| This page describes the final channel select filter and matching
oscillator used for the loop filter. Both the filter and oscillator
have center/resonant frequencies which are adjustable, which gives the
channel selection. The filter has a nominal center frequency of 5.5Mhz
with a Bandwidth of 1Mhz. The Oscillator has a nominal resonant frequency
of 4 times that of the filter, or 22Mhz. Both circuits operate at
a low IF frequency which requires large components. It is difficult
to build large inductors on chip and maintain a good quality, so a different
approach must be taken. The circuits described here use a Gm-C technique
to "simulate" an inductor. This approach seems to work well based
on simulation. Below are a series of links which describe our process
from design to implementation. It begins with the basic circuit topology,
moves to design issues and then to the layout and ends by recommending
improvements. |
