# High pass filter seems not to work

I have designed this basic RC high pass filter circuit in LtSpice IV:

It should have cutoff frequency around 10 kHz. When I perform the ac small signal analysis the output is as expected.

But if I give as input tree AC sinusoidal signals each with different frequency(400 Hz, 4 kHz and 18 kHz), and perform transient analysis, it seems that filter doesn't work.

Here's the procedure...

I made desired input using summing amplifier. Circuit is shown belove.

Then I perform transient analysis. The output and input signals are:

Output signal seems to have all tree frequency components.

Then I have done FFT on both input and output signals and it confirmed that both signals contain same frequencies, just like the high pass filter didn't worked.

By inspecting the magnitude axes (the one in dB) I noticed that the signals have been suppressed, but as it seems not significantly. If you look at output signal in time domain this becomes apparent.

What I'm doing wrong?

How could I design a filter that will keep only 18 kHz signal?

• Are you aware that you compare two C-R highpass sections which have DIFFERENT corner frequencies?
– LvW
May 10, 2014 at 15:58
• I uploaded the wrong picture of circuit. All analysis are done on circuit with correct RC values. I'm now editing question. Thank you. May 10, 2014 at 16:00
• It looks like it worked perfectly to me. The 4 kHz signal is down by about 8 dB, and the 400 Hz signal is down by about 28 dB, which is a 20 dB/decade slope, exactly what I'd expect for a single-pole filter. What did you expect? May 10, 2014 at 16:06
• If you want to select only 18kHz signals, you will need a bandpass filter. Also, getting a frequency response from time domain data is very hard, you would need to run the simulation for a very looong time to allow all the start up transients to die out. These start up transients are still easily visible in your time domain plot, so the FFT will be inaccurate. May 10, 2014 at 16:07