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

IV.5 Frequency Limitations
As said, the Rauch filter has originally been designed to be tunable between 45 and
240MHz. The use of this filter at higher operating frequencies results in using the OA with a
reduced voltage gain. Indeed, due to its finite GBW, the gain of the OA decreases with
frequency [IV.7].
since
Furthermore, a lower OA gain leads to a higher K and so to a Q-factor enhancement
( Z 0 + Z f )
− ( Z + Z )
A(
jω)
K(
jω
) =
. (V.31)
A(
jω)
Z
0
Indeed, the fixed capacitor decrease to 6pF in the capacitors banks so that the central
frequency increases as well leads to a Q factor of 6 at 450MHz, and to a gain of 15dB. Q is
enhanced due to a lower OA gain. Moreover, due to this smaller voltage gain, the input RF
signal is more subject to distortions from the OA. This is why IIP3 strongly decreases as
frequency increases. At 450MHz, in this configuration, IIP3 falls down to -3dBm.
One possibility to counter the change in the value of K is to implement a tunable
capacitor Cf (or C0) in order to keep K constant. This partially fights the drop of the OA gain
and gives the results described in Table 30 where 5dBm IIP3 (for 10dB gain) are obtained at
450MHz for a 3.5 Q factor value.
Last point preventing us from using the Rauch structure at higher frequencies is the
trade-off selectivity versus filter gain. As explained in introduction, a more demanding
selectivity is required in UHF in order to reject adjacent channels in a sufficient way. This is
only possible increasing the gain of the filter. However, this solution would require
amplifying less on the front-end LNA and it would result in challenging noise constraints on
this LNA as well [IV.3].
0
- 156 -
f