Title:
LUMPED CONSTANT QUADRATURE COUPLER WITH IMPROVED PARASITIC SUPPRESSION
United States Patent 3723914


Abstract:
A lumped constant quadrature hybrid coupler having a wide band of directivity is disclosed. The coupler is constructed from lumped constant elements including a pair of closely magnetically coupled conductors with a lumped capacitor connecting them at a central portion. Each of the two respective ends of each of the pair of conductors serve as one of the four ports of the resulting coupler. Three printed circuit embodiments are disclosed which incorporate the teachings of this invention.



Inventors:
CAPPUCCI J
Application Number:
05/221012
Publication Date:
03/27/1973
Filing Date:
01/26/1972
Assignee:
CAPPUCCI J,US
Primary Class:
International Classes:
H03H7/48; (IPC1-7): H01P5/12; H01P3/08; H01P5/14
Field of Search:
333/10,11,84,84M
View Patent Images:
US Patent References:



Primary Examiner:
Saalbach, Herman Karl
Assistant Examiner:
Nussbaum, Marvin
Claims:
What is claimed is

1. A quadrature hybrid coupler including:

2. The quadrature hybrid coupler as defined in claim 1 also including an insulating substrate having first and second sides and said first and second conductors are maintained in close magnetic relationship by being held on said first and second sides, respectively, of said insulating substrate.

3. The quadrature hybrid coupler as defined in claim 2 in which said first and second plates of said capacitor are formed on said first and second side of said substrate.

4. The quadrature hybrid coupler as defined in claim 3 in which said first and second conductors are wrapped about a center to provide self-inductance increasing the inductance per unit length thereof.

5. The quadrature hybrid coupler as defined in claim 4 in which said first and second plates of said capacitor are formed at said center on said first and second sides of said substrate and form a central portion of said first and second conductors respectively.

6. The quadrature hybrid coupler as defined in claim 3 in which said first and second conductors are spirally arranged to provide self-inductance increasing the inductance per unit length thereof.

7. The quadrature hybrid coupler as defined in claim 4 in which said first and second plates of said capacitor are connected to said first and second conductors, respectively, by conductors affixed to said first and second sides, respectively, of said insulating substrate.

8. The quadrature hybrid coupler as defined in claim 2 in which said first and second plates of said capacitor are formed at said center on said first and second sides of said substrate and from a central portion of said first and second conductors respectively.

Description:
FIELD OF THE INVENTION

This invention relates to quadrature hybrid couplers and particularly to lumped constant parameter quadrature hybrid couplers.

BACKGROUND OF THE INVENTION

Lumped constant quadrature hybrid couplers are disclosed in U.S. Pat. No. 3,452,301 which issued to Joseph D. Cappucci et al. on June 24, 1969 and is entitled "Lumped Parameter Directional Coupler" and U.S. Pat. No. 3,452,300 which issued to Joseph D. Cappucci et al. on June 24, 1969 and is entitled "Four Port Directive Coupler Having Electrical Symmetry With Respect to Both Axes." These patents teach that directive quadrature couplers can be fabricated from lumped constant elements (substantially less than a quarter wavelength at the operating band of frequencies) if the input admittance thereof is the dual of the input impedance thereof at each of the ports when properly terminated.

U.S. Pat. No. 3,452,300 discloses a coupler formed from two end sections and a pair of uncoupled T sections interconnecting them. Each of the end sections are formed by a pair of conductors in close magnetic registration with a capacitor connecting one of the ends of one conductor in each pair to one of the ends of the other conductor respectively in that pair.

U.S. Pat. No. 3,452,301 teaches that a pair of closely magnetically coupled conductors may form a lumped constant quadrature hybrid coupler by having the distributed capacity between the two conductors form a major portion of the capacitance of the circuit to provide the above-described conditions of duality with respect to the desired terminating impedances. In both of these coupler constructions substantial improvement over the prior couplers was achieved, both from an economic view point and from the fact that they were substantially less than a quarter wave in length and therefore smaller for any particular operating band of frequencies than prior couplers.

U.S. Pat. No. 3,506,903 which issued to H. R. Beurrier on Apr. 14, 1970 and is entitled "Quadrature Hybrid Coupler" discloses a coupler substantially as taught in U.S. Pat. No. 3,452,301 in which the pair of couplers are held in registration by being bonded to a printed circuit board. The coupler of the Beurrier patent incorporates the teaching of duality of employs the capacity between the two conductors to form a substantial portion of the capacity of the coupler.

If any of these structures could be built with no parasitic capacitance or leakage inductance, the couplers would appear as solely inductive to an even mode excitation and purely capacitive to an odd mode excitation. However, in the real world this cannot be done. Therefore, the parasitic and leakage reactances have been taken into account analytically by treating them as compensated end sections at each of the ports of the couplers. This adds phase shifts to the signals applied to the coupler but no differential phase shift with respect to the input or output ports. This theoretical treatment of the parasitics holds up for the structures thus far devised so long as the leakage inductance is small and the frequency of operation is not too far from the center operating frequency of the coupler. When either of these conditions fail, however, the input admittance ceases to be the dual of the input impedance at each of the ports when appropriately terminated and therefore the device is no longer directive. To the extent that coupler is no longer directive, it does not serve the intent for which it was designed.

Therefore, it is an object of this invention to provide a new and improved lumped constant element quadrature hybrid coupler.

It is a further object of this invention to provide a lumped constant element quadrature hybrid coupler which has a greater band width of directivity than such couplers heretofore known.

It is still another object of this invention to provide a lumped constant element quadrature hybrid coupler which is less sensitive to parasitic reactances than such couplers heretofore known.

BRIEF DESCRIPTION OF THE INVENTION

With these and other objects in view the present invention contemplates a quadrature hybrid coupler in which first and second conductors are held in close magnetic relationship to provide coupling therebetween while a capacitor is connected between the ends thereof to render the coupler self-dual at each of its ports when terminated in an appropriate impedance.

The capacitor is connected midway between the ends of the first and second conductors respectively to segregate leakage and parasitic reactances symmetrically on either side of the capacitor so that they will physically behave as terminating end sections over a relatively large frequency range. The capacitance of the capacitor is large compared with the capacitance between the first and second conductors to maximize the effect of segregating the leakage and parasitic reactances into end sections.

By maximizing the amount of capacity in the lumped capacitor to maintain the desired relationship of duality, the thickness of the conductors is minimized. This tends to increase the inductance per unit length thereof, thereby further reducing the size of the device. In several embodiments the conductors are printed on opposite sides of an insulating substrate and are wrapped about themselves creating self-coupling, further increasing the inductance per unit length, further reducing the size of the device.

In these embodiments the capacitor may also be printed on opposite sides of the same insulating substrate further reducing the manufacturing costs of the devices constructed. In one embodiment the conductors are configured to allow the capacitors printed therewith to be connected thereto by additional printed circuitry on the substrate further reducing the manufacturing cost thereof.

DESCRIPTION OF THE FIGURES

A more complete understanding of the invention can be had by reference to the following figures and detailed description in which:

FIG. 1 is a diagram partially in block diagram form and partially in schematic form showing a coupler constructed in accordance with this invention connected into an operating system;

FIG. 2 is a top view of a printed circuit embodiment of this invention in which a capacitor is formed from a central portion of a printed conductor;

FIG. 3 is a top view of a second printed circuit embodiment of this invention in which a pair of oppositely wound spirals form the conductors and a pair of printed metalic areas form a capacitor connected directly thereto; and

FIG. 4 is a top view showing a third printed circuit embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 we see a lumped constant quadrature hybrid coupler 10 having an input port 1, a coupled output port 2, a transmitted output port 3 and an isolated port 4. The transfer characteristics (scattering parameters) of the coupler 10 are as taught in U.S. Pat. No. 3,452,301. The coupler 10 of FIG. 1 of this invention is analogous in its transfer characteristics to the coupler 10 of FIG. 1 of the above-mentioned U.S. Pat. No. 3,452,301.

A signal generator 11 having a source impedance Zo (the characteristic impedance of the coupler 10) is connected by a cable 12 also having the characteristic impedance Zo to the port 1. The ports 2 and 3 of the coupler 10 are connected each by cables 13 and 14, respectively, each having a characteristic impedance Zo to a utilization device 16. The utilization device 16 terminates the cables 13 and 14 in the characteristic impedance Zo. A resistor 17 having a value of Zo terminates the terminal 4.

In operation the signal applied by the signal generator 11 to the terminal 1 is divided according to the characteristics of the coupler 10 between terminals 2 and 3 and applied therefrom to the utilization device 16. The phase of signals appearing on the terminals 2 and 3 are in quadrature and utilized therefore in the utilization device 16 as is well known.

The coupler 10 of this invention is formed from a pair of conductors 18 and 19 held in close magnetic registration and a capacitor 21 connecting the mid-sections of the conductors 18 and 19. The values of the inductance of the conductors 18 and 19 and the total capacity of the coupler 10 including the capacitor 21 and the interconductor capacity between the conductors 18 and 19 are chosen in accordance with the teaching of U.S. Pat. No. 3,452,301 to provide the duality taught therein when terminated in the appropriate impedance Zo.

As mentioned above the transfer or scattering parameters of the coupler 10 are analogous to the transfer or scattering parameters of the coupler as taught in U.S. Pat. No. 3,452,301. However, the band of frequencies over which these parameters actually represent the performance of the device is substantially greater than in any lumped quadrature hybrid couplers heretofore in existence. It should be particularly appreciated at this point that the band shape (amplitude versus frequency) is not altered by placing the capacitor in the center rather than at the ends or distributed along the conductors 18 or 19. Rather the directivity versus frequency is altered. This means that the coupler 10 of this invention is a lumped constant quadrature hybrid coupler over a wider band of frequencies than previous lumped quadrature hybrid couplers. This characteristic renders the coupler 10 of this invention useful in systems and circuits which prior lumped constant quadrature hybrid couplers were not.

The reason that the coupler 10 with the capacitor 21 centrally located has a wider band width of directivity than heretofore known lumped constant quadrature hybrid couplers is that the leakage inductance and parasitic capacity is now symmetrically disposed on either side of the major portion of the capacity of the coupler 10. Therefore the leakage inductance and parasitic capacity performs in fact as an end section for the coupler 10. The value of the leakage inductance while not being eliminated can be thereby compensated for so that the end sections are constant K half sections having characteristic impedances of Zo. Therefore, phase shift is introduced thereby but the overall characteristic of the coupler 10 is not altered.

The directivity of the coupler 10 therefore does not fail until the cutoff frequency of the end section formed by the parasitic capacity and leakage inductance is reached. In prior couplers where the capacity of the system was either lumped at the respective ends of the conductors such as conductors 18 and 19 or distributed therebetween, the parasitic capacity and leakage inductance (while occasionally thought of as end sections) was physically in fact interspersed within the actual coupler structure. Therefore the prior art structures failed in directivity as the frequency moved away from the center operating frequency thereof. This was because the leakage inductance and parasitic capacity tended to defeat the duality of the coupler. In contradistinction to this by lumping the majority of the capacity at the central portion of the conductors 18 and 19, duality is maintained over a much broader frequency band thereby maintaining the properties of directivity heretofore discussed.

Referring now to FIG. 2 we see a first printed circuit embodiment of the coupler 10. The conductors 18 and 19 are held in close magnetic registration by being adhered to opposite sides of an insulating substrate 22. The capacitor 21 is formed by an enlargement of the conductors 18 and 19 at the central portions thereof so that the plates of the capacitor 21 are formed intricately in the conductors 18 and 19. It should be noted that the conductors 18 and 19 are wrapped around the portions thereof which form the capacitor 21 to provide self-inductance which increases the inductance per unit length thereof typically shortening the length of conductors 18 and 19 necessary by 40 to 50 percent. It should be further noted in this regard that by lumping the capacity at the center of the conductors 18 and 19 rather than distributing it in a printed circuit coupler, as was taught by U.S. Pat. No. 3,506,932, the width of the conductors 18 and 19 are reduced again increasing the inductance per unit length thereof resulting in shorter couplers. It should be noted that the couplers of the instant invention are shorter even without wrapping.

FIGS. 3 and 4 show additional printed circuit embodiments of the coupler 10 in which the conductors 18 and 19 are formed on opposite sides of an insulating substrate 22. In each of the embodiments in FIGS. 3 and 4 the conductors 18 and 19 are formed in two spirals to further increase the inductance per unit length. In FIG. 4 the two spirals are wound in opposite senses so that direct connection to the ports of the coupler 10 can be made from the ends of the conductors 18 and 19. The capacitor 21 is formed adjacent to the conductors 18 and 19 and are connected thereto at the center of the spirals by jumpers 23 and 24 on the side shown and analogous jumpers on the opposite side. It should be noted that the jumpers 23 and 24 are conductively connecting the centers of the two spirals together as well as attaching them to the plate of the capacitor 21 on the visible side of the substrate 22.

In the embodiment of FIG. 4 the conductors 18 and 19 are wound in two spirals having similar senses providing a mutual inductance further increasing the inductance per unit length resulting in, again, shorter devices than even the embodiment shown in FIG. 4. In this embodiment the capacitor may be formed by two areas on each side of the insulator 22 and can be directly connected to the central portion of the conductors 18 and 19 by printed circuit lines 26 and 27 on the visible side of the substrate 22 and analogous printed circuit lines on the opposite sides thereof. In the embodiment of FIG. 4 jumper leads 28 and 29 are required to connect the centers of the spiral on the visible side of the substrate 22 to the ports of the coupler 10. Analogous jumpers are used on the opposite side of the substrate 22.

It should be appreciated that while this invention has been disclosed with respect to four specific embodiments thereof, numerous others will become obvious to those of ordinary skill in the art in light thereof.