Bandpass Filter for Sawtooth wave

Discussion in 'The Projects Forum' started by simply_me, Jun 8, 2010.

  1. simply_me

    Thread Starter Member

    May 6, 2010
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    Hello,

    I've a bandpass filter centered at 40kHz. The filter is working nice when I send it a sinusoid/square wave/triangle wave, but when it receives the actual signal that I need it to filter which is a sawtooth wave the filter converts it to a triangular wave (which is part of the reason I'm using a filter) but it cuts the frequency by half. In turn, my transmitter filters the 20kHz signal. Does someone know how to modify the bandpass filter to receive a 40kHz output?

    Thank you.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Without a schematic, we don't have a clue how to help you.
     
  3. simply_me

    Thread Starter Member

    May 6, 2010
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    Do you prefer a .pdf or a Multisim file?
     
  4. simply_me

    Thread Starter Member

    May 6, 2010
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    Here are the schematics and the incoming wave.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    .png file formats are preferred; they are compact and require no extra software. Screen capture -> MSPaint -> Save as .png
    .pdf is OK, but is slower to load.

    Many people don't have Multisim.
     
  6. simply_me

    Thread Starter Member

    May 6, 2010
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    Here is the .png, sorry I was attaching the previous files when you posted the .png comment.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Have a look at this Wiki entry: http://en.wikipedia.org/wiki/Sawtooth_wave
    You'll note that sawtooth waves are comprised of both the fundamental frequency, plus all of the even and odd harmonics of the fundamental frequency.

    A pure sine wave consists of only the fundamental frequency.

    You will need to convert your sawtooth wave to a sine wave prior to it entering the bandpass filter.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Keep in mind that the LM358 is a very old and very slow opamp. It might work OK for audio frequency sine waves, but not for anything broadband.
     
  9. simply_me

    Thread Starter Member

    May 6, 2010
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    Thank you. It's for an ultrasonic range finder, so I think the LM358 will be sufficient.
     
  10. simply_me

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    May 6, 2010
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  11. SgtWookie

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    You'll eliminate most of the information in the sawtooth wave.

    Also, your sawtooth wave is not symmetrical; there's a period of time where it holds at a steady level before it starts ramping up. That changes the fundamental frequency and the harmonics included.
     
  12. simply_me

    Thread Starter Member

    May 6, 2010
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    Well, if I convert it before I'm sending it out it should be fine, I think. Maybe I'll skip the conversion and just filter the received signal.
     
  13. simply_me

    Thread Starter Member

    May 6, 2010
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    I've added an LC before the filter. Does it look okay to you?
    It oscillates a little too much (should be between 39-41kHz), but I think it'll do the job.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    Your output is a triangle-wave instead of a sine-wave because the old LM358 has slew-rate limiting above 5kHz.
    At your high frequency its output is ramping as fast as it can go which is too slow to produce a sine-wave at that high frequency.

    Use a TL071 opamp instead for frequencies up to 100kHz.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    The LF411 is obsolete.
    The LM358 should be obsolete, but it works OK for some low frequency stuff.

    You need to use better opamps.

    The TL071 is a much faster opamp, but it cannot "see" within 3v of the negative rail, or about 1.5v of the positive rail. Therefore, you would need to use a split supply with it.

    Since I really don't know what you are trying to do, I'll let you figure that out and then maybe you can try to explain it better.

    See the attached; it's a simulation of your signal, along with an FFT analysis of the waveform content. Notice all of the harmonics.
     
  16. simply_me

    Thread Starter Member

    May 6, 2010
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    @SgtWookie,
    Thank you for the analysis. I'm going to get a TL082 and try to design the filter. I'm building an ultrasonic rangefinder using a linear ramp (the LF opamp you've seen in the schematics). I wanted to convert the sawtooth wave to a sinusoid (ideally, if not a triangle) to avoid harmonics. Every bandpass filter that I try isn't good enough (for 1. filter & 2. convert the sawtooth to a sinusoid). Idk, I might just leave it as is.
    @AudioGuru, thanks for the advice it was exactly the problem. I'm going now to get a few TL082 to replace the 358 and test it. I'm not sure I'm able to redesign the filter to do what I need it to do.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    I have never made a Multiple-Feedback-Bandpass Filter but the ratios for the parts determine the bandwidth. You can make it wideband with a low voltage gain or a narrow bandwidth with a high voltage gain.
     
  18. simply_me

    Thread Starter Member

    May 6, 2010
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    That's actually a great idea. On the receiving end I'll have a narrow bandpass and on the transmitting end I'll have a wide bandpass mainly to convert the waveform.This is my modified "wide band" bandpass, I've cleaned most of the clipping. Do you think it's sufficiently good to transmit w/o picking up too much noise?
     
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  19. Audioguru

    New Member

    Dec 20, 2007
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    Much of the bottom of your "new" transmitted waveform is clipped away. Why?
     
  20. simply_me

    Thread Starter Member

    May 6, 2010
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    I know, I can't get rid of it, but I'm still trying.
    I really appreciate your help throughout this thread by the way.
     
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