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

The capacitors bank is made of several capacitors in parallel that can be switched-on,
as illustrated in Figure 61, so that a capacitance is added to the initial capacitance. This is a
very efficient way of synthesizing a tunable capacitance since the switchable capacitance can
have a same quality factor (tuned by the Ron of the switches).
Figure 61. a) Capacitors banks, b) Capacitance evolution of a capacitors bank
Figure 62 shows the value of the central frequency given in Equation (II.10) when
varying the capacitance value. This graph is plotted for different inductance values.
It is interesting to notice the great dependency of f0 for small C values and large L
values. Hence, it means that the filter f0 is very sensitive to capacitance values. Since
capacitances are small, such a filter may be difficult to centre accurately at a given frequency.
It may also be very sensitive to capacitive parasitics. Moreover, it requires very large
capacitances to reach low frequencies.
Hence, this graph underlines the L versus C values trade-off and highlights the
difficulty of having a single LC filter for the whole 45-1002MHz band.
Figure 62. Central frequency versus C for various L values
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