The Ultimate Audio Filter

Posted on Feb 7, 2014

This project is an extension of two projects - first is the work on adding an APF-type high-Q bandpass feature to the 2nd RX of the FT-2000. More on that project may be found by CLICKING HERE. And the second driver is the memory of the fun I had as a kid with active filters. An active audio filter is in many ways comparable to the digital variety

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without the complexity. It has the performance and flexibility advantages without the need for specialized DSP hardware or coding implementations. There are two areas that the DSP is superior - in current rig implementations, the DSP filter is typically part of the AGC loop where a back-end AF only filter is not. In addition, the ringing performance of the DSP can be better than the analog variety at the expense of higher demands on DSP hardware. If any active filter project can be called a Beast, it`s possibly this one. Based on the Maxim MAX262 universal 4-pole switched capacitor filter (SCF) to provide the BPF function. That is followed by a pair of MAX7400 8-pole elliptical response LPF modules. These filters are adjustable for performance like cutoff frequency and (in the BPF filter case) Q. Total pole count here is 24. In addition, fixed LPFs on the input, output and amplification modules add an additional 6 poles of noise rejection roll off. 15 IC total. The design objectives were to allow for maximum flexibility of settings. The disadvantage of traditional fixed active filters are that their performance is not adjustable. Yet often times, band conditions would allow for a different Q or center frequency setting and with a fixed filter, that is not possible. The low end filtering was not considered because the 1K-mod combined with the IF shift on the 2nd RX allows for the similar steep roll-off on an adjustable...

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