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

II.2.d Passive LC Filters from the Literature
It has previously been demonstrated that for our application, the most interesting
passive LC filters are second order bandpass filters, sometimes cascaded to obtain a higher
order filter. As explained, they are mostly based on a parallel LC resonator, often referred as
an LC tank. According to the inductor nature, papers can be sorted into two main categories:
- Discrete inductor based filters
- On-chip inductor based filters
The performances of several passive LC filters are summarized in a table that may be
found in APPENDIX C. Among them, the most recent one [II.4] describes two cascaded first
order passive LC filters, as shown in Figure 68. Figure 68 also describes how frequency and
gain tuning is realized. The two LC tanks are fed in current by transconductances having a
built-in first order low-pass filter which allows higher harmonic frequencies rejections. The
130MHz to 1GHz band is divided into two sub-bands. The higher inductance value is realized
by means of an off-chip inductor while parasitic inductances from PCB traces and bonding
wires realize a small inductance value, as depicted in Figure 69.
This filter exhibits in band 18dB NF and 20dBm IIP3, for a Q-factor of 5 as it may be
seen in Figure 70. The strong asset of this filter is the harmonic rejection which is higher than
36dB over the whole tuning range.
Figure 68. Cascade of two second order bandpass filter
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