Basic Knowledge catv (Community Antenna Television) and hfc ( Hybrid Fiber Coax ) Network Outline

Basic Knowledge CATV (Community Antenna Television) and HFC ( Hybrid Fiber Coax ) Network

Poor frequency response. Some part of the received band had notably lower levels than other parts. This is particularly true as the frequency increases. In other words, there was fairly severe amplitude distortion. Thus equalization became necessary.
The mixing of two or more RF signals in the system caused intermodulation products and “beats” (harmonics), which degraded reception.
When these TV signals carried modulation, cross-modulation (Xm) products degraded or impaired reception.

A CATV headend places multiple TV and FM (from 30 to 125) carriers on a broadband coaxial cable trunk and distribution system.
The objective is to deliver a signal-to-noise ratio (S/N) of 42–45 dB at a subscriber’s TV set.

We define 0 dBmV as 1 mV across 75 ohm impedance. Note that 75 is the standard impedance of CATV, coaxial cable, and TV sets.
the electrical power law, we have
where Pw is the power in watts, E the voltage in volts, and R the impedance, 75 ohm
By definition, then, 0.0133 W = +60 dBmV
If 0 dBmV = 0.0133 × 10−6 W and 0 dBm = 0.001W,and gain in dB = 10 log(P1/P2), or, in this case, 10 log[0.001/(0.0133 × 10−6)], then 0 dBm = +48.76 dBmV.

Remember that, when working with dB in the voltage domain, we are working with the E2/R relationship, where R = 75 . With this in mind the definition of dBmV is

Let the bandwidth, B, of an NTSC TV signal be rounded to 4 MHz. The open circuit noise voltage for a 75- resistor is
a 2.2-μV noise generating source (resistor) connected to a 75- (noiseless) load. Only half of the voltage (1.1 μV) is delivered to the load. Thus the noise input to 75 is 1.1 μV or −59 dBmV. This is the basic noise level, the minimum that will exist in any part of a 75- CATV system.

S/N can be related to C/N on CATV systems as
with an NTSC 4.2-MHz TV signal. Adding noise weighting improvement (6.8 dB), we find
With most CATV systems, coaxial cable trunk amplifiers are identical. This, of course,eases noise calculations. We can calculate the noise level at the output of one trunk amplifier. This is
where NF is the noise figure of the amplifier in decibels.

In the case of two amplifiers in cascade (tandem), the noise level (voltage) is
If we have M identical amplifiers in cascade, the noise level (voltage) at the output of the last amplifier is

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 resistive. What is the thermal noise level (voltage) at the last amplifier output?
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,

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?
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:

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.

CATV spectrum based on Ref. 1, showing additional upstream and downstream services.Note the imbalance between upstream and downstream.
CATV spectrum with equal upstream and downstream bandwidth for other services.

Data over Cable Service Interface Specification (DOCSIS) is a family of specifications
that permits CATV systems to have two-way capability: voice, data, and broadcast-type television downstream and data and voice upstream.