Aerospace & Defense Technology - September 2022 - 40

RF & Microwave Technology
1: Transmission
2: Reflection
dB
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1
Log Mag
Log Mag
1
2 3
8 dB/Ref
4 dB/Ref
0.00 dB
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C
C
2
Start 20000.000 MHz
Mkr1
Mkr2
Mkr3
27950 MHz
27650 MHz
28260 MHz
-4.960 dB
-7.766 dB
-7.766 dB
Figure 2. mmW-FH bandpass filter, substrate integrated waveguide design, on high-performance RF/microwave
thermoset substrate.
sion-line construction with low insertion
loss and high power-handing
capability but with the tradeoff of the
largest size of any RF filter topology. A
smaller RF filter will suffice at lower signal
power levels in a system with coaxial
components. In small satellites and
mobile systems, which require even less
RF signal power, a filter housed in a surface
mount technology (SMT) package
may provide the channelization needed
while also achieving demanding system
miniaturization goals.
Including an RF/microwave filter
within an A&D system enables selection
and rejection of high-frequency
signals for communications systems,
radar systems, and systems that detect
adversarial radar. High-frequency filters
can be designed with various responses,
such as bandpass filters, notch filters
(also known as band-reject), lowpass
filters, and highpass filters. Each
allows a desired set of frequencies to
pass while others are rejected.
Multiple filter responses are combined
into diplexers, triplexers, and
40
multiplexers to process multiple signal
channels with minimal filtering hardware.
A diplexer is a three-port RF component,
making it possible to connect
two transmitters at different frequencies
to a single antenna. With an additional
diplexer connected to the antenna
and two receivers, a single antenna
can transmit on one frequency channel
and receive on a different frequency
channel. A triplexer allows connecting
three transmitters or receivers operating
at different frequencies to the same
antenna. A multiplexer is the means,
within a reasonable channel count, to
connect many communications transmitters
and receivers to one or a limited
number of antennas.
Diplexers, triplexers, and multiplexers
sort transmitted and received
signals in A&D electronic systems
for communications, radar, spectrum
monitoring, and surveillance. Within
each component, RF filter circuitry in
various forms separates and isolates the
desired frequency channels from the
many other EM signals in the operatmobilityengineeringtech.com
Stop
40000.000 MHz
ing environment. Several other RF
filter-based components combine
multiple filters to save physical size
in a system, including the combination
of RF filters with electromagnetic
interference (EMI) filters. EMI
filters help prevent EM energy from
serving as noise within an electronic
system, such as a communications
radio. A combination RF/EMI
filter eliminates the need for separate
components, saving size and
weight.
As newer A&D system designs
seek increased functionality from
smaller system sizes through smaller
components, RF filters are among
the components being designed into
smaller volume packaging. Today's
RF filters must meet demanding
SWaP requirements without sacrificing
electrical performance. Larger
systems may require the power-handling
capabilities of waveguide RF
filters or coaxial-packaged RF filters
based on traditional microstrip,
stripline, or coplanar waveguide
(CPW) printed-circuit-board (PCB)
structures. But at lower power levels
and signal frequencies, drop-in
or SMT filters often provide the RF
filtering functions while supporting
SWaP targets. Lower-power filters for
processing 1W (+30 dBm) or less signal
power include filters based on resonant
materials such as surface-acoustic-wave
(SAW) and bulk-acoustic-wave (BAW)
materials.
SAW filters use various piezoelectric
materials to convert input electrical
energy to acoustic energy (where filtering
is performed) and then back to
electrical energy at the output utilizing
interdigital transducers. They typically
operate with 1W signal power or less
and with RF signals as high as 2 GHz.
BAW filters can cover the frequency
range from about 1.5 to 6.0 GHz with
slightly more signal power than SAW
filters and fewer performance variations
over wide operating temperature
ranges. Both filter types can be housed
in miniature drop-in and SMT packages
for ease of mounting on PCBs.
Passive RF filters are also fabricated
on low-temperature-cofired-ceramic
(LTCC) substrates, while active RF filAerospace
& Defense Technology, September 2022

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Aerospace & Defense Technology - September 2022

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