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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<strong>III</strong>.4.b Filter Limitations<br />
In the following, Filter 2 has been simulated at higher frequencies in or<strong>de</strong>r to quantify<br />
the <strong>de</strong>gradation of the performances when increasing the tuning range of the filter.<br />
Performances are summarized in Table 16.<br />
Table 16. Filter 2 frequency limitations<br />
Frequency Band (MHz) Performances<br />
40 – 240<br />
240 – 470<br />
470 – 750<br />
750 – 1000<br />
- 116 -<br />
Gain = 6dB<br />
Q = 4<br />
IIP3 = 7 to 12.5dBm<br />
NF = 16dB<br />
30mW<br />
Gain = 7-8dB<br />
Q = 4.5<br />
IIP3 = 1dBm at 650MHz<br />
NF = 17dB<br />
30mW<br />
Poor adjacent channels rejection<br />
Gain = 8.5dB<br />
Q = 5-6<br />
IIP3 = -0.5dBm at 1GHz<br />
NF = 17dB<br />
30mW<br />
Poor adjacent channels rejection<br />
This table has been established using the same 10mS transconductances. Higher<br />
frequencies are reached using a smaller fixed capacitance. It comes out that Q increases with<br />
frequency, which explains the origin of the NF increase. This increase of the Q-factor is due<br />
to the higher Q-factor of the fixed capacitance compared to the Q of the other capacitances<br />
due to the Ron of the switch. At 650MHz and up to 1GHz, IIP3 falls down to 0dBm for a Qfactor<br />
which increases up to 6 and a NF of 17dB.<br />
Linearity strongly <strong>de</strong>creases with frequency. In<strong>de</strong>ed, using smaller capacitances to<br />
reach higher frequencies of operation makes the Coff of the switches being relatively more<br />
important and this <strong>de</strong>gra<strong>de</strong>s the filter linearity.<br />
Hence, the dynamic range is consi<strong>de</strong>rably <strong>de</strong>creased. Furthermore, a Q-factor of 6 in<br />
UHF is very poor in terms of adjacent channels rejection since N±5 channels are rejected by<br />
only 1dB.