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i am working with LTSpice simulation. And i am creating a bodeplot of an special circuit with the AC Simulation. How can i use the meas. command to measure the amplitude by -135° phase? I already tried using .meas AC bw FIND v(out) AT phase(V(out)=-135° but it failed. I don't find any answers researching this problem in the internet.

Thank your for your help!

enter image description here

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  • \$\begingroup\$ I'm not familiar with phase measurements, but I notice your command is missing a ), so maybe that's why it failed. \$\endgroup\$
    – Hearth
    Commented Dec 22, 2020 at 16:26
  • \$\begingroup\$ Thx for your answer, but also with a ")" it doesnt work. :/ \$\endgroup\$
    – Pr0metheux
    Commented Dec 22, 2020 at 17:00

1 Answer 1

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There are two small problems with your simulation. First the command you are looking for is ph and not phase. Second, this approach is only going to work if there is a frequency point which produces the given phase. You can try the following command and ensure that your frequency sweep has many points:

.meas AC bw FIND V(out) WHEN ph(V(out))=-89.9

circuit_1

UPDATE #1

If you using a bandpass filter, instead of tracking the phase you can check the two points where the filter attenuates the magnitude by 3dB (or 0.707) using the following measurements:

.meas AC f1 find V(out) when mag(V(out))=0.707 cross=1
.meas AC f2 find V(out) when mag(V(out))=0.707 cross=2

In order for it to work, you have to make sure that the magnitude is normalized, meaning that the pass band should have a magnitude equal to 0dB (or 1).

Below is a simple simulation:

circuit_2

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  • \$\begingroup\$ Thank you very much vtolentino ! It works :) \$\endgroup\$
    – Pr0metheux
    Commented Dec 23, 2020 at 15:00
  • \$\begingroup\$ Now, i want to use these point .meas AC bw FIND V(out) WHEN ph(V(out))=-89.9 to get the bandwidth: .meas AC BAW TRIG V(out=bw/sqrt(2) RISE=1 TARG V(out)=bw/sqrt(2) Fall=last . But it failed :/ These methode worked when i measured the maximal magnitude. But in my special case i cant use these methode because i have resonance peaks so. Do u have a solution when i found the -135° point? \$\endgroup\$
    – Pr0metheux
    Commented Dec 23, 2020 at 15:21
  • \$\begingroup\$ If you have a close look at the measurement, it shows already the bandwidth for the given phase. In my example it is 91.189kHz. \$\endgroup\$
    – vtolentino
    Commented Dec 23, 2020 at 16:13
  • \$\begingroup\$ Yes, it does, but that only works for an low/high pass filter. But my circuit is an bandpassfilter with resonance frequence \$\endgroup\$
    – Pr0metheux
    Commented Dec 23, 2020 at 16:25
  • \$\begingroup\$ This should still work for a bandpass filter provided that you measure the phase margin twice. Could you share your spice simulation? \$\endgroup\$
    – vtolentino
    Commented Dec 23, 2020 at 18:10

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