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Figure 1:
Simulation of band pass response combing XHF-292M+ with XLF-73+.
Suitability or
Reflectionless Filters for
UWB RF Front End
While UWB technology has shown
much potential, many design
challenges remain in bringing
the technology to a stage of
wider industry adoption and
commercialization. One of those
challenges has been developing RF
filters with a wide enough passband,
flat response over the whole band,
and sufficient selectivity to meet FCC
specifications. Several approaches
have been studied to achieve the
desired response utilizing microstrip
technology [2] [4] [5]. While these
approaches have achieved varying
degrees of success, they each come
with drawbacks. Microstrip UWB
filter designs typically occupy greater
than a square inch of board space
and tend to be costlier than practical
for volume production.
Mini-Circuits’ reflectionless filters
present an attractive alternative to
existing approaches for UWB filters.
Because reflectionless filters absorb
and terminate stopband signals
rather than reflecting them back to
the source, they give designers the
ability to cascade filters in multiple
sections without generating standing
waves due to impedance mismatch
betweenstagesandotherundesirable
effects. This characteristic allows
combination of low pass and high
pass filters to create a bandpass
response, a technique that becomes
useful for the purpose of designing
UWB filters.
In addition to their intrinsic
cascadability, reflectionless filters are
uniquely suited for UWB filter designs
for at least three reasons. First,
reflectionless high pass filters have
broad enough passbands to achieve
the desired bandwidths for UWB;
most other filter technologies do not.
Second, the low pass filters offer
cut-offs that extend high enough in
frequency to achieve 3 dB bandwidths
well above 100%. Finally, the good
impedance match at the band edges
allows multiple filters to be cascaded
in series without causing distortion
of the passband shape, whereas
cascading conventional filters can
often create standing waves between
stages and introduce passband ripple
and phase instability.
Moreover,
while
competing
approaches employ transmission
lines, reflectionless filter topologies
are based on lumped elements and
produced using MMIC technology
resulting in much smaller size, lower
cost, and excellent repeatability,
making them suitable candidates
for volume production. Models are
available in package sizes as small
as 2x2mm and in bare die format for
chip and wire integration.
With these advantages in mind,
this article explores possibilities for
use of reflectionless filters in UWB
filter design. Five case studies
are presented using standard
reflectionless filter models available
off the shelf from Mini-Circuits.
Simulation results and measured
data are presented to illustrate the
various advantages of reflectionless
filters in UWB applications. Finally,
a design is presented that meets
UWB bandwidth requirements and
conforms to the specifications of the
FCC spectral mask.
Case 1: General Proof of
Concept
To illustrate the technique, two
reflectionless filters were combined to
create a bandpass response. In this
case, high pass model XHF-292M+ and
low pass model XLF-73+ were used.
The preliminary simulation shown
in figure 1 exhibits a 3 dB passband
from 2.3 to 9.7 GHz (4.2:1 or 123%
bandwidth).
To validate these results, the filters
were mounted on a test board as shown
in figure 2. Insertion loss and input /
output return loss were swept from 0.1
to 40 GHz and again from 45 MHz to
2 GHz, the later with fine resolution to
capture the low frequency details. The
measurement was then corrected for
the fixture by subtracting the measured
loss of a straight thru-line.
The measured data for this filter is
plotted in figure 3, with insertion
loss in black. The response confirms
the simulation results, exhibiting a 3
dB bandwidth of roughly 2.4 to 9.7
GHz (121% or 4:1). As expected,
cascading the units shows no effect
on the flatness of the passband.
The higher rejection on the low end
is due to the two-section design of
32 l New-Tech Magazine Europe