US2201938A - Image rejector circuit for radio receivers - Google Patents

Description

R. B. ALBREGHT IMAGE REJECTOR CIRCUIT FOR RADIO RECEIVERS Filed Jan. 5, 1938 AM a; 2/0 510$ W A a M Q Patented May 21, 1940 ATE T OFHE IMAGE REJECTOR CIRCUIT FOR RADIO RECEIVERS Robert Benjamin Albri signer to ght, Philadelphia, Pa, as- .Philco Radio and Television Corporation, Philadelphia, Pa, a corporation of Dela- Ware Application January 5,

1938, Serial No. 183,542

In Great Britain January 9, 1937 10 Claims.

This invention relates to novel means for suppressing or attenuating undesired signals in radio receiving systems and, more particularly, to means for attenuating so-called image signals in radio receivers of the superheterodyne type.

The principal object of the invention is to provide a novel device of this character which may be employed to advantage in a superheterodyne multiband receiver, that is a receiver adapted to receive signals in a plurality of frequency bands, and which device embodies a minimum number of circuit elements and utilizes certain elements in a dual manner.

Another object of this invention is to provide a multiband receiver ofthe superheterodynetype in which the coil or coils of an unused wave band are employed to attenuate undesired image signals associated with the desired signals in the particular wave band being used, and in which the same coils are used for the transfer of desired signals in their particular bands, thus permitting the coils to serve a dual purpose.

Still another object of the invention is to provide a novel antenna circuit for a radio receiver of this type, which circuit has associated with it the novel image suppression device provided by.

the invention.

Although the principles of the invention are applicable in any instance where it is desired to attenuate undesired signals and to transfer desired signals substantially unaffected, as stated above, the invention is particularly adapted for use in a superheterodyne multiband radio receiver and, therefore, it will be described herein with particular reference to such specific application.

As is now well known, any superheterodyne receiver is responsive to signals of two different frequencies which represent the sum and difference, respectively, of the oscillator frequency and the intermediate frequency of the receiver. To a lesser extent, the receiver is also responsive to signals whose frequencies represent the sum and difference, respectively, of the intermediate free quency and harmonics of the oscillator frequency. One of the signals to which the receiver is responsive is commonly known as the image signal. The presence of the image signal is particularly undesirable in certain instances, for example in the reception of signals in the long wave band, that is signals having a frequency in the frequency range between and 350 kilocycles, since the image signals corresponding thereto lie in the standard broadcast band which extends from 550 kilocycles to 1600 kilocycles within which there are present signals emanating from powerful transmitting stations. For example, if the intermediate frequency of a receiver is 450 kilocycles, the image frequency range corresponding to signals in the long wave band will extend from 750 kilocycles to 1250 kilocycles. Therefore, in order to receive signals in the long Wave band efficiently, it is necessary that the strong image signals present in the broadcast band be' effectively attenuated.

The present invention accomplishes this desired result by providing a signal transfer path between the antenna and first tuned circuit which path includes an impedance in shunt across the path which impedance has a high value and therefore good transfer characteristics for desired signals but a low value and therefore poor transfer characteristics for the undesired signals. Moreover, the invention utilizes for this purpose certain elements which are used for reception of signals in the broadcast band, thus causing such elements to serve a dual purpose. In this manner, the desired image suppression is effected employing a minimum number of parts or elements.

Other objects and features of the invention will be apparent from consideration of the drawing and specification. The single figure of the drawing is a diagrammatic illustration of one embodiment of the invention.

Referring now to the drawing, there is shown a multi-band radio receiver which is adapted to receive signals in the usual frequency bands, viz., the long wave band, the broadcast band and the short wave band. To this end, there are provided three tuning coils L1, L2 and L3 which are respectively designed to be tuned to signals in the long wave band, the broadcast band and the short wave band by means of the tuning condenser C: and the padding condensers Cpl, C 2 and 0133 associated respectively with the coils L1, L2 and La. By means of the three-position switch S1, any one of these coils may be selected to adapt the receiver for reception of signals in a particular frequency band, as will be well understood, As illustrated, the selectable contacts I, 2 and 3 of the switch S1 are connected respectively, to the coils L1, L2 and L3. Preferably there should be some coupling between L1 and L2 as indicated by M in the figure. Suitable grid bias or automatic volume control bias may be supplied to the grid of the control tube V by means of the isolating resistance R connected, to the common point of L1, L2 and L3 as shown.

Signals may be supplied to the several tuned circuits either from a conventional high impedance antenna A or from a low impedance source such as the transmission line T. L. which may be connected to an associated antenna as well understood. When it is desired to receive signals from the antenna A, the antenna may be connected to the terminal 4 and the signal transfer will then take place through the capacity coupling comprising condensers C1, C2 and Ca. When it is desired to receive signals from the transmission line T. L., the line may be connected to the terminals 5 and 6, which are connected to the primary winding P of transformer T, the midpoint of the primary winding being connected to ground so as to balance the transmission line. Secondary S of the transformer has one end coninected to ground and its other end connected to a point between condensers C2 and C3 so that the transformer secondary is shunted about the condenser C3. The signal transfer takes place through the coupling transformer T which builds up a signal across the condenser C; from whence the signal is supplied to the selected tuned circuit.

The wave band switch S1 is connected to the input element of a vacuum tube V so that the switch is adapted to connect the selected tuned circuit to the said input element. The tube V may be a conventional detector-oscillator or it may be a conventional tube of the mixer type. In any case, there will be associated with the tube V and oscillator 0 having a tuning condenser Cu, which condenser may be ganged with the tuning condenser Ct for simultaneous operation of the two tuning condensers, as indicated by the broken-line representation. It will be understood that the oscillator is a conventional device which requires no further illustration than the diagrammatic representation of the drawing.

The output of the tube V may be supplied to any suitable utilization means such as the intermediate frequency amplifier of a conventional superheterodyne radio receiver.

In accordance with the present invention, there is provided a second switch S2 which may be a three-position switch similar to the switch S1 as illustrated, although only one position of switch S2 is utilized in the manner to be presently described. Preferably, the switch S1 and S2 are ganged or connected together for simultaneous operation as indicated by the broken-line representation. The stationary contact I of switch S2 is connected to the stationary contact 2 of switch S1 as illustrated. A condenser C4 has one side connected to the movable arm of the switch S2 and its other side connected to ground. It will be seen, therefore, that when the switch S2 is in its position I, the coil L2 is connected to the condenser C4. When the switch S2 is in either of its positions 2 and 3, however, it is ineffective since its contacts 2 and 3 are dead contacts.

Suppose now that it is desired to receive signals in the long wave band, as described above, the switch S1 will be thrown to its position I so as to select the long wave band coil L1. At the same time, the switch S2 will, of course, be thrown to its position I, thus connecting the condenser C4 to the broadcast band coil L2. The coil L1 is now energized by the signal across the condensers C2 and C3. It will be noted, however, that the series circuit comprising coil L2 and condenser C4 is placed in shunt relation with the condensers C2 and C3. Remembering that the coils L1 and L2 are adapted respectively to be tuned to receive signals in the long wave band and the broadcast band by means of the common tuning condenser Ct, it will now be apparent that the condenser C4 may be designed in such relation to the coil L2 that the series circuit L204 may be made to attenuate image signals in the broadcast band. By designing the condenser C4 so that it has a capacity equal to about one-half the maximum capacity of the tuning condenser Cr, the series circuit L2C4 may be made resonant at a frequency in the lower end of the image frequency band associated with the long wave hand. For example, the resonant frequency may be in the neighborhood of 750 k. 0. At low frequencies corresponding to desired signals in the low wave band (i. e., -350 k. c.), the series circuit will have a very high capacitive reactance and hence will not deleteriously effect the transfer of desired signals. With respect toimage signals, however, the series circuit will substantially completely suppress those signals having a frequency in the lower part of the image frequency range since, for signals of such frequencies (1. e., near the resonant frequency of L204), the impedance of the series circuit L204 is very small. For undesired signals in the high end of the image frequency band, the impedance of condensers C2 and C3 will be small and will prevent the transfer of such signals to a considerable extent. However, by coupling L1 and L2 slightly in accordance with the invention, further suppression of undesired image signals in the high part of the image signal band may be had. This obtains from the fact that for these higher image frequencies, the image frequency current in L2 will induce a voltage in L1 which will oppose the undesired image signal in that circuit. Complete balancing out may be obtained at one frequency, for example, the middle of the image band, and for the remainder of the band the two signals will oppose one another and nearly completely balance out.

For reception of signals in the broadcast band, the switches S1 and S2 are moved to position 2 and the circuit L2Ct may be tuned to the desired signal in the broadcast band in the normal manner, Reception of signals in the short wave band likewise obtains by moving S1 and S2 to position 3. It will be understood of course that the padding condensers Cpl, CD2 and C 3 will be adjusted so that the tuning condenser Cr will track in each band.

Generally speaking, it has been found that by using the circuit of the invention as above described, it is necessary to provide only two tuning condensers instead of three such condensers as used heretofore to obtain satisfactory image signal attenuation. Thus, in this respect also the invention simplifies the radio receiver and effects economy in the manufacture thereof.

In a specific embodiment of the invention, the following values were found to be satisfactory. The coil L1 may have an inductance of 2580 microhenries. The coil L2 may have an inductance of 221 microhenries. The coil L3 may have an inductance of 1.9 microhenries. The tuning condenser Cr may be of sufficient capacity to tune these coils over the above-mentioned frequency bands. The condensers C1, C2 and C3 may each have a capacity of .01 microfarad. The tube V may be of any suitable type such as the 6A'7G frequency converter. or a simple tetrode. The intermediate frequency of the radio receiver may be 451 kilocycles.

The coils L1 and L2 may be wound on the same form. In the above specific embodiment, L1 comprises 365 turns of 3-40 Litzendraht wire and L2 comprises 82 turns of 7-41 Litzendraht wire, both windings being of the universal type and in the same direction. The coils were wound on a form and spaced The beginnings of each winding were connected together and to C2. The end of the winding L1 was connected to Ct and the end of the winding L2 to C4, by means of the- I switches S1 and S2, respectively.

While the invention has been described with respect to a single embodiment thereof, it will be understood that various modifications may be made as will be apparent to those skilled in the art and consequently the invention is not limited to the specific form shown in the figure.

I claim:

1. In a multiband superheterodyne radio receiver, a signal channel, av tuning condenser, a coil tunable to a desired signal in one wave band by said condenser, a second coil tunable to a desired signal in a different wave band by said condenser, each of said coils having one end connected in the signal channel, a wave band switch for connecting the other end of one coil in the signal channel for transferring desired signals in its Wave band and for alternatively connecting the other end of the second coil in the signal channel for the transfer of desired signals in the second wave band, a condenser of predetermined capacity relative to the inductance of one of said coils, and means including a switch for serially connecting said last-mentioned coil and con,- denser in shunt with the signal channel to suppress undesired signals associated with the desired signals. upon operation of said wave band switch to connect the first-mentioned coil in the signal channel.

2. In a multiband superheterodyne radio receiver, a signal channel, a tuning condenser, a coil tunable to a desired signal in one wave band by said condenser, a second coil tunable to a desired signal in a different wave band by said condenser, each of said coils having one end connected in the signal channel, a. wave band switch for connecting the other end of one coil in the signal channel for transferring desired signals in its wave band and for alternatively connecting the other end of the second coil in the signal channel for the transfer of desired signals in the second wave band, a condenser of predetermined capacity relative to the inductance of one of said coils, and means including a switch for serially connecting said last-mentioned coil and condenser in shunt with the signal channel to suppress undesired signals associated with the desired signals upon operation of said wave band switch to connect the first-mentioned coil in the signal channel, said last-mentioned condenser having a value between the minimum and maximum values of said tuning condenser.

3. In a multiband superheterodyne radio receiver, a source of signals, a tuning condenser, a plurality of coils, each of said coils being tunable by said tuning condenser to transfer desired signals in different wave hands, a common connection between one end of each of said coils, connections for transferring signals from said source to said common connection, an amplifier, means for connecting the tuning condenser in shunt with the input circuit of said amplifier, a wave band switch for alternatively connecting the other ends of said coils to the tuning condenser to transfer desired signals in the respective wave bands, a condenser of predetermined capacity relative to the inductance of one of said coils, and switch means for serially connecting last-mentioned coil and condenser in a shunt path across said signal source upon operation of said wave band switch to connect another of said coils in the signal channel, the said seriesconnected elements in said shunt path being resonant to undesired signals associated with the desired signals transferred by said other coil.

4. In a multiband superheterodyne radio receiver, a source of signals, a tuning condenser, a plurality of coils, each of said coils being tunable by said tuning condenser to transfer desired signals in different wave bands, there being some mutual coupling between said coils, a common connection between one end of each of said coils, connections for transferring signals from said source to said common connection, an amplifier, means for connecting the tuning condenser in shunt with the input circuit of said amplifier, a wave band switch for alternatively connecting the other ends of said coils to the tuning condenser to transfer desired signals in the respective wave bands, a condenser of predetermined capacity relative to the inductance of one of said coils, and switch means for serially connecting said last-mentioned coil and condenser in a shunt path across said signal source upon operation of said wave .band switch to connect another of said coils in the signal channel, the said seriesconnected elements in said shunt path being resonant to undersired signals associated-with the desired signals transferred by said other coil.

5. In a multiband radio receiver, a source of signals, a vacuum tube having at least a cathode, a control grid, and an anode, a tuning con-. denser connected between said cathode and said control grid, a plurality of coils, each of said coils being tunable by said tuning condenser to signals in different wave bands, a capacitor of predetermined capacity relative to the inductance of one of said coils, and switching means for connecting another of said coils serially between said signal source and said control grid, while simultaneously connecting said one coil and said capacitor in series across said signal source.

6. In a multiband radio receiver, a source of signals, said source having a high potential side and a low potential side, a vacuum tube having at least a cathode, anode and control grid, a tuning condenser connected between said cathode and said control grid, a connection between the low potential side of said signal source and said cathode, a first coil capable of being tuned over a certain frequency band by said condenser, a second coil capable of being tuned over a second frequency band by said condenser, switching means for selectively connecting either one of said coils serially between the high potential side of said signal source and said control grid, whereby the selected coil may be tuned to series resonance by said tuning condenser, a capacitor having a predetermined capacity relative to the inductance of said second coil, and additional switching means operative when said first coil is switched into circuit for connecting said second coil and said capacitor in series across said signal source.

7. In a multiband superheterodyne radio receiver, a source of signals, said source having a high potential side and a low potential side, a vacuum tube having at least a cathode, anode and control grid, a tuning condenser connected between said cathode and said control grid, a connection between the low potential side of said signal source and said cathode, a first coil capable of being tuned over a certain frequency band by said condenser, a second coil capable of being tuned over a second frequency band by said condenser, switching means for selectively connecting either one of said coils serially between the high potential side of said signal source and said control grid, whereby the selected coil may be tuned to series resonance by said tuning condenser, a capacitor having a predetermined capacity relative to the inductance of said second coil, and additional switching means operative when said first coil is switched into circuit for connecting said second coil and said capacitor in series across said signal source, said second coil and said capacitor eing adjusted to give series resonance at some point in the image frequency band associated with the band covered by said first coil.

8. In a multiband radio receiver, a source of signals, said source having a high potential side and a low potential side, a vacuum tube having at least a cathode, anode and. control grid, 9, tuning condenser connected between said cathode and said control grid, a connection between the low potential side of said signal source and said cathode, a first coil capable of being tuned over a certain frequency band by said condenser, a second coil capable of being tuned over a second frequency band by said condenser, said coils being inductively coupled, switching means for selectively connecting either one of said coils serially between the high potential side of said signal source and said control grid, whereby the selected coil may be tuned to series resonance by said tuning condenser, a capacitor having a predetermined capacity relative to the inductance of said second coil, and additional switching means operative when said first coil is switched into circuit for connecting said second coil and said capacitor in series across said signal source.

9. In a multiband radio receiver, a source of signals, said source having a high potential side and a low potential side, signal transfer means having a high potential side and a low potential side, a tuning condenser connected between the high and low potential sides of said transfer means, a connection between the low potential side of said source and the low potential side of said transfer means, a plurality of coils, means connecting one end of each coil to the high potential side of said source, switching means for selectively connecting the other ends of said coils to the high potential side of said transfer means, the said coils being tunable respectively by said condenser over different frequency hands, a capacitor having a predetermined capacity relative to the inductance of one of said coils, and means for serially connecting said capacitor and said one coil in shunt with said source upon operation of said switching means to connect another of said coils to said transfer means.

10. In a multiband radio receiver, a source of signals, signal transfer means, a tuning condenser connected across said transfer means, a connection between one side of said source and one side of said transfer means, a plurality of coils, means connecting one end of each coil to the other side of said source, switching means for selectively connecting the other ends of said coils to the other side of said transfer means, the said coils being tunable respectively by said condenser over different frequency bands, a capacitor having a predetermined capacity relative to the inductance of one of said coils, and means [or serially connecting said capacitor and said one coil in shunt with said source upon operation of said switching means to connect another of said coils to said transfer means.

ROBERT BENJAMIN ALBRIGHT.

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