TABLE OF CONTENTS We have examined some of the most widely used techniques for designing fully integrated continuous-time filters, and, of equal importance, methods by which these filters can be automatically tuned by on-chip circuitry. The frequency range over which these filters may be applied in wireless transceivers is extremely wide, ranging from audio frequencies to the RF signal frequency, which is normally in the UHF range, and with the general availability of deep-submicron IC processes is being extended to several gigahertz.
Integrated filter architectures can be divided into second-order cascade and multiple loop feedback types; a subset of the MLF filters includes a range of passive LC ladder simulation techniques. The second-order cascade offers simplicity of design with a modular circuit structure; however it possesses relatively high sensitivity compared to the MLF structures. Filters based on simulations of LC ladders are probably the most popular for integrated filter design. Their low sensitivity helps to offset the difficulties caused by loose integrated component tolerances. These may utilise component substitution and circuit transformation techniques to eliminate inductors from the prototype ladder design, or be based on leap-frog structures which simulate ladder filter behaviour at a system level.
The OTA-C filter has important benefits for high-frequency operation. In this structure, all high-impedance nodes have grounded capacitors connected to them to define the filter response. This allows the inevitable stray capacitances to be absorbed into these tuning capacitors, without introducing additional unwanted parasitic poles into the response. The operational transconductance amplifiers operate in an...
