Multilayer balun transformer structure

Details Multilayer balun transformer structure Multilayer balun transformer structure

2001-12-21

2003-08-05

Nguyen, Tuyen T. (Department: 2832)

Inductor devices

Coil or coil turn supports or spacers

Printed circuit-type coil

C336S083000, C333S025000

Reexamination Certificate

active

06603383

ABSTRACT:

BACKGROUND
FIELD OF THE INVENTION
The present invention relates to balun transformers, in particular it relates to balun transformers with one or two balanced ports and an unbalanced port which have a small size, low microwave losses and which most particularly are suitable for Monolithic Integrated Circuit (MMIC) applications. Particularly they should be small enough for applications at low frequencies, particularly below about 10 GHz.
A balun (balanced-to-unbalanced) transformer is a (passive) two or three-port electronic circuit with a functionality of converting an unbalanced signal (unbalanced in relation to ground) into balanced signals and vice versa of converting balanced signals into an unbalanced signal. It is generally used for conversion between balanced (symmetrical) and unbalanced (non-symmetrical) transmission lines. A signal incoming to an unbalanced port may be divided between two balanced ports providing signals which have the same amplitude but phases differing 180° in relation to one another. Baluns may for example be used in transmitting circuits and receiving circuits of mobile communication devices, for construction of balanced amplifiers, mixers, VCOs (Voltage Controlled Oscillators), antenna systems etc.
Generally baluns are widely used in microwave devices and microwave systems. As referred to above, a balun can be used both as a two-port component and as a three-port component.
In
FIG. 1
a (state of the art) two-port balun, or rather the lumped circuit equivalent of such a balun, is illustrated. The balanced port is formed between terminals T
3
and T
4
respectively, at which terminals the potentials are V
3
and V
4
respectively, of a primary coil whereas the unbalanced port is formed by terminals T
1
and T
2
respectively of a secondary coil. The potential at T
2
of the unbalanced port is zero whereas T
3
is not balanced, here having a potential denoted V
u
. The unbalanced port may be connected to a microstrip, coplanar waveguide (CPW) or some other kind of unbalanced lines or components. For a two port component, the balanced port may be connected to components which have no ground plane on the back side of the substrate such as an inductor or a coplanar strip waveguide (CPS).
FIG. 2
shows a lumped circuit equivalent of a balun with a ground plane on the back of the substrate, i.e. of a three-port component. Again terminals T
1
and T
2
form the unbalanced ports with V
2
=0 and V
1
=V
u
, T
2
being connected to ground. A first balanced port, port P
b1
, formed by terminals T
3
at potential V
3
=V
b
and T
5
at potential V
5
=0 (however, T
5
does not have to be grounded, it may also be a fixed voltage) whereas the second balanced port, port P
b2
, is formed by T
4
and T
5
, wherein the potential at T
4
is V
4
=−V
b
. The potentials at terminals T
3
and T
4
are 180° out of phase with respect to the terminal T
5
and T
5
corresponds to the ground plane on the backside of the substrate on which the balun is provided. For one three-port configuration component, T
5
should be in galvanic contact with the ground plane. The balun of
FIG. 2
may also be used in a two port configuration where the balanced port is formed between terminals T
3
and T
4
, whereas terminal T
5
may be galvanically connected to the ground plane, (although this is not necessarily the case). It may also be connected to a DC source.
In the past several attempts, using different approaches, have been made to reduce the size of a balun. For microwave frequency applications it is known to realize baluns on sections of transmission lines. In “Miniaturised Lumped-Distributed Balun for Modern Wireless Communication Systems, IEEE, MTT-S′77, pp. 1347-1350” by Ojha et al, a distributed microstrip balun is suggested which incorporates lumped capacitors to reduce the balun size. However, the balun is still large, it has a length exceeding 5 mm and it is also a narrow band (0.864-0.896 GHz) balun. It is also disadvantageous that the lumped capacitors have to be wire bonded which makes the design expensive.
In “A reduced Size Planar Balun Structure for Wireless Microwave and RF Applications, IEEE MMT-S′96, pp. 526-529” by Preetham et al, another design based on coupled microstrip lines is discussed in which the effective length enhancement of transmission lines is used by employing capacitive effects. In such a design no lumped capacitors are needed but the balun will still be a large, narrow band device which further is not compatible with MMIC technology.
In “Modelling and design of Novel Passive MMIC Components with Three and More Conductor Levels, IEEE MTT-S′94, pp. 1293-1296; Improved compaction of multilayer MMIC/MCM baluns using lumped element compensation, IEE MTT-S′97, pp. 227-280” by R. Jansen et al, baluns based on lumped inductors are proposed. Although it is possible to achieve a substantial reduction in size, which makes it possible to fabricate the baluns on semiconductor chips (MMIC) the losses would still be high if low resistivity silicon substrates were used.
Baluns may also be realized by transformers based on lumped inductors. It is for example known to realize baluns by ferrite transformers. However, the microwave losses would be high for such baluns and they are not suitable for applications in MMIC technology. Lumped inductor based transformers may be used to make small on-chip baluns in MMIC technology for low frequencies. However, for conventional silicon MMICs, the losses due to the high conductivity of the silicon used in standard manufacturing technology would be high.
Baluns based on sections of transmission lines, microstrip lines, CPW (Coplanar Waveguide) etc. generally have low microwave losses. However, they are large and that limits the possibilities of using them in MMIC applications, especially at low frequencies (less than 10 GHz). U.S. Pat. No. 5,061,910 shows a distributed balun. This balun requires a plurality of conductor elements of &lgr;/4 length. This means that the balun will be large. Thus, also the balun disclosed therein is not small enough and not applicable at low frequencics. Further yet it is not appropriate for MMIC implementations.
SUMMARY
What is needed is therefore a balun transformer which is small and which has a good performance. The performance is given by the insertion losses, the reflection losses and the bandwidth. Thus a balun is needed which has low insertion losses, low reflection losses and a large bandwidth. Particularly a balun transformer is needed which is small enough to enable practical applications in MMIC. Still further a balun is needed which can be used for microwave frequencies which are low, particularly below about 10 GHz. Still further a balun transformer is needed which is easy and cheap to design, and fabricate, and which is compatible with MMIC technology for frequencies below 10 GHz. Particularly a small size, low loss balun is needed which is compatible with standard silicon MMIC production technology without requiring introduction of additional masks and processing steps. Generally a wideband balun transformer is needed which has a small size, and which can be produced on low resistivity substrates, i.e. substrates with a high conductivity such as for example silicon as used in conventional IC production.
Therefore a multilayer balun signal transformer is provided which comprises a coil system comprising a first coil and a second coil providing at least one balanced signal port at one side of the balun transformer and an unbalanced signal port at another side of the balun transformer, wherein the at least one balanced signal port is provided by a first balanced signal terminal and a second balanced signal terminal formed by the ends of the first coil, the unbalanced (single-ended) signal port being provided by a first unbalanced signal terminal and a second unbalanced signal terminal. The balun is a discrete component and it is formed on a low resistivity, e.g. a semiconductor, substrate layer, and the first and the second

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