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75 Í2 Resistor generating noise

75 Í2 Resistor generating noise

75 Q Load

75 Q Load

Figure 17.4 Minimum noise model.

Let the bandwidth, B, of an NTSC TV signal be rounded to 4 MHz. The open circuit noise voltage for a 75-Q resistor is en = (4 x 75 x 4 x 10-16)1/2 = 2.2 ^V rms.

Figure 17.4 shows a 2.2-^V noise generating source (resistor) connected to a 75-Q (noiseless) load. Only half of the voltage (1.1 ^V) is delivered to the load. Thus the noise input to 75 Q is 1.1 ^V or -59 dBmV. This is the basic noise level, the minimum that will exist in any part of a 75-Q CATV system. The value -59 dBmV will be used repeatedly below (Ref. 3).

The noise figure of a typical CATV amplifier ranges between 7 and 9 dB (Ref. 3).

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