US1945427A - Coupled circuits - Google Patents

Description

P. o. FARNHAM Jan. 30, 1934.

I COUPLED CIRCUITS Filed March 21,1932

I I I I I I I I I I I I I I l I YL Patented Jan. 30, 1934 UNITED STATES COUPLED cmourrs Paul 0. Farnham, Boonton, N. J., assignor to Radio Frequency Laboratories, Incorporated, Boonton, N. J a corporation of New J ersey Application March 21,1932. Serial No. 600,358

10 Claims. 101113-44) This invention relates to coupled circuits particularly to methods of and circuit arrangements for varying the coupling coeflicient between two circuits without varying the natural resonant frequencies of the circuits and/or of the transmission line in which said circuits are incorporated. Transmission systems which include two circuits tuned to the same frequency and coupled with somewhat more than critical coupling act as band pass filters as the resonance curve of the coupled circuits is not a single sharp peak but comprises a relatively fiat-topped curve resulting from the combination of two spaced resonance curves. The mean or natural" frequency of the coupled circuits is the resonant frequency of the separate circuits and the two peaks of the resonance curve are spaced from the natural resonant frequency by an amount that varies with the coeflicient of coupling of the two circuits. With less than critical shape coupling, the resonance curve has only one peak but the condition still obtains that the width and shape of the curve depend upon the coeflicient of coupling;

An object of the present invention is to provide coupled circuits capable of adjustment to vary the width of the resonance curve without affecting the natural resonant frequency. A further object is to provide tuned circuits that are coupled by a tuned impedance that may be effectively removed from the network to change the coupling efficiency. More specifically, an object is to provide a system of two circuits tunable to the same frequency and which include coupling impedances that are also tuned to the same fre quency, and switches for short-circuiting the coupling impedances, thereby to alter the coupling coefficient of the two circuits without affecting the natural resonant frequency of the system.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompanying drawing wherein:

Fig. 1 is a circuit diagram of an embodimentof the invention, and

Fig. 2 is a curve sheet illustrating theresonance curves obtainable with a coupling system such as shown in Fig. 1. I

In the drawing, the reference numerals 1, 1

. and 2, 2 identify, respectively, the input and the A capacitive the coupling impedances of the circuits and, as

indicated by the broken line 4, these switches are preferably mechanically connected for simultaneous operation.

v When the switches are closed, the coupling between circuits I and II is preferably very weak, approaching zero when C and M are zero, and the resonance curve has a single peak, as shown by curve A of Fig. 2. For simplicity of explanation, it will be assumed that both circuits are tuned to the same frequency. Under these conditions, the resonant frequency ,f is given by the equations: a

In accordance with the invention, the values of the reactances of the short-circuited coupling system are so related as to satisfy the equation:

Under these conditions, opening of the switches 3 to include the coupling impedances in the circuits I and II will not alter the resonant fre- 100 than critical shape coupling, but it will be apparent that the same the natural frequency absence of any change in i will attend the application of the invention to circuits in which the coupling is less than critical shape.

As indicated in Fig. 1, a magnetic coupling -M1 may be provided between the i'nductances La, Lb of the coupling impedance. With magnetic coupling present, the total coupling controlled by switches 3 will be:

X,,,- :hwM we (4) To secure economy and tolerance, the sign of the magnetic coupling should be negative in order that the mutual inductive coupling will aid the capacitive coupling Cm. Inductive coupling is not necessary, however, so long as Equation (3) is satisfied.

With the coupling impedances La, Lb, Cm and, if employed, -M1, so related that the natural resonant frequency is not altered by the position of switches 3, it will be apparent that the relatively permanent couplings C and/ or M may be adjusted for less than critical shape coupling, i. 'e. to give a comparatively sharp peak for the resonance curve. This adjustment is, of course, effected with the switches 3 closed to short-circuit the complex coupling impedance. The opening of switches 3 introduces a further coupling to broaden the resonance curve without changing the natural resonant frequency. The magnitudes of the coupling impedance may be chosen to give any desired broadening of the resonance curve.

The invention is applicable not only to coupling systems in which the separate circuits are tuned to the same frequency i, but also to systems in which the individual circuits are tuned to different frequencies. In addition to its obvious use for varying the characteristics of a circuit during the transmission of signals, the invention may be used to facilitate the adjustment of the several elements of a circuit. The alinement of the intermediate frequency circuits of a super heterodyne receiver is quite difficult when more than critical shape coupling is employed in band pass circuits. Circuits designed as above described are readily adjusted by first short-circuiting the coupling impedances and thentuncircuits I and II to the intermediate frequency f on which the oscillatortracking is based. The low coupling efficiency makes this adjustment practical as the tuning of one circuit does not aifect the resonant frequency of the other circuit.

*hen the main circuits are to be tuned over a band of frequencies, the coupling condenser Cm maybe mechanically connected to the tuning condensers to maintain the relationships expressedby the above equations throughout the tuning, range of the circuits.

I claim:

1. The combination with two circuits, and a reactive coupling between said circuits, of reactive means for varying the coefficient of coupling between said circuits while maintaining their natural resonant frequency unchanged; said reactive means comprising serially connected ing each of the reactances coupling said circuits and resonating at the said natural frequency, and means for alternatively including said reactances in and excluding the same from said circuits.

2. A pair of circuits each including main and auxiliary reactances, in each circuit the main and auxiliary reactances being resonant at the same frequency, a. coupling between the auxiliary reactances of said circuits, and means for changing the coefficient of coupling between said circuits without altering the natural resonance frequency of said coupled circuits.

3. The invention as set forth in claim 2, wherein said means comprises switch means for including said auxiliary reactances in or excludingthe same from their respective circuits.

4. A pair of resonant circuits each including reactances coupled to provide less than critical shape coupling, and means adapted to increase the coupling to more than critical shape coupling, said means comprising coupling reactances and switch means for connecting said coupling reactances into the respective circuits, said coupling reactances including a common portion and portions for introduction into the respective circuits, and each of said last portions resonating with said common portion at the resonant frequency of the circuit into which the respective last portions may be introduced.

5 In a transmission circuit, the combination with a tuned circuit and a second tuned circuit coupled thereto, of a coupling impedance including a reactance of one type and a .pair of reactances of the opposite type, and switch means for connecting said coupling impedance into said two circuits.

6. In a transmission circuit, the combination with a tuned circuit and a second tuned circuit coupled thereto, of a coupling impedance including a reactance of one type and. a pair of reactances of the opposite type, andswitch means for connecting said first reactance into both of said circuits and for connecting the reactances of said'pair into the respective circuits.

7. The combination with apair of circuits tuned to the same frequency and coupled to resonate as a whole at that frequency, of means for varying the coefficient of coupling between said circuits, said means comprising an impedance having two branches each resonating at the said same frequency.

8. A pair of tuned circuits each including inductance and a capacity, a second capacity common to and coupling said circuits, of means for short-'circuiting said common capacity and such portions of the inductanceof each of said circuits that the short-'circuited reactances resonate at the natural resonance frequency of the coupled circuits.

9. The invention as set forth in claim 8, wherein the said portions of said inductances are coupled magnetically.

10. The invention as set forth in claim 8, wherein the said portions of said inductances are coupled magnetically to add to the capacitive coupling of said common capacity.

PAUL O. FARNHAM.

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