Radio Frequency Integrated Circuit Design - Webs

9
High-Frequency Filter Circuits
9.1 Introduction
The need for filters in a radio has already been discussed extensively in Chapter
2. They must be included in a radio to prevent blockers from overloading the
receiver and to provide image rejection. Traditionally, these filters have used
off-chip passive filters, for example, LC circuits or surface acoustic wave devices;
however, these off-chip filters can add to the overall cost of a radio substantially,
so recently there has been much interest in trying to move these filters on chip.
On-chip filters do come with many challenges, however. Often, to make them
practical, they must have active circuitry to enhance the Q. Thus, on-chip
filters require power and have potential problems with linearity. Due to process
variations, they will also require some form of feedback in many cases to set
their frequency of operation and to set the Q precisely. In addition, if care is
not taken, filters with active Q enhancement can become unstable due to their
high Q.
At the time of this writing, on-chip filters are still not very common, but
more and more researchers are starting to investigate all aspects of these circuits,
and it will only be a matter of time before these filters start to appear in products,
thus removing one more barrier to higher levels of integration. We will focus
on image reject filters (to replace or complement the image reject mixers discussed
in Chapter 7); however, filters can be used to solve other problems as well,
such as harmonic filtering or blocker rejection. While the filters presented here
are only second order, it is understood that more complex filters may be needed
for a specific application.
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