Cable Length Measurement Systems Based on Time Domain Reflectometry

Transcription

1 Cabe Length Measurement Systems Based on Time Domain Refectometry Jianhui Song *, Yang Yu, and Hongwei Gao Schoo of Information Science and Engineering, Shenyang Ligong University, Shenyang, , P.R. China Abstract. Based on the principe of time domain refectometry(tdr) cabe ength measurement, the principe error of the TDR cabe ength measurement is anayzed. Methods for reducing cabe ength measurement error are anayzed. Two cabe ength measurement systems based on the TDR principe are designed. The high-precision time interva measurement modue is the core of the first measurement system. The samping oscioscope with a computer is the core of the second measurement system. The experimenta resuts show that the measurement systems deveoped in this paper can achieve high cabe ength measuring precision. Keywords: TDR, time interva measurement, measurement system. 1 Introduction The issue of measuring precision in the wire and cabe market becomes more and more prominent. It is significant to measure the cabe ength precisey, rapidy and economicay. Compared with the traditiona measurement methods, time domain refectometry (TDR) technoogy has an advantage of non-destruction, portabiity and high-precision, which is an idea cabe ength measurement method[1~3]. In order to achieve high cabe ength measuring precision and study the key technoogies of the cabe ength measurement based on TDR, two cabe ength measurement systems based on different patforms are deveoped. The TDR Cabe Length Measurement Theory TDR is a very usefu measuring technoogy based on high-speed puse technoogy. The cabe ength measuring principe is very simpe. The test votage puse is injected into one end of the cabe, and the puse wi be refected at the end of the cabe. By measuring the time interva between the injection puse and the refection puse, the cabe ength can be obtained by assuming the veocity as constant[4,5]. The formua for ength measurement is v Δt = (1) * Corresponding author. S. Lin and X. Huang (Eds.): CSEE 011, Part I, CCIS 14, pp , 011. Springer-Verag Berin Heideberg 011

2 Cabe Length Measurement Systems Based on Time Domain Refectometry 397 Where, cabe ength, v signa propagating veocity, Δ t time interva between the injection puse and the refection puse. According to the inear superposition aw of error propagation, the cabe ength error can be expressed as formua () 1 δ = ( t δv+ v δt ) () Where, v signa propagating veocity in cabe t time interva between the injection puse and the refection puse δ v propagation veocity error δ t time interva measuring error. The direct puse counting method is the most basic method of time interva measurement and it is amost the basic of a time interva measurement methods. Many methods combine the direct puse counting method with other methods to achieve high precision measurement. Therefore, the TDR cabe ength measuring accuracy based on the principe of direct puse counting time interva measurement are anayzed. According to the principe of direct puse counting, δ t can be expressed as formua (3) Where, f count puse frequency N count puse number δ f count puse frequency error δ N count puse number error. δ 1 N t = N f f δ f δ (3) Therefore, formua () can be expressed as formua (4) δv δn δ f δ = + (4) v N f The cabe ength is measured n times. Taking into account of the random measuring errors and according to the of random error accumuation, the average measuring error can be cacuated by formua (5): n δv 1 δ f δ = + δn v n N f (5) i= 1 The ± 1 quantization error can be expressed as: 1 i = =... = ( ) = (6) N N N N n

3 398 J. Song, Y. Yu, and H. Gao 9 At present, the crysta osciator temperature stabiity is better than 10, therefore the count puse frequency error can be ignored. At this point the TDR cabe ength measuring uncertainty u can be expressed as u u v v f n v = ( ) + ( ) (7) Where, u v propagation veocity standard uncertainty u f reference frequency standard uncertainty n number of measurements. It can be seen from formua (7) that when the veocity is constant, the count puse frequency and number of measurements are the key factors to the TDR cabe ength measuring uncertainty. 3 The TDR Cabe Length Measurement Systems In order to achieve high cabe ength measuring precision and study the key technoogies of the cabe ength measurement based on TDR, two cabe ength measurement systems based on different patforms are deveoped. The schematic diagram and photo of measurement system I and II are shown in Fig. 1 and Fig.. The first measurement system is composed of signa transmitting and receiving modues, the high precision time interva measurement modue, temperature measurement modue, dispay circuit, contro pane and microprocessor. Different ampitude and different puse width signa is transmitted into the cabe under test based on the actua measurement situation. Then the time interva between the transmitted puse and refected puse is shaped into the gate signa and measured by high precision time interva measurement modue. The time interva measurement resut is sent into the microprocessor and fitered to eiminate the gross errors. According to the ambient temperature measured by the temperature measurement modue, the propagation veocity is compensated. Finay the ength of the cabe under test is cacuated and sent to the dispay circuit to dispay. Fig. 1. Schematic diagram and photo of measurement system I

4 Cabe Length Measurement Systems Based on Time Domain Refectometry 399 computer signa generator samping oscioscopes temperature measurement modue Cabe under test Fig.. Photograph of measurement system II The second measurement system is composed of signa generator, samping oscioscopes temperature measurement modue and computer. Different ampitude and different puse width signa generated by signa generator is transmitted into the cabe under test based on the actua measurement situation. The waveform data of the transmitted puse and refected puse is coected by the samping oscioscope and sent into the computer processing. Firsty, the measurement data is fitered by the system software. Then the refected wave is identified and detected by the refected wave recognition agorithm. According to the ambient temperature measured by the temperature measurement modue, the propagation veocity is compensated. Finay the ength of the cabe under test is cacuated. 4 Experiment and Anaysis A ength of m RVV 300/300V PVC sheathed fexibe cabe is measured by measurement system I. The count frequency of the high precision time interva measurement modue is 1.GHz. The cabe ength is measured 56 times. The 8 propagation veocity is 10 m/s. The measurement data are anayzed. There are nine measurement resuts. The histogram is shown in figure 3. The standard deviation of the TDR cabe ength measurement system is s 56 i (8) i= 1 ( ) = = 0.1 m N 1

5 400 J. Song, Y. Yu, and H. Gao The standard deviation of the cabe ength measurement arithmetic mean is s ( ) t 0.01( m) Δ = = (9) N The coverage factor k is, then the expanded uncertainty of the cabe ength measurement is ( ) U = ks( Δ ) = s( Δ ) = 0.0 m (10) It can be seen from formua (8) to (10) that the measurement system deveoped in this paper can achieve high precision cabe ength measurement. Occurrences number of measurement resuts Cabe ength(m) Fig. 3. Random error of cabe ength measurement system 5 Concusions Based on the principe of time domain refectometry(tdr) cabe ength measurement, Two cabe ength measurement systems based on the TDR principe are designed. The high-precision time interva measurement modue is the core of the first measurement system. The samping oscioscope with a computer is the core of the second measurement system. The experimenta resuts show that the measurement systems deveoped in this paper can achieve high cabe ength measuring precision. References 1. Dodds, D.E., Shafique, M., Ceaya, B.: TDR and FDR Identification of Bad Spices in Teephone Cabes. In: 006 Canadian Conference on Eectrica and Computer Engineering, pp (007). Du, Z.F.: Performance Limits of PD Location Based on Time-Domain Refectometry. IEEE Transactions on Dieectrics and Eectrica Insuation 4(), (1997)

6 Cabe Length Measurement Systems Based on Time Domain Refectometry Pan, T.W., Hsue, C.W., Huang, J.F.: Time-Domain Refectometry Using Arbitrary Incident Waveforms. IEEE Transactions on Microwave Theory and Techniques 50(11), (00) 4. Langston, W.L., Wiiams, J.T., Jackson, D.R.: Time-Domain Puse Propagation on a Microstrip Transmission Line Excited by a Gap Votage Source. In: IEEE MTT-S Internationa Microwave Symposium Digest, pp (006) 5. Buccea, C., Feiziani, M., Manzi, G.: Detection and Locaization of Defects in Shieded Cabes by Time-Domain Measurements with UWB Puse Injection and Cean Agorithm Postprocessing. IEEE Transactions on Eectromagnetic Compatibiity 46(4), (004)