The Electromagnetic Spectrum Innovator
WHITE PAPER
www. apitech.com | 855.294.3800 | info@apitech.com |
1
High Power Switched Filter Banks Raise the Temperature on
Design Challenges
By: Jon Scoglio, Engineering Manager, APITech
www.linkedin.com/company/api-technologies-corp-/
An application that has driven the early development
of high power SFBs is communications band signal
jamming in the VHF and UHF frequencies. These
devices have predominantly been used to prevent
communications band signals to remotely triggered
explosives and to deny cellular service near secure
governmental and military facilities. Nonetheless,
technology that has initially been leveraged to mitigate
communications may be a solution that enhances
communications in modern and future trending
applications.
Assembly chassis for high power SFBs must be designed to
incorporate design elements to optimize electrical, RF, and
thermal performance.
The Dierence Between High Power and
Low Power SFBs
High power SFBs are generally judged by the same
performance parameters as lower power SFBs. These
parameters include size, weight and cost. Electrical
considerations, such as passband, roll o, insertion loss,
and out of band rejection are also as signicant in high
power SFBs. Achieving a lower insertion loss and higher
out of band rejection may require more design eort
for high power SFBs, as heat dissipation from a high
insertion loss and attenuating high power harmonics are
often key considerations.
The main dierentiations between the low power and
high power SFB requirements include high power SFBs’
heat-dissipation characteristics and the high voltage
handling needs. These two factors of high power
operation induce greater voltage and thermal stresses
on the switch elements and components directly in
the signal chain. Additionally, higher voltages and
power increase the impact of nonlinearities in system
components. These factors demand a detailed analysis
of each component’s performance over a wide range of
operational parameters.
Both a at passband with low insertion loss and steep drop o on
high power SFB lters is critical to enable the 100s of watts of RF
power that are passed and ltered by these devices.
High Power SFB Design Challenges
Often, the SFB is considered a less critical component
compared with high power amplier system. However,
an assembly that wasn’t designed upfront with the
considerations of the SFB may lead to an underutilization
of the amplier module’s capability or signicant
amounts of costly redesign.
The physical demands by the latest applications also
encourage much lower size, weight, and reduced cost
structure, without sacricing functionality. This is hard
to achieve without optimizing the physical and electrical
The Electromagnetic Spectrum Innovator
WHITE PAPER
www. apitech.com | 855.294.3800 | info@apitech.com |
2
High Power Switched Filter Banks Raise the Temperature on
Design Challenges
By: Jon Scoglio, Engineering Manager, APITech
www.linkedin.com/company/api-technologies-corp-/
design of the SFB section in an integrated assembly. The
power and thermal factors also form a trade-o with
frequency and bandwidth, as high frequency RF signals
tend to generate greater thermal dissipation in signal
chain components in much smaller dimensions.
Additionally, high power, voltage, current, and thermal
stresses can exceed the maximum operating specications
for many components not designed specically for high
power operation. A complete understanding of the signal
characteristics presented to the assembly establishes
design requirements so that each component can be
optimized to withstand the various stresses associated
with the applied power. For example, various continuous
and pulsed power conditions can dramatically
inuence the thermal consideration and transient
voltage/current handling parameters of many signal
chain components.
Hot switching in high power SFBs poses many challenges, as the
impedance shift from on-state to o-state in PIN diode switches
could induce undesired reections and loading of the switch
circuitry.
Another electrical consideration is the increased
harmonics from nonlinear components whose harmonic
products scale with input power. The active switching
elements, such as PIN diode and FET switches, fall under
this category, as well as any nonlinear driver and bias
circuitry. Higher RF power also leads to increased reverse
bias voltages that can aect diodes, drivers, resistors,
and interconnect components. This in turn, increases the
thermal stresses experienced by those components. The
switching speed is also limited by the power and thermal
stresses experienced during switching.
Component & Device Limitations and
Considerations
Every component and device in the signal chain of a high
power SFB also brings limiting factors, parasitics, and
design challenges. For example, switches and inductors
are critical to design performance. For inductors, the
ability to carry high RF power requires an increase in
wire thickness to minimize thermal concerns from
resistive losses. An increase in wire diameter also
increases the parasitic capacitances—interwinding
capacitance and shunt capacitance to nearby
grounds—and overall inductor size and inductance,
which ultimately limits the diversity of lter
topologies and quality factor of the lter stage.
The latest 3D CAD tools enable more optimal component
placement and more compact designs in a rapid design cycle.
The switches in a high power SFB are burdened with the
task of both allowing and blocking hundreds of watts
of RF power without exceeding power, voltage, current,
and thermal operating parameters. For these reasons, it
is generally infeasible to achieve the necessary device