High frequency balun provided in a multilayer substrate

Baluns according to the present invention use both distributed and discrete elements connected together in a multilayer dielectric structure. As distributed elements, coupled striplines are provided in the multilayer dielectric structure. The discrete components are placed on the surface of the multilayer structure and connected with the distributed elements through via-holes. The operating frequency of the balun can be changed by changing values of the discrete components without changing the multilayer structure itself.

BACKGROUND 
The present invention is generally directed to baluns and, more 
particularly, to baluns which are implemented as part of a multilayer 
structure. 
A balun (which term comes from the phrase BALanced to UNbalanced) is a 
passive three port electronic circuit used for conversion between 
symmetrical and nonsymmetrical transmission lines. The signal, for example 
incoming to a nonsymmetrical port, is divided between two symmetrical 
ports providing signals which have the same amplitude but with phases 
which are 180 degrees offset relative to one another on their outputs. 
Baluns are used, for example, in the construction of balanced amplifiers, 
mixers and antenna systems. 
The balun construction depends on the intended operating frequency range. 
In the microwave frequency range, where the size of the structure is 
comparable to the wavelength of the signal, distributed element circuit 
technology is commonly used. In lower frequency ranges, e.g., up to 2500 
MHz, coupled wire transformer solutions are common in which wires are 
wound spirally around a highly permeable magnetic core. These conventional 
balun configurations suffer from a number of problems. 
These transformer solutions, using the phenomenon of magnetic coupling 
between wires, are theoretically wide band circuits. In practice, however, 
compensation for eigen capacitances is needed, especially in the frequency 
range 400 to 2500 MHz. This means that the physical construction of the 
transformer-type baluns has to be specifically optimized for operation 
within its operating frequency bandwidth. Additionally, it is difficult to 
accurately set the length of the wires to be wound about the core so that 
baluns which are designed to be the same, actually have substantially the 
same electrical characteristics. 
Most existing baluns operating in the high (e.g., greater than 2500 MHz) 
frequency range, give good balun performance only if both symmetrical 
ports are well matched. In many applications, power matching of the 
symmetrical ports is not desirable for other reasons. For example, power 
matching on the symmetrical inputs of mixers or amplifiers worsens their 
noise parameters. Thus, a compromise between power matching and noise 
matching is needed. 
Moreover, ongoing miniaturization of electronic structures is, in turn, 
causing the miniaturization of baluns. For example, baluns used in the 
frequency range 400 to 2000 MHz are not usually bigger than about 20 
mm.sup.2 and are designed for automatic surface mounting onto the end 
product. However, during the production of the baluns themselves, manual 
mounting is still used because the wires require manual winding around the 
core and the ends of the wires need to be inserted into electrical 
connectors on the end product. Manual mounting is expensive, time 
consuming and causes spread in the parameters of the end product, e.g., a 
radio receiver, of which the balun is just one of many components. 
Thus, it would be desirable to provide a balun having better symmetry when 
working with unmatched loading, which do not require different physical 
constructions to handle different operating frequency ranges, and which 
are less expensive to manufacture by allowing automatic integration of the 
balun with other circuit components as opposed to manual mounting. 
SUMMARY 
These and other drawbacks and limitations of conventional baluns are 
overcome according to exemplary embodiments of the present invention. 
Baluns according to the present invention use both distributed and 
discrete elements connected together in a multilayer dielectric structure. 
As distributed elements, coupled striplines are provided in the multilayer 
dielectric structure. The discrete components are placed on the surface of 
the multilayer structure and connected with the distributed elements 
through via-holes. This provides a number of advantageous balun 
characteristics. 
For example, within a range bounded by the characteristics of the 
dielectric material used in the multilayer structure, the operating 
frequency range of the balun can be adjusted simply by changing values of 
the discrete components used to fabricate the balun. In this way, the 
operating frequency of the balun can be easily changed without 
necessitating a completely new balun construction. This is a great 
advantage as compared to, for example, transformer-type baluns for which a 
completely new construction design was required for different operating 
frequency ranges. 
By fabricating the balun as a multilayer structure, the distributed 
elements can be provided in a layer which is embedded below that on which 
the discrete components are mounted. This allows the top layer surface 
area required for the balun to be reduced which further promotes 
miniaturization of the products in which the balun is incorporated.

DETAILED DESCRIPTION 
An example of a multilayer structure in which baluns according to the 
present invention can be implemented is shown (in a top view) as FIG. 1. 
Therein, a coupled pair of striplines S1 and S2 are illustrated as hidden 
(i.e., by way of the dotted lines) since the coupled striplines are 
embedded in a lower layer of the multilayer structure 10. A stripline is a 
well known transmission line which can be formed as a conductive metal 
trace placed in a dielectric media with two parallel ground planes on both 
sides of the dielectric surface. A coupled stripline is a structure using 
two striplines having a constant distance between them. In a multilayer 
structure, coupled striplines can be made as two parallel traces on the 
same layer with ground planes on layers above and below the layer with 
traces, or as parallel traces placed on two adjacent layers. The remaining 
components illustrated in FIG. 1 are on the surface or top layer of the 
multilayer structure 10. For example, the multilayer structure 10 could 
include surface mounted devices 3 and 5. Surface mounted devices 3 and 5 
can be electrically connected to the coupled striplines S2 and S1, 
respectively, using vias (thru holes) 7 and 9, respectively. As is well 
known in the art, vias are apertures formed in multilayer structures which 
are plated with conductive material to establish electrical connections at 
desired points between different layers in the multilayer structure. 
Additionally, the exemplary multilayer structure 10 shown in FIG. 1 
includes microstrips 11 and 13 on the surface or top layer. As is well 
known in the art, microstrips are controlled impedance microwave frequency 
transmission lines which are formed as conductive metal traces on one side 
of a dielectric surface with a ground plane on the other side of the 
dielectric surface. Microstrips 11 and 13 are connected to one of the 
coupled striplines S2 by via-holes 15 and 17, respectively. Also 
illustrated in FIG. 1 is a via 19 which connects the coupled stripline S1 
and S2 with a ground plane. 
FIG. 2 illustrates a side view of an exemplary multilayer structure in 
which baluns according to the present invention can be implemented. 
Although the side view portrays a slightly different component 
configuration than the multilayer structure of FIG. 1, similar reference 
numerals are used to refer to similar elements. For example, the top layer 
(denoted Layer 1 in FIG. 2) includes a surface mounted device 3. This 
surface mounted device 3 is connected to one of the coupled striplines S2 
which have been fabricated in Layer N-1. In this example, the multilayer 
structure has four layers, although any number of layers which are equal 
to or greater than four can be used. The via 7 which interconnects surface 
mounted device 3 with coupled striplines S1 and S2 is isolated from the 
ground planes as seen, for example, at point 20 on Layer N which 
illustrates a separation between the via 7 and the metallized ground plane 
portions 22. Another conductive via 24 provides a connection between the 
ground plane Layers N and N-2. 
Each of the four conductive layers illustrated in FIG. 2 are separated from 
adjacent layers by a layer of dielectric material. Moreover, the discrete 
electrical components provided on Layer 1 of the multilayer structure are 
electrically isolated from the electrical components provided on Layer 
N-1, e.g., coupled striplines S1 and S2, by a ground plane provided as 
Layer N-2. The ground plane can, for example, be a copper layer of about 
17.5 mm in thickness. This helps to ensure that the operation of the 
components provided on Layer 1 is not affected by the provision of 
electrical impulses to the components on Layer N-1, e.g., by capacitive or 
inductive coupling effects. 
The operating frequency of the balun is bounded by the electrical 
parameters of the dielectric layers provided in the multilayer structure 
(e.g., dielectric constant, dielectric losses (loss tangent) and 
dielectric thickness). For example, if a typical glass-fiber resin 
material (e.g., having a dielectric constant of 4.25, a loss tangent of 
0.02 and a layer thickness of 5 mm) is used for the dielectric layers of 
FIG. 2, an operating frequency of baluns according to the present 
invention can be set to be between 100 MHz and 2.5 GHz. The lower value 
is, in practice, limited by the lengths of the stripline structure. The 
higher frequency value is limited by losses in dielectric layers and 
higher wavelength transmission modes which are associated with increasing 
frequency (given a constant dielectric layer thickness). A significant 
feature of the present invention is that the operating frequency of the 
balun may be changed within the bounded range by changing the values of 
the discrete components, without changing the multilayer structure itself. 
These and other benefits of baluns according to the present invention will 
become more apparent after reviewing the detailed description of an 
exemplary balun circuit configuration provided below. 
An exemplary balun according to the present invention is shown in FIG. 3. 
In FIG. 3, vias are depicted using shaded circles. As seen by the examples 
in this figure, some of the vias are connected to a ground plane (e.g., 
Layer N-2 and Layer N) while others are connections between the top 
surface of the multilayer structure and an embedded layer. Port 30 is the 
nonsymmetrical port of the balun, while ports 32 and 34 are the 
symmetrical outputs. Thus, ports 32 and 34 each provide an output having 
the same amplitude, but whose phases differ by 180 degrees (if the balun 
is perfectly symmetrical). Port 36 is optionally provided (as is 
transmission line S7) if an external bias is to be connected for biasing 
ports 32 and 34. Port 36 can be used, for example, to connect active 
devices (e.g., active amplifiers or active mixers) to bias the symmetrical 
output ports 32 and 34. Transmission line S7 provides electrical isolation 
between common node 38 and port 36. When connected to passive devices, 
port 36 (and transmission line S7) can be omitted. 
The distributed element part of the balun includes two sections S 1 and S2 
of coupled transmission striplines. Section S1 includes striplines S11 and 
S12, while section S2 includes striplines S21 and S22. Both sections S1 
and S2 can have identical characteristic impedances for even and odd modes 
and can have identical electrical lengths and be coupled together at 
common node 38 (represented by a dotted line in FIG. 3). This can be 
accomplished by making each stripline S11, S12, S21 and S22 of the same 
length (e.g., 6.4 mm), same width (e.g., 0.2 mm), same thickness and 
providing a uniform spacing between sections S1 and S2 (e.g., 0.15 mm). 
Node 40 of stripline S12 is connected to the ground planes and node 42 of 
stripline S11 is connected to capacitors C1 and C2. The capacitors C1 and 
C2 have values that are chosen based upon the desired operating frequency 
of the balun to provide proper matching and impedance transformation for 
the nonsymmetrical output of the balun. Nodes 44 and 46 of striplines S21 
and S22 are connected to striplines S3 and S5 and, together with 
capacitors C3 and C4, give proper impedance transformation at nodes 48 and 
50, respectively. Symmetrical output ports 32 and 34 are connected to 
nodes 48 and 50 with lines S4 and S6. The proper choice of impedance 
values and electrical lengths for transmission line elements S3 and S4 and 
capacitance value of C3 on one side and substantially the same impedance 
values and electrical lengths for transmission line elements S5 and S6 and 
capacitance value for C4 on the other side, determines an output impedance 
of the terminating balun circuit. Those skilled in the art will appreciate 
that the output impedance can be varied so that the symmetrical ports 
provide maximal gain (power matching), minimal noise (noise matching) or a 
compromise between the two competing objectives depending upon the 
application. The value selected for capacitor C5 gives a proper symmetry 
of the balun so that the symmetrical outputs have substantially the same 
amplitude and are as close to 180 degrees offset in phase as possible. 
This capacitance value depends on the characteristic impedances of the 
coupled striplines for even and odd modes as well as the electrical 
lengths of sections S1 and S2 at the operating frequency of the balun. If 
used, the characteristic impedance and electrical length of transmission 
line S7 and the impedance in port 36 have to be taken into account to 
determine the appropriate capacitance value for C5 to maintain the balun 
symmetry. 
As mentioned above, coupled stripline sections S1 and S2 are embedded 
within a multilayer structure, e.g., structure 10, while the discrete 
components (e.g., capacitors C1-C5) are provided on Layer 1 or the top 
surface of the structure. Depending on the design constraints of the 
balun, striplines (in an embedded layer) or microstrips (on a surface 
layer) can be used for transmission line elements S3, S4, S5, S6 and S7. 
In the example shown in FIG. 3, S3 and S5 are fabricated as striplines in 
an embedded layer (e.g., Layer N-1) along with coupled striplines S1 and 
S2. Transmission line elements S4 and S6 are fabricated as microstrips on 
the surface layer along with capacitors C1-C5. 
According to exemplary embodiments of the present invention, baluns are 
constructed in a multilayer structure using distributed and discrete 
components such that the operating frequency of the balun can be easily 
adjusted. For example, by adjusting values of capacitors C1-C5, the 
operating frequency of the balun can be changed within the range dictated 
by the dielectric media used (e.g., at least within an octave of an 
originally designed operating frequency between 100 MHz and 2.5 GHz) 
without changing the multilayer structure or adjusting the coupled 
striplines which are embedded therein. For example, using the values in 
Table 1 below, a balun according to the exemplary embodiment of FIG. 3 can 
operate within the range of 935-960 MHz. By changing the values of 
capacitors C1-C5 to those shown in Table 2, the same balun structure can 
instead operate at between 425-430 MHz. Those skilled in the art will 
appreciate that other capacitance values can be used to achieve other 
operating frequency ranges. 
TABLE 1 
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C1 1.8 pF 
C2 0.47 pF 
C3 4.7 pF 
C4 4.7 pF 
C5 3.3 pF 
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TABLE 2 
______________________________________ 
C1 8.2 pF 
C2 12 pF 
C3 27 pF 
C4 27 pF 
C5 33 pF 
______________________________________ 
The above-described exemplary embodiments are intended to be illustrative 
in all respects, rather than restrictive, of the present invention. Thus 
the present invention is capable of many variations in detailed 
implementation that can be derived from the description contained herein 
by a person skilled in the art. For example, the nonsymmetrical port can 
be used as either an input or an output port, while the symmetrical ports 
can be used as output or input ports, respectively. All such variations 
and modifications are considered to be within the scope and spirit of the 
present invention as defined by the following claims.