Abstract:
The trend in electronics has been toward reducing the size of circuits. This trend has led to the development of integrated circuits. Since the ferromagnetic effect cannot be obtained in semi-conductors, the size of inductors cannot be reduced as can the size of capacitors and resistors, for integrating circuits, therefore, active RC filters were developed. The active element in the RC filter has taken the place of the inductor. Today, complete systems are being integrated by using large scale integration techniques, (LSI). Since filters may be required on a single chip, in addition to other circuitry, it is desirable that the filter be fully integrated, require no trimming and use as little silicon area as possible. Active RC filters require too much silicon area and the resistors must be trimmed to obtain precision RC time constants. (3). Using metal oxide semiconductors (MOS) integrated circuit technology, voice band filter realizations can be completely integrated on a silicon chip, The switched-capacitor approach makes use of MOS integrated circuit technology to circumvent the active RC problem. MOS technology has several advantages over bipolar technology. One of the most important is that it offers the ability to store charge on a node over a period of many milliseconds and the value of the charge can be sensed continuously and non-destructively. (1) Another advantage is the reduced number of processing steps leading to higher density and lower cost. Switched-capacitor filters are analog sampled-data filters and can be realized by recursive filter structures or infinite impulse response (HR) filters. Specifically, discrete-time state-variable structures are used almost exclusively; therefore, highly selective filters or high Q filters can be realized. (5) It is shown in Chapter 2 that the switched-capacitor resistor can take the place of a resistor with the equivalent resistance being equal to T/C, where T is the sampling period. If the equivalent resistor is used in an active RC integrator, a building block component is achieved for the design of any order filter. Since R = T/C for the resistor and the transfer function of the integrator is -(s RC)^-1, its equivalent transfer function is -(s T C/C1 )^-1. Therefore, the transfer function coefficients are determined by the clock frequency as well as the ratio of the capacitors in the integrator. In Chapter 3 we use sampled-data techniques to find the exact transfer function of switched-capacitor circuits. The switched-capacitor integrator is found to have the same phase shift as the analog integrator when it has a one half clock cycle of delay in its forward path, This delay is obtained by sensing the output of the integrator as soon as it changes value. If the output is sensed one half cycle later, there will be a full clock cycle delay in the forward path of the integrator and the phase shift differs from that of the analog integrator, Therefore, as will be shown, proper phasing of the switches must be observed to avoid these phase shift errors. Three z-domain integrators will he studied in Chapter 4: the Forward Difference, the Backward Difference and the Bilinear integrator. It is shown that the s-domain integrator can be replaced by any one of these z-domain integrators; therefore, discrete-time filters can be obtained from existing continuous-time filters. In Chapters 5A and 5B the Reticon R5610 programmable switched-capacitor filter is studied. The Reticon R5610 contains on one chip four second order filter sections and the required digital logic circuitry needed to program the filter sections. The center frequency and quality factor of each filter section may be independently programmed. Thirty two values of Q (Quality factor) are made possible by selecting various amounts of feedback capacitance from a thirty-two capacitor array. Similarly, thirty two values of center frequency per octave of frequency is obtained. Seven octaves can be covered resulting in 7 x 32 = 224 different center frequencies possible. The same thirty two values of Q are possible in each octave. The clock frequency is common to all filter sections. Higher order filters and biquadratic transfer functions are obtained with the use of the Reticon R5610. In Chapter 6, the design of a low pass, a band pass and an elliptic filter using the Reticon R5610 is considered.
Description:
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