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

Table 33. Performances Comparison
Filter [VI.1] Filter 1 Filter 2 Units
Linearization tech. DSD* DSD* MGTR -
Tuning range 50 - 300 45 - 385 45 - 450 MHz
Q factor 6 3.5 3.7 -
Gain 6 10 6 dB
NF 20 18 16.3 dB
IIP3 2 to 6 -4 to 2 8 to 10 dBm
Power
consumption
7.6 16.5 30 mW
Supply 1.2 2.5 2.5 V
CMOS Technology 130 65 65 nm
From the Gm-C filters design, it comes out that the main limitation of the performance
is the gyrator. Indeed, the RF signal undergoes strong distortion and degradation when
flowing through the active inductor. It has also been assessed the ability of the Gm-C filter to
be used at higher frequencies. However, linearity is strongly degraded as soon as frequency
increases. This is due to the use of smaller capacitances in the capacitors banks ensuring the
frequency tuning. Indeed, the switch capacitances non-linearity of the switches becomes non
negligible compared to the classically capacitances. Furthermore, a selectivity of 4 in UHF
only rejects adjacent channels by 1dB or less. A higher Q is demanded but enhancing the
quality factor would result in higher NF.
Rauch filters
To overcome Gm-C limitations due to the signal flowing through the gyrator,
operational amplifier (OA) based filters are investigated. The purpose is to take advantage of
both a high loop gain and a filter response introduced by to the RC network. Hence, we are
aiming at designing a highly linear filter while not compromising noise at the same time.
However, RC filters are more selective at high gain. Among the structures found in the
literature, the positive feedback Rauch topology presents the highest selectivity for a given
gain. It also presents limited sensitivity to passive components, making it a more robust
solution. It has been chosen to limit the filter gain to 10dB, which corresponds to a quality
factor of 3 when optimizing the dimensioning of the Rauch filter. Indeed, if the gain on the
RF filter stage is too high, this would result in very high noise constraints on the LNA.
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