Demodulator circuit

Discussion in 'The Projects Forum' started by Daniel2004, Jun 5, 2015.

  1. Daniel2004

    Thread Starter New Member

    Jun 5, 2015

    I try simulate demodulator based on full wave. The carrier frequency is 8MHz, Amplitude 1V. The modulated signal frequency is 100Khz, level is 100mV.

    I attach my simulation created by LTSPICE, schematics, and waveform at de-modulator output.(Zoom out and zoom in)

    The input signal is differential and U4 modify it to single ended, this is vout1.

    Vout2 is the output of the full wave demodulator comprised of U1,U3, and you can see the signal is distorted.

    I use LTC6247, gbw=180MHz, slew rate is 90V/us.

    So I think the decvice fit.

    The diodes are BAT54.

    At lower frequency VOUT2 look O.K.

    Please advise if this is circuit problem or devices selection.

  2. DickCappels


    Aug 21, 2008
    It works. Just put a low pass filter in series with the output. Fc = 200 kHz; 10k from the output then 82pf to ground. The signal across capacitor C3 looks pretty good (see images at the bottom).

    The ratio of baseband signal to residual carrier can be improved by using more poles in the low pass filter, using a resonant filter or increasing the ratio of carrier frequency to baseband frequency.


  3. Daniel2004

    Thread Starter New Member

    Jun 5, 2015
    Hi DickCappels, thanks for your answer!!!

    I know I can filter the 100KHz with RC and it looks O.K.

    My concern is the waveform at U3 output (Vout2).

    Is it O.K it is little distorted ?(Maybe due to high frequency)