Selective coupling circuits

2,083,901

June 15, 1937. p. o. farnham

SELECTIVE COUPLING CIRCUITS

Filed Nov, 13, 1931

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Patented June 15, 1937

2,083,901

UNITED STATES PATENT OFFICE

2,083,901

SELECTIVE COUPLING CIRCUITS

Paul O. Farnham, Boonton, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware

Application November 13,1931, Serial No. 574,864

Claims. (Cl. 250—20)

This invention relates to selective coupling circuits, and more particularly to coupling circuits which have the property of attenuating a disturbing frequency to a degree greater than that obtainable by tuning the known coupling circuits to the frequency which it is desired to transmit.

While not limited thereto, the invention will be described as applied to a superheterodyne receiver, in which type of receiver it operates as an “image” suppressor. In the operation of a superheterodyne receiver, two beat frequencies are produced when the local oscillation is combined with a received signal and, conversely, two signal frequencies may beat with the local oscillations to produce the same intermediate frequency. When a signal of a given frequency produces the required intermediate frequency, a signal of a frequency differing from the desired frequency by twice the intermediate frequency will also give rise to an intermediate frequency signal. This second frequency is commonly known as the “image” frequency since it produces an undesired intermediate frequency signal superposed upon the desired signal.

Selectivity effective to suppress the image frequency may be obtained by cascaded stages of tuned radio frequency in advance of the first detector, but, since adequate selectivity against signals other than one at the image frequency is provided in the intermediate frequency amplifier, it is not economical to include a large number of radio frequency stages in a superheterodyne receiver. The requirements for satisfactory operation in the broadcast band call for a minimum of two tuned radio frequency stages in advance of the first detector and, for nearly complete suppression of image frequencies, three tuned stages should be employed to secure the necessary selectivity.

In accordance with the present invention, the coupling system between two radio frequency amplifier tubes is so designed as to obtain an image frequency suppression that, with the known circuits, could be obtained only by the use of an additional tuned carrier wave amplifier stage.

An object of the invention is to provide a coupling circuit which will suppress an undesired signal frequency to a greater extent than a coupling circuit of the previously known types. An object of the invention is to provide an image frequency suppressor which will attenuate an undesired frequency to a degree not obtainable by tuning a conventional coupling circuit to resonance at the desired signal frequency. More specifically, an object is to provide a tuned cou pling circuit which includes a tuned suppressor circuit for by-passing undesired signals. A further specific object is to provide a superheterodyne receiver system in which the particular image frequency corresponding to a desired signal will be 5 substantially attenuated when the coupling system is tuned to the desired frequency.

These and other objects of the invention will be apparent from the following specification, when taken with the accompanying drawing, in 10 which,

Fig. 1 is a circuit diagram of a coupling system embodying the invention, and

Fig. 2 is a fragmentary circuit diagram of a superheterodyne receiver embodying the inven- 15 tion.

In the circuit diagram of Fig. 1, the reference characters L, Li identify two inductances which are coupled magnetically by a mutual inductance M, the inductance L being included in circuit. ₂θ with an alternating current source E, and the inductance Li being shunted by a tuning condenser C. If the high potential terminals of the inductances are coupled by a capacity Ci, it will be apparent that two main paths are pro- _2gvided for the current i from source E. A part it will flow through coil L, and a second part 22 will flow through Ci. The current 22 then takes two paths, a part 13 flowing through coil Li as the remainder it returns to source E through con- ₃₀denser C.

If, however, the magnetic coupling M is so chosen that the voltage induced across Li by the flow of current 21 through L is equal and opposite to the voltage drop across Li due to the flow of 35 current 23, the net voltage drop across Li due to current flow from source E is zero, and current flow from source E does not affect the voltage across the terminals A of the coupled circuits. It should be pointed out that the sign of the ₄₀mutual inductance for this condition is that given by having the coils L, Li wound and connected in the same sense.

A mathematical analysis of the circuit will show that the frequency for which the circuit 45 has a maximum attenuation is:

A² where 50

M k = -j==== coefficient of magnetic coupling. V 1*^2

It is to be noted that this equation does not include the capacity of condenser C, and there- 55

2,083,901 fore the attenuation due to the opposing actions of the capacitive and the inductive couplings is not a function of the magnitude of the tuning capacity C. In other words, as condenser C is ad-, justed to tune the coupling system to a desired frequency, the undesired frequency most effectively suppressed by the capacitive coupling Ci remains constant and the attenuation of all other undesired signals is only that due to the tuning of Li C to a particular frequency.

In the operation of a superheterodyne receiver, the image frequency varies with the tuning and is spaced from the desired frequency by a definite finite value equal to twice the interme15 diate frequency. It Is therefore apparent that, to suppress the image frequency, some factor appearing in Equation (1) must be varied as the magnitude of condenser C is varied to tune to the desired signal within a frequency band.

An inspection of Equation (1) indicates that the coefficient of coupling between inductances L, Li, or the values of the inductances may be varied to effect a maximum attenuation at a frequency which varies with the magnitude of ca25 pacify C, but the most convenient method is to vary the magnitude of the coupling condenser Ci. A mathematical analysis of the problem presented will show that, to effect a maximum suppression at a frequency ω' (equal to the signal 30 frequency, ω, plus twice the intermediate frequency), the conditions obtaining are:

(2)

In accordance with the invention, the adjustable elements of the condensers C and Ci are related by Equation (2) thus showing that to provide maximum attenuation at the image frequency, the condenser Ci varies in the same 40 direction as the tuning condenser C, but at a somewhat slower rate.

In the fragmentary superheterodyne circuit shown in Fig. 2, the coupling from the collecting structure I to the first carrier frequency ampli45 fier 2 may be of any desired form. The novel features are to be found in the coupling system between tubes 2 and 3, the tube 3 being either an additional carrier wave amplifier or a first detector. The coupling system is substantially 50 identical with that shown in Fig. 1, but the coupling condenser Ci is mechanically connected to the tuning condenser C as is indicated by the dotted line 4.

In one particular circuit arrangement, the sev55 eral elements had the following values:

L=approximately 5.8 millihenries

Li=approximately 200 microhenries

C=maximum, approximately 400 micromicrofarads

Ci=from about 8 micromicrofarads at 550 kilocycles to about 1.5 micromicrofarads at 1500 kilocycles.

By thus varying the value of the coupling condenser Ci as the circuit is tuned over a frequency C5 band by adjustment of condenser C, signals of all frequencies other than the resonant frequency are attenuated and a further suppression of the image frequency is obtained.

The invention provides an additional suppres70 sion of image frequencies which is comparable to that obtained by the use of an additional tuned carrier amplifier stage. It has, however, the great advantage of low cost since, for equal suppression of the image frequency, an ampli75 fier constructed in accordance with the inven tion requires but one small variable condenser instead of a complete tuned amplifier stage.

While I have illustrated the embodiment of the invention which now seems most practical, it will be apparent that, without departing from the 5 invention as set forth in the following claims, one or more of the other factors which control the relationship between the desired and image frequency may be varied in addition to, or in place of, the described variation of the capacitive 10 coupling.