filters and cut off frequencies

Discussion in 'General Electronics Chat' started by slimnick, Sep 4, 2010.

  1. slimnick

    Thread Starter Member

    Nov 25, 2007
    im designing a circuit using op amps. this stages in this design is a second order low pass butterworth filter, that it then cascaded with 2 single RC low pass filters, and the final stage is a comparator(op amp) to change the signal into a square wave.
    my question is, i want the whole system to have a cut off frequency of 1kHz, would this by simply setting each stage to a cut off frequency of 1kHz(ie, butterworth to 1Khz, and each single order RC low passes to 1kHz)? or is there i resultant cutoff frequency of these? ( becuase i really need cut off frequency of 1kHz).
  2. Ghar

    Active Member

    Mar 8, 2010
    First off how are you defining cutoff frequency?
    Generally the cutoff frequency is defined as -3dB (70%) of the input.
    Filters are never perfect, they do not cut off exactly at 1 kHz, this is why you go with higher order filters.

    So, what's your definition? How much attenuation do you need at what frequency?

    If you set every filter's -3dB frequency to 1kHz the overall -3dB frequency will be lower than 1 kHz since each filter will be attenuating 1 kHz by some factor and it adds up.
  3. slimnick

    Thread Starter Member

    Nov 25, 2007
    there is only one butterworth second order low pass filter(op amp) the other 2 are just a resistor and capacitor arranged as a low pass filter. basacially when im finish this circuit will be a forth order low pass.

    ai am using these as a signal generator, feeding the output back to the input, but i want to produce a frequency of 1kHz, and all other harmonics are attenuated. my problem right now is not the attenuation , my problem is i dont kno whtehr if i put each stage to cut off frequency of 1kHz, will the overall circuit be also cut off of 1Khz, and if not any ideas on how can i make it 1kHz.
    so basically i want the highest frequency to be fully passed to be 1kHz :p
  4. Ghar

    Active Member

    Mar 8, 2010
    So basically you're saying the problem is the attenuation but you have no quantified requirements for it whatsoever.
    Also, by saying you want 1kHz "fully passed" that means the cut off frequency should be above 1kHz.

    I already said that cascading multiple 1kHz filters does not give an overall 1kHz cutoff.
    The only point of a high order filter is to control the rate of attenuation, if you don't know any requirements for that it's pretty pointless to discuss where the cut off is exactly.
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
    We just call it the cutoff frequency, nothing is ever "cut off" so to speak. It's when the dB level is "cut" by 6 dB (per filter pole)
  6. Papabravo


    Feb 24, 2006
    It is also the case that filter sections load each other because the output impedance of one does not match the input impedance of the next. To see how this works you might want to consider running a simulation with four cascaded RC filters each having a 1 kHz cutoff to see what you get.
  7. gootee

    Senior Member

    Apr 24, 2007
    Is this a phase-shift oscillator or something like that? OR, are you only needing to eliminate the harmonics of 1 kHz?

    Do you just want it to work or do you want to be able to calculate everything?

    If you can't simulate it, you could try setting each filter stage's cutoff frequency to 1.5 kHz, for example, and see what happens.

    You might need a unity-gain opamp buffer amplifier before and after each simple RC stage.