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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RF selectivity challenges<br />
In this thesis, several topologies have been assessed. The purpose is to find which one<br />
best suits the abacus created with the rejections specifications of the RF filter. It comes out<br />
that the second or<strong>de</strong>r bandpass topology is the best. It actually needs few components which<br />
make it an attractive structure for easy tunability. Moreover, it is also possible to tune the<br />
quality factor Q of such a filter. Q <strong>de</strong>scribes the selectivity of the filter and is <strong>de</strong>fined as the<br />
ratio of the central frequency by the -3dB bandwidth. The higher the quality factor, the more<br />
selective the filter.<br />
Once the topology chosen, the literature has been assessed to find most appropriate<br />
solutions. First, LC passive filters have first been studied as a comparison basis to active<br />
solution studied further. A figure-of-merit has also been introduced to handle fair comparisons<br />
with current state-of-the-art. As far as fully-active structures are concerned, the literature<br />
mainly consi<strong>de</strong>rs Gyrator-C and <strong>Gm</strong>-C filters. Gyrator-C filters consist in emulating an<br />
inductive behaviour at RF frequencies by a gyrator, allowing the replacement of inductors by<br />
active counterparts. <strong>Gm</strong>-C filters actually consist in synthesizing a transfer function by means<br />
of integrators, and so of <strong>Gm</strong>-cells. For simple topologies such as the second or<strong>de</strong>r bandpass,<br />
these two kinds of filter may lead to the same structure.<br />
<strong>Gm</strong>-C filters<br />
The studied <strong>Gm</strong>-C filter structure is based on an analogy with a parallel LC resonator<br />
where the inductor is replaced by a gyrator. Tuning the gyrator value and tuning the<br />
capacitance in parallel, a large tuning range is obtained compared to a passive LC filter. The<br />
theoretical study of noise and linearity limitations leads to the choice of a mo<strong>de</strong>rate quality<br />
factor since as soon as Q increases, RF performances are strongly <strong>de</strong>gra<strong>de</strong>d. This study also<br />
<strong>de</strong>monstrates that high RF performances are required on all <strong>Gm</strong>-cells. It has been chosen to<br />
use large transconductance values (2.5 and 10mS) in or<strong>de</strong>r to <strong>de</strong>crease the overall noise of the<br />
filter. However, such transconductances have to be linearized to reach the linearity<br />
specification set on the <strong>Gm</strong>-cell.<br />
Several transconductor linearization techniques from the literature have been assessed.<br />
Only two of them exhibit an interesting linearity versus noise tra<strong>de</strong>-off: the Dynamic Source<br />
Degeneration (DSD) which is also used in the literature for a TV reception RF filter [VI.1],<br />
and Multiple Gated Transistors (MGTR) which show the most promising results at the <strong>Gm</strong>cell<br />
level. <strong>Gm</strong>-C filters based on <strong>Gm</strong>-cells <strong>de</strong>signed with these linearization techniques have<br />
been simulated in 65nm CMOS technology. Table 33 summarizes the obtained performances<br />
compared to literature results. The filter with MGTR-based <strong>Gm</strong>-cells presents the best<br />
performances. For 6dB gain and a Q-factor close to 4, it exhibits an <strong>III</strong>P3 above 8dBm up to<br />
450MHz, with a 16dB NF. However, this structure appears to be very sensitive to bias as well<br />
as process and mismatch variations, where a robust solution is required for our applications.<br />
That is why no test-chip has been realized based on this architecture.<br />
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