In this experiment I have obtained the first diagram at the oscilloscope when th

Question

In this experiment I have obtained the first diagram at the oscilloscope when the cables are matched, and the second diagram when cable impedances are mismatched. Is there reflection at the source in either case? Please provide beginner explanation of the time domain response specifying why the graph shape obtained is as above with cables of the same impedance (from the signal generator to the oscilloscope and after the t connector) . I know why there is varying delay time as length increases, so please dont include this in the answer. Please also explain the effect of mismatched cables on the time domain response.

Thank you

Experiment Set up the function generator to produce a 40ns pulse with very small duty cycle so that reflected pulses can be separated from the next pulse. (One way to do this is to use a 10kHz square wave with a duty cycle of 0.04%) Use a T junction to connect the function generator via a short coaxial cable to the input of the oscilloscope. Then connect the transmission line (different lengths of coaxial cable) to the other side of the T connector. Part 1 speed of waves on a transmission line. Vary the length of the transmission line and record the results. From these results find the dielectric constant of the coaxial cable. (Take care with the coaxial lines that you are using you are provided with two types of cable, one with 50 Qimpedance and the other with 750. Think carefully about what happens if you mix cables of different impedance.)

Explanation / Answer

In the second diagram there is reflection of signal. During transmission through a coaxial cable the entire coaxial system including all the connectors should have matched impedances otherwise reflections occur which may distort the information being transferred through the cable and at times may even lead to cable breakdown. If impedances are not matched it can even cause attenuation where the transmitted signal is either hidden in the output or not seen at all.

For maximum power transfer and minimum signal reflection source impedance should be equal to load impedance, in real cases usually the cable impedance sending end impedance and receiving end impedance all have the same value to achieve maximum power transfer

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