Abstract:
A balun transformer having two series connected transformers with each having a primary loop conductor disposed in a stacked configuration. One portion of each primary loop conductors is in a first layer and these two portions of the two primary loop conductors are connected in series. The second portions of the primary loop conductors are in a second layer that is spaced from the first layer with the secondary loop conductors interlaced with these portions of the primary loop conductors in the second layer.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to balun transformers and, in particular, to the construction of balun transformers fabricated as part of an integrated circuit. 
     BACKGROUND OF THE INVENTION 
     It is common practice to use differential circuits in highly integrated radio frequency integrated circuits to improve signal-to-noise ratio. Two signals of opposite phase, developed from an input signal supplied from an antenna, are combined by the differential circuit, so that the two signals are added to produce the desired signal, while undesired noise is cancelled. 
     Frequently, in radio frequency integrated circuits that include such differential circuits, unbalanced components are connected to balanced components. Unbalanced components usually are large and expensive filters, power amplifiers, etc. Making such components balanced, though possible, results in large and costly components. 
     Power matching requires the use of balanced impedance transformers. Passive versus active balun transformers are used primarily because they do not require additional current. However, passive balun transformers are physically large. 
     Further, high power differential circuits require low loss balun transformer combiners to be power efficient. The two main loss mechanisms in an on-chip balun transformer are conductor loss and substrate loss. In low impedance systems and at lower radio frequencies, conductor loss dominates. At higher frequencies and, in particular, in high impedance systems, where the balun transformer port impedance is on the order of the substrate impedance, substrate loss dominates. 
     As the loadline impedance is high in a differential circuit to reduce current consumption, high substrate loss is a problem. This problem can be solved by providing a means to reduce substrate loss. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a new and improved balun transformer. 
     It is another objective of the present invention to provide a new and improved balun transformer fabricated as part of an integrated circuit. 
     It is a further objective of the present invention to provide a balun transformer fabricated as part of an integrated circuit having reduced substrate loss. 
     A balun transformer, constructed in accordance with the present invention, includes a first layer having a substantially flat first conductor portion of a first primary loop conductor and a substantially flat first conductor portion of a second primary loop conductor connected in series with the substantially flat first conductor portion of the first primary loop conductor. This balun transformer also includes a second layer spaced from the first layer. The second layer has a substantially flat second conductor portion of the first primary loop conductor having a width less than the width of the substantially flat first conductor portion of the first primary loop conductor and a substantially flat second conductor portion of the second primary loop conductor having a width less than the width of the substantially flat first conductor portion of the second primary loop conductor. The second layer also has a substantially flat first secondary loop conductor interlaced with the substantially flat second conductor portion of the first primary loop conductor and a substantially flat second secondary conductor loop conductor interlaced with the substantially flat second conductor portion of the second primary loop conductor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be best understood from the following detailed description when read in conjunction with the accompanying drawings. 
     FIG. 1 is a perspective view of one-half of a balun transformer constructed in accordance with the present invention. 
     FIG. 2 is a schematic cross-section view of the FIG. 1 balun transformer. 
     FIG. 3 is a schematic circuit diagram of the FIG. 2 balun transformer. 
     FIG. 4 is a schematic cross-section view of the FIG. 1 balun transformer as part of an integrated circuit. 
     FIG. 5 is a perspective view of a balun transformer constructed in accordance with the present invention. 
     FIG. 6 is a circuit diagram of a balun transformer constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1,  2  and  3 , a balun transformer, constructed in accordance with the present invention, includes a first layer  10  having a substantially flat first conductor portion  12  of a first primary loop conductor and a second layer  14  spaced from first layer  10  and having a substantially flat second conductor portion  16  of the first primary loop conductor and a substantially flat first secondary loop conductor  18 , shown by dotted lines, interlaced with second conductor portion  16  of the first primary loop conductor. The input terminals of the first primary loop conductor are identified by reference numeral  20  and  22  and the output terminals of the first secondary loop conductor  18  are identified by reference numerals  24  and  26 . As will be explained in connection with FIGS. 5 and 6, first layer  10  also has a substantially flat first conduct or portion of a second primary loop conductor connected in series with the first conductor portion  12  of the first primary loop conductor and second layer  14  also has a substantially flat second conductor portion of the second primary loop conductor and a substantially flat second secondary loop conductor interlaced with the second conductor portion of the second primary loop conductor. 
     FIGS. 2 and 3 indicate that a substrate  28 , typically a lossy dielectric substrate made from, for example, silicon, on which the balun transformer is formed has a capacitance  30  and a resistance  32 . In addition, there is parasitic capacitance  34  between first conductor portion  12  of the first primary loop conductor and substrate  28 , parasitic capacitance  36  between first conductor portion  12  of a first primary loop conductor and second conductor portion  16  of the first primary loop conductor, parasitic capacitance  38  between first conductor portion  12  of the first primary loop conductor and first secondary loop conductor  18 , parasitic capacitance  40  between second conductor portion  16  of the first primary loop conductor and first secondary loop conductor  18 , and parasitic capacitance  42  between second conductor portion  16  of the first primary loop conductor and substrate  28 . Resistances  44 ,  46 , and  48  shown in FIG. 3, are the winding resistances of first conductor portion  12  of the first primary loop conductor, second conductor portion  16  of the first primary loop conductor and the first secondary loop conductor  18 , respectively. 
     As shown by FIG. 4, first layer  10  has a first insulation layer  52 , made from, for example, silicon dioxide, and a first circuit layer  54  that is positioned over first insulation layer  52  and has the first conductor portion  12  of the first primary loop conductor. As will be explained in connection with FIGS. 5 and 6, first circuit layer  54  also has the first conductor portion of the second primary loop conductor that is connected in series with the first conductor portion  12  of the first primary loop conductor. 
     A spacer layer  56  is positioned over first circuit layer  54 . Spacer layer  56  is made from, for example, silicon dioxide. 
     Second layer  14  has a second insulation layer  58 , made from, for example, silicon dioxide, and positioned over spacer layer  56  and a second circuit layer  60  that is positioned over second insulation layer  58  and has the second conductor portion  16  of the first primary loop conductor. As will be explained in connection with FIGS. 5 and 6, second circuit layer  60  also has the second conductor portion of the second primary loop conductor. As shown in FIG. 4, the width of second conductor portion  16  of the first primary loop conductor is less than the width of first conductor portion  12  of the first primary loop conductor. 
     FIGS. 5 and 6 indicate that a balun transformer, constructed in accordance with the present invention, includes a second half balun transformer, identical to the one illustrated in FIG. 1, connected in series with the first balun transformer. In particular, first layer  10  further includes a substantially flat first conductor portion  60  of a second primary loop conductor connected in series with the first conductor portion  12  of the first primary loop conductor with the junction at which first conductor portion  12  of the first primary loop conductor is connected to first conductor portion  60  of the second primary loop conductor grounded. Second layer  14  further includes a substantially flat second conductor portion  62  of the second primary loop conductor and a substantially flat second secondary conductor loop conductor  64  interlaced with second conductor portion  62  of the second primary loop conductor. Second secondary loop conductor  64  is connected in series with first secondary loop conductor  18 . As with the first primary loop conductor, the width of second conductor portion  62  of the second primary loop conductor is less than the width of first conductor portion  60  of the second primary loop conductor. 
     The present invention reduces the substrate loss mechanism, namely the effect of resistances  32  in substrate  28 . This is accomplished by the series connection of the two balun transformers to form a single balun transformer. By providing an A-C ground at terminal  22 , as shown in FIGS. 5 and 6, the substrate loss is reduced because the parasitics are shorted out. As the loadline impedance is high in a differential circuit to reduce current consumption, a means to reduce substrate loss is required. In accordance with the present invention, a new balun structure is used to “shield” itself from the substrate. 
     With the ability to design a low loss/high impedance balun at high frequencies (e.g., &gt;1 GHz), this power combining function can be done on-chip. With an on-chip balun, the parasitics can be controlled more easily. Dealing with high transmission line impedances at the board level at high radio frequencies is very difficult due to board parasitic capacitances. Current consumption can be reduced significantly with a low loss monolithic balun constructed in accordance with the present invention. 
     Although described and illustrated above with reference to certain specific embodiments, the present invention is nevertheless is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.