BALANCED TO UNBALANCED CIRCUIT

Abstract: The present invention relates to a balun circuit (1B) comprising a first sub-circuit (10) and a second sub-circuit (20) which each correspond to, or essentially to, a lambda /4-wave guide. The first sub-circuit (10) includes a first conductor (10U), a second conductor (10L) and a dielectric layer disposed between said first and second conductors, said conductors being connected together capacitively and inductively. The second sub-circuit (20) includes a first conductor (20U), a second conductor (20L) and a dielectric layer disposed between said first and second conductors, said conductors being connected together capacitively and inductively. A first side on the first conductor (10U) in the first sub-circuit (10) is connected to an input port (P1). A second side on the first conductor (10U) in the first sub-circuit (10) is connected to a first side on the first conductor (20U) in the second sub-circuit (20) via a connecting conductor (15). A second side on the second conductor (10L) in the first sub-circuit (10) is connected to a first output port (P2). A first side on the second conductor (20L) in the second sub-circuit (20) is connected to a second output port (P3). A first open terminating lambda /4-wave guide (30) is connected to a second side on the second conductor (10L) in the first sub-circuit (10), and a second open terminating lambda /4-wave guide (40) is connected to a second side of the second conductor (20L) in the second sub-circuit (20).

Description: BALANCED TO UNBALANCED CIRCUIT TECHNICAL FIELD The present invention relates to a balanced to unbalanced circuit (BALUN) according to the preamble of Claim 1. High frequency electric signals can be transmitted in two often occurring ways, namely either balanced or unbalanced. In the case of balanced transmission there is used two conductors in which electric currents are constantly in antiphase. Unbalanced transmission, on the other hand, uses only one signal conductor and the signal (the current) is returned via earth. The balanced transmission is differential in nature and therewith less sensitive to disturbances and interference than the unbalanced transmission. Balanced and unbalanced transmissions are often mixed in radio systems. It is therefore necessary to be able to convert a balanced signal to an unbalanced signal and vice versa with the smallest possible losses. Balun circuits are used to this end. The properties of the balun circuit depend on impedance difference and phase difference for odd and even modes in the high frequency electric signal. A typical balun is the Marchand-balun which includes four λ/4 waveguides connected in pairs. In many cases, a balanced port shall be connected to an input or to an output of a differential amplifier. Normally, it is necessary to DC-bias the amplifiers and consequently a DC-wise short circuit of the balanced port to earth cannot be accepted. This has earlier been solved by including two capacitors between the actual balanced circuit and the balanced load. The capacitors are discrete capacitors which are chosen so that their resonance frequency coincides with the signal frequency. The capacitance is then balanced by the own parasite inductance of the capacitor and ideally behaves transparent at the frequency concerned. A problem with discrete capacitors is that they are relatively bulky and cannot be implemented readily when integrated in multilayer printed circuit boards or ceramic substrates. When biasing the differential amplifiers it is normally necessary to connect present day balun circuits to a current source or voltage source via additional discrete components. This applies particularly to the Marchand balun and also constitutes a problem. SUMMARY OF THE INVENTION An object of the present invention is to at least reduce the aforesaid problem. This object is achieved in accordance with a first aspect of the invention by means of a device according to Claim 1. One advantage afforded by the present invention is that performance is improved with regard to loss reductions and better phase characteristics of the balanced signal. Another advantage is that it can be simulated more readily than in the case of existing solutions, since it is not necessary to rely on discrete component models. The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a principle diagram of a classic Marchand-balun. Figure 2 is a principle diagram of one embodiment of a Marchand-balun constructed in accordance with the present standpoint of techniques so as to avoid DC-wise short circuiting to earth of the balanced output signal. Figure 3 is a principle diagram of one embodiment of an inventive Marchand-balun. DESCRIPTION OF PREFERRED EMBODIMENTS In order to provide a better understanding of the particular features of the inventive device, reference is made first to Figures 1 and 2. Figure 1 illustrates an embodiment of a classic Marchand-balun 1, which includes a first and a second sub-circuit 10 and 20 respectively. The first sub-circuit 10 includes an upper conductor 10U, a lower conductor 10L and a discrete layer disposed between said conductors. The upper conductor 10U and the lower conductor 10L in the first sub-circuit 10 are connected together capacitively and inductively, with a given coupling constant. The first sub-circuit 10 corresponds to, or essentially to, a first λ/4-wave guide. Similarly, the second sub-circuit 20 includes an upper conductor 20U and a lower conductor 20L and a dielectric layer disposed between said conductors. The upper conductor 20U and the lower conductor 20L are connected to one another in said second sub-circuit 20 capacitively and inductively, with a given coupling constant. The second sub-circuit corresponds to, or essentially to, a second λ/4-wave guide. An input Pi is connected to a first side on the upper conductor 10U in the first sub-circuit 10. A second side on the upper conductor 10U in the first sub-circuit 10 is connected to a first side on the upper conductor 20U in the second sub-circuit 20 via a connecting conductor 15. A second side on the upper conductor 20U in the second sub-circuit 20 is open. A first side on the lower conductor 10L in the first sub-circuit 10 is connected to earth. A second side of the lower conductor 10L in the first sub-circuit 10 is connected to a first output port P2. A first side on the lower conductor 20L in the second sub- circuit 20 is connected to a second output port P3. A second side of the lower conductor 20L in the second sub-circuit 20 is connected to earth. Figure 2 illustrates a Marchand-balun circuit 1A. The sole difference between the Marchand-balun circuit 1A and the classic Marchand-balun 1 shown in Figure 1 is that the Figure 2 circuit includes two capacitors 50 and 60 which prevent the balanced output signal from being short circuited DC-wise to earth. A first capacitor 50 is arranged between the output port P2 and the second side of the lower conductor 10L in the first sub-circuit 10. A second capacitor 60 is arranged between the output port P3 and the first side of the second conductor 20L in the second sub-circuit 20. Figure 3 illustrates an embodiment of an inventive balun circuit IB. The illustrated embodiment of the inventive balun circuit is shown in stripline form, in other words the mutually connected conductors lie in different planes. The inventive balun circuit IB includes a first and a second sub-circuit 10 and 20 respectively. The first sub-circuit 10 includes an upper conductor 10U, a lower conductor 10L and a dielectric layer disposed between the said conductors. The upper conductor 10U and the lower conductor 10L in the first sub-circuit 10 are connected together capacitively and inductively with a given coupling constant. The first sub-circuit 10 corresponds to, or essentially to, a first λ/4-wave guide. Similarly, the second sub-circuit 20 includes an upper conductor 20U and a lower conductor 20L and a dielectric layer disposed between said conductors. The upper conductor 20U and the lower conductor 20L in the second sub-circuit 20 are connected together capacitively and inductively with a given coupling constant. The second sub-circuit corresponds to, or essentially to, a second λ/4-wave guide. An input PI is connected to a first side of the upper conductor 10U in the first sub-circuit 10. A second side on the upper conductor 10U in the first sub-circuit 10 is connected to a first side on the upper conductor 20U in the second sub-circuit 20 via a connecting conductor 15. A second side on the upper conductor 20U in the second sub-circuit 20 is open. A first side on the second conductor 10L in the first sub-circuit 10 is connected to a first side of a first open terminating λ/4-wave guide 30. A second side on the lower conductor 10L in the first sub-circuit 10 is connected to a first output port P2. A first side on the lower conductor 20L in the second sub-circuit 20 is connected to a second output port P3. A second side on the lower conductor 20L in the second sub-circuit 20 is connected to a first side on a second open, terminating λ/4-wave guide 40. The dielectric material disposed between the upper conductors 10U and 20U and the lower conductors 10L and 20L in the first and the second sub-circuits is disposed in a layer structure, a stripline structure. It will be understood, however, that the dielectric layer can be disposed in the same plane as the upper and the lower conductor, microstructure . The electric conductors may be linear in accordance with Figure 1, or of the spiral type. A point 70 between the third λ/4-wave guide 30 and the lower conductor 10L in the first sub-circuit 10 functions as an RF- wise earth point. A point 80 between the fourth λ/4-wave guide 40 and the lower conductor 20L in the second sub-circuit 20 functions as an RF-wise earth point. The λ/4-wave guides in the illustrated embodiment of the balun circuit IB may be made of metal, for instance from a silver alloy, copper, tungsten or aluminium. The dielectric material may comprise a ceramic material, polymeric material, an electrically non-conductive organic material, silicon dioxide or silicon nitride. Although the inventive balun circuit will function for all wave lengths, the length of each λ/4-wave guide must be manageable for purely practical reasons. The balun circuit IB may be of the microstrip or stripline kind. It will be understood that the invention is not restricted to the aforedescribed and illustrated embodiments thereof, and that modifications can be made within the scope of the following claims.

Claims: CLAIMS 1. A balun circuit (IB) comprising a first sub-circuit (10) and a second sub-circuit (20) each of which corresponds to, or essentially to, a λ/4-wave guide, wherein the first sub-circuit (10) includes a first conductor (10U) , a second conductor (10L) and a dielectric layer disposed between said first and second conductors, wherein said conductors are connected together capacitively and inductively, wherein the second sub-circuit (20) includes a first conductor (20U) , a second conductor (20L) and a dielectric layer disposed between said first and said second conductors, said conductors being connected together capacitively and inductively, wherein a first side on the first conductor (10U) in the first sub-circuit (10) is connected to an input port (Pi) , a second side on the first conductor (10U) in the first sub-circuit (10) is connected to a first side on the first conductor (20U) in the second sub-circuit (20) via a connecting conductor (15), a second side on the second conductor (10L) in the first sub-circuit (10) is connected to a first output port (P2) and a first side on the second conductor (10L) in the second sub-circuit (20) is connected to a second output port (P3), characterized in that a first open, terminating λ/4-wave guide (30) is connected to a second side on the second conductor (10L) in the first sub-circuit (10), and in that a second, open terminating λ/4-wave guide (40) is connected to a second side of said second conductor (20L) in the second sub-circuit (20) . 2. A balun circuit according to Claim 1, characterized in that the balun circuit is of the stripline kind. 3. A balun circuit according to Claim 1, characterized in that the balun circuit is of the microstrip kind.

Inventors: WESTBERG DAVID, WESTBERG, DAVID
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON (PUBL)
Application year: 2000
Main classification: H01P5
IPC codes: H01P5