Tunable Active Filter Research Articles

Digitally controlled, conductance tunable active filters

The use of digitally controlled variable conductances (DCVC's) to control the center frequency and selectivity of an active state variable filter is described theoretically and experimentally. Advantages of this approach are: zero sensitivity of selectivity with respect to the control conductances, excellent tuning linearity (1 percent over one decade) can be achieved with 1 percent part accuracies, a common digital control can be used to drive simple three-element variable conductances, and the input signal can be very low (about 0.5 mV) without any adverse effect on the tuning accuracy or linearity.

Low-sensitivity high-frequency tunable active filter without external capacitors

An active filter is discussed which uses only resistors and two operational amplifiers represented by their single-pole model, but no external capacitors. The filter has very low sensitivities to all circuit parameters, is tunable over exceptionally wide frequency ranges, and is suitable for high-frequency and medium- Q applications. The usable frequency range is limited only by the validity of the single-pole model of the amplifiers used. The circuit may be used both as a lowpass and as a bandpass. Extensive room-temperature experimental results are presented.

A radio-frequency tunable active filter

A design approach for a tunable active filter of constant bandwidth and center-frequency gain is described. The approach is based on a feedback configuration with a complex zero pair in the feedback transfer function. Center frequency tuning is achieved with the variation of only one element, a potentiometer, which controls the feedback loop gain. A circuit realization using two commercial video amplifiers with RC embedding is presented. A tuning range of 25 percent at 5 MHz, with a nominal bandwidth of 90 kHz or 180 kHz, and a variation of center-frequency gain of less than /spl plusmn/1 dB over the tuning range, is obtained.

Active bandpass filtering with bucket-brigade delay lines

Bucket-brigade delay lines have been used to build a tunable active bandpass filter. Experimental results showing the dependence of center frequency and bandwidth on the delay-line clock frequency, and the dependence of the center frequency and the Q of the filter on internal gain parameters are presented.

Electronically tunable time-multiplexed active filters

A simple modification of the time-multiplexed active filter allows the transfer function to be electronically tuned. The method is to allow the switched memoryless active subnetwork to be multiplexed in such a way that a variable time slot is allocated to each stage of the filter. Electronic variation of this time slot achieves a corresponding tuning of the transfer function. In terms of circuitry, a simple modification to the logic drive circuitry is required to make tuning possible.

On a completely tunable active filter using fixed RC elements

Linear analog computing elements may be employed to realize a voltage transfer function whose poles and zeros are to be independently controlled. The method is based on the dummy variable technique widely used in analog computer simulation. It requires at most (n + 4) operational amplifiers, n of which are connected as integrators, where n is the degree of the denominator polynomial.

A voltage-controlled tunable active null network

A realization of Reshef's voltage-controlled tunable active filter is presented. By placing constraints on the input voltages, independent control of the null frequency is achieved through variation of a single parameter.

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A completely tunable active filter using fixed RC elements

An arbitrary open-circuit voltage transfer function whose poles and zeros may be independently controlled is realized using RC elements and at most (2n + 2) operational amplifiers, with adjustable gains, where n is the higher of the degrees of the numerator and denominator polynomials of the transfer function.

A voltage-controlled tunable active filter

An extension to Yanagisawa's active synthesis is given which stresses the importance of tunability. The approach is to use multiple inputs, each one differing from the other by a constant. Using this approach, controllability of the zeros of transmission is achieved without changing the values of the elements in the synthesized network, and precorrection for the internal resistance of the voltage source is possible in most cases.