JPH10335913A - Improvement of microwave directional coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microwave directional coupler without the use of expensive materials. SOLUTION: A microwave device incorporating a directional coupler 20 is constituted, including a coupler dielectric board 22 which is vertically mounted on a parent circuit board 24. Upper interdigital elements are arranged on the facing surfaces of the board 22. The couplings among the upper interdigital elements decide the odd mode impedance Zoo of the microwave directional coupler 20. Lower interdigital elements are also arranged on the facing surfaces of the board 22. The lower interdigital elements are connected to the ground and arranged, so that the couplings among the lower interdigital elements and among the upper interdigital elements decide the even mode impedance Zoe of the coupler 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to microwave devices, such as directional couplers, and to interdigital (interdigital) devices.
digital) A coupler having a coupling line element.

[0002]

BACKGROUND OF THE INVENTION Microwave directional couplers are microwave devices with four ports that are used as power dividers or combiners. When a signal is input to port 1, it is output to port 4
Instead, they are coupled to ports 2 and 3. Port 4
Are also coupled to ports 2 and 3 instead of to port 1. Because there is no coupling between ports 1 and 4, these ports are known with respect to each other as uncoupled or isolated ports. Signals can be input to ports 2 and 3 or can be derived from reflections at these. The signal input to port 2 is coupled to ports 1 and 4, but not to port 3. On the other hand, the signal input to port 3 is coupled to ports 1 and 4, but not to port 2. Thus, ports 2 and 3 are isolated ports.

In a radar system, a radar transmitter is coupled to port 1 of a directional coupler. Next, the antenna is coupled to port 2, the microwave power detector is coupled to port 3, and port 4 terminates in a matched load. With this configuration, the reflected power propagation to port 2 measured at port 3 as the transmitter power output does not result in any antenna impedance mismatch. This is true. This is because ports 2 and 3 are isolated ports, and no signal input to port 2 is coupled to port 3.

A microstrip directional coupler is disclosed in US Pat. No. 4,823,097, and comprises a first dielectric board that stands upright on top of a parent dielectric board. Coupling line portions are located on each side of the first dielectric board. Leads are placed on the top surface of the parent dielectric board and connected to both ends of each coupling line section. The ground plane is formed by a conductive layer located on the underside of the parent dielectric board.
This physical configuration results in the even and odd mode impedance of the coupler determined by both the coupling between the coupling line portions and the coupling between each coupling line portion and the ground plane. Since the impedance is determined in part by coupling to the ground plane, the configuration and physical dimensions of the parent dielectric board will affect the impedance. Therefore,
Coupler design requires knowledge and consideration of the type of parent dielectric board to which the first dielectric board will be mounted.

Another embodiment of a microstrip directional coupler is disclosed in US Pat. No. 5,373,266.
The coupler is a pair of microstrip coupling elements having straight, spaced edges and a curved edge having adjacent edges inverted in a curvature, e.g., a sine or semicircular pattern. And those following the path. These coupling elements are arranged on one surface of the dielectric substrate. A ground plane conductive layer is disposed on the opposite surface of the dielectric substrate. In this coupler, the even mode impedance is formed by the coupling between the linear spaced edge of the microstrip coupling element and the ground plane.

A directional coupler comprising comb electrodes having elongated bus bars with a plurality of spaced teeth is disclosed in US Pat. No. 4,394,630. In one embodiment, the first comb electrode is located on one side of the dielectric substrate and the second comb electrode is located on the other side. Usually, a rectangular, hollow conductor incorporates and supports the dielectric substrate. A filling material having predetermined dielectric properties is used to fill the volume between one side of the dielectric substrate and the adjacent side of the hollow rectangular conductor. In the same way, a filling material is used to fill the volume between the other side of the dielectric substrate and the adjacent side of the hollow rectangular conductor. The even and odd mode impedance of the coupler is determined by the composition and physical dimensions of both the dielectric substrate and the fill material.

[0007]

A disadvantage common to these prior art couplers is that the conductor line pattern forming the coupling element is printed on a parent printed wiring board containing other peripheral electronics. That is to say. Design conflicts arise in this situation, i.e., where the directional coupler requires expensive dielectric material to achieve the required performance, while the peripheral circuitry requires less expensive material. Because the coupler and peripheral circuits are housed on the same circuit board, meeting the coupler design criteria requires the use of expensive materials for the entire board.

[0008]

SUMMARY OF THE INVENTION The present invention includes a microwave device including a directional coupler, the microwave device comprising a parent circuit board having opposing surfaces, and a coupler dielectric board, A first and a second upper interdigitated electrically conductive element comprising an opposing surface and the opposing surface mounted substantially perpendicular to one of the opposing surfaces of the parent circuit board; A first and a second lower interdigitated electrically conductive element respectively disposed on opposing surfaces of the coupler dielectric board, each disposed on opposing surfaces of the coupler dielectric board. And wherein the upper interdigital electrically conductive elements are electrically coupled to each other. The coupler may be presented with an odd mode impedance having a value that varies as a function of the coupling, wherein each of the lower interdigital electrically conductive elements is spaced apart from each upper interdigital electrically conductive element. And the lower interdigital electrically conductive element and the upper interdigital electrically conductive element are electrically coupled to each other,
The coupler has an even mode impedance,
It is characterized in that it has a value which varies as a function of the coupling between the lower interdigital electrically conductive element and the upper interdigital electrically conductive element.

Next, the present invention will be described by way of embodiments with reference to the accompanying drawings. A microwave directional coupler 20 is shown in FIGS. The coupler 20 comprises a coupler dielectric board 2 having flat surfaces 22a and 22b.
2 inclusive. Coupler 20 has flat surfaces 24a and 24a.
mounted on the parent circuit with b. Coupler 20
Are the flat surfaces 24a and 2a of the parent circuit board 24.
Mounted with its flat surfaces 22a and 22b perpendicular to 4b. Although not shown in FIGS. 1-3, other peripheral circuits associated with this circuit, in which coupler 20 is used, are also mounted on parent board 24. Coupler 20 has surface 22a
And 22b, respectively, including interdigitated coupling elements 26 and 28 disposed thereon. The interdigital coupling element 26 includes an upper interdigital element 26a and a lower interdigital element 26b. The upper interdigital element 26a comprises two lateral sections 30 and 32, each of which is connected to a longitudinal section 34 having a predetermined length and width. The longitudinal section 34 has a straight upper edge 36 and a lower edge 38 with teeth 40. Repetitively finger-like engraved on the tooth 40 is the lower interdigital element 26
b are the teeth 42 formed on the upper edge 44 of FIG.

The interdigitated coupling element 28 is symmetrically arranged on the flat surface 22b. The interdigitated coupling element 28 includes an upper interdigital element 28a.
And a lower interdigital element 28b. The upper interdigital element 28a is 2
Comprising side portions 46 and 48, each of
It is connected to one end of a longitudinal portion 50 having a predetermined length and width. The longitudinal section 50 has a straight upper edge 52 and a lower edge 54 with teeth 56. Interdigitated with tooth 56 is tooth 58 formed on upper edge 60 of lower interdigital element 28b.

Although the coupling elements 26 and 28 are shown and described symmetrically, this need not be the case. For example, the width of the upper interdigital elements 26a and 28a can be varied to adjust the desired coupling factor of the coupler 20. Upper interdigital element 26a
And 28a and lower interdigital elements 26b and 28b (shown as W2 in FIG. 2) are also adjusted. These lengths are set as a function of the desired operating frequency of coupler 20.

The interdigitated coupling elements 26 and 28 are formed by etching the surface of the coupler dielectric board 22 according to a very accurate photographic master. Prior to etching, the surface of coupler dielectric board 22 is covered on its flat surfaces 22a and 22b with a rolled or electrodeposited copper film. The alignment marks on the photo master ensure a high degree of back-to-front alignment of the pattern.

In addition, conductive traces must be placed on parent circuit board 24 to facilitate mounting of coupler 20. Four conductive traces 62, 64, 66 and 68 are disposed on the flat surface 24a of the parent circuit board 24. Traces 62, 64, 66 and 68
Are the parts 30, 32, 4 respectively with minimal edge effects
Provides direct connection to 6 and 48. This provides a good match between the conductive traces and the associated sides. In addition to the conductive traces 62, 64, 66 and 68, two ground planes 70 and 72 are also located on the flat surface 24a of the parent circuit board 24. Grounding surface 7
0 and 72 make the width W2 the same as the lower interdigital coupling elements 26b and 28b.
The lower ground plane 74 is the flat surface 2 of the parent circuit board 24.
4b. Each ground plane 70 and 72 is connected to a lower ground plane 74 by a plurality of passages 76. This lower ground plane 74 is connected to the system ground potential, and the passage 76 results in ground planes 70 and 72 which are also at system ground potential.

The directional coupler 20 is connected to the coupler dielectric board 2.
2 are connected to the parent circuit board 24 by soldering the conductive traces located on the parent circuit board to those located on the parent circuit board. Sides 30 and 32 are soldered to conductive traces 62 and 64, respectively, at lower edge 78 of coupler dielectric board 22. Sides 46 and 48 are similarly soldered at lower edges 78 to conductive traces 66 and 68, respectively. The lower interdigital coupling element 26b is soldered to the ground plane 70 at the lower edge 78, while the lower interdigital coupling element 28b is
At 8, it is soldered to the ground plane 72. All these solder connections form a fillet solder joint 80.
Other methods, for example, a suitable conductive adhesive may be used instead of the fillet solder joint 80.

During soldering or other connection means,
Proper mechanical alignment of coupler dielectric board 22 is accomplished through the use of mounting pegs 82 and 84. Each mounting peg 82 and 84 is mounted on the parent circuit board 24.
Fit into the relevant mounting holes (not shown) above. The use of mounting pegs 82 and 84 for alignment and the use of fillet solder joints 80 for mechanical and electrical connections is known in the art as "pick-and-place".
d-place) "should be easily automated using assembly techniques.

The electrical characteristics of the directional coupler 20 are determined by the coupling factor C of the coupler. An equal amount of current in the same direction
When flowing through both coupling elements 26a and 28a, an even mode impedance, represented by Zoe, exists between each element and ground. Similarly, a current of equal magnitude, but in the opposite direction, is applied to the upper interdigital coupling element 26.
a and 28a, there is an odd mode impedance represented by Zoo between each element and ground. The even and odd mode impedances for a coupled pair of lines are given by the following equations:

[0017]

(Equation 1)

[0018]

(Equation 2) Where C is the coupling factor and Zo is the characteristic impedance of the transmission line (ie, conductive traces 62-68) to which coupler 20 is connected. Equations (1) and (2) describe the coupling factor C and the even and odd mode impedances Zoe and Zoo.
The functional relationship between them is shown. In designing the coupler 20, a desired coupling factor C is selected (for example, 3 dB). From the selected coupling factor C, equations (1) and (2) derive the even and odd mode impedances Zoe and Zoo required to achieve that coupling factor. The required even and odd mode impedances Zoe and Zoo must also satisfy the relation Zo = (ZoeZoo) 1/2 to meet the impedance matching requirements, as is known in the art. Equations (1) and (2) assume that the phase velocities of the radio waves associated with both the even and odd mode impedances Zoe and Zoo are equal. If these phase velocities are not equal, the specific impedance of each mode must be used to determine the associated impedance. If not, the bandwidth and directivity of the directional coupler will be adversely affected by the different phase velocities.

FIGS. 5 and 6 illustrate the causes of the different phase velocities. The general formula of the phase velocity is as follows.

[0020]

(Equation 3) Where 4 is the speed of light in free space (ie, 3 ×
108 m / sec), and ∈eff is the effective dielectric constant of the medium in which the electric field propagates. As shown in FIG. 5, the electric field 86 associated with the even mode impedance Zoe propagates partially in the coupler dielectric board 22 and partially in the air 88. As a result, the effective permittivity ∈eff is the permittivity of free space ∈o and the relative permittivity of the dielectric board 22 ∈
exists somewhere between r. However, the effective permittivity ∈eff for the even mode impedance Zoe
Is the odd mode impedance Zo, as shown in FIG.
o is different than that for o. The electric field 90 associated with the odd mode impedance Zoo is mainly propagated to the dielectric board 22 and thus has an effective permittivity ∈eff,
It will have something closer to the relative permittivity Δr of the dielectric board. The equations for the even and odd mode impedances Zoe and Zoo as a function of the associated phase velocities are:

[0021]

(Equation 4)

[0022]

(Equation 5) Where Vpo is the phase velocity associated with the odd mode impedance Zoo, Vpe is the phase velocity associated with the even mode impedance Zoe, and Coo is the effective per unit length of one line to ground for the odd mode impedance. Capacitance and Coe
Is the effective capacitance per unit length of one line to ground for even mode impedance.

In order to equalize the propagation delay, the radio waves associated with the even mode impedance Zoe are transmitted to the teeth 40, 42,
The gap created by 56 and 58,
Meandering around what forms the interdigitated pattern. The propagation delay for radio waves associated with the even mode impedance Zoe is a function of their tooth number or periodicity. In practice, equations (4) and (5) cannot be used to directly determine Zoo and Zoo. It is the phase velocities Vpo and V
This is because pe cannot be easily measured. Instead, it must be determined by modeling, mapping or calculating the capacitances Coo and Coe. Therefore, the determined capacitance Coo
And Coe are used to approximate the odd and even impedances Zoo and Zoe. The phase velocity is then adjusted by changing the physical layout of the interdigitated pattern until the required Zoo and Zoo are achieved and the desired coupling factor C is obtained.

One embodiment of the directional coupler 20 according to the present invention described above was constructed and tested. The physical parameters of this embodiment were as follows. Average length of upper interdigital coupling element 58.6 mm Length of lower interdigital coupling element 46.0 mm Tooth width 1.0 mm Tooth gap 2.5 mm Total thickness of foil forming coupling element .038 mm Coupler dielectric Board thickness .813 mm Coupler dielectric board constant 2.5 Coupler dielectric board dissociation factor 0.0025 at 10 GHz Coupler dielectric board 22 in this embodiment was prepared from a woven glass fiber / PTFE composite.

The test results of the preferred embodiment are shown in FIGS. In FIG. 7, lines 92 and 94 are graphs, which show the output from ports 2 and 3, respectively, when the signals input to port 1 and port 4 are determined in a matched load. The vertical axis is the power represented by each vertical section 1 dB. The horizontal axis represents frequency, which is from 450 Mhz to 95
0 Mhz. Above the full 500 Mhz frequency range, the graph of FIG. Approximately 1/2 of the power input to port 1 is port 2
And 1/2 are coupled to port 3. At the center frequency 700 MHz indicated by arrow 98, the power at port 3 is about -2.76 dB, while the power at port 2 is about -3.45 dB.

[0026]

FIG. 8 shows that coupler 20 exhibits good return loss. In FIG. 8, the vertical axis represents power again, and the horizontal axis represents frequency. Each vertical section represents 10 dB, and 100 is a reference level of 0 dB. Line 102 is a graph of the reflected power resulting from the input signal to the other one when three of the ports of coupler 20 are terminated with matched loads. Ideally, the reflective power should be zero for this configuration. Over the entire frequency range,
Line 102 is shown at a level whose reflection power is less than about -25 dB. At a center frequency of 700 MHz, the reflected power level is -27.917 dB, as indicated by arrow 104.

FIG. 9 illustrates that the coupler 20 exhibits good isolation and thus good directivity over the relevant frequency range. The vertical axis again shows power with each vertical section 10 dB. The horizontal axis is again frequency. Arrow 106 is 0d
This is the B reference level. Line 108 is a graph for the power output from port 4 when the signals input to port 1 and ports 2 and 3 are terminated in a matched load. Recall that ideally ports 1 and 4 are isolated. This means that no power is coupled to ports 1 through 4 (ie, infinite directivity). At a center frequency of 700 MHz indicated by an arrow 110,
The power at port 4 is about -47.015 dB.
Over the entire frequency range, line 108 indicates that the power at port 4 is at a level below -38 dB, which is excellent isolation and directivity for this type of coupler.

[Brief description of the drawings]

FIG. 1 is a microwave directional coupler according to the present invention.

FIG. 2 is a side view showing one side of the coupler dielectric board 22 of FIG. 1;

FIG. 3 is a side view showing another side of the coupler dielectric board 22 of FIG. 2;

FIG. 4 is a cross-sectional view showing a microwave directional coupler.

FIG. 5 is a cross-sectional view of the microwave directional coupler showing the electric field associated with the even mode impedance Zoe.

FIG. 6 is a cross-sectional view of a microwave directional coupler showing an electric field associated with an odd mode impedance Zoo.

FIG. 7 is a graph showing power output from a coupled port of a microwave directional coupler as a function of frequency.

FIG. 8 is a graph showing the input return loss of a microwave directional coupler as a function of frequency.

FIG. 9 is a graph showing microwave directional coupler isolation as a function of frequency.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 20 Microwave directional coupler 22 Coupler dielectric board 22a, 22b Flat surface 24 Parent circuit board 24a, 24b Flat surface 26, 28 Coupling element 26a, 28a Upper interdigital element 26b, 28b Lower interdigital coupling element 62 , 64, 66, 68 conductive traces 70, 72 ground plane 74 lower ground plane 76 passage

Claims (10)

[Claims] 1. A parent circuit board having an opposing surface, and a coupler dielectric board, the opposing surface having an opposing surface mounted substantially perpendicular to one of the opposing surfaces of the parent circuit board. And first and second lower interdigital electrical conductive elements disposed on opposing surfaces of the coupler dielectric board, respectively. Electrically conductive elements, each of which is disposed on an opposing surface of the coupler dielectric board, wherein the upper interdigital electrically conductive elements are electrically coupled to each other, A coupler with odd mode impedance that has a value that varies as a function of coupling As shown, each of the lower interdigital electrically conductive elements is spaced from a respective upper interdigital electrically conductive element, and the lower interdigital electrically conductive element and the upper interdigital electrically conductive element. Are electrically coupled to each other,
The coupler has an even mode impedance,
A microwave device including a directional coupler, exhibiting a value that varies as a function of coupling between the lower interdigital electrically conductive element and the upper interdigital electrically conductive element. . 2. The apparatus of claim 1 wherein said first and second upper interdigitated electrically conductive elements are symmetrical, wherein said first and second lower interdigitated electrically conductive elements are symmetrical. . 3. The apparatus according to claim 1, wherein said coupler dielectric board has a dielectric constant ∈, said parent circuit board has a dielectric constant ∈r, and these dielectric constant values ∈ and Δr are not equal. 4. Means for electrically connecting said first and second lower interdigitated electrically conductive elements to ground potential, and wherein said first and second upper interdigitated electrically conductive elements are provided. 2. The method of claim 1, wherein the sexual elements have preferably unequal widths.
The device according to any one of claims 1 to 3, wherein 5. The method of claim 1, wherein the first and second upper and lower interdigital electrically conductive elements have a scaled length as a function of the desired operating frequency. Apparatus according to any of claims 1 to 4. 6. A parent circuit board having an opposing surface and a coupler dielectric board, the opposing surface being provided and the opposing surface being mounted substantially perpendicular to one of the opposing surfaces of the parent circuit board. And first and second lower interdigital electrical conductive elements disposed on opposing surfaces of the coupler dielectric board, respectively. An electrically conductive element disposed on opposing surfaces of the coupler dielectric board, wherein the upper interdigital electrically conductive element is disposed on a straight top edge and disposed thereon. A bottom edge having a plurality of teeth, wherein the upper interdigital electrically conductive elements are mutually alternating. When coupled to the coupler, the coupler presents an odd mode impedance having a value that varies as a function of the coupling, each of the lower interdigital electrically conductive elements being Remote from the upper interdigitated electrically conductive element, the lower interdigitated electrically conductive element has a straight bottom edge and a top edge having a plurality of teeth disposed thereon; The teeth are repeatedly finger-fed for teeth located on each bottom edge of the upper interdigitated electrically conductive element, and the lower interdigitated electrically conductive element and the The upper interdigital electrically conductive elements are electrically coupled to each other. Is grayed,
The coupler has an even mode impedance,
A microwave device including a directional coupler, exhibiting a value that varies as a function of coupling between the lower interdigital electrically conductive element and the upper interdigital electrically conductive element. . 7. The number of teeth located on the bottom edge of said upper interdigital electrically conductive element is even, and in this case, the number of teeth located on the top edge of said lower interdigital electrically conductive element. Apparatus according to claim 6, wherein the number is preferably odd. 8. The apparatus of claim 6, wherein said first and second upper interdigitated electrically conductive elements are symmetrical, and wherein said first and second lower interdigitated electrically conductive elements are symmetrical. 9. The method according to claim 6, wherein the coupler dielectric board has a dielectric constant Δr, and the parent circuit board has a dielectric constant ∈, and the dielectric constants ∈r and ∈ are not equal. The described device. 10. Apparatus according to claim 6, including means for electrically connecting the first and second lower interdigitated electrically conductive elements to ground potential.