## Thermal Noise in CATV Systems

We remember from Section 3.3.3 that thermal noise is the most common type of noise encountered in telecommunication systems. In most cases, it is thermal noise that sets the sensitivity of a system, its lowest operating threshold. In the case of a CATV system, the lowest noise levels permissible are set by the thermal noise level—at the antenna output terminals, at repeater (amplifier) inputs, or at a subscriber's TV set—without producing snowy pictures.

Consider the following, remembering we are in the voltage domain. Any resistor or source that looks resistive over the band of interest, including antennas, amplifiers, and long runs of coaxial cable, generates thermal noise. In the case of a resistor, the thermal noise level can be calculated based on Figure 17.3.

To calculate the noise voltage, en, use the following formula:

where en = rms noise voltage,

B = bandwidth (Hz) of the measuring device (electronic voltmeter, V), and k = a constant equal to 40 x 10-16 at standard room temperature.2

Figure 17.3 Resistor model for thermal noise voltage, en.

2This value can be derived from Boltzmann's constant (Chapter 3) at room temperature (68°F or 290 K) is assumed.

Figure 17.3 Resistor model for thermal noise voltage, en.

2This value can be derived from Boltzmann's constant (Chapter 3) at room temperature (68°F or 290 K) is assumed.