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The programme demonstrates some of the important electronic characteristics of coaxial and parallel twin feeder transmission lines.
Metadata describing this Open University video programme
Module code and title: T321, Telecommunication systems
Item code: T321; 04
First transmission date: 27-03-1976
Published: 1976
Rights Statement:
Restrictions on use:
Duration: 00:34:00
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Producer: Tony Jolly
Contributors: Gary Alexander; David Crecraft
Publisher: BBC Open University
Keyword(s): `Lumped line'; Coaxial; Delay; Parallel twin feeder; Phase shift; Sinusoidal signals; Standing waves; Transmission lines
Footage description: David Crecraft uses a spectrum analyser to measure the frequency response of a short piece of coaxial cable. Crecraft introduces the programme. He goes on to look at the frequency response of a large reel of properly terminated cable and works out the 3dB band width of the cable. Crecraft connects a pulse generator to the cable. Taking measurements from the oscilloscope screen, he works out the signal delay which he uses to calculate the length of the cable. Crecraft looks at and discusses the delay and rise times of the pulses, pointing out that the cable has the properties of a filter. Crecraft removes the 75 Ohm termination at the end of the cable to create an open circuited line. He notes the results - a pulse at the receiving end of double amplitude and a reflected pulse. Crecraft places a short circuit across the end of the cable and notes the results - the end pulse disappears and the reflected pulse is inverted. Gary Alexander demonstrates the standing wave pattern on a short circuited transmission line with a sinusoidal signal on it. He places a fluorescent tube near the transmission line. Alternate bright and dark areas are displayed by the tube. Alexander next demonstrates the "lumped model" of a transmission line. He explains how the model is constructed and then looks at the progress of a pulse down the line. The pulse appears progressively more delayed. He then short circuits the line and notes an inverted reflected pulse at the sending end with a delay corresponding to twice the length of the line. Alexander demonstrates the "lumped model" again to show why an inverted reflected pulse is obtained in a short circuit. Alexander uses the "lumped model" to demonstrate a sinusoidal signal sent down the line. Increased delay at points increasingly distant from the source is seen. Alexander short circuits the line to obtain a standing wave pattern. He explains what is happening and then repeats the experiment to reinforce his points. Crecraft sums up.
Master spool number: 6HT72005
Production number: 00525_5223
Videofinder number: 1309
Available to public: no