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the high-pass filter used in this case.
Input and output return loss are also
plotted in red and blue, respectively,
simply to highlight the good
matching in the passband and the
stopband, which is a differentiating
feature of these filter designs.
Case 2: Maximizing
Bandwidth
Case 1 establishes the viability of the
technique by combining high pass
and low pass reflectionless filters to
create an ultra-wideband bandpass
response. By employing the same
technique, we can now experiment
with different models to shape the
response and achieve desirable
characteristics. In this case, we
seek to create the widest passband
possible with models available,
combining two-section, high pass
model XHF-581M+ and low pass,
three-section model XLF-312H+.
In addition to the wide bandwidth,
because this filter incorporates two
and three section designs, we also
expect to see very high rejection in
the upper and lower stopbands.
A simulation combining these two
models in series is shown in figure 4,
exhibiting a 3 dB passband from 450
MHz to 5.7 GHz (12.7:1 or 171%
bandwidth). Note that a logarithmic
frequency scale is used to better
show the shape of the response. Also
note the lower stopband rejection
greater than 30 dB and upper
stopband rejection reaching the 50
to 60 dB range, again a function of
the two- and three-section designs
used in this case.
The filters were mounted on the
test board shown in figure 5, and
again insertion loss, input and
output return loss were swept from
0.1 to 40 GHz and from 450 MHz to
2 GHz at fine resolution to capture
more detail at low frequency. The
measurements were corrected for
the fixture by subtracting the loss
of a straight thru-line.
The measurement plots are
shown in figure 6. Again, note
the logarithmic frequency scale to
better represent the filter behavior.
Figure 2:
Test board for XHF-
292M+ and XLF-73+
Figure 3:
Measurement plots of S21 (black), S11 (red), and S22 (blue)
for combined XHF-292M+ and XLF-73+, exhibiting a bandpass response
with roughly 121% bandwidth.
Figure 4:
Simulation of band pass response combining XHF-581M+ and
XLF-312H+
New-Tech Magazine Europe l 33