Performance Limitations with Typical RF Filters
Tunable, lumped element band-reject filter require multiple poles in order to achieve
steep frequency roll-os. Additional components increase parasitic losses, which in turn degrade
the overall filter quality factor and reduce tunable range. Fixed-frequency, commercial
filter designs in 1-3GHz RF bands can satisfy stopband rejections in the range of 20-40dB.
The challenges of a notch filter design for a chosen 0.2% proximity (!S
!J
) are highlighted in
Fig. 3.5, where !0 = !J (absolute accuracy). In order to achieve 40dB SIR improvement
better than 57dB rejection is warranted when Q=30; while fixed-frequency microwave filter
have trouble meeting such specification, the task becomes impractical with tunable ones..