12704_SCTE_Broadband_Nov2016_COMPLETE_lowres

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The net result is that standing waves can be set up which may cause a severe ripple in the frequency response of the signal at the receiver input. The resistive tee may be improved by adding a resistor as in Fig. 21.

In addition to the components shown, most spur units contain extra components. These are the various capacitors used for RLR correction and balancing of the transformer reactances and also line power routing components. The line power components usually take the form of chokes from input to outputs combined with suitable blocking capacitors. These circuits form low pass filters which route the 50Hz line power to the appropriate point. It is not possible to pass high AC currents through the ferrite-cored components used in the RF splitters as saturation of the magnetic material can occur. This will result in amplitude modulation of the RF envelope at twice the supply frequency (hum modulation). 4. The Tap-Off or Tee unit The tee unit has the same basic purpose as the splitter units. However, the side losses are normally greater (between 12 and 40dB). One of the most important requirements in a cable system is to provide adequate isolation between adjacent receivers. This isolation is required to prevent oscillator signals from one receiver passing to other receivers via the relay network. For this reason, isolation is normally a regulated system parameter. A typical isolation figure for a relay network would be 33dB between any two outlets. If non-directional tees are used, then the lowest loss tee which could be employed would be 33/2 dB i.e. 16.5dB. This value may be reduced by the use of directional tee units based on the principle outlined earlier for spur units. Using the directional technique, the lowest tap value which can be used is that recorded when the reverse loss of the first tee plus the forward loss of the following tee is 33dB (see Fig.19).

The extra resistor provides good back matching and reduces the standing wave problem, but adding the resistor reduces the signal level by 6dB. This in turn means that the series resistor ‘R’ must be reduced to overcome the extra loss and the through loss of the tap is consequently increased. Even so, the resistive tee unit is widely used because of its low cost. The transformer tap-off is used mainly for frequencies up to VHF. A typical circuit is shown in Fig. 22.

This tap is an auto-transformer with the side loss determined by the tapping point on the transformer, together with the loss incurred in the series resistor “R”. This resistor is included to maintain a good back match at the tap which otherwise would be of very low impedance. This type of tee is suitable for all values from 18-40dB. The shunt resistance of the transformer has so far precluded the use of this type of transformer tap at UHF. 5. The Mechanical Aspects of Equalisers, Spurs and Tee Units Robust, reliable housings must be provided for tee units. The choice of casing seems to narrow to either a die-cast case or sheet steel can be suitably plated and provided with a plastic weatherproof housing. The design requirements for the housing are made more difficult when it is understood that the unit must accommodate a large variety of cable sizes. Indeed, the specification for a tee or spur unit which is presented to the design engineer usually takes the following form: 1) Size - must be small enough to fit into the smallest conduit box that the main contractor can get away with... 2) Weatherproofing and rust proofing - must withstand salt spray, acid, wind, rain, hosepipes and dogs. 3) Mounting - must be possible to mount in ten seconds by an engineer perched on a 100-foot ladder in a Force 9 gale.

The design employed for higher value tee units can be made less complex than that required at low values. The simplest technique is the use of a simple resistor tapped onto the centre conductor of the through cable (see Fig. 20).

The main disadvantage of this simple tap is that the unit does not present the correct impedance to the television receiver input. If all television receivers were well matched then no reflections would be set up. Most receivers are matched for best noise figures and not best impedance and their input return loss ratio is very poor.

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Vol. 38 No. 4 - November 2016 Issue