Mumbai university > Electronics and telecommunication Engineering > Sem 6 > Television Engineering

Marks: 10

Years: May 2016

• The system used in individual homes for direct reception of TV from a satellite is illustrated in the figure below:

• The preferred band for this service is K or Ka band. The size of the reflector would be as small as the headlight of a car.

Outdoor unit:

• It consists of a receiving antenna, low noise amplifier (LNA) and convertor.

• The receiving antenna is a parabolic reflector with a horn as the active element. The horn can be directly in front of the reflector or it may use an offset feed as shown in the above figure.

• The reflector diameter may be 0.6m for 11GHz and still smaller for K and Ka bands.

• The low noise block consists of a low noise wideband amplifier followed by a convertor.

• The output of the convertor consists of a signal of UHF frequency ranging from 950-1450MHz (500MHz bandwidth).

• The advantage of using UHF frequency is that a low cost coaxial cable can be used as feeder from the outdoor unit to the indoor unit.

• LNB (low noise block) cannot be kept indoor because long cable between the horn and the first amplifier (LNA) will cause substantial degradation of the overall noise figure of the set.

• Cable after the LNA cannot add any significant noise as the dominant noise is due to the first amplifier.

• The casing of LNB is such that it is able to withstand hostile environmental conditions outside.

Indoor unit:

• The wideband signal from the LNB is fed to an RF amplifier (turned to UHF). The amplified signal is fed to a channel selector circuit which selects the wanted signal.

• The selected channel is down converted to a fixed IF of 70MHz by a local oscillator and mixer.

• IF amplifier amplifies the signal which, then goes to an FM detector.

• The detector recovers original baseband signal consisting of a composite video signal and audio signal.

• The video part of the baseband signal modulates an RF carrier using AMVSB technique.

• The audio part modulates another RF carrier using frequency modulation.

• Both carriers pertain to a VHF/UHF channel of the terrestrial TV system. These modulated signals are fed to the normal domestic TV receiver which after due processing reproduces picture and sound.