Cutoff frequency of low pass filter

Discussion in 'General Electronics Chat' started by Janani_Rajkiran, Dec 9, 2016.

  1. Janani_Rajkiran

    Thread Starter Member

    Nov 1, 2016
    46
    0
    Hi!

    I need to calculate the cut-off frequency of a low pass filter.

    there is a resistor divider network from the input after which the low pass filter is present

    When i simulate it using LTspice the cut-off frequency is much lower than the calculated value

    when I remove the Resistor divider network and supply the RC Circuit directly then the cut-off frequency is the same as the calculated value.

    Which value is correct the calculated value or the simulated value with the resistor divider?
     
  2. #12

    Expert

    Nov 30, 2010
    18,076
    9,690
    With no circuit diagram, no component values, and no frequency values for the result, how are we supposed to know?
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Simulated is correct.
     
  4. EM Fields

    Active Member

    Jun 8, 2016
    583
    154
    They're both correct; the problem is that you forgot to add the voltage divider's resistance contribution to the filter's RC.

    On the attached graphic, note that the cutoff frequency of the filter with a high-resistance divider is lower than the cutoff frequency of the same filter with a lower resistance divider.
    Lowpass.png
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
    4,667
    745
    Assuming all resistors are identical.
    V_{out}=\frac{0.5}{sCR+1}V_{in}
     
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