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Tunable Filter for Frequency Analyzer

IP.com Disclosure Number: IPCOM000046527D
Original Publication Date: 1983-Aug-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 39K

Publishing Venue

IBM

Related People

James, DT: AUTHOR

Abstract

The time constant of an integrating filter is varied by chopping its input in response to a variable mark-space ratio switching signal. This switching signal is produced by applying a high frequency clock signal to a fixed ratio divider and to a presettable counter. The outputs of the divider and counter define the rise and fall of the switching signal, the mark-space ratio being determined by the presetting of the counter. As shown in the drawing, a 2-pole active filter 10 comprises two tunable integrators 11 and 12. The RC time constant of each integrator is controlled by an analog switch 13, 14 in series with an input resistor RIN . The switches are driven by the constant frequency variable mark-space waveform shown.

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Tunable Filter for Frequency Analyzer

The time constant of an integrating filter is varied by chopping its input in response to a variable mark-space ratio switching signal. This switching signal is produced by applying a high frequency clock signal to a fixed ratio divider and to a presettable counter. The outputs of the divider and counter define the rise and fall of the switching signal, the mark-space ratio being determined by the presetting of the counter. As shown in the drawing, a 2-pole active filter 10 comprises two tunable integrators 11 and
12. The RC time constant of each integrator is controlled by an analog switch 13, 14 in series with an input resistor RIN . The switches are driven by the constant frequency variable mark-space waveform shown. The effective input resistance then becomes (t1 + t2)/t1 RIN, where t1 and t2 are mark and space times, (t1 + t2) being fixed. By altering the mark-space ratio of the switching waveform, the center frequency of the filter can be altered. The switching waveform is derived from a high frequency crystal-controlled clock 15, of period tc . The clock frequency is at least twenty times higher than the filter frequency. The clock output drives a fixed ratio asynchronous counter 16, which divides the input by a factor N.

Output pulses QN from the counter clock are latched by a 'D' type latch 17 which has been preset by the negative phase of the clock waveform. The falling output of latch 17 sets a further R-S latch 18...