3 channel spectrum analyser questions

Discussion in 'The Projects Forum' started by Neil Groves, Dec 6, 2011.

  1. Neil Groves

    Thread Starter Member

    Sep 14, 2011
    125
    3
    Hi Guys...i am building the circuit that i have supplied a link to but can't find any info about how it works, setting up or testing, so far i have built the input stage and the top filter (u1a and u1b) respectively, on feeding the input with a gorgeous sinewave, everything looks good untill the signal comes out of pin 9 where is is ringing badly? i don't know what to expect at the output, i am assuming that as the frequency changes, the voltage at the output rises and falls respectively as it does on mine, i expected a clean rising and falling D.C voltage at though instead of a peaky waveform at the Cathode of D1 though, can anyone give any insight to this circuit please?

    I am aiming to be able to hook up my signal generator and sweep though 40Hz-16Khz and seeing the leds sweep through from bottom to top as the frequency rises.....or am i mistaken about what this circuit does?

    Neil.

    http://www.aaroncake.net/circuits/3chspec.asp

    http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=8847&

    Neil.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Hi again, Neil.
    I think that type of peaking filter has a bandpass that is too narrow to do what you want.
    You need filters that have a flat bandpass then complementary slopes like the crossover network for a speaker.
     
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  3. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Just to show you what AudioGuru is talking about, here's an AC analysis on the part of the circuit before the LED drivers.

    See the attached schematic & simulation.
     
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  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A few minutes ago, I posted this on the other website:
     
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  5. Neil Groves

    Thread Starter Member

    Sep 14, 2011
    125
    3
    These graphs show me EXACTLY what i was seeing on the scope as i varied the input frequency, a sudden spike instead of a gradual rise and fall....you guys are so bright!!!!!

    i have a couple of projects i really need to finish then i am going to sit down with a book or two and learn this stuff, is there a formulae for calculating filter frequency response?

    Neil.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    There are lots of different types of filters.

    For this type of thing, you need three different types:
    1) A low-pass filter for the low frequencies - everything below a certain frequency can go through.

    2) A bandpass filter for the midrange frequencies - usually specified by a center frequency and a width. Everything outside the band is rejected. This consists of a high pass with a low frequency roll-off, and a low pass with a high frequency roll-off. They can be combined into one stage.

    3) A high-pass filter for the high frequencies - everything below a certain frequency is attenuated.

    These ranges don't have to be absolute. A Butterworth type filter will provide smooth transitions between the various frequency ranges.
     
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