Page 194 - System on Package_ Miniaturization of the Entire System

168 Cha pte r F o u r

passband while having only one input and one output port. Such filters are very feasible
in the design of diversity-type WLAN (IEEE 802.11a/b/g) where low economics and
small form factor designs are of prime importance. Several dual-band filters can be
found in the literature [30–33] where dual behavior transmission line resonators (DBRs)
are used to synthesize different passbands. The same theory can be used to implement
filters with three passbands [33]. Current solutions for a single-band front-end filter use
high-quality-factor (Q) packaging technologies based on ceramics and polymers with
2
filter sizes in the range of 6 to 14 mm . For a dual-band filter to be used in commercial
wireless products it should, in addition to small insertion losses, occupy a smaller area
compared to using two single-band filters. One of the major requirements in the design
of dual-band filters, because of the large number of resonators, is the need for high Q
technologies. Transmission line based dual-band filters [30–33] are often too large in
size to be employed in commercial multimode systems since the physical length of the
transmission line has to be at least comparable to half a wavelength, which is large in
the frequency band from 1 to 10 GHz, a frequency band where most of the mobile
consumer applications are expected to function.
A reduction in the size of the filter is possible by replacing the transmission lines by
lumped-element dual-band resonator sections. Figure 4.13 shows the schematic of a dual-
band filter that is an efficient method to obtain dual-passband characteristics. It consists
of two, fourth-order resonators that are capacitively coupled to each other and to the
input-output terminals. The filter is matched using series input capacitors (C and C )
1 2
that also control the two center frequencies. The bandwidth and the passband ripple of
the filter at the lower passband are adjusted by controlling the capacitive coupling (C )
c
between the two shunt fourth-order resonators. Since the filter uses the same set of
passives to operate over a wide frequency range (∼3 to 5 GHz), passives with high
quality factors (Qs) and stable electrical characteristics over a broad frequency range are
required. The basic idea is to synthesize two different inductances from the fourth-order

L f
C f

C c
C 1 C 2

L c
L a C a L a C a

Port 1 Port 2

L b C b L b C b

FIGURE 4.13 Dual-band fi lter.