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

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

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27.<br />

This leads to the downconversion of odd harmonics to IF, as it is <strong>de</strong>picted in Figure<br />

Figure 27. Downconversion of LO harmonics to IF<br />

Therefore, a certain harmonic rejection level is required so that the wanted signal is<br />

not corrupted by these harmonics.<br />

I.3.b Roles of RF Selectivity<br />

At first sight, a LNA and a mixer are sufficient to get the reception functionality.<br />

Nevertheless, in case of TV receivers, the architecture has to be optimized in or<strong>de</strong>r to reach<br />

the required high RF performances. In particular, RF selectivity is essential to relax the<br />

constraints on the LNA and on the mixer. Moreover, this RF filtering is a strong asset to be<br />

able to keep the tuner performances in the presence of strong interferers, when handling<br />

terrestrial TV signals.<br />

Within the tuner, the RF filter can be located between the LNA and the mixer, if the<br />

LNA presents high enough performances, as illustrated in Figure 28. As said previously, it has<br />

to be frequency tunable, so as to be centered on the wanted channel. The central frequency of<br />

the filter has to be tunable over the 45-1002MHz band, in or<strong>de</strong>r to be used both for cable and<br />

terrestrial receivers.<br />

Figure 28. TV tuner architecture<br />

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