Abstract:
A characteristic impedance of an electric transmission line is measured by way of extraction. In the method, a first probe and a second probe are provided, wherein the first probe and the second probe are separable and independently operable probes. A first characteristic impedance of a first circuit where a first terminal of the first probe and a first terminal of the second probe are directly interconnected to each other is first measured. Then a second characteristic impedance of a second circuit where the first terminal of the first probe and the first terminal of the second probe are connected to opposite terminals of the electric transmission line, respectively, is measured. The characteristic impedance of the electric transmission line can then be obtained according to the first characteristic impedance and the second characteristic impedance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for measuring a characteristic impedance of an electric transmission line, and more particularly to a method for measuring a characteristic impedance of an electric transmission line by way of extraction. 
       BACKGROUND OF THE INVENTION 
       [0002]    As known, high-frequency electronic devices play an important role in modern technologies. With substantial increase of operational frequencies, a variety of problems are possibly caused in printed circuit boards (PCBs) themselves, configuration of integrated circuits (ICs) in PCBs, and interconnection structures such as back planes or connectors for physically connecting to active devices or power supplies. Due to the electric properties of the signal transmission structures, problems in power integrity and electromagnetic interference, etc., might occur or get worse. Therefore, manufacturing difficulty and hardware cost would rise. 
         [0003]    For solving the above-mentioned problems, factors associated with high-frequency signal transmission are studied and controlled. For example, characteristic impedance of a transmission line is an important controllable factor for improving signal transmission efficiency. 
         [0004]    So far, a variety of measuring techniques adapted for high frequency signals have been proposed to control characteristic impedance of a transmission line. For example, in one of the measuring techniques, a Sub Miniature A (SMA) connector is disposed at an edge of a PCB, and used as an input and output point of a measurement signal. Unfortunately, the design of the connector is confined due to the size requirement. As known to those skilled in the art, for SMA I/O connectors with 3.5 mm or 2.92 mm female connectors, there should be a clearance of 12 mm or more between terminals of two connectors to avoid unexpected contact. In another example, for Bayonet Neill-Concelman (BNC) coaxial cable connectors, a terminal of a connector needs to be kept 20 mm or more away from a terminal of another connector to make connection feasible. 
         [0005]    To avoid such confining conditions, a high frequency and low cost probe means is proposed to measure properties of high-speed PCB signals lines. Please refer to  FIG. 1 . A probe  10  includes an SMA connector  12  and a needle  14 . When a Vector Network Analyzer (VNA) is used to execute measurement of characteristic impedance, e.g. S-parameter, two probes  10  are disposed at two ends of a high-speed/high-frequency signal line, respectively, wherein one of the two probes  10  serves as an input point and the other serves as an output point. Accordingly, it is understood that the measured characteristic impedance would contain mixed characteristic impedances of internal elements, connecting coaxial cables, connectors, probes and high-speed PCB signal lines. 
         [0006]    Since it is necessary to specifically realize the characteristic impedance of a specific target to be measured, e.g. a high-speed PCB signal line, calibration of the instrument is essential to measurement precision. When conducting calibration, a Vector Network Analyzer measures an object whose characteristic impedance is well known or partially known first. The measured data are then referred to for correcting deviation of the system so as to isolate the desired characteristic impedance of a high-speed PCB signal line from the mixed one containing the characteristic impedances of internal elements, connecting coaxial cables, connectors and probes. In other words, as illustrated in  FIG. 1 , a reference plane can be established at the needle tip  14 A of the probe  10 . 
         [0007]    Conventionally, four measuring ways, including open-circuit, short-circuit, applying load and transmitting thru, are used for measurement of characteristic impedance for calibration in order to move the reference plane to the needle tip of the probe. Generally, the measuring means are provided by VNA manufacturers, and can only be used with some specific probes for calibration unless specifically prepared carriers are used. The carries need to be specifically designed for different circuit boards, so time and cost would be additionally spent. Furthermore, the carriers might not be adapted for measurement of other kinds of objects. 
         [0008]    In other words, the conventional measuring means are limited in many ways and lack of flexibility. 
       SUMMARY OF THE INVENTION 
       [0009]    Therefore, the present invention provides a method for measuring characteristic impedance of an electric transmission line in a flexible manner while saving time and cost for calibration. 
         [0010]    The present invention further provides a method for measuring a characteristic impedance of an electric transmission line, which comprises: providing a first probe and a second probe, wherein the first probe and the second probe are separable and independently operable probes; measuring a first characteristic impedance of a first circuit where a first terminal of the first probe and a first terminal of the second probe are directly interconnected to each other; measuring a second characteristic impedance of a second circuit where the first terminal of the first probe and the first terminal of the second probe are connected to opposite terminals of the electric transmission line, respectively; and obtaining the characteristic impedance of the electric transmission line according to the first characteristic impedance and the second characteristic impedance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram illustrating a structure of a probe that is conventionally used for measurement of characteristic impedance; 
           [0013]      FIG. 2A  is a flowchart schematically illustrating a method for measuring characteristic impedance of an electric transmission line according to an embodiment of the present invention; 
           [0014]      FIG. 2B  is a flowchart schematically illustrating a method for measuring characteristic impedance of an electric transmission line according to another embodiment of the present invention; 
           [0015]      FIG. 3A  is a schematic diagram illustrating an embodiment of measuring means for executing a certain stage of the method of  FIG. 2A  according to the present invention; 
           [0016]      FIG. 3B  is a schematic diagram illustrating an embodiment of measuring means for executing another stage of the method of  FIG. 2A  according to the present invention; 
           [0017]      FIG. 4  is a schematic circuit block diagram illustrating an embodiment of measuring means for executing a certain stage of the method of  FIG. 2B  according to the present invention; and 
           [0018]      FIG. 5  is a schematic circuit block diagram illustrating an embodiment of measuring means for executing another stage of the method of  FIG. 2B  according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    The invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0020]    An embodiment of a method for measuring characteristic impedance of an electric transmission line according to the present invention will be illustrated hereinafter with reference to the flowchart of  FIG. 2A  as well as measuring means as illustrated in  FIG. 3A  and  FIG. 3B . In this embodiment, a first probe  30  and a second probe  32  are used for measuring characteristic impedance of the electric transmission line  36 . 
         [0021]    Please refer to  FIG. 3A , in which the measuring means working at a first stage, i.e. Stage S 10  as shown in the flowchart of  FIG. 2A , is schematically illustrated. In this stage, a needle tip  30 B of the first probe  30  and a needle tip  32 B of the second probe  32  are in direct contact to form a physical circuit (hereinafter, a first circuit) in Step S 102 . Then in Step S 104 , an electric variation curve is obtained by measuring an electric property of the first circuit through heads  30 A and  32 A of the probes  30  and  32 . The resulting electric variation curve is referred to as a first real measured curve. For executing the above operation, the probe  10  as shown in  FIG. 1 , or any other suitable probe having a head portion equivalent to the head  12 A of the probe  10  and a tail portion equivalent to the needle tip  14 A of the probe  10 , may serve as either or each of the first probe and the second probe to configure interconnected needle tips  30 B and  32 B, and conduct measurement via heads  30 A and  32 A. 
         [0022]    Subsequently, the first real measured curve obtained in Step S 104  is based on to measure characteristic impedance in Step S 106 . It is to be noted that the measured object at this stage is the first circuit, and the characteristic impedance of the first circuit is measured to obtain a first characteristic impedance. In this embodiment, Step S 106  for obtaining the first characteristic impedance is executed immediately after the first real measured curve is obtained in Step S 104 , but it may alternatively be executed later before Step S 112  for obtaining a second characteristic impedance. 
         [0023]    It is to be noted that the first characteristic impedance measured with two interconnected individual probes  30  and  32  would include, in addition to impedance of the probes  30  and  32  themselves, impedance resulting from discontinuous electric properties at contact faces of the interconnected probes  30  and  32 , which vary with practical working conditions. The impedance resulting from discontinuous electric properties at contact faces can be considered as a result of poor grounding. 
         [0024]    Please further refer to  FIG. 3B , which schematically illustrates the embodiment of measuring means working at a second stage, i.e. Stage S 12  as shown in the flowchart of  FIG. 2A . In this stage, terminals at opposite ends of the electric transmission line  36 , whose characteristic impedance is to be measured, are electrically connected to the first probe  30  and the second probe  32 , respectively, so as to form another physical circuit (hereinafter, a second circuit) as illustrated in  FIG. 3B  in Step S 108 . In practice, the first probe  30  and the second probe  32  are electrically connected with the terminals of the electric transmission line  36  with respective needle tips  30 B and  32 B. Then in Step S 110 , an electric variation curve is obtained by measuring an electric property of the second circuit through heads  30 A and  32 A of the probes  30  and  32 . The resulting electric variation curve is referred to as a second real measured curve. In Step S 112 , the second real measured curve is referred to for calculating gross characteristic impedance of the second circuit. Afterwards, in a third Stage, i.e. Stage S 14  as shown in the flowchart of  FIG. 2A , net characteristic impedance of the electric transmission line  36  can be extracted by removing the component of the first characteristic impedance realized previously. 
         [0025]    Practically, the gross characteristic impedance of the second circuit includes three parts, i.e. the characteristic impedance of the electric transmission line  36 , the characteristic impedance of the probes  30  and  32  themselves and the characteristic impedance caused by the poor grounding. Therefore, the characteristic impedance of the electric transmission line  36  can be realized by removing the component of the first characteristic impedance from the whole second characteristic impedance. 
         [0026]    In summary of this embodiment, in the first stage S 10  including Steps S 102 , S 104  and S 106  for measuring the first characteristic impedance of the directly interconnected probes, reference characteristic impedance is realized. Then in the second stage S 12  including Steps  108  and S 110  for measuring the second characteristic impedance of the assembly of the electric transmission line clamped between the probes, gross characteristic impedance is realized. Finally, in the third stage S 14  including Step S 112 , net characteristic impedance of the electric transmission line is calculated based on the reference characteristic impedance obtained in the first stage S 10  and the gross characteristic impedance obtained in the second stage S 12 . 
         [0027]    By way of the above-described method and measuring means according to the present invention, characteristic impedance of an electric transmission line can be readily realized. Only are a couple of measurement and calculation procedures involved. There is neither specially designed carrier nor particular type of probe required. Therefore, time and cost spent in the prior art for inspection and calibration with a carrier can be saved. 
         [0028]      FIG. 2B  is a flowchart schematically illustrating a method for extracting characteristic impedance of an electric transmission line according to another embodiment of the present invention. In this embodiment, Step S 202  through Step S 206  are identical to Step S 102  through Step S 106  of the embodiment illustrated in  FIG. 2A , and Step S 210  through Step S 212  are identical to Step S 108  through Step S 110  of the embodiment illustrated in  FIG. 2A . Repetitive descriptions would not be redundantly presented herein. 
         [0029]    The embodiment illustrated in  FIG. 2B  differs from that illustrated in  FIG. 2A  in introducing an equivalent circuit model. In practice, a first impedance circuit model correlating to the first characteristic impedance obtained in Step S 206  is built by a software operation in Step S 208 . In this embodiment, Step S 208  for obtaining the first characteristic impedance model is executed immediately after the first characteristic impedance is obtained in Step S 206 , but it may alternatively be executed later before Step S 214  for building a second characteristic impedance model. 
         [0030]    Please refer to  FIG. 4 , in which an embodiment of the first characteristic impedance model is schematically illustrated. As shown, the first characteristic impedance model  40  includes a first equivalent circuit model  42  and a second equivalent circuit model  44 . Each of the first equivalent circuit model  42  and the second equivalent circuit model  44  is built with equivalent electronic elements such as resistor(s), conductor(s), capacitor(s) and/or inductor(s). It is required that the characteristic impedance of the whole first impedance circuit model  40  calculated by a software operation substantially equal to the first characteristic impedance realized in the preceding procedure. Furthermore, when probes of the same type are used, the impedance of the first equivalent circuit model  42  and the impedance of the second equivalent circuit model  44  are made identical. 
         [0031]    In this embodiment, a second equivalent circuit model  50  is further built in Step S 214 , which is schematically illustrated in  FIG. 5 . The second equivalent circuit model  50  contains the first equivalent circuit model  40  built in Step S 208 , and additionally, an electric transmission line model  52 . As shown in  FIG. 5 , the electric transmission line model  52  is electrically coupled to the first equivalent circuit model  42  and the second equivalent circuit model  44  of the first equivalent circuit model  40  at opposite sides thereof. The parameters of the electronic elements constituting the first equivalent circuit model  42  and the second equivalent circuit model  44  illustrated in  FIG. 4  and the first equivalent circuit model  42  and the second equivalent circuit model  44  illustrated in  FIG. 5  are made identical. In this embodiment, the electric transmission line model  52  is a variable impedance circuit model. By adjusting the characteristic impedance of the electric transmission line model  52  in a software operational way, the electric variation curve simulated with the second impedance circuit model  50 , hereinafter the simulated electric variation curve, would vary accordingly. In other words, Step S 216  in this embodiment can have the contour of the simulated electric variation curve of the second impedance circuit model  50  approximate the second real measured curve obtained in Step S 212  as much as possible by adjusting the characteristic impedance of the electric transmission line model  52 . At last, as a result of proper adjustment, the contour of the simulated electric variation curve approximates the second real measured curve, and meanwhile, the characteristic impedance of the adjusted electric transmission line model can be outputted as the second characteristic impedance of the electric transmission line (Step S 218 ). It is to be noted that the criteria for determining approximation may vary with designs and practical requirement. 
         [0032]    It should be understood that the above-described simulating and approximating operations may be used not only for realizing the characteristic impedance of the electric transmission line model  52 , but also for realizing the characteristic impedance of the first impedance circuit model  40 . In this case, the first equivalent circuit model  42  and the second equivalent circuit model  44  are adjusted to have the simulated electric variation curve of the first impedance circuit model  40  approximate the first real measured curve. 
         [0033]    Compared with the embodiment illustrated in  FIG. 2A , the embodiment illustrated in  FIG. 2B  utilizes parameter adjustment by way of simulation software instead of mere calculation. Accordingly, similar objectives can be achieved in an alternative way. 
         [0034]    The measuring methods and measuring means described above may be used together with a variety of fixtures and tools to measure electric properties of electric elements such as circuit on wafer, IC pins and circuit on PCB. The probes required for executing measurement according to the present invention may be used for measuring electric properties of more than one channel and transmission line independently, in parallel connection, or in a general or differential way. 
         [0035]    According to the present invention, once the characteristic impedance of the probe-incorporated circuit, i.e. the first characteristic impedance, is known, simulation of circuit can be made based on the first characteristic impedance to readily realize the characteristic impedance of the electric transmission line in a circuitry. Furthermore, the calibration of probes with a carrier required in the prior art can be omitted. Therefore, time and cost spent for measuring characteristic impedance of an electric transmission line in a circuitry can be minimized. 
         [0036]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. For example, in spite hardware devices are exemplified as above in order to practice the redundancy power supply system and the power control circuit according to the present invention, hardware/software hybrid modules or firmware designs may also adopted as alternatives of the devices when appropriate.