Abstract:
A balun is constructed from a parallel pair of lines (coaxial or bifiler) that are bent to form a U-shape including a more curved (bent) section with two ends each of which are connected to a respective less curved (straight leg) section. One or more single hole ferrite beads are threaded over each respective pair of less curved sections of the pair of lines.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of broadband communication signal processing such as in cable access television CATV and more specifically to the field of broadband baluns for use in line amplifiers. 
     BACKGROUND 
     A balun is a component used in radio frequency systems to transfer signals between an unbalanced transmission line and a pair of balanced transmission lines. In a balanced transmission line a pair of lines carry equal current signals that are 180° out of phase. Twisted wire pairs are typically used for balanced transmission lines. In an unbalanced transmission line, such as a coaxial cable, the currents in the two conductors are unbalanced with respect to a symmetrical ground plane. 
     Such baluns are often produced by winding a coil of fine bifiler wire (e.g. #34 or #36 wire) or fine coaxial cable around a small toroidal or rod-shaped core of ferrite. The performance of this form of balun is highly dependent on the position of the wires with respect to each other and with respect to the core, so that additional tuning and rework are usually required. For such wound baluns, insertion loss is typically approximately 0.8 dB at 900 MHz due to losses in the ferrite core and in the fine wire that is used. Excessive insertion loss can result in reduced efficiency and higher distortion levels when the balun is used with push-pull amplifiers 
     Alternately, baluns may be produced using cylindrical ferrite beads having single axial holes through which bifiler wire or bare coaxial cable is threaded. The length of the wire should be less than approximately −/4 at the highest frequency of operation, and the number of beads should be sufficient to provide adequate low frequency performance. Ferrite beads for such application is described in “Ferrite Beads, Balun and Broadband Core”, in Power Conversion International, Vol. 6, No. 4, pp. 44-50, July-August 1980. An example of a beaded balun is described in “Transforming the Balun” by John S. Belrose in QST, June 1991, pp. 30-33. That citation describes a balun with 50 beads of 73 ferrite (e.g. Amidon No. FB-73-2401) threaded over a 12 inch portion of Teflon dielectric coaxial cable for use from 1.8 to 30 MHz. 
     A typical application for such baluns is in push-pull operation of a linear amplifier in a broadband network. In such an amplifier, the power of the signal in both lines of a balanced transmission line is amplified. “Broadband Transformer Design for RF Transistor Power Amplifiers” by Octavius Pitzalis Jr. and Thoman Couse in ECOM-2989, pp. 207-216, U.S. Army Electronics Command, Fort Monmouth, N.J., July 1986, describes such an amplifier. A first balun converts an unbalanced signal in a coaxial cable into a balanced signal in a balanced transmission line pair, then the balanced signal is amplified by several amplifier stages, and then a second balun converts the balanced signal into an unbalanced signal for transmission through another coaxial cable. In modern wideband GaAs FET linear amplifiers, the frequency response is often limited by the baluns that are used. 
     U.S. Pat. No. 5,808,518 to McKinzie, III et. al. describes a 4:1 balun for use with signal frequencies between 100 MHz and 10 GHz. In that citation, a circuit board has a window and two narrow elongate cantilever portions of the circuit board extend from a first side of the window. The cantilever portions carry straight printed sections of a U-shaped conductor of the circuit board. Four beads are threaded over each of the cantilever portions, and wires are connected from the free ends of the cantilever portions to the circuit board at a second side of the window to form the balun. U.S. Pat. No. 5,379,006 describes baluns formed by threading beads over coaxial cable for use in ultra wide band (e.g. DC to GHz) transmission. That balun is supported by a circuit board with a window through which a portion of the balun extends onto the other side of the circuit board. U.S. Pat. No. 5,742,205 to Cowen et. al. describes FET CATV line amplifiers that utilize input and output baluns. 
     Transmission line transformer theory and example designs are described in “Transmission Line Transformers”, 1st edition, by Jerry Sevick, Amateur Radio Relay League, Newington Conn., 1987; and in “Building and Using Baluns and Ununs” by Jerry Sevick, from CQ Communications Inc, Hicksville, N.Y., 1994. 
     All of the above citations are hereby incorporated herein in whole by reference. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a 1:1 balun for high performance broadband applications (i.e. in a frequency range from 45 MHz to 1005 MHz). 
     It is an object of the invention to provide such a balun with performance that insensitive to manufacturing tolerances in the position of the wires with respect to each other or with respect to the ferrite cores, so that additional tuning and rework is minimized and automated manufacturing of the balun is easily achievable. 
     It is an object of the invention to provide such a balun in which the input terminals are proximate to the output terminals of the balun for compact circuitry. 
     It is an object of the invention to provide such a balun that is lightweight and compact. 
     It is an object of the invention to provide such a balun that can be easily and securely mounted on a circuit board in automated manner. 
     It is an object of the invention to provide a linear amplifier circuit for push-pull operation in such broadband applications that utilizes such baluns for input and output. 
     It is an object of the invention to provide such a balun with low insertion power loss (e.g. less than approximately 0.2 dB at 900 MHz); 
     It is an object of the invention to provide such a balun with low return loss (e.g. approximately 20 dB minimum from 45 to 1005 MHz); and 
     In the invention, a balun is constructed from a transmission line pair that are bent to form a U-shape including two less curved (straight leg) sections connected at one end by a more curved (bent) section. Single hole ferrite beads are threaded over the less curved sections of the lines. This balun is simple, very compact, and easily adapted for automatic manufacture. Since the input and output of the balun are proximate, the design of circuitry utilizing the balun is simplified. The balun provides high quality signal processing and is useful for high performance broadband push-pull amplifiers in CATV distribution systems especially for signal amplification prior to laser transmission. 
     The invention includes a high-performance compact push-pull amplifier for use in broadband networks. The compact amplifier includes an input balun for converting an unbalanced signal in a single line into balanced signals in a balanced transmission line pair. A cascade of amplifier stages, in each conductor of the transmission line, amplifies the input signal. An output balun converts the amplified balanced signals in the balanced pair of conductors of the transmission line into an unbalanced signal in a single output line. 
     The invention also includes a CATV distribution network in which the compact high-performance push-pull amplifier of the invention is used in the head-end and nodes of the network for amplification of the feed to a laser diode transmitter. This provides high quality optical signals in the forward and return directions in the network. 
     Those skilled in the art can understand the invention and additional objects and advantages of the invention by studying the description of preferred embodiments below with reference to the following drawings that illustrate the features of the appended claims: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a specific embodiment of the balun of the invention using a coaxial cable and positioned for attachment on a circuit board; 
     FIG. 2 is a section of the embodiment of FIG. 1 following the center of the coaxial cable after attachment to the circuit board. 
     FIG. 3 schematically illustrates a compact push-pull amplifier of the invention utilizing the balun of FIGS. 1 and 2. 
     FIG. 4 schematically illustrates an alternate connection of the balun of FIG. 1 for use in the push-pull amplifier of FIG.  3 . 
     FIG. 5 is a block diagram schematically illustrating a CATV distribution system using the compact push-pull amplifier of the invention. 
     FIG. 6 shows the measured insertion loss for back-to-back coaxial baluns of FIG. 1 from 45 to 1005 MHz. 
     FIG. 7 shows the measured phase linearity for back-to-back coaxial baluns of FIG. 1 from 45 to 1005 MHz. 
     FIG. 8 shows the measured input return and output return losses for back-to-back coaxial baluns of FIG. 1 from 45 to 1005 MHz. 
     FIG. 9 shows the measured input return loss for the coaxial baluns of FIG. 1 from 45 to 1005 MHz. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates relevant portions of a specific embodiment  100  of the compact balun of the invention. A transmission line  102  is bent into a U-Shape with a curve  104  and straight legs  106  and  108  extending from the curve. The transmission line may be bifiler wire or a twisted pair of wires, but preferably the wires are a semi-rigid bare coaxial cable which has an intrinsic impedance of 75 Σ, is about 2.5 mm thick, and has Teflon insulation between the inner and outer conductors. Ferrite beads  110 - 113  are threaded over each of the legs  106 ,  108  of the coaxial cable. Preferably, there are 1 to 4 beads on each leg, more preferably 2 beads on each leg. The beads are approximately 4 to 8 mm long and preferably about 6 mm long; and the beads are 4 to 8 mm in diameter and preferably 5 mm in diameter. The ferrite may be No. 61 or 65 material and such beads are well known and available, for example, from Fair-Rite Products Corp., Wallkill N.Y. The radius of the curved section may be 3 to 8 mm and the legs may be 10 to 30 mm long. The ends of the legs that are remote from connecting bend  104 , are positioned on contacts  116 - 119  of circuit board  122 . More specifically, for leg  106 , end  124  of the outer conductor of the coaxial cable is positioned over conductive pad  116 , and end  126  of the inner conductor of the coaxial cable is positioned over conductive pad  117 . Also, curve  104  is positioned over contact pad  128 . Fiberglass epoxy circuit boards with copper wiring layers and methods for their manufacture are well known in the art. 
     Contact pads  116 - 119  are interconnected, for example, to micro-strips  130 - 13  or plated through holes of the circuit board in FIG. 2 described below. The microstrips may be, for example, conductors of the circuits shown in FIGS. 3 and 4. Contact  123  is isolated and is only used for structural support, that is, it is not connected to any other circuit board wiring. The circuit board has a window  134  through the thickness of the circuit board and into which the bottom parts of beads  110 - 113  extend, as shown, to reduce the profile of the balun over the surface of the circuit board and to hold the beads in place on the transmission line legs. 
     FIG. 2 is a section of the balun of FIG. 1 sliced through the central axis of the coaxial cable from the center of pad  128  to end  126  of leg  106 , after the balun has been connected to the circuit board. Material  136  connects end  124  of the outer conductor of coaxial cable  102  to pad  116 , material  137  connects end  126  of the inner conductor of the coaxial cable to pad  117 , and material  138  connects the outer conductor at the U-bend  104  to contact  128 . The material may be conductive adhesive or more preferably, solder alloy such as Sn-Pb solder. 
     FIG. 3 illustrates relevant portions of compact push-pull amplifier  200  of the invention. An input signal is provided to the amplifier from single signal line  202 . Compact balun  204  converts the signal into a balanced signal in a balanced transmission line  206 ,  208 . Each conductor of the transmission line includes a similar cascade of amplifier stages  210 ,  212 ,  214 . Also, interstage network (e.g. compensators, equalizers, phase shifters, and/or predistorters)  216  and  218  may be provided between each amplifier stage to remove distortions caused by the amplifiers. After the signal has been amplified, second compact balun  220  converts the balanced output signal in transmission line conductors  206  and  208  into an unbalanced output signal in output line  222 . Preferably, the amplifiers utilize Gallium Arsenide Field Effect Transistor (GaAs FET) technology as described in U.S. Pat. No. 5,742,205 to Cowen discussed above. 
     This compact high-performance push-pull amplifier of the invention is capable of providing high power amplification with very low distortion, which is especially useful in amplification of signals immediately prior to laser transmission. 
     FIG. 4 illustrates another embodiment  230  of the compact balun of the invention. The balun converts between an unbalanced signal in single line  232  and a balanced signal in a balanced transmission line pair  234 ,  236 . Either or both of baluns  204  and  220  in FIG. 3 can be replaced by balun  230  of FIG.  4 . 
     FIG. 5 illustrates relevant portions of CATV distribution system  300  using the compact push-pull amplifiers of the invention. A head-end  302  includes first compact push-pull amplifier  304  of the invention herein, to amplify a forward signal prior to laser diode  306  converting the electronic forward signal to an optical forward signal. The head-end also includes second compact push-pull amplifier  308  that amplifies a return signal after optical detector  310  converts an optical return signal to the electronic return signal. 
     Optical cable  312  conducts the optical forward signal from head end  302  to a plurality of nodes, only node  316  of which is shown. Also, optical cable  314  conducts optical return signals from the nodes back to the head end. 
     The node includes optical detector  318  to convert the optical forward signal into an electronic forward signal and conventional amplifier  320  to amplify the forward signal. The forward signal is provided to a coaxial cable network  322  which distributes the forward signal to a multitude of customer interfaces, such as at  324 . A return signal from the customer interfaces is separated from the forward signal in the coaxial cable network by diplex filter  326  of node  316 . The node also includes compact push-pull amplifier  328  of the invention herein, for amplifying the return signal before laser diode  330  converts the return signal from an electronic signal to an optical signal for transmission through optical fiber  314 . 
     Customer interface  324  includes compact push-pull amplifier  340  for amplifying a television signal sufficient for distribution into the customer&#39;s video equipment (not shown). The customer interface also includes interface equipment for producing return signals such as a telephone modem  342  for bi-directional telephone communications and computer modem  344  for bi-directional computer communications through the CATV system. 
     Line amplifiers, such as at  350 , are required to extend the range of the forward and return electronic signals through coaxial cable network  322  between the node and respective customer interfaces. Depending on the distance of the distribution path between the node and the customer interface, some distribution paths may not require any line amplifiers while other distribution paths may require several line amplifiers. 
     FIGS. 6-9 show measured test data for the balun shown in FIG.  1 . 
     The invention has been disclosed with reference to specific preferred embodiments, to enable those skilled in the art to make and use the invention, and to describe the best mode contemplated for carrying out the invention. Those skilled in the art may modify or add to these embodiments or provide other embodiments without departing from the spirit of the invention. Thus, the scope of the invention is only limited by the following claims.