III. Gm-C Filtering - Epublications - Université de Limoges

The transmission of the signal through several paths can also be a drawback. Indeed,
before being received with a single antenna, terrestrial signals are subject to multipath
propagation and also to echoes, especially in mountain areas. Degradation and delay of the
signal due to these phenomena may cause interferences between one transmitted symbol and
its subsequent symbols, thus degrading their correct detection. This phenomenon is called
intersymbol interferences (ISI) and may strongly degrade the reception of the signal. To
reduce the impacts of ISI, more complex modulations than those previously described are
used, such as multiple carriers (COFDM) or modulations which eliminate spectral
redundancies (8VSB).
I.1.d General Description of a TV Receiver
The receiver is an essential piece in a television. Composed of a tuner and a
demodulator, it allows the conversion of the RF television transmission into audio and video
signals, which can further be processed to produce sound and to display colored pictures. This
is illustrated in Figure 15. Hence, people are able to select a TV channel.
Figure 15. TV reception chain
In more details, the RF television signal is first received and processed by the tuner, to
select one channel within the spectrum. This front-end part of the receiver has to transmit the
cleanest possible signal to the input of the demodulator, without introducing degradation nor
interferences with other channels. Indeed, distortions or degradation of the wanted TV signal
may result in demodulation errors leading to wrong pixel colors on the TV screen.
That is why very high performances are required for the tuner. It has to deal with the
received broadband RF signal. As explained in more details further, the signal has then to be
downconverted from RF frequencies to lower frequencies, and finally, the tuner has to filter
the wanted channel, so that only one is transmitted to the demodulator. Besides, all these steps
have to be performed with as little degradation of the signal as possible.
From this, three main characteristics of the TV tuner can be highlighted. It has to be:
- sensitive, in order to be able to receive even weak wanted signals;
- selective, so that unwanted interferers are strongly rejected;
- accurate, to return the exact emitted signal.
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