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The filter achieves a 3 dB bandwidth
from roughly 500 MHz to 5.2 GHz
(10:1 or 165% bandwidth). The
measured data exhibits a slightly
narrower passband than the
simulation, but still achieves full
decade of bandwidth. As expected
lower stopband rejection is between
30 and 40 dB, and upper stopband
rejection ranges from 40 to greater
than 60 dB. The passband shows
excellent flatness with no distortion
from adverse interactions between
the filter stages.
Case 3: Confirming
Stopband Rejection up to
40 GHz without Re-Entry
It’s clear from Cases 1 and 2 that
combining reflectionless filters
can achieve ultra-wide passbands,
allowing bandwidth at least up to a
full decade, amply supporting the
bandwidth requirements of UWB
applications. Another concern for
system designers building UWB
transmitters is the potential for
“re-entry” out of band at higher
frequencies. Such unintentional
radiation can potentially interfere
with signals at neighboring
frequencies and violate the FCC
rules. Therefore, UWB filters must
exhibit good stopband rejection
without re-entry up to a very high
frequency.
Reflectionless
filters
offer
advantages in this regard. In part
due to their fabrication using MMIC
technology, reflectionless low-pass
filters provide stopband rejection
extending up to 40 GHz. Many
conventional filter approaches
would suffer re-entry over this
bandwidth. This characteristic
allows us to create a UWB filter
response that meets the FCC power
mask up to 40 GHz without re-entry.
In this case, we combine high pass
model XHF-23+ and XLF-73+, both
single section designs. S-parameter
simulation for these models is
shown in figure 7, exhibiting a 3
dB passband from 1.6 GHz to 10
GHz (6.25:1 or 145% bandwidth).
Stopband rejection remains better
than 15 dB up to 40 GHz without
re-entry.
Figure 9 shows measured insertion
Figure 5:
Test board for XHF-
581M+ and XLF-312H+
Figure 6:
Measurement plots of S21 (black), S11 (red), and S22 (blue)
for combined XHF-581M+ and XLF-312H+, exhibiting a bandpass
response with roughly 165% bandwidth.
Figure 4:
Simulation of band pass response combining XHF-23+ and
XLF-73+
34 l New-Tech Magazine Europe