed condenser I5 and coil I6 which, in this case, operate to eliminate or to reduce the amplitude of oscillations in the first tuned circuit due to the local source.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described,

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but that manymodications may be made with- I5 out departing from the scope of my invention as set forth in the appended claims.

What I claim is:-

1. A supersonic heterodyne wireless receiver comprising an input circuit, a second circuit and a coupling between these circuits such that oscillations are transferred from said input circuit to'said second circuit, at least one of said circuits being tunable to the frequency of a wanted oscillation, auxiliary coupling means between said circuits comprising an inductive coupling and a capacitive coupling said auxiliary coupling means acting to transfer energy from the input circuit to the second circuit in phase opposition with respect to the energy transferred by the first named coupling and of such intensity to substantially buck out unwanted oscillations transferred from the input circuit to the second circuit through the rst named cou pling.

2. A wireless receiver according to claim I wherein the magnitudes of said capacitive and said inductive couplings are made such that substantially complete suppression of oscillations of the unwanted frequency is obtained over two bands of frequencies within the range of frequencies to be received.

3. A supersonic heterodyne wireless receiver comprising an input circuit, a second circuit and a coupling between these circuits such that oscillations are transferred from said input circuit to said second circuit, at least one of said circuits being tunable over a predetermined band of frequencies for signal selection, auxiliary coupling means between said circuits arranged so as to feed into the second circuit oscillations in phase opposition to the oscillations transferred thereto through the rst named coupling means, said auxiliary coupling means having inherent frequency discriminating characteristics due to which the intensity of the energy transferred therethrough is different for different portions of said frequency range and energy transference means between the primary circuit and the second circuit for compensating for the frequency discriminating effect-s of the auxiliary circuit.

4. A wireless receiver, according to claim 3, wherein said second coupling is of a substantially wholly inductive nature.

5. A wireless receiver, according to claim 3, wherein said compensating means comprises a further coupling of a substantially wholly capacitive nature.

6. In a superheterodyne receiver, an input circuit, a second circuit, coupling means between the circuits such that oscillations are transferred from the input circuit to the second circuit, at least one of said circuits being tunable over a predetermined band of frequencies for signal selection, auxiliary coupling means between said two circuits comprising an inductive coupling and a capacitive coupling, said auxiliary coupling means acting to transfer energy from the input circuit to the second circuit in phase opposition to the energy transferred through the first named coupling means, the energy transferred through the capacitive coupling portion of the auxiliary coupling means being of such intensity as to substantially completely suppress unwanted oscillations transferred from the input circuit to the second circuit through the first named coupling means over a range of frequencies which is near one end of said frequency range, the energy transferred through the inductive portion of the auxiliary coupling means being of such intensity as to substantially completely suppress unwanted oscillations transferred from the' input circuit to the second circuit through the first named coupling means over a range of frequencies which is near the other end of said frequency range.

7. In a superheterodyne receiver, an input circuit, a second circuit, means for coupling said circuits so that oscillations are transferred from the input circuit to the second circuit, at least one of said circuits being tunable over a, predetermined band of frequencies to provide signal selection, auxiliary coupling means between said circuits arranged so as to feed into the second circuit oscillations in phase opposition to the oscillations transferred thereto through the first named coupling means, said auxiliary coupling means having inherent frequency selective characteristics due to which the intensity of the energy transferred therethrough varies for different portions of the said frequency range and means connecting the input circuit and the second circuit for compensating for the frequency discriminating effects of the said auxiliary circuit.

8. In a superheterodyne receiver a primary circuit, a secondary circuit, means for coupling said two circuits so that oscillations are transferred from the primary circuit to the secondary circuit at least one of said circuits being tunable over a predetermined band of frequencies to provide signal selection, auxiliary coupling means between said two circuits comprising an inductive coupling and a capacitive coupling each of said couplings being separately adjustable so as to separately control the amount of energy transferred therethrough, said auxiliary coupling means being arranged so as to transfer energy from the primary circuit to the secondary circuit in phase opposition with respect to the energy transferred through the first named coupling means, the energy transferred through the capacitive coupling portion of the auxiliary coupling means being of such intensity as to substantially suppress unwanted oscillations transferred from the primary circuit tothe secondary circuit through the first named coupling means over a range of frequencies which is near one end of said frequency range, the energy transferred through the inductive portion of the auxiliary coupling means being of such intensity as to substantially completely suppress unwanted oscillations transferred from the primary circuit to the secondary circuit through the first named coupling means over a range of frequencies which is near the other en`d of the frequency range.

RUPERT I. KINROSS.

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