Balun-transformer

[0002] 1. Field of Invention

[0003] The present invention relates to a balun-transformer composed of a balun transition and an impedance matching circuit.

[0004] 2. Related Art

[0005] The design of microwave circuits often need to convert from balanced lines to unbalanced lines and, at the same time, solve the problem of impedance matching. They are usually made according to experience and experiments. The most popular circuits with balun-transformer include the push-pull amplifiers and the sampling phase detectors.

[0006] Generally speaking, the following methods are often used to solve the above problems.

[0007] Coaxial-Line Type Balun-Transformer: Because of the good performance in power handling, it is often used in high power components. The coupling length is one fourth the wavelength at the usage frequency. The coupling length is already greater than three inches between 400 MHz and 500 MHz. It is too large to be realized in circuits. This type of transformer needs very good soldering technology to fix and ground it.

[0008] Ferrite-Toroid-Core Type Balun-Transformer: Its volume is smaller and can be put into the SMT package. The soldering technology is easier but the power handling capability is poor. Most of all, it has a higher cost.

[0009] In the actual design process of the disclosed balun-transformer, an odd mode analysis is first employed to design accurately and a commercial microwave software is then used to simulate.

[0010] The disclosed balun-transformer has two preferred embodiments. The first preferred embodiment chooses a conductor-backed coplanar waveguide (CBCPW) to conductor-backed coplanar stripline (CBCPS) transition to convert balanced line (CBCPW) to unbalanced line (CBCPS). An impedance matching circuit is designed following this CBCPW to CBCPS Transition. This kind of balun-trasfomer can be used from several MHz to 10 GHz.

[0011] A second preferred embodiment of the invention is a simplified form of the previous embodiment, wherein only the CBCPS is used. One node of its input port is grounded. The signal of the CBCPS output nodes is kept at a 180° phase difference. The impedance matching circuit is further employed. This simplified design can be used between several MHz and 2 GHz.

[0012] The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:

[0013] FIG. 1 shows the electromagnetic structure according to a first preferred embodiment of the invention;

[0014] FIG. 2 shows the schematic diagram of the odd mode analysis method according to a first preferred embodiment of the invention;

[0015] FIG. 3 shows the schematic diagram of the complete circuit structure according to a first preferred embodiment of the invention;

[0016] FIG. 4 shows the electromagnetic structure according to a second preferred embodiment of the invention;

[0017] FIG. 5 shows the schematic diagram of the complete circuit structure according to a second preferred embodiment of the invention;

[0018] FIG. 6 shows the simulated return loss of the second preferred embodiment of the invention; and

[0019] FIG. 7 shows the measured return loss of the second preferred embodiment of the invention.

[0020] The disclosed balun-transformer has two preferred embodiments, which are described hereinafter in further detail.

[0021] With reference to FIG. 1, a conductor-backed coplanar waveguide (CBCPW) to conductor-backed coplanar stripline (CBCPS) transition is employed to convert a balanced line (CBCPW) to a unbalanced line (CBCPS). The impedance of the transition can be designed to have a constant value. Its output resistance R160 and system source resistance are usually 50Ω. The load impedance R270 on the other terminal is determined according to the devices used, and usually has an extremely low resistance.

[0022] According to FIG. 1, the input port is comprised by I110, G220, and G330. The I110 is signal node of the input port. G220,and G330 are the ground nodes of the input port. The signal between the CBCPS output nodes 0440 and 0550 is kept at a 180° phase difference. Using the odd mode analysis, the original circuit can be simplified into the circuit shown in FIG. 2. The ground is a RF virtual ground and the resistances R1/2 and R2/2 are half those of R1 and R2.

[0023] An impedance matching circuit composed of a first capacitor 80, a second capacitor 82, a first inductor 90 and a second inductor 92 is inserted between the half output resistor R1/2 and the half load resistor R2/2. The order of the matching circuit (N) can be any positive integer. Usually, the bigger the level N is, the wider the bandwidth is. For example, the bandwidth of the impedance ratio 8.5:1 can reach more than 50% when N=2. It can satisfy the most of the practical purposes. Finally, the obtained capacitance is divided by 2 and put in a symmetric structure between the output terminals 40, 50. The finally circuit is shown in FIG. 3. This balun-transformer works in the range between several MHz to 10 GHz.

[0024] With reference to FIG. 4, a second preferred embodiment is obtained by removing the CBCPW from the electromagnetic structure of the first embodiment. It contains only the CBCPS. I1 is the input nodes 12 and the G2 terminal 22 is grounded. The signal between the CBCPS output nodes 0442 and 0552 is also kept at a 180° phase difference. Of course, the simplified transition impedance is changed. However, if the CBCPS length is far less than the wavelength used, the impedance becomes unimportant. There is no need to take care for the impedance. Instead of calculating impedance, The structure can be placed in commercial microwave simulation software, as shown in FIG. 5. A simple microstrip coupled line 62 model is used to replace the CBCPS. The simulation can give extremely good results by adjusting the first capacitor 80, the second capacitor 82, the first inductor 90, the second inductor 92, the third inductor 94, and the fourth inductor 96.

[0025] Using the above simulation result, a balun-transformer of order 2 with a central frequency of 50 MHz and the impedance transforming ratio of 8.5:1 is designed. FIG. 6 shows simulated return loss of the invention, and FIG. 7 shows measured return loss of the invention.

[0026] From FIGS. 6 and 7, one sees that the simulated and measured results are pretty close. The return loss parameters are smaller than −18 dB and the bandwidths are close to 50%. The transformer is applicable in microwave circuits below 2 GHz. Since the transformer impedance is designed from 6Ω and 50Ω and impedance of the network analyzer ranges between 50Ω and 50Ω, the insertion loss cannot be measured. However, according to the law of energy conservation, if the return loss is greater than 18 dB, the capacitances Q are all greater than 50 and the current embodiment does not have any resistive element, the insertion loss should be smaller than 0.5 dB.

[0027] The above-mentioned two preferred embodiments disclose two kinds of balun-transformers applicable to very high frequency (VHF) and ultra high frequency (UHF) microwave circuits. The first preferred embodiment has a balun accompanied with an impedance matching circuit. The second embodiment is a simplified version of the first embodiment. In comparison with various methods used in the industry, its volume is smaller (irrelevant to the quarter wavelength) and its manufacturing-process only uses simple chemical etching, which is compatible with the common microwave circuit manufacturing processes. The present invention does not use a ferrite toroid core, thus greatly lowering the work time and manufacturing cost. Furthermore, the invention can work at a relatively large power, which depends on the working voltage and current of the matching capacitors and inductors respective.

[0028] In the actual design process, we first uses the odd mode analysis method to do an accurate design and then use commercial microwave software to do simulations. It is more accurate and more efficient than the conventional method to make balun-transformers that can only be design by experience and experiments.

[0029] Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments, will be apparent to persons skilled in the art. It is, therefore, contemplated that the appended claims will cover all modifications that fall within the true scope of the invention.