square wave -->sine using passive bandpass filter


I'm designing a circuit that will take a square wave generated by a microprocessor and feeds it through a bandpass filter to get a sine wave. The application will be to provide a tone for morse code to a handheld ham radio on board a small satellite. Right now im considering remaining simple by using a 1st order passive bandpass filter that uses a capacitor and inductor in series. Assuming this I have a few questions.

1) How should I choose the impedance of the bandpass filter? I do not know the impedance of the GPIO of the microprocessor. Does this impedance matter much?

2) What is the difference between the parallel and series configuration for the passive 1st order bandpass filter?

3) Should I filter more to get a truer sine wave? Any experienced hammers out there? I will be transmitting the morse code tone from a ham radio from a 400km orbit (LEO). I will then feed the tone through a morse decoder to convert the morse to text using a freeware program.

4) Finally, I wanted to use a passive filter to save power, however, I'm open to an active filter that uses an opamp. Are there any advantages to this?

Any guidance would be greatly appreciated!



  • 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.

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