LOW PASS FILTER DESIGN works fine in multisim but not in the lab

Discussion in 'The Projects Forum' started by rjcast, Dec 9, 2006.

  1. rjcast

    Thread Starter New Member

    Dec 9, 2006
    2
    0
    I designed a LPF with the requirements below; everything seems to work fine using MULTISIM but when I test the circuit in the lab it gives me different results. <Please see my attachment of my design>

    Specifications:
    Output impedance; Zo = 50 ohms +-20% at 5 KHz
    Input impedance; Zi = 50 ohms +-20% at 5 KHz
    3 dB cutoff frequency = 20 Khz
    Roll of of at least -6dB

    ANY SUGGESTIONS WILL BE GREATLY APPRECIATE IT!
     
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  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    I can't comment on Multisim, as I don't have or use it. I plugged your parameters into a designer at http//www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter and got different values for components. For the caps, use .16 microfarad and the inductor value is .8 mH. I checked that the corner is at 20 Khz, but not the rolloff. With three poles, the response has to be better than 6 dB/octave.

    The Zi & Zo values are set by R1 & 2, so worrying about the impedance at 5 kHz is not too useful, unless you want to mess with wirewound - and therefore inductive - resistors.
     
  3. rjcast

    Thread Starter New Member

    Dec 9, 2006
    2
    0
    Thanks a lot; that helped a lot but am not sure why the amplitude starts at -6 db and not at 0?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Not quite right. Signal amplitude at frequencies below the cut-off may be measured and set to 0 dB as a reference level. Then, at the knee (or operating point), the signal amplitude will be down by 3 dB. That is at the 20 kHz frequency. The signal will decrease by 6 dB/octave as the frequency increases past the filter's operating point.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Consider where the output is with reference to your two resistors ...
     
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