Active Filters LP. 3º, and 4º order.

Discussion in 'General Electronics Chat' started by Audiofilo, Sep 12, 2008.

  1. Audiofilo

    Thread Starter Member

    Jul 15, 2008
    Hi friends

    I need modify (and verify) some low pass active filters, It use one Op.Ampl, but are 4º order, I know they are named Elipthical, or Cauer.
    I have diagram (from a magazine) it have 4 capacitors and one Op Amp, but I want modify frequency.

    I could not find information about that, all web site only speak about 2º order, or connect two Op Ampl in cascade for get 3º, or 4º order (I do not want adds more IC).

    Please if someone can help me. I need formulas, and design criteria.

  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Can you post a schematic or a link to a schematic of the active filter that you are interested in analyzing?

  3. bertus


    Apr 5, 2008
  4. Audiofilo

    Thread Starter Member

    Jul 15, 2008
    Thank you my friends, I will take care about that webpage Bertus, anyway I attached the Filters I have. I get it from a electronic magazine, is a Low pass for 5KHz, but as you know, generally in magazines the values and connections sometimes are wrong. That filter must filters all frequency above 5KHz (it goes after a balanced modulator), I have a little noise at output so I think the filter maybe is not fine.
  5. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Wow, that's quite the circuit!

    My approach would be to search desperately for a similar filter topology under the title "elliptical 4th order filter", etc. If you cannot find a similar circuit, you will need to analyse the circuit.

    I recommend using Laplace Circuit analysis and to assume an ideal opamp. You should be able to extract the filter poles from the transfer function. This would be a fairly complex analysis, hopefully this is review ! (well, I never have done a 4th order system, I should mention...)

    One other thing I will mention. You mentioned you were getting noise from this circuit. Obvious sources of noise are the opamp input voltage/current noise, then the values of the resistors used in the filter. Your V/2 should be filtered before application and should be derived from a temperature stable and clean reference. There are other sources of noise, but these are the main ones to watch out for assuming good design practices.

    Last edited: Sep 16, 2008
  6. Audioguru


    Dec 20, 2007
    I think the circuit is a Twin-t notch filter.
    Maybe the modulator is a suppressed carrier type and the filter notches out the carrier frequency.
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You can scale the frequency response by scaling either the resistors or the capacitors. Below is a Bode plot with the resistors scaled from 0.1 (10 times original cutoff frequency) to 1 (original cutoff frequency).

    Elliptic filter.asc will run on LTspice.
  8. Audiofilo

    Thread Starter Member

    Jul 15, 2008
    RonH, very good your help. In bode diagrams I saw the first plot is cutoff exactly at 5Khz, but, Is it for resistor at original value ? I do not understnad. , and, How did you get that plots ? (LT Spice ?)Audioguru, you are right it is a carrier supresed modulator, so the noise present is because filter do not filter it good.Thanks
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Yes. I explained that the lowest frequency cutoff was with k=1. All resistor values are multiplied by k (except the feedback resistors, which can remain constant), as you can see in the schematic. Note that the notch frequency is 5kHz, not the cutoff frequency.
    Yes. I said that elliptic filter.asc would run in LTspice.