1、1,BALUN,Mar. 01, 2012,2,Contents,Why Do We Need Balun BALUN Basic Information BALUN Parameter Measurement Common BALUN Information and Design BALUN Applications,3,Differential circuits are becoming more widely used in RF circuits for the same reason that they have been used for years in lower freque
2、ncy circuits. The benefits of using differential circuits include immunity to electromagnetic interference, power supply noise and ground noise, even-order harmonic suppression, and tolerance to less than perfect RF grounds.,Thus, baluns are employed in many modern circuit designs.,1. Why Do We Need
3、 Baluns,Why do we need baluns(1),4,Balun principle is according to the antenna theory, dipole antenna is balance antennas, and coaxial cable is not balanced transmission lines, if its direct connection, the coaxial cable skins have high frequency current through (But by coaxial cable transmission pr
4、inciple, high frequency current flow should be in cable, the cortical is as shielding layer ), and thus, it will affect the antenna radiation .,So, we need add an unbalanced to balanced convertor between antenna and coaxial, the high frequency current flow will be suppressed nicely.,Why do we need b
5、aluns(2),5,1. BALUN Basic Information,The word “balun” is a portmanteau term formed from the words “balanced” and “unbalanced” in which “balanced” implies a differential configuration and “un-balanced” represents a single-ended configuration.,As shown in Figure1 , a balun is a transformation block b
6、etween a single-ended stage and a differential pair, either from the single-ended block to the differential pair.,Sometimes, people refer to the balun type in Figure1(a) as a “splitter” and the balun type in Figure1(b) as a “combiner”. The “splitter” is a reversed “combiner”, and vice versa. From no
7、w on, we will only focus on the “splitter” unless otherwise noted.,6,(a) From single-ended block to differential pair,(b) From differential pair to single-ended block,Figure1 The balun is a block between a single - ended block and a differential pair.,7,The characteristics of an ideal balun and can
8、be outlined as follows:,The Characteristics of a Balun(1),1)There are three terminals: one single-ended and two differential. However, the two differential terminals impedances must be kept equal.,8,The Characteristics of a Balun(2),3)Total insertion loss from the single-ended terminal to the differ
9、ential terminals is zero.,2)At the two differential terminals, the magnitudes of the signal are equal but their phases are kept at a 180difference.,9,Some baluns provide impedance transformation in addition to conversion between balanced and unbalanced signal modes; others provide no impedance trans
10、formation. For 1:1 baluns (no impedance transformation), the input and output are usually both 50 ohms or 75 ohms. The most common impedance-transformation ratio is 1:4 (alternatively 4:1). Some baluns provide other impedance-transformation ratios, such as 1:9 (and 9:1), 1:10 (and 10:1), or 1:16 (an
11、d 16:1).,Impedance-transformer baluns with larger ratios are used to match high-impedance balanced antennas to low-impedance unbalanced wireless receivers, transmitters, or transceivers.,The “balanced“ terminals of some baluns can be connected to an unbalanced system.,10,Unbalanced feed current affe
12、cts feed point impedance of the antenna this is a special headache when trying to measure the input impedance of a high impedance antenna or other device.,a) Input impedance of 1 dipole with and without feed cable,b) The effect of feed cable on VSWR of a /2 folded dipole.,11,Balance circuitry archit
13、ecture,UI 分别经过低通、高通得到反相的UO1 、UO2,由计算可知UO1 、UO2始终有180度相差（Differential）适合在Balance系统上传输。,12,In balanced systems the signals can be divided into a common mode and a differential mode. The balanced output of an ideal balun (or an ideal unbal./bal. SAW filter) are 180 phase shifted and are half the amplit
14、ude of the input signal:,13,As we see, an ideal balun cancels the common mode completely. Therefore, the common mode rejection is infinite. Now, what happens if we have a real balun with non-ideal properties? The phase difference is not any longer 180, represented by a phase error j in one of the ou
15、tput paths. Furthermore, the envelope amplitudes of the two output paths are not equal, represented by an amplitude ratio r. Therefore, the amplitude of one output is r times the amplitude of the other output, but the sum of both has to stay equal. These conditions are put into formulas:,14,Now, the
16、 power common mode rejection ratio (CMRR) is being defined as the ratio of the input signal power to the common mode signal power for a loss-less balun:,15,CMRR VS Phase &Amplitude Error,16,3. BALUN Parameter Measurement,It is difficult to provide accurate characterization of baluns for two primary
17、reasons: it is a 3-port device, and the input an output typically have different impedances from each other, making network analyzer calibration more difficult. In order to characterize baluns, we commonly uses two types of test methods. The “A” type test board is designed to have two baluns of the
18、same part number mounted on it in a back-to-back configuration. It has two 50 ohm I/Os whereby one I/O is connected to the unbalanced port of one of the baluns, and the other I/O is connected to the unbalanced port of the other balun. The balanced ports of one of the baluns are directly connected to
19、 the balanced ports of the other balun. The through insertion loss can be determined by dividing by two.,The “B” type test board is designed to have one balun mounted on it and has three I/Os (one unbalanced, two balanced).,Measurement method,17,Measurement process,1.Insertion Loss,Insertion Loss =
20、-1/2 * dB(S21),18,2.Return Loss,Return Loss = -dB(S11),For the return loss test, a 50, 100, or 200 ohm chip resistor (value dependent on the transformation ratio of the DUT) is mounted between the balanced ports in place of the second balun.,19,3. Measure Amplitude and Phase,The Amplitude and Phase
21、Imbalance are measured in a setup per the following: The “B” style board can be used for this measurement. The values of the resistors are 25, 50, or 100 ohm chip resistors (values being dependent on the transformation ratio of the D.U.T.). The component schematically shown as a switch represents sw
22、itching done in test either by a 3-Port Network Analyzer, or by hardwiring in each respective position.,20,A variety of RF baluns have been developed in past decades. In this chapter, three types of baluns are emphasized: the transformer, LC , and micro strip line.,4. BALUN Types and Designs(1),1) I
23、n the simulation stage of RF circuit design, the ideal transformer balun is a good candidate for a transformation block between single - ended and differential pair blocks. The greatest advantage of the transformer balun is its frequency response with an almost infinite bandwidth. In addition, the i
24、nsertion loss of the balun can be set to zero since it is an ideal balun.,21,2) In the testing stage, the LC and micro strip line baluns are recomm-ended. The greatest advantage of an LC balun is its simplicity: it can be implemented in a laboratory quickly and easily. Surprisingly, instead of a nar
25、row - band response as might be expected, the LC balun behaves with a reasonably wide - band frequency response.,(a) LC Splitter,(b) LC Combiner,22,3) Compared with the LC balun, the frequency response in a micro strip line balun is wider. In addition, its outstanding advantage is low cost, since it
26、s basic parts are micro strip lines.,微带线A的长度为0.5个波长，微带线B的长度为0.25个波长，波长为在实际基板上的信号波长，需要考虑基板的介电常数。,巴伦的性能和所用的节数有关系，节数越多，频宽越宽，不过节数越多，尺寸也越大.,23,LC Balun,最简单的LC巴伦仅有两个相同的电感和电容构成，如下图所示：,L C元件值的计算公式如下：,Rout和Rin为单端和输出端口的阻抗,4. BALUN Types and Designs(2),24,设计一个工作频率在100MHz巴伦，单端和差分阻抗为50欧姆.,按照上述公式计算得到 L=113nH，C=22
27、.5pF.,S参数仿真结果：,在100MHz频率上幅度和相位和设计值基本吻合，不过LC对工作频率较为敏感，因此，该balun的工作带宽不宽，基本只能工作在100MHz附近。,26,Micro Strip Line Balun,在频率较高的情况下（大于1GHz），LC巴伦由于电感，电容的寄生效应，自谐振频率等影响，性能将变差，而在高频上，用微带线设计的巴伦在性能，尺寸上都比较理想.,S参数仿真结果：,28,Radio and television,In television, amateur radio, and other antenna installations and connectio
28、ns, baluns convert between 300 ribbon cable or 450 ladder line (balanced) and 75 coaxial cable (unbalanced) or to directly connect a balanced antenna to (unbalanced) coaxial cable. To avoid feed line radiation, baluns are typically used as a form of common mode choke attached at the antenna feed poi
29、nt to prevent the coaxial cable from acting as an antenna and radiating power. Match 300 twin-lead cable to 75 coaxial cable.,6. BALUN Applications,Impedance-transformer baluns having a 1:4 ratio are used between systems with impedances of 50 or 75 ohms (unbalanced) and 200 or 300 ohms (balanced). M
30、ost television and FM broadcast receivers are designed for 300-ohm balanced systems, while coaxial cables have characteristic impedances of 50 or 75 ohms.,29,In audio applications, baluns convert between high impedance unbalanced and low impedance balanced lines. Except for the connections, the thre
31、e devices in the image are electrically identical, but only the leftmost two can be used as baluns. The device on the left would normally be used to connect a high impedance source, such as a guitar, into a balanced microphone input, serving as a passive DI unit. The one in the centre is for connect
32、ing a low impedance balanced source, such as a microphone, into a guitar amplifier. The one at the right is not a balun, as it provides only impedance matching. In power line communications, baluns are used in coupling signals onto a power line.,Audio,30,BALUN used in some projects,WCDMA AWS Band RX path,WCDMA Cell Band RX path,GSM Low Band RX path,
