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Passive Components for Cable Television Systems Part 2 Aseries of articles, edited by R. J. Seacombe, dealing with the general principles of Cable TV systems. THE BASICS OFCABLETVENGINEERING 8

by Chris Swires (Fellow) Swires Research & Development, Hornchurch, Essex.

In Part 1 of this article, published in Vol. 11 No. 4 (February 1979) of Cable Television Engineering, the author dealt with the design considerations and system applications of attenuators and equalisers, with particular emphasis on the importance of good impedance matching. In Part 2 he deals with the important parameters of the other passives for cable distribution systems. He discusses the design and operation of splitters, subscriber's taps, and system outlets.

3. Spur or Splitter Units The spur or splitter is used to divide the signal into a number of paths or lines. The splitter is either an equal split, dividing power equally between two or more lines, or a non-symmetrical split used to tap off a small portion of the power on a through line. The theoretical minimum loss when dividing signal two ways is 3dB in a matched system. The simplest form of splitter uses a resistive star network to divide power into two output lines. (Fig.13).

It can be seen therefore that any variation in R 1 affect the impedance of both ports of the splitter. It can also be shown that the power absorbed by the resistors in the splitter is the same as that delivered to the load. This means that the total loss to each port is 6dB as opposed to a theoretical minimum loss of 3dB. This loss means that although the resistive splitter is low in cost and can be made to operate over a very wide bandwidth, half of the power entering the unit is lost. The availability of modern low loss ferrites has made possible the design of transformer splitter units with very low inherent losses. By carefully matching the impedances in these ferrite splitters, the signal losses to the output ports can approach the theoretical minimum. The equal splitter gives a loss of about 3.5dB to each of two lines and is almost universally configured, as shown in Fig. 14. , will seriously The input transformer is used to transfer the impedance from 75 ohm to 37.5 ohm. This is achieved by a √2:1 turns ratio. The output cables effectively appear in parallel at the centre of T 2 (i.e. 2 x 75 ohm in parallel gives an impedance of 37.5 ohm) with the two output signals in phase. The output transformer T 2 is used to provide isolation between the two output ports. A signal applied at output 1 appears in antiphase at output 2 and no current flows in R. In the event of any imbalance between the two outputs, a current flows in R and the unbalanced power is absorbed. In an extreme case when one output line is short-circuited, output is still maintained on the other output port. Although some loss is incurred in R, this is only of the order of 1dB on a practical unit.

Looking into the input, the impedance is that of R 2 + R L

in parallel

with R

+ R

L plus the resistance of R 1 .

3

where R 1

= R

= R

3 this can be simplified to:

2

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Vol. 38 No. 4 - November 2016 Issue