A New Recursive Structure for Very High-Q Digital and SC-CTD Filters.

The second-order recursive fundamental section up to now, has larger coefficient sensitivities of the amplitude at the filter central frequency and the central angular frequency itself. This makes it difficult to realize high-Q bandpass characteristics. This paper proposes a new structure with a feedback loop containing sampled-data filter with medium-Q bandpass filter and a long-time delay element. When a second-order bandpass filter is used as the feedback filter, the transfer function of the new structure is a pseudo-second-order function. Then the approximate relation is derived between Q of the feedback filter and Q of the new structure filter. The validity of the expression is verified by numerical calculations. It is shown also that when realizing a bandpass filter of the same Q, to what extent the new structure is superior to the traditional second-order structure from the viewpoints of coefficient sensitivity and round-off noise. The design procedure for the new structure is shown also. In the digital filter, the long-time delay can be realized by using a register or a memory. In the analog sampled-data filter, it is realized by a charge-transfer device. Thus, the new structure can be realized by a digital filter, or by combining switched capacitor circuits and a charge-transfer device. Finally, an experimental example of a high-Q filter by the latter method is shown. [ABSTRACT FROM AUTHOR]