US2151814A - Superheterodyne receiving circuits - Google Patents

Description

March 28, 1939. B. 0. H. TELLEGEN SUPERHETERODYNE RECEIVING C IRGUITS Filed Dec. 31, 1956 INVENTOR B. D. H. TELLEGEN ATTORNEY Patented Mar. 28, 1939 PATENT OFFICE SUPERHETERODY NE RECEIVING CIRCUITS Bernardus D. H. Pellegen, Eindhoven, Netherlands, assignor to N.

V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands, a corporation of the Netherlands Application December 31, 1936, Serial No. 118,455 In the Netherlands January 6, 1936 9 Claims.

This invention has reference to a superheterodyne receiving circuit in which the local frequency is higher than the frequency of the oscillations received and in which the oscillations received are supplied to a high frequency amplifying valve or detector valve through a band filter. 1

Ina superheterodyne receiving circuit it is common practice to supply the oscillations received jointly with the oscillations generated by a local oscillator to a detector valve so that in the anode circuit of the detector valve are set up oscillations the frequency of which is equal to the sum or to the difference of the frequencies of the oscillations received and those locally generated. In this case the anode circuit is coupled to a. circuitor band-filter tuned to the difference frequency by which the oscillations of the difference frequency are selected and supplied to an intermediate frequency amplifier. If the frequency of the oscillations to be received is equal to f and if the frequency to which the intermediate frequency amplifier is tuned is equal to f'm, the frequency of the local oscillator may be equalized with f+fm which causes oscillations of the frequency jm to be set up in the anode circuit of the detector valve. the difficulty that oscillations of the frequency f+2fm will also yield jointly with the locally generated frequency f-l-fm the difference frequency fm. In this case, not only oscillations of the desired frequency I but also oscillations of the frequency f+2fm are received, the latter frequency being referred to as image frequency.

It ispossible to obviate thisldifii'culty in part by supplying the oscillations received to the first detector valve, or to a radio frequency'amplifying valve preceding it, through a band filter tuned to the desired frequency. This band filter will, however, nevertheless transmit, even though it may be to a low degree, oscillations of the image frequency so that disturbances as a result of the reception of oscillations of the image frequency are not entirely avoided.

According to the invention, in a superheterodyne receiving circuit in which the frequency of the locally generated oscillations is higher than that of the oscillations received and in which the oscillations received are supplied to a radio frequency amplifier valve or detector valve through a band filter constituted by two or more mutually coupled tuned circuits the first of which is coupled to the antenna or to the output circuit of a radio frequency amplifier valve and a point (Q) of said first circuit being connected to the There is however earth terminal and/or to the cathode of the radio frequency amplifier valve or detector valve, a further suppression of theimage frequency is ensured by connecting the antenna terminal or the anode of the radio frequency amplifier valve preceding the band filter through a condenser to a point (P) of the first circuit of the band filter which is connected to the point (Q) through circult elements which substantially determine the tuning of the circuit and on the other hand through circuit elements which are coupled to the second circuit of the band filter. 1

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully, by way of example, with reference to the accompanying drawing wherein Fig. 1 shows a circuit embodying the invention which may form the input to the first detector or to the radio frequency amplifier of a superheterodyne receiver, Fig. 2 shows the invention applied to a receiver adapted for the reception of a plurality of bands, and Fig. 3'is a modification of a circuit shown in Fig. 2.

Referring to Figure l, oscillations received by the antenna are supplied to the valve V through a band filter constitutedby two mutually coupled tuned circuits. The first circuit L1, L2, C5, C1 is coupled to the antenna inductively by the mutual inductance of the coils Lo and L1 and also capacitively by the condenser C3. The signof the inductive coupling between the coils Lo and L1 is so chosen that this coupling assists the action of the capacitive coupling by the condenser C3. The point Q of the first circuit is connected to the earth terminal E. -The second circuit L3, C5, C2 of the band filter is coupled to the first circuit both inductively by the mutual inductance of the coils L2 and L3 and capacitively by the condenser C5. The coupling between the first and the second circuit of the band filter may, however, be effected otherwise. The two circuits are tuned by means of the variable condensers C1 and C2 to the frequency of the oscillations to be received. The resonance frequency of the first circuit is substantially determined by the inductance L1 and the capacity C1, or in other words the inductance L1 is high relatively to the inductance L2 and the capacity C5 is high relatively to the capacity C1. According to the invention the point P of the first circuit, which is situated between the coils L1 and L2, is connected through the condenser 04 to the antenna terminal A, the condenser C4 being so proportioned that the image frequency oscillations occurring in the second circuit are suppressed.

The first circuit of the band filter may be coupled to the antenna in any manner. Generally, however, for any antenna coupling the value of the capacity C4 at which the image frequency is suppressed in the second circuit of the band filter will be different for every frequency. The manner of coupling shown in Figure 1 according to which the first circuit of the band filter is coupled to the antenna both inductively and capacitively in such manner that the two couplings assist the action of one another presents the particular advantage that when the two couplings are properly chosen a suppression of the image frequency in the second circuit of the band filter can be obtained for the same value of the capacity C4 throughout a wide band of frequencies.

The operation of the circuit arrangement may be explained as follows. Since the image frequency is higher than the frequency to which the band filter is tuned, the reactance of the condenser C1 for the image frequency will be lower than the reactance of the coil L1. Thus, the condensers C3 and C1 behave for the image frequency as a capacitive potentiometer so that the potential of the point B relatively to the point Q will be substantially in phase with the potential of the antenna terminal A relatively to the point Q. In addition, as a result of the inductive coupling to the coil L0, the coil L1 has induced in it an'E. M. F. which is also in phase with the potential of the antenna terminal A relatively to the point Q. The circuit formed by the coils L1 and L2 and the condenser C constitutes for the image frequency an inductive impedance so that a current of the image frequency passes through the coil L1 to the point P which lags through an angle of some 90 with respect to the voltage between the points A and Q. The impedance of the circuit formed by the inductance L2 and the capacity C5 will be small for the image frequency as compared with the impedance of the circuit formed by the inductance L1 and the capacity C1. The circuit formed by the condenser C4, the coil L2 and the condenser C5 constitutes for the image frequency a capacitive impedance so that a current of the image frequency will pass through the condenser C4 to the point P which leads through an angle of some 90 relatively to the voltage between the points A and Q. It is obvious that if the condenser C4 is correctly chosen, the two currents of the image frequency that pass through the point P will neutralize each other wholly or in part.

The point P is preferably chosen so as to be connected to the point Q via circuit elements (Li, G1) which are only coupled to the antenna and on the other hand via circuit elements (L2, C5) which are only coupled to the second circuit of the band filter. It is, however, found that in superheterodyne receiving circuits for more than one wave-band in order that it may be possible for the same value of C4 to sufi'ice for the band of the longest waves to be received, it may be favorable if the path connecting the point P to the point Q which comprises the circuit elements coupled to the antenna, includes in addition one or more circuit elements coupled to the second circuit. If, for example, the condenser C4 is so chosen that for the band of wave lengths from 200 to 600 meters a satisfactory suppression of the image frequency is obtained it is found at given values of the circuit elements that if the same condenser C4 is connected to 2. corresponding point of the first circuit, overcompensation occurs for the band of the long broadcast waves.

The measure described hereinbefore also ensures satisfactory suppression of the image frequency for the band of the longest waves to be received.

Figures 2 and 3 show receiving circuits according to the invention which are suitable for the reception of two bands of wave lengths. In the circuit of Fig. 2 there has been added the coil L"0 in the antenna circuit, the coil L"1 in the first tuned circuit and the coil L"3 in the second tuned circuit, the latter coil forming a resonant circuit with an additional condenser 0"5. In the closed position of the switches S which are connected across the above mentioned additional elements the circuit of Fig. 2 is almost identical with that shown in Fig. 1, the coil L4 functioning in the same manner as the coil L2 in Fig. 1. The difference in Fig. 2 over Fig. 1 is the series connection of the coil L"1 and the resonant circuit formed by the coil L": and the condenser C"5. This series connection is shunted by the coil L4. When the band of the shortest waves is being received, that is in the closed position of the switches S, the aboveseries connection relatively to the coil L4 forms a large inductive impedance which is negligible in comparison to L4. In the open position of the switches S the band of longest waves will be received, and the first resonant circuit is then formed by the variable condenser C1, the self inductance coils L1 and L1 and the fixed condensers 0'5 and C"5. The second resonant circuit contains the variable condenser C2, the self inductance coils L: and L"3 and the fixed condensers C's and C"5. Both resonant circuits are capacitively coupled by the condensers C's and 0"5. The inductive coupling between the coil L4 and the coil L's, used when the band of shortest waves is being received, has no influence when receiving the band of longest Waves.

The circuit of Fig. 3 differs from that of Fig. 2 in the addition of the two coils L"2 and U2 in the first tuned circuit which are inductively coupled respectively to the coils L3 and L's included in the second tuned circuit. The coil L2 is equivalent to the coil L2 in Fig. 1 and to L4 in Fig. 2. In the closed position of the switches S the circuit arrangement of Fig. 3 is identical with that shown in Fig. 1.

In the circuit of Fig. 2 the condenser C4 by which the image frequency is to be suppressed when correctly dimensioned for the band of shortest waves gives an over-compensation when receiving the band of longest waves. This overcompensation can be avoided by connecting condenser C4 to the adjacent terminals of the condensers C's and 0"5. In Fig. 3 the same purpose is attained by connecting the condenser C4 to the adjacent terminals of the coils U2 and L"2 instead of to the adjacent terminals of the coils L1 and L"2. Thus the inductively transmission of the image frequency to the second resonant circuit is decreased.

What I claim is:

1. In a coupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a source of signal frequencies, a pair of tunable circuits, a condenser connected between said source and a point on one of said tunable circuits which divides said circuit into two parallel paths each including an inductance and a condenser, means for coupling the signal source to one of said paths, and means for coupling the other tunable circuit to the other of said paths.

2. In a coupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a source of signal frequencies, a pair of tunable circuits, a condenser connected between said source and a point on one of said tunable circuits which divides said circuit into two parallel paths, one path including a large inductance and a tuning condenser, the other path including a small inductance and a fixed condenser, the signal source having an inductance which is coupled to the large inductance, and the other tunable circuit including an inductance which is coupled to the small inductance of the first tunable circuit.

3. In acoupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a source of signal frequencies, a first tunable circuit, a fixed condenser connected between said source and a point on said tunable circuit which divides said circuit into two parallel paths, one path including a large inductance and a tuning condenser, the other path including a small inductance and a fixed condenser, the signal source having an inductance which is coupled to the large inductance, and a second tunable circuit including a tuning condenser, the fixed condenser of the first tunable circuit and an inductance which is coupled to the small inductance of the first tunable circuit.

4. In a coupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a. first tunable circuit resonant to desired signal frequencies constituted by a pair of parallel paths, each including an inductance and a condenser, a second tunable circuit also resonant to the desired signal frequencies coupled to one of the parallel paths of the first tunable circuit, a circuit responsive to signal frequencies coupled to the other of said parallel paths of the first tunable circuit, and a connection including a capacity connected between the signal responsive circuit and a point common to the two parallel paths of the first tunable circuit.

5. In a coupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a first tunable circuit resonant to desired signal frequencies constituted by a pair of parallel paths, each including an inductance and a condenser, a second tunable circuit also resonant to the desired signal frequencies coupled both magnetically and capacitively to one of the parallel paths of the first tunable circuit, a circuit responsive to signal frequencies coupled both magnetically and capacitively to the other of said parallel paths of the first tunable circuit, and a connection including a capacity connected between the signal responsive circuit and a point common to the two parallel paths of the first tunable circuit.

6. In a coupling network for the suppression of image frequencies in a superheterodyne receiver capable of receiving signals in a high frequency band and in a low frequency band, the

combination of a first tunable circuit resonant to desired signal frequencies in the high frequency band constituted by a-pair of parallel paths, each including an inductance and a condenser, a second tunable circuit also resonant to the desired signal frequencies in said band coupled to one of the parallel paths of the first tunable circuit, a circuit responsive to signal frequencies in said band coupled to the other of said parallel paths of the first tunable circuit, a connection including a capacity connected between the signal responsive circuit and a point common to the two parallel, paths of the first tunable circuit, additional reactances in each of said circuits, and means for short-circuiting said reactances in the reception of signals in the high frequency band, said reactances being in circuit for the reception of signals in the low frequency band, the first tunable circuit having a reactance which is coupled to the additional reactance of the signal responsive circuit and another reactance which is coupled to the additional reactance of the second tunable circuit.

7. A coupling network comprising a circuit responsive to signal frequencies and including an inductance, a tunable circuit including an inductance which is magnetically coupled to the first inductance, a coupling condenser connected between the high potential ends of said inductances, a tuning condenser connected between ground and the high potential end of the inductance included in the tunable circuit, a second coupling condenser connected between the other end of the last mentioned inductance and the high potential end of the first inductance, another inductance and a series-connected condenser con.- nected between ground and said other end of the tuning circuit inductance, and a second tunable circuit constituting the input of a vacuum tube coupled to the first tunable circuit.

8. A coupling network according to claim 7 wherein the second tunable circuit includes a tuning condenser, the series-connected condenser of the first tunable circuit, and an inductance which is coupled to the inductance which is connected to said series-connected condenser.

9. In a coupling network for the suppression of image frequencies in a superheterodyne receiver, the combination of a source of signal frequencies, a tunable circuit comprising an: inductive reactance branch and a shunt capacitive reactance branch, an intermediate point on said last branch being grounded, a condenser connected between said source and an intermediate point on said inductive reactance branch thereby dividing the same into two portions, means for coupling the signal source to one of said inductive branch portions, and means for coupling a second tunable circuit to the other of said inductive branch portions.

BERNARDUS D. H. TELLEGEN.

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