Design Methodologies of LTCC Bandpass Filters, Diplexer, and ...

720 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006
Fig. 8. Fabricated 2.4-GHz bandpass filter with transmission zero at the
frequency of 1.9 GHz. (a) 3-D structure. (b) Measured and EM simulated
results.
Two filters mentioned above are fabricated with the substrate
of Dupont 951. Their dielectric constant and loss tangent are 7.8
and 0.0045, respectively. The 2.4-GHz LTCC filter is designed
on one upper layer with the sheet of 1.57 mil, six middle layers
with the sheet of 3.6 mil, and three lower layers with the sheet
of 1.57 mil. Its overall size is 110 mil 92 mil 28 mil. After
circuit simulation, these values are converted into the LTCC
structure. The simulation is carried out with the assistant of the
full-wave electromagnetic (EM) simulator Sonnet from Sonnet
Software Inc., North Syracuse, NY. In the 3-D structure, the parallel-coupled
line is placed on the lower layer to reduce the coupling
effect with other capacitors. Fig. 8(a) shows the three-dimensional
(3-D) structure of 2.4-GHz LTCC bandpass filter.
The on-wafer tester has been chosen to improve the accuracy of
measurement. The network analyzer Agilent N5230A PNA_L
is used to measure, and the short-open-load-through (SOLT) is
adopted to calibrate.
Fig. 9. Fabricated 2-GHz bandpass filter with transmission zero at the
frequency of 2.5 GHz. (a) 3-D structure. (b) Measured and EM simulated
results.
As shown in Fig. 8(b), the frequencies of measured and EM
simulated transmission zeros are 1.84 and 1.9 GHz, respectively.
At the frequency of 2.4 GHz, the measured and EM simulated
insertion losses are less than 1.44 and 1.3 dB, respectively; the
return losses are greater than 15 and 17 dB, respectively.
A 2-GHz bandpass filter is given as another design example.
This 2-GHz LTCC filter is designed on five upper layers with the
sheet of 3.6 mil, four middle layers with the sheet of 1.57 mil,
and two lower layers with the sheet of 3.6 mil. Its overall size
is 107 mil 102 mil 32 mil. The 3-D structure of the 2-GHz
LTCC bandpass filter is shown in Fig. 9(a). In the 3-D structure,
the parallel-coupled line is the same placed on the lower layer
to reduce the coupling effect with other capacitors. As shown
in Fig. 9(b), the frequencies of the measured and EM simulated
transmission zeros are 2.45 and 2.5 GHz, respectively. At the
frequency of 2 GHz, the measured and EM simulated insertion
losses are less than 1.45 and 1.26 dB, respectively; the return
losses are greater than 19 and 39 dB, respectively.
The LTCC technology is a kind of thick-film process. In order
to realize the physical 3-D circuit, the parasitic effect among ca-