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

Figure 43. Second order Low-pass Filters Topologies
Figure 44. H3 and N+5 rejections according to the second order topology
II.1.b.ii Assessment of the Topology
The major advantage of low-pass filters is their ability to reject harmonic frequencies.
For a given filter, the rejection of harmonic frequencies remains constant while tuning the cutoff
frequency, as it may be observed in Figure 42.
However, as it may also be seen in Figure 42, first and second order filters do not
reach the desired 20dB H3 and H5 rejection. A third order is then required, meaning at least
three reactive elements. Tuning the cut-off frequency by tuning the reactive elements values
while keeping a constant set of coefficient in the transfer function may become difficult as
well.
This graph in Figure 42 also highlights the poor adjacent channels rejection of lowpass
filters, especially when reaching high frequencies: less than 1dB rejection of the N+5 at
500MHz., even for third order topologies Furthermore, adjacent channels located below the
cut-off frequency are amplified by up to 3dB.
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Topologies comparison
for a same fc