US1960984A - Radio frequency receiver - Google Patents

Description

y H. A. WHEELER 1,960,984

RADIO FREQUENCY RECEIVER Filed April 5, 1933 RADIO RECf/ YER 19f! PONY/K. T0 1. E

[9 AVC INVENTOR HAROLD A.WHEELER ATTORNEYS Patented May 29, 1934 PATENT OFFICE 7 1,960,984 namo FREQUENCY RECEIVER Harold A; Wheeler, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application April 5, 1933, Serial No. 664,480

10 Claims. (Cl. 250-20) The present invention relates to the selection of a radio-frequency signal being transmitted on one carrier frequency and the simultaneous reduction of interference being transmitted on other frequencies not greatly .different from the frequency of the desired signal, More particularly, this invention relates to an image frequency suppression system for permitting substantially uniform image frequency suppression in two or more frequency hands.

This application is a continuation in part of the copending application of Harold A. Wheeler for Selective circuits, Serial No. 590,173, filed February 1, 1932.

The invention is particularly applicable to superheterodyne radio receivers. It is well known that the superheterodyne receiver is particularly sensitive to voltages of two frequencies when set to receive any given frequency. The

the amount of the intermediate frequency to which the intermediate-frequency amplifier is permanently tuned.

It is common practice to employ a signal-selecting system to amplify the desired frequency of the two frequencies to which the superheterodyne receiverresponds, the desired frequency herein being called the signal frequency, and which signal-selecting system attenuates the undesired carrier frequency or disturbance occurring at the other frequency to which the recelver is particularly responsive, herein called the image frequency.

The attenuation of the image frequency relative to the signal frequency may be increased by simultaneously tuning several selective circuits to the signal frequency. However, if the station transmitting at the image frequency comes in rather strong, it is difficult to differentiate between the signal and image frequencies. Furthermore, in a receiver which is adapted to operate over several frequency bands, the necessity for altering'the constants of several radiofrequency tuned circuits increasesthe disadvantage of this type of image frequency attenuation.

Itv is the object of the present invention to overcome the above-noted objections to the superheterodyne radio receiver and provide a cirtwo frequencies differ from each other by twice.

frequency suppression throughout each of said bands.

These and further objects of this invention will become apparent from the following specification and claims taken in connection with the appended drawing.

For accomplishing the objects of this invention, there is provided between the antenna circuit and the input terminals of the receiver, a pair of transfer means for each of the several '65 ranges through which the receiver is adapted to be tuned. The first transfer means includes a tunable circuit which selectively transfers a voltage of the signal frequency and, incidentally, also transfers a voltage of the undesired image v frequency, though the latter is considerably attenuated. The second transfer means non-selectively transfers voltages of all image frequencies corresponding to the signal frequencies within the band through which the receiver is adjusted to operate. The intensity of this image frequency voltage from the second transfer means is proportioned to be the same as that of the image frequency voltage incidentally transferred through the selective transfer means. 30 The image frequency voltages from the two transfer meansare applied in opposite polarities to the input terminals of the first tube of the receiver, and therefore image frequency response is balanced out in said tube. Although the sec- 35 0nd transfer means, incidentally, also transfers a voltage at the signal frequency, the first transfer means effects a much greater transfer than the second at this frequency, so that the second means has but little effect upon the transfer of the desired signal frequencies. Also, the signal frequency voltages transferred by these two means are not in opposition at the desired signal frequency.

The circuits are so designed and proportioned that the frequency of complete suppression varies automatically with the resonant frequency of the tunable circuit.

The theory underlying this part ofthe invention has'been described in complete detail in the mo copending application above referred to and will not therefore be further described in this application.

Provision is made for altering the tuninsband of the tunable transfer means so that the variable 5 tenna circuit to make the antenna circuit broadly resonant in the selected signal frequency band. The non-selective image transfer between the antenna and the input terminals is thus altered by the second provision to give optimum or nearly optimum suppression of image interference with signals in the band selected'by the first provision.

Optimum suppression is that obtained when substantially equal and opposite image voltages are transferred by the two respective transfer means.

This system results in a great attenuation of the' image frequency over and above thatrdue to the selectivity of the tunable circuit in each of the several bands through which the receiver is designed to operate with image suppression. It is an inherent feature of this invention that the incidental non-selective transfer of signal frequency voltage generally causes substantially no change in the sensitivity ofthe receiver to the desired signal.

Attention is now invited to the accompanyingdrawing in which the single figure shows the antenna circuit and tunable circuit associated with a superheterodyne receiver.

In this figure, the operation of which will first be described with the switches 25 and 26 open, and the system operating in the lower of its two frequency bands, the antenna circuit includes the antenna 10, the trap circuit 12, series condenser 13, series coils 14 and 15, fixed coupling condenser 16, and the ground terminal 11. The trap circuit 12 is tuned to the intermediate frequency to prevent the direct reception of currents of that frequency. The resistor28 is connected across part of the antenna circuit. Constants of the antenna circuit are so chosen that the'entire antenna circuit is broadly resonant at about the middle of the lower frequency band,

the broadness of resonance being determined largely by the resistor 28.

The variable tuning circuit 17 comprises the coil 18 and the variable condenser 19, and in addition, the coil 15 and the condenser 16 which are common to the antenna circuit. The fixed condenser 16 has a maximum reactance which is about one-tenth to one-thirtieth the minimum reactance of either of elements 18 or 19 of the parallel tunable circuit 17, in each frequency band.

The coupling between the antenna circuit and the tunable circuit 17 is composed of the coil 15 and the condenser 16. These two couplingcomponents are in opposition by virtue of their opposite kinds of reactance. At all image frequencies, corresponding to the tuning range, the coil 15 has a reactance less than that of the condenser 16. The effect of the coil 15 is material at the higher frequencies; but is small or negligible at the lower frequencies.

The parallel tunable circuit 17, coupling coil 24, coil 22, and biasing resistor and condenser 21 comprise the input circuit of the receiver and are connected in series between the input terminals A-A of the receiver, namely, the grid and cathode of the modulator or first detector tube 20. The parallel tunable circuit constitutes the first or selective transfer means between the antenna and input circuits.

The coil 22 is inductively coupled to a voltage source 23, the frequency of which is varied, with the tuning of the circuit 17 by the condenser 19, but which remains at a fixed frequency difference relative thereto equal to the intermediate frecal linkage indicated by the dotted lines.

quency to which the intermediate-frequency amplifiers of the receiver are tuned. The'frequency .of the source 23 is preferably higher than the coupling constitutes the non-selective transfer means. 1

By this arrangement, two transfer means are provided between the antenna and the modulator tube. The first means includes the parallel tunable circuit 17 which is adapted to selectively transfer a voltage of the desired signal frequency and which attenuates voltages of the image frequency. The second transfer means comprises the mutual inductance between coils 14 and 24, which means non-selectively transfers a voltage from the antenna circuit to the input terminals of the modulator 20. The characteristics of these two transfer means are so chosen that the voltages thus impressed between the cathode and the tunable circuit 17 are opposed to those transferred through the tunable circuit 17 and are equal at the image frequency.

The relative proportions of the two components of the coupling between the antenna and the parallel tunable circuit 17 are such that the alignment between the image frequency and the frequency which is suppressed to the greatest degree by means of the two transfer means above described is exact or nearly exact throughout the entire tuning range, thus giving optimum or nearly optimum image suppression.

The circuit and features above described are, as has been mentioned above, those which are involved when the receiver is adjusted to operate in the lower of two frequency ranges. When it is desired to operate the same circuits over the higher of two frequency ranges, the switches 25 and 26 are simultaneously closed by a mechani- Closure of switch 25 short-circuits a portion of the coil 18, to permit the condenser 19 to tune the parallel tunable circuit 17 over the high frequency band, and closure of switch 26 connects the auxiliary image frequency coil 27 substantially in parallel with the coil 14. The inductance of coil 14 is substantially greater than that of coil 27, and the larger portion of the current will fiowthrough the latter coil and coils 14 and 15 will have relatively little effect on the operation. The total mutual inductance, however, is chosen to give substantially optimum image suppression with the switches 25 and 26 in the closed position, for permitting operation in the highfrequency band. The image frequency suppression is obtained in this band in substantially the same manner as described above for the low frequency band. However, the inductance of the coil 27 is so chosen that when it is included in the antenna circuit as when operating within the high-frequency band, the entire circuit will be broadly tuned at about the middle of the highfrequency band.

The output of the modulator 20 is connected to the input of the radio receiver 29 which is, as indicated, responsive to currents of the intermediate frequency. This portion of the receiver, which is well known in the art and therefore need not be further described, amplifies and reproggces the signals supplied to it by the modulator The junction of the coil 15 and condenser 16 of grid-bias voltage in the receiver 29. This source is preferably varied automatically in an automatic volume control (AVG) system, the spe-' ciflc details "of which constitute no part of the present invention.

With the circuit which has been'described, it is possible to obtain image frequency suppression throughout either of two frequency ranges and to alter simultaneously the resonance characteristics of the antennaand tunable circuits as desired by the simple manipulation of two switches connected for uni-control operation.

The-frequency controlling element of the generator circuit 23 may be connected to be operated in a. uni-c ontrol manner with the variable condenser 19 of the tunable circuit 1'7.

Appropriate current may be supplied to heat the cathode of the modulator tube 20, preferably by the usual power supply means included in the receiver 29'.

Th circuit shown and described can be designed to operate over any two frequency bands and the bands need not necessarily be contiguous. However, in a specific arrangement constructed in accordance with this invention, the lower frequency band comprised 550 kilocycles to 1400 kilocycles and the next adjacent higher frequency band comprised 1400 kilocycles to 3550 kilocycles. An intermediate frequencyof, 450 kilocycles was utilized. The image frequency was 900 kilocycles higher than the signal frequency.

Furthermore, the principles involved in this invention may be utilized in a receiver designed to operate over three or more frequency bands.

The expression nearly optimum as used in the claims refers to circuit relations which are optimum or which approach optimum nearly enough to secure a substantial advantage in the suppression of image interference.

There is claimed: a

- 1. In combination with a superheterodyne receiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling anantenna to said receiver and suppressing image interference over both of said bands; which comprises a parallel tuned circuit and a coupling coil connected in series between theinput terminals of said receiver, an antenna ceiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to said receiver and suppressing image interference over both of said bands which comprises a parallel tuned circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna circuit including a series coil, a condenser common to and coupling said antenna and parallel tuned circuits, image-suppressing mutual inductance between said coils, and switching means for simultaneously adjusting said tuned circuit to operate overeither of said bands, adjusting said series coil to resonate the antenna circuit within the selected band, and selecting. a. predetermined value of said mutual inductance which gives nearly optimum image suppression in the same band.

3. In combination with a superheterodyne receiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to said receiver andsuppressing image interference over both of said bands,

which comprises a parallel tuned circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna cirwhich gives nearly optimum image suppression in the same band.

4. In combination with a superheterodyne receiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to said receiver and'suppressing image interference over both of said bands,

which comprises a parallel tuned circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna circuit including a series coil, a condenser common to and coupling said antenna and parallel tuned circuits, image-suppressing mutual inductance between said coils, and switching means for adjusting said tuned circuit to operate over either of said bands, and for selecting a predetermined value of said mutual inductance which gives nearly optimum image suppression in the same band.

5. In combination with a superheterodyne receiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to'said receiver and suppressing image frequency interference over both of said bands, which comprises an input circuit including a parallel tunable circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna circuit including a series coil and, alternatively, a second coil in shunt therewith, a condenser common to and coupling saidantenna and parallel tunable circuits, the coils of said antenna circuit being inductively coupled'to the coupling coil in the input circuit, the coupling thus provided between the antennaand the input circuits being proportioned to balance out image frequency voltages transmitted to said input circuit through the coupling between said antenna-and parallel tunable circuits, a switch for altering one of the constants of said parallel tunable circuit to permit tuning thereof over each of said frequency bands, and a second switch operatively connected to said first-mentioned switch for alternatively connecting the second series'coil of said antenna circuit in parallel with the first coil thereof and thereby simultaneously altering the constants of said antenna circuit to cause it to be broadly resonant within the selected band and selecting the proper inductive coupling between said antenna.

and input circuits to give nearly optimum image suppression in the same band.

6. In combination with a superheterodyne receiver operative over a high-frequency band and its a low-frequency band, the arrangement for couimage frequency interference over both of said bands, which comprises an input circuit including a parallel tunable circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna circuit including a series coil, and a condenser common to and coupling said antenna and parallel tunable circuits, the series coil of said antenna circuit being inductively coupled to the coupling coil in the input circuit, the coupling thus provided being proportioned to balance out image frequencyvoltages transmitted to said input circuit through the common coupling condenser, a second series coil for alternative connection in parallel with the first series coil of said antenna circuit, a switch for altering one of the constants of said parallel tunable circuit to permit tuning thereof over each of the frequency bands, and a second switch operatively connected to said first-mentioned switch for alternatively connecting said second series coil in parallel with the first series coilof said antenna circuit and for simultaneously altering the constants of said antenna circuit to cause it to be broadly resonant within the selected band and selecting the proper inductivev coupling between said antenna and input circuits to give nearly optimum image suppression in the same band.

7. In combination with a superheterodyne receiver operative over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to said receiver and suppressing image frequency interference over both of said bands, which comprises an antenna circuit and an input circuit, a tunable transfer means coupling said circuits, an untuned transfer means also coupling said circuits, the characteristics of said untuned transfer means being so chosen as to transfer voltages at the image frequency which are-equal and opposite to those incidentally transferred through said tunable transfer means, a switch means for altering a characteristic of said tunable transfer means to permit tuning over either of said frequency bands, and a second switch means operatively connected to said first switch means for simultaneously altering the characteristics of said untuned transfer means to give nearly optimum'image suppression in the selected band and to tune the antenna circuit to be broadly resonant within the same band.

8. In combination with a superheterodyne receiver operative selectively over a high-frequency band and a low-frequency band, the arrangement for coupling an antenna to said receiver and suppressing image frequency interference over both of said bands, which comprises an input circuit having a parallel tunable circuit and a coupling coil connected in series between the input terminals of said receiver, an antenna circuit includ'ng a series coil, and a condenser common to and coupling said antenna circuit and said pling an antenna to said receiver and suppressing parallel tunable circuit, the series coil of said antenna circuit and the coupling coil of said input circuit being inductively coupled thus providing two coupling paths between. said antenna and input circuits whereby the image frequency currents transferred through said inductive coupling will balance out those transferred through said parallel tunable circuit, and a switch connected to said circuits for simultaneously altering said inductive coupling and the constants of said antenna circuit to cause it to be broadly resonant within the selected'band and to give nearly optimum image suppression in the same band.

9. In combination with a superheterodyne receiver selectively operative over a plurality of frequency bands, the arrangement for coupling an antenna to. said receiver and suppressing image frequency interference over each of said bands, which comprises an antenna circuit and an input circuit, a tunable transfer means coupling said circuits, an untuned transfer means also coupling said circuits, the characteristics of the untuned transfer means being so chosen as to transfer voltages at theimage frequency which are equal and opposite to those incidentally transferred through said tunable transfer means, and a-switch means connected to said circuits for simultaneously altering .the characteristics of said untuned transfer means to give nearly optimum image suppression in any desired frequency band and to tune the antenna, circuit to be broadly resonant within the same band.

10. In combination with a superheterodyne receiver selectively operative over a plurality of frequency bands, the arrangement for coupling an antenna to said receiver and suppressing image frequency over any of said bands, which comprises an input circuit having connected in series between the input terminals of said receiver a parallel tunable circuit and a coupling coil, an antenna'circuit including a series coil, and a condenser common to and coupling said antenna and parallel tunable circuits, the coil of said antenna circuit being inductively coupled to the series coil in the input circuit, the coupling thus provided being proportioned to balance out image frequency voltages transmitted to said input circuit through the common coupling condenser, a second series coil for alternative connection in parallel with the first series coil of said antenna c'rcuit, and a switch connected to said circuits for connecting said second series coil in parallel with the first series coil to said antenna circuit and thereby simultaneously altering the constants of said antenna circuit to cause it to be broadly resonant within the selected band and simultaneously selecting the proper inductive coupling between said antenna and input 'circuits to give nearly optimum image suppression in the same band.

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