BALUN基础介绍

1、BALUNBALUNMarMar. . 01 01, 20, 201212Confidential2ContentsWhy Do We Need BalunBALUN Basic InformationBALUN Parameter MeasurementCommon BALUN Information and DesignBALUN ApplicationsConfidential3Differential circuits are becoming more widely used in RF circuits for the same reason that they have been

2、 used for years in lower frequency 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 circ

3、uit designs.1. Why Do We Need BalunsWhy do we need baluns(1)Confidential4Balun 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

4、 (But by coaxial cable transmission principle, 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 b

5、e suppressed nicely. Why do we need baluns(2)Confidential51. 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 i

6、n Figure1 , a balun is a transformation block between 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

7、 a reversed “combiner”, and vice versa. From now on, we will only focus on the “splitter” unless otherwise noted. Confidential6(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 differenti

8、al pair. Confidential7The characteristics of an ideal balun and can 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.Confidential8The Characteristics of a

9、 Balun(2) 3)Total insertion loss from the single-ended terminal to the differential terminals is zero. 2)At the two differential terminals, the magnitudes of the signal are equal but their phases are kept at a 180 difference. Confidential9Some baluns provide impedance transformation in addition to c

10、onversion between balanced and unbalanced signal modes; others provide no impedance transformation. 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 o

11、ther impedance-transformation ratios, such as 1:9 (and 9:1), 1:10 (and 10:1), or 1:16 (and 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 o

12、f some baluns can be connected to an unbalanced system.Confidential10Unbalanced feed current affects 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

13、feed cable, b) The effect of feed cable on VSWR of a /2 folded dipole.Confidential11Balance circuitry architectureLCCLUIUO1UO2Rdiff11111IoIUj CCUUj LLj CC211IoIUj LLUUj LLj CCUI 分别经过低通、高通得到反相的UO1 、UO2由计算可知由计算可知UO1 、UO2始终有始终有180度相差度相差（Differential）适合在适合在Balance系统上传输。系统上传输。Confidential12In balanced sy

14、stems 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 amplitude of the input signal:)(2121)()()()(, 2, 1tfeetftftfwtjjwtidealidealDM02121)()()()(, 2, 1wtjjwtideali

15、dealCMeetftftfConfidential13As 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

16、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:(),1,2,1( )(

17、 )( )11jwtj wtDM realidealidealftftftee(),1,2,1( )( )( )11jwtj wtCM realidealidealftftfteeConfidential14Now, the 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:2222(),2222( )CMRR1( )1111111111CMRR

18、jwtPowerjwtj wtCM realjwtjwtjjwtjPowerjef tfteeeee eeeConfidential15CMRR VS Phase &Amplitude Error Confidential163. BALUN Parameter MeasurementIt is difficult to provide accurate characterization of baluns for two primary reasons: it is a 3-port device, and the input an output typically have dif

19、ferent 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 same part number mounted on it in a back-to-back configuration. It has two

20、 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 the balanced ports of the other balun. The through insertion loss can be

21、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 methodConfidential17 Measurement process1.Insertion LossInsertion Loss = -1/2 * dB(S21) Confidential182.Return Loss Return Loss = -dB(S11)

22、 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.Confidential193. Measure Amplitude and PhaseThe Amplitude and Phase Imbalance are measured in a setup per the fol

23、lowing: 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 switching done in test either by a 3-Port Netwo

24、rk Analyzer, or by hardwiring in each respective position.Confidential20 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) In the simulation stage of RF circu

25、it 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 insertion loss of the balun can be

26、set to zero since it is an ideal balun. Confidential212) 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 narrow - band response as

27、might be expected, the LC balun behaves with a reasonably wide - band frequency response.(a) LC Splitter(b) LC CombinerConfidential22 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 its basic parts

28、are micro strip lines.微带线微带线A A的长度为的长度为0.50.5个波长，微带线个波长，微带线B B的长度为的长度为0.250.25个波长，波长为在个波长，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。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，尺寸也越大多，尺寸也越大. . Confidential23LC 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： 22*12*inoutinou

29、tfLRRCRRRout和Rin为单端和输出端口的阻抗 4. BALUN Types and Designs(2)Confidential24设计一个工作频率在设计一个工作频率在100MHz巴伦，单端和差分阻抗为巴伦，单端和差分阻抗为50欧姆欧姆. 按照上述公式计算得到按照上述公式计算得到 L=113nH，C=22.5pF.ConfidentialS参数仿真结果：参数仿真结果： 在在100MHz频率上幅度和相位和设计值基本吻合，不过频率上幅度和相位和设计值基本吻合，不过LC对工作频对工作频率较为敏感，因此，该率较为敏感，因此，该balun的工作带宽不宽，基本只能工作在的工作带宽不宽，基本只能工

30、作在100MHz附近。附近。 Confidential26Micro 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. ConfidentialS参数仿真结果：参数仿真结果： Confidential28Radio and televisionIn television, amateur radio, and other antenna installations and connections, baluns convert between 300 ri

31、bbon 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 point to prevent the coaxial cable from acting as an antenna and radiating power. Match 300 twin-lead cable to 75 coaxial cable.6. BALUN ApplicationsImpedance-transformer baluns having a 1:4 ratio are used between systems with impedances of 50 or 75 ohms (unbalanced) and 200 or 300 ohms (balan