how to design low pass filter 0.15Hz

Discussion in 'The Projects Forum' started by fird, Apr 8, 2009.

  1. fird

    Thread Starter New Member

    Feb 12, 2009
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    Hai guys,
    I'm Fird,
    I would like to ask you how to design low pass filter for 0.15Hz.
    Currently I already design my circuit but I still can't get that range of frequency.

    [​IMG]

    this is my circuit.thanks
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Your image post didn't work
     
  3. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Hi Fird. I designed a bunch of 0.2 Hz highpass filters just recently, and it's kind of fun working so close to DC. It's just a matter of scaling, whatever circuit you use. The trick is to choose sensible values of Rs and Cs - keep the resistors between (ideally) 1k and 100k, and the caps above 100p. You won't have any choice with the latter here.

    A good starting point for your circuit would be to make the caps about 10 μF, then the resistors should work out reasonably. There isn't much choice for decent caps in the 10 μF range (don't use a tantalum or an electrolytic), but fortunately ceramic caps are available in larger sizes these days. They compromise a bit on non-linearity and microphonic pickup, but they're cheap. Use an X7R dielectric, not a nasty Y5V or Z5U, and make the cap working voltage much higher than the maximum signal voltage to minimise non-linearities/harmonic distortion.

    If you want a very low noise circuit, use polypropylene caps. The downside is that they're expensive and physically huge.

    There are a number of VLF filters on the internet, and most of them are deeply flawed, so watch out there.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    A note on posting images, you can upload images using the advanced option. From there you can post it on your post full size, but the first step is to upload the attachment.

    If you're interested in the gory details they can be found here.
     
  5. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    There's nothing fundamentally different about designing a ULF filter than there is for any other type. How sharp a cutoff do you need? If it's no more than 3 dB/octave, you can probably just use a 1uf capacitor and a 10 ohm resistor. If there's a specific type you need, such as Butterworth, Chebychev, or whatever, you can scale the circuit from known prototypes.

    There is a program called Elsie, available from ARRL, that can design just about any filter shape you want.

    Eric
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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