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loss, input and output return loss
for the two filters on the test board
shown in figure 8. The same
measurement method was used
as in the previous cases. The
measured response shows a 3 dB
passband from roughly 1.7 to 9.3
GHz (5.5:1 or 138% bandwidth),
with stopband rejection well above
15 dB up to 40 GHz. This result
confirms that this technique can be
used in UWB applications without
unintentional emissions out of band
due to re-entry.
Case 4: Combining
Reflectionless Filters and
LTCC Filters to Sharpen
Selectivity
We’ve shown that reflectionless
filters can be combined to achieve
ultra-wide passbands and that
this approach provides excellent
stopband rejection up to 40 GHz
without re-entry. To come closer
to the real-world requirements
of a UWB system under FCC
specifications, it may be necessary
to sharpen the transition to conform
to the FCC spectral mask.
The absorptive characteristic
Figure 8:
Test board for XHF-
23+ and XLF-73+
Figure 9:
Measurement plots of S21 (black), S11 (red), and S22 (blue)
for combined XHF-23+ and XLF-73+, exhibiting a bandpass response with
roughly 138% bandwidth and good stopband rejection up to 40 GHz.
Figure 4:
Simulation of band pass response combining XHF-292M+ and
LFCW-1062+
with FCC spectral mask for UWB indoor transmissions (dotted line and
right axis).
of reflectionless filters means
that they’re not only cascadable
with other reflectionless filters,
but also with all manner of
conventional filters. This hybrid
approach allows us to achieve the
desired wideband response while
incorporating the selectivity of
another filter technology. In this
case, we combine a two-section,
high pass reflectionless filter (XHF-
292M+) with a low pass LTCC filter
(LFCW-1062+) to take advantage
of the greater selectivity of the
latter. Simulation results of this
combination are shown in figure
10 along with the FCC spectral
mask for indoor UWB transmissions
(dotted line and right axis).
This combination exhibits a
passband from 2.4 to 10.9 GHz
(4.5:1 or 128% bandwidth). The
deep rejection at the lower stopband
below 2.4 GHz keeps transmissions
New-Tech Magazine Europe l 35