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Tuning of Switched Capacitor Filters With a Voltage-Controlled Oscillator

IP.com Disclosure Number: IPCOM000046084D
Original Publication Date: 1983-May-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 65K

Publishing Venue

IBM

Related People

Zumbado, JA: AUTHOR

Abstract

MOS switched capacitor filters have been described in prior work, such as R. W. Broderson, et al., "MOS Switched Capacitor Filters," Proceed -ings of the IEEE, January 1979, pages 61-75. Switched capacitor filters can operate as either high-pass or low-pass analog filters which are driven by a high frequency clock source. A precise clock frequency is required to obtain a precisely defined passband for the filter. A circuit is disclosed herein to provide an adjustable passband filter utilizing switched capacitor filters. The circuit employs voltagecontrolled oscillators which can be selectively set to any one of several desired frequencies to obtain the variable passband characteristics for the resultant filter.

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Tuning of Switched Capacitor Filters With a Voltage-Controlled Oscillator

MOS switched capacitor filters have been described in prior work, such as R.
W. Broderson, et al., "MOS Switched Capacitor Filters," Proceed -ings of the IEEE, January 1979, pages 61-75. Switched capacitor filters can operate as either high-pass or low-pass analog filters which are driven by a high frequency clock source. A precise clock frequency is required to obtain a precisely defined passband for the filter. A circuit is disclosed herein to provide an adjustable passband filter utilizing switched capacitor filters. The circuit employs voltagecontrolled oscillators which can be selectively set to any one of several desired frequencies to obtain the variable passband characteristics for the resultant filter.

Fig. 1 illustrates the circuit. An analog input at terminal 1 is input to the buffer amplifier 2 and passes through the summing amplifier 4 to the input of the presampling filter 5. The other input 3 to the summing amplifier 4 is a test input which can be selectively used to provide a known analog signal input to enable testing of the circuit. The presampling filter 5 is a low-pass filter for providing an anti-aliasing function, since switched capacitor filters are sampled data systems.

The output of the presampling filter 5 is input to the high-pass (HP) switched capacitor filter (SCF) 6. The switched capacitor filter 6 has its clock input connected to the output of the voltagecontrolledoscillator (VCO) 7. The voltage-controlled oscillator 7 has a precise voltage input applied to it from the reference voltage source 10 which can supply, for example, a 10,000-volt level. A voltage divider 9 is provided which will precisely divide the reference voltage output from the source 10 into a predetermined number of predefined precise voltage levels, each one of which can be uniquely switched through the multiplexer (MUX) 8 to the input of the voltage-controlled oscillator 7.

Reference to Fig. 2 will illustrate the selective properties of the high-pass switched capacitor filter 6. When the multiplexer 8 selects the voltage V/l/ from the voltage divider 9, the voltage V/1/ is applied to the input of the voltage- controlled oscillator 7, which, in turn, outputs a clock frequency FCLK/1/. This clock frequency, when applied to the switched capacitor filter 6, provides a high- pass fil...