Understanding filters.

Discussion in 'General Electronics Chat' started by nepdeep, Dec 17, 2012.

  1. nepdeep

    Thread Starter Active Member

    Sep 14, 2011
    I have been learning filters from my first year in electronics. Now I hear a lot of people talking about them. In my simplest understandings, when low pass filter is employed in the circuit, it filters out the high frequency component.
    Now, suppose I have sine wave of 2V peak to peak, 100KHz and a dc offset of 1V. and yes, suppose I have lowpass filter with bandwidth of 50KHz.

    Now, my filter would totally block high frequency component(100KHz). Does that mean it would totally block the whole sine wave?? and pass only DC.??
    Not sure if I am clear but, does blocking the high frequency component, also means blocking its amplitued too??
    :confused:Its hard question even to ask..Hope you can understand my question..:confused:
  2. MrChips


    Oct 2, 2009
    But not such a difficult question to answer.

    A sine wave is characterized by three things, its amplitude, frequency and phase.

    A filter does not block signals. It attenuates the signal. That is, it reduces the amplitude.

    Hence your low pass filter, regardless of its bandwidth, will reduce the amplitude of the sine wave.

    How much the amplitude is reduced depends on where the frequency of the sine wave falls on the frequency spectrum with respect to the frequency response of the filter.
  3. nepdeep

    Thread Starter Active Member

    Sep 14, 2011
    Mr Chips,
    Still in a bit of confusion. Help please :)
    IMG_0210 (Copy).JPG
  4. MrChips


    Oct 2, 2009
    The gain factor is never 1 or 0.

    A simple RC low pass filter is a voltage divider. (An RC filter is also considered a first-order filter).

    In a simple resistor voltage divider, the gain factor is

    R2/(R1 + R2)

    For finite values of R1 and R2, the gain is never 0 or 1.

    For an RC filter, the gain is

    Zc/(R + Zc)

    where R and Zc are the impedances of R and C respectively,

    Zc = j/(ωC)

    where ω = 2πf

    (If you need to see circuit diagrams I can add these later.)
    Last edited: Dec 17, 2012
    nepdeep likes this.
  5. JMac3108

    Active Member

    Aug 16, 2010
    As Mr Chips said, your filter will REDUCE the amplitude of your signal, not make it go away completely.

    The amount that the amplitude is reduced depends on the filters transfer function. For example, a single pole low pass filter rolls off at 20dB per decade. This means that the amplitude of the signal is reduced by 20dB for every decade change in frequency.
    nepdeep likes this.
  6. Veracohr

    Distinguished Member

    Jan 3, 2011
    Also, the roll-off of a single pole filter in octaves is 6dB per octave. Since 100kHz is one octave above 50kHz, you should expect a 100kHz signal coming out of a 50kHz single pole lowpass filter to be 6dB below the input.
  7. crutschow


    Mar 14, 2008
    In case you don't know, a 6dB rolloff is 0.5, so the output is reduced to 1/2 of the input.