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


1 Answer 1


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
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.