ne is generated by the intervening sections of coaxial cable. Example 1. A CATV system has 30 amplifiers in tandem; each amplifier has a noise figure of 7 dB. Assume that the input of the first amplifier is terminated in 75 So resistive. What is the thermal noise level (voltage) at the last amplifier output? Use Eq. (17.8): Nv = -59 dBmV + 7 dB + 10 log 30 = -59 dBmV + 7 dB + 14.77 dB = -37.23 dBmV. For carrier-to-noise ratio (C/N) calculations, we can use the following procedures. To calculate the C/N at the output of one amplifier, C/N = 59 dBmV - NFdB + input level (dBmV) (17.9) Example 2. If the input level of a CATV amplifier is +5 dBmV and its noise figure is 7 dB, what is the C/N at the amplifier output? COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Use Eq. (17.9): C/N = 59 dBmV - 7 dB + 5 dBmV = 57 dB. With N cascaded amplifiers, we can calculate the C/N at the output of the last amplifier, assuming all the amplifiers are identical, by the following equation: C/NL = C/N (single amplifier) - 10 log N. (17.10) Example 3. Determine the C/N at the output of the last amplifier with a cascade (in tandem) of 20 amplifiers, where the C/N of a single amplifier is 62 dB. Use Eq. (17.10): C/NL = 62 dB - 10 - log 20 = 62 dB - 13.0 dB = 49 dB. 17.3.5 The Problem of Cross-Modulation (Xm) Many specifications for TV picture quality are based on the judgment of a population of viewers. One example was the TASO ratings for picture quality given earlier. In the case of cross-modulation (cross-mod or Xm) and CTB (composite triple beat), acceptable levels are -51 dB for Xm and -52 dB for CTB. These are good guideline values (Ref. 1). Xm is a form of third-order distortion SO typical of a broadband, multicarrier system. It varies with the operating level of an amplifier in question and the number of TV channels being transported. Xm is derived from the amplifier manufacturer specifications. The manufacturer will specify a value for Xm (in dB) for several numbers of channels and for a particular level. The level in the specif