re-emphasis) and an opposite charac- A coaxial line is required to meet an average noise contribution teristic at the receiving terminal end (de-emphasis) the noise con- per circuit of better then 1.5pWOp per km. In order to achieve this tributions can be flattened SO that all the channels have a similar with a limited DC power budget optimal designs have to be used. contribution and the worst channels are 3 to 4dB improved. Line level verses frequency Line level verses frequency with pre-emphasis without pre-emphasis Frequency Frequency Noise contributions without Noise contributions with pre-emphasis pre-emphasis Overall Overall Thermal contribution Intermodulation Thermal contribution Intermodulation contribution contribution Frequency Frequency Figure 20.21 Principles of pre-emphasis Repeatered cable line equipment 20/21 Cable Impedance 1:1:n Zb = nzo Figure 20.22 Hybrid input circuit In other words, the shape of the pre-emphasis balances the multi- build up, a requirement to maintain the error to within 1dB at any channel load and thermal noise contributions SO that these are frequency over a 280km system is necessary. equally distributed over the line spectrum and all channels have ideally the same performance as Figure 20.21 20.14.3.1 Regulation range 20.14.2 Thermal noise The change in loss of typical coaxial cable is approximately 0.004% To optimise the noise figure of the repeater and also to provide a per °C. The change in buried cable temperature in the UK is +10°C. good input impedance matching to the line produces conflicting Thus for a typical cable section loss of 40dB at the regulating pilot requirements at the amplifier input. A good input impedance match frequency the maximum diurnal change is approximately +1.6dB. to the cable is required with reflection losses normally kept to below Over two such sections the loss change would be 3.2dB and with 55dB and at the same time effective noise impedance matching is reference to Figure 20.15 it can be seen that a change in transmission essential to gi