III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
III. Gm-C Filtering - Epublications - Université de Limoges
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These adjacent channel rejection requirements can even become more stringent in the<br />
coming years. In<strong>de</strong>ed, with the analog switch-off, frequency bands formerly occupied by<br />
analog TV become free. They are called white spaces. These frequency allocations may be<br />
used in the future for various purposes such as home wireless LAN for instance. Thus, their<br />
emitting power and frequencies could become a problem if TV tuners are not selective<br />
enough.<br />
Furthermore, the LTE, standing for Long Term Evolution, which is the most recent<br />
GSM/UMTS norm, is looking for emitting within the 800-900MHz range. Since mobile<br />
communications require high emitting powers, this may cause interference from mobile<br />
service base stations and handsets overloading the sensitive TV tuner front end circuitry,<br />
preventing existing viewers from seeing a picture [I.9].<br />
Such a filter has also to be accurately centered on 6 to 8 MHz large channels. That is<br />
to say that frequency calibration has to be taken into account when <strong>de</strong>signing the filter,<br />
through a way of correcting the central frequency if it <strong>de</strong>viates from the wanted one.<br />
Moreover, the frequency tuning range has to be divi<strong>de</strong>d in frequency steps which are 6MHz<br />
maximum in or<strong>de</strong>r to be able to center the filter on all the different channels, as <strong>de</strong>picted in<br />
Figure 30.<br />
Figure 30. Maximum frequency step of the filter<br />
This RF filtering is a first step of selectivity. It does not aim at selecting sharply only<br />
one channel with high RF performances. This sharp filtering is performed after<br />
downconversion to IF, to take advantage of working at lower frequencies. However, the RF<br />
filtering has other assets which are essential to reach the high performances required by the<br />
TV tuner.<br />
I.3.c Selectivity Specifications<br />
Selectivity specifications are summarized in Table 2. As said, both high selectivity and<br />
broadband tunability are required. Specifications on adjacent channels rejection are set on<br />
most critical points (N±5 and N±6 from ATSC A/74). Thus, it enables to get a shape for the<br />
filtering which rejects all other adjacent channels sufficiently. Note that, in the following, H3<br />
and H5 respectively stand for the third or<strong>de</strong>r and the fifth or<strong>de</strong>r harmonic frequencies.<br />
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