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I have been busy with a frequency shift keying modulation circuit in LTspice. The modulator works but I have this horrible DC-offset! I have tried high-pass filters and DC-blocking capacitors but I cannot get it to work correctly. Can someone point out my mistake? I a screenshot of the circuit below. As you see the signal is not oscillating around 0V but instead on top of 0V. My filter or DC-blocking cap work fine when I use a spice signal generator with offset just not on this signal and I don't understand it. Any help would be appreciated.

LTspice circuit with high-pass filter

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The time constant of \$10k\Omega\$ and \$100\mu F\$ is 1s. Your simulation is 5ms long. The high pass filter hasn't had time to settle. Either:

  • change the filter to a shorter time constant
  • increase the length of the simulation
  • put an initial condition on the capacitor
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  • \$\begingroup\$ with LTspice, you can ask it to simulate without output for a while, to settle time constants like this, then start output later. \$\endgroup\$
    – Neil_UK
    Commented May 31, 2019 at 5:43
  • \$\begingroup\$ @Neil or set sn initial condition \$\endgroup\$
    – Matt Young
    Commented May 31, 2019 at 10:56

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