LTspice DC-blocking capacitor not working

Question

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.

Matt Young · Accepted Answer · 2019-05-31T10:59:16.920

5

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

edited May 31 '19 at 10:59

answered May 31 '19 at 02:16

Matt Young

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with LTspice, you can ask it to simulate without output for a while, to settle time constants like this, then start output later. – Neil_UK May 31 '19 at 05:43
@Neil or set sn initial condition – Matt Young May 31 '19 at 10:56

LTspice DC-blocking capacitor not working

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