Page 23 - EngineerIT July 2022

ADVANCED FILTER TECHNOLOGY

Figure 9: The group delay flatness affects the deviation from the linear phase: (a) showing 2.24 ns group delay flatness vs. (b) showing 0.8 ns flatness
resulting in more consistent phase change vs. frequency.

Traditional filter technologies for RF communications periodic characteristics. In some cases, lumped components
When designing a filter for RF communications systems, there are are added to improve/miniaturise the distributed filter.
a variety of technologies available to implement the classical filter • Ceramic resonator filters use multiple ceramic resonators
types. Traditionally, RF engineers relied upon discrete lumped (which are a distributed element) that are coupled via
element implementations with surface-mount components or lumped elements. The coupling element is typically a
distributed element filters containing transmission lines printed on capacitor but sometimes inductors are also used. This type of
PCB materials. However, in recent years, filters have been designed filter is a hybrid of distributed and lumped elements.
on semi-conductor processes that allow for precise temperature • Cavity filters are implemented with distributed elements
stable reactive components with improved quality factors. (rods) enclosed within a conducting box. They are known for
Additionally, the semi-conductor processes allow for switched being able to handle high amounts of power with little loss
and tunable reactive elements that can be more challenging to but at the expense of size and cost.
implement in the discrete lumped element implementations. • BAW and SAW technologies can provide excellent
There are other technologies as well, such as the bulk acoustic performance but they tend to be frequency selective and not
wave (BAW), surface acoustic wave (SAW), low temperature co- suitable for wideband applications.
fired ceramic (LTCC), cavity filters, or ceramic resonators. • LTCC filters are implemented by combining many layers of
distributed transmission lines within a ceramic package,
Trade-offs exist with each approach and technology: which is similar to a distributed filter and can serve a number
• Lumped LC filters are implemented with surface-mount of applications but are fixed. Since they are 3D stacked, they
inductors and capacitors on a PCB. The benefit is the ease of end up taking little space on the PCB.
assembling and then changing the performance of the filter • Lastly, filters integrated into semiconductors support a wide
by swapping out values. frequency range with recent advances in semi-conductor
• Distributed filters are designed as resonant pieces of a performance. The ability to easily integrate digital control
transmission line implemented on a dielectric (either elements into these components aids in the adoption into
integrated into the PCB or standalone on a separate software-defined transceivers. In general, the trade-off
dielectric) and are oriented to behave as quasi-inductors between performance and integration provides a compelling
or quasi-capacitors in some frequency range. They exhibit value to designers of wideband systems.

Table 1: Filter type comparison
Frequency Range Tunability Size Cost Q-factor
Lumped LC <6 GHz Difficult to implement Medium $ Medium
Distributed <50 GHz Fixed Medium $$ Medium/high
Ceramic Resonator <6 GHz Fixed Large $$ High
Cavity <40 GHz Fixed Large $$$ High
SAW/BAW <6 GHz Fixed Small $ High
LTCC <40 GHz Fixed Small $ Medium
Semiconductor <50 GHz Digital tuning integrated Small $$ Medium

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