square wave 2 sine wave convertion

  
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I would like to convert a TTL square wave into sine wave. The frequency range of the square wave is: 4Hz-60Hz (yes its those evil low frequency experiments that I plant to take over the world with One approach is to filter the square wave with a high order filter. But if the frequency varies then you need a tracking filter whose center frequency v
square wave 2 sine wave convertion - schematic

aries with the input frequency. That`s not trivial to do. Another technique is to run the square-wave through two integrators in series. The first integrator generates a triangular wave and the second generates a sine wave. The problem is that the output amplitude is inversely proportional to the square of the frequency so you need some sort of AGC circuit to maintain a constant amplitude. For a frequency range of 4Hz-60Hz the amplitude would vary by a factor of 225 or 47dB. A switched-capacitor Butterworth lowpass filter IC uses a clock that is 100 times the cutoff frequency. The clock can be the VCO of a phase-locked-loop IC and a digital divider produces the fundamental frequencies. You need to clock them many times faster than the output frequency and you tell them the frequency digitally, so it is not a direct conversion, but that might help. A switched-capacitor Butterworth lowpass filter IC uses a clock that is 100 times the cutoff frequency. The clock can be the VCO of a phase-locked-loop IC and a digital divider produces the fundamental frequencies. I tried reading the datasheet for MAX7400-7407 but I am confused. Most of the information presented on the datasheet is new to me. From what I understood, the output frequency will = Fclk/100. So I made some adjustments to my circuit below: This would make it less work for the filter. The problem is that the attentuation is greater at higher frequencies than lower...



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