20.14.1 Pre-Emphasis
The noise contributions from a single repeater are shown in Figure 20.21(a). The greatest contribution is thermal noise at high frequencies as the highest gain is required at this end to overcome to cable loss.
By applying deliberate shaping to the transmission path at the transmitting terminal end (pre-emphasis) and an opposite characteristic at the receiving terminal end (de-emphasis) the noise contributions can be flattened so that all the channels have a similar contribution and the worst channels are 3 to 4dB improved.
In other words, the shape of the pre-emphasis balances the multichannel load and thermal noise contributions so that these are equally distributed over the line spectrum and all channels have ideally the same performance as Figure 2fl.21(b).
20.14.2 Thermal noise
To optimise the noise figure of the repeater and also to provide a good input impedance matching to the line produces conflicting requirements at the amplifier input. A good input impedance match to the cable is required with reflection losses normally kept to below 55dB and at the same time effective noise impedance matching is essential to give a low noise figure F.
Use of a Hybrid circuit at the input as shown in Figure 20.22 (Bell, 1971) provides a method of achieving both these requirements. The noise matching is improved by about 3dB compared to a conventional amplifier.
This technique can be extended further by including a hybrid transformer at the output and bridging the feedback between the input and output hybrid transformers.
20.14.3 Regulation
The gain shaping of a repeater is designed to match the loss characteristics of the cable. This is characterised by Equation 20.40, the loss being in dB.
Thus if the loss of the cable is known at one frequency f (normally the regulating pilot frequency) the loss at other frequencies can be deduced.
The change of the cable loss with temperature obeys the same law and thus correction for temperature is applied with a root f shape across the line frequency spectrum with the greatest correction at the highest frequency.
This equalisation which is controlled by the regulating pilot is not exact and small errors accumulate in the gain frequency response at each repeater equalisation point. The error systematically adds along the repeatered cable and to avoid excessive systematic error build up, a requirement to maintain the error to within ldB at any frequency over a 280km system is necessary.
20.14.3.1 Regulation range
The change in loss of typical coaxial cable is approximately 0.004% pert. The change in buried cable temperature in the UKis±10°C.
Thus for a typical cable section loss of 40dB at the regulating pilot frequency the maximum diurnal change is approximately ±1.6dB. Over two such sections the loss change would be 3.2dB and with reference to Figure 20.15 it can be seen that a change in transmission load of 3.2dB produces only a small increase in NPR.
Thus regulation points in the UK are established every two repeatered sections as in Figure 23(a) and allow for a loss change of ±4dB. A higher temperature shift would require regulation at every repeatered point with the greater cost and power feeding requirements this would entail.
An alternative method of regulation, applying pre-regulation to halve the diurnal changes in repeater levels, is shown in Figure 20.23(b).