square wave -->sine using passive bandpass filter

Replies

• 

  reachrkata

  Hi Windell,
  
  To answer your questions -
  
  1) IMO there should be no influence of impedance of the filter on the microprocessor. Just make sure that the impedance is not too low to exceed the Max port source current of the processor
  
  2) Series LC normally is used in band pass filtering, while parallel LC is band reject.
  
  3) Yes of course, the more you filter the more closer the output is to a true sine. But from theory its never possible to get a true sine just by filtering a square wave.
  
  4) No particular advantage, except that you can add in a gain component to the filter to get a higher output. An active filter can also support higher loads.
  
  My suggestion - If your need is to get a proper sine output, I would suggest to use an oscillator triggered with the square wave. That way you have a pure sine at Level HIGH and no output at Level LOW of the square wave.
  Another idea is to use a VCO, by which you have say sine output of frequency f1 for Level HIGH and f2 for Level LOW.
  
  - Karthik
  😁
• 

  windell747

  How do I calculate the gain of a passive series LC filter? Is it just the coefficient, 4/pi * square wave peak amplitude, of the forier transform of the square wave?
• 

  reachrkata

  I don't think its that simple.
  The gain of the circuit is 1/(impedance of LC circuit)
  So depending in the LC, the gain would be the highest at the center frequency of the filter (assuming narrow band filter design).
  Of course the gain is also different for different frequencies.
  
  So depending on which sine wave frequency you need at the output the gain is to be calculated.