The Research Of Impulse Source And The Key Technology Of Sampling For TDR

TDR(Time Domain Reflectometry) is an important instrument for testing the impedance of PCB which can detect whether the cable is damaged accurately and effectively. In order to have a better role in the impedance measurement-field, it is necessary to improve the performance of the instrument. The performance of TDR is reflected in the horizontal resolution and system bandwidth, the former is decided by the rising edge of the impulse source, while the latter is controlled by the sampling system and the impulse source. Therefore, the impulse source and the sampling system are the key parts to improve the performance of the instrument.The main work and contributions are listed as follows:(1)The matching-transmission and reflection of electrical signals were analyzed in this thesis, then TDR impedance tester was introduced and the factors which affect the performance of characteristic impedance tester were analyzed too.(2)The design of impulse source. The difficulty is how to find the balance between the rising time of the edge and the low demand of ring and so on. Two design schemes were discussed in this thesis. One was that the tunnel effect of tunnel diodes can be used to adjust the pulse edge to get the pulse waveform with rapid rising edge. The second solution was to complete the reshaping of pulse edge based on the variation characteristic of SRD in the positive and negative bias case and the comparison of the Comparator. And the second solution was adopted because it had faster rise time and more flat roof in the pulse signal.(3)The research of key technologies in the sampling system. The preliminary studies were made about sampling system based on the difference principle and the key technologies were analyzed such as bypassing effection and sampling pulse in this thesis. The program which the power divider was fitted by adder and substracter circuit was designed for the bypassing effection. Two design schemes were given in this thesis for the sampling pulse. The one was the combination of SRD and microstrip line whose difficulty were the generation of pulse signal with high amplitude and the design of Pulse Forming Network(PFN). The other was based on the avalanche transistor whose difficlulty were parameters coordination and the set of working condition of snow slide. But the latter has smaller bottom width and higher bandwidth and was adopted in this this thesis.In the end, all the programs were applied to the characteristic impedance tester. After testing, it could be found that rising edge of impulse source was 57 ps and the bottom width of sampling pulse was 230 ps. The system rise time is about 140 ps and the bandwidth is about 2.5GHz, it made a big difference on the performance of the TDR. Therefore, it is of great value in engineering application.