12704_SCTE_Broadband_Nov2016_COMPLETE_lowres

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4) Cables - must take every cable from a main trunk to a job lot of downlead cable (or even wet string). 5) Terminals - easily secured by an engineer with bananas for fingers.

6) Return loss - infinity. 7) Through loss - zero.

8) Price - about half that of the very best unit on the market! Needless to say we haven’t yet reached the point where we can meet all these requirements simultaneously. 6. System Outlets Two basic types of outlets are available for use on VHF/UHF. These are for loop wiring, where the distribution cable passes through the outlet system and for tee wiring where a single cable comes into the outlet plate from the tee or tap-off unit. The loop wired outlet plate, incorporates the tap-off unit within the outlet plate. Whilst there is no theoretical disadvantage in the loop wired type of outlet plate it suffers from a serious practical disadvantage. Because the distribution cable must pass into the subscribers home, any disturbance to the outlet can cause the failure of all outputs on that distribution line. Access to the cable in a subscriber’s home is frequently difficult and if the subscriber is out, faults are difficult to rectify. It is not unknown for a subscriber on a distribution line to “improve” his own signal by removing the tee resistor thus raising his signal level by about 30dB! For this reason, the use of loop wired units is not recommended currently although in some situations the conduit layout leaves no alternative. The normal outlet used for tee wired systems consists of a socket isolated from the input cable by two or more capacitors (Fig. 23). These capacitors provide safety isolation of the receiver from the cable system and thus other connected receivers. Current regulations for VHF/UHF systems dictate that the isolation components of the outlet plate must withstand a DC flash test of 3KV for one minute without breakdown.

In addition to this flash test, certain clearances must be maintained in the design of the outlet. From a mechanical standpoint, the plate must be robust and is normally made to match the electrical fittings in the dwelling. Where TV and FM outputs are required, a simple diplexing circuit is built into the outlet plate. On low cost units, the diplexer may simply be an extra resistance in series with the FM output so that the greater power goes to the TV outlet. Performance requirements for passive network accessories The choice of good quality network accessories can considerably enhance the performance and reliability of a cable network. Unfortunately, as accessories are required in large quantities there is a temptation for system installers to choose on the basis of cost rather than performance. If expensive failures are to be avoided, the life of low-cost housings must be studied in relation to the projected life of the network. The electrical performance of the accessories should be carefully examined. A higher-cost passive unit may have lower through loss and where a large number are cascaded, the cumulative reduction in loss can be very significant. It may even allow the use of fewer amplifiers or lower-cost amplifiers, thus more than paying for the higher cost of the passive accessories. As in all calculations of this nature, it is the cost- effectiveness of the components which must be studied. Some network accessories are easier to install than others and there can be considerable cost savings during installation. Afive-hundred point system may well have over a thousand network accessories, but only fifteen amplifiers. It is not difficult to see that the right choice of passive accessory is of fundamental importance to the performance of the cable network.

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Vol. 38 No. 4 - November 2016 Issue