Electronic – LTSpice: why does the .MEAS 3dB calculation disagree with the cursors on the plot

ltspicesimulation

I'm exploring the .MEAS command with the following simple BJT Class-AB amplifier:

straightforward BJT Class-AB amplifier

And am getting the following results from my .MEAS commands in the error log:

vout_max: MAX(mag(v(vout)))=(27.4778dB,0°) FROM 10 TO 1e+010
vout_3db: vout_max/sqrt(2)=(24.4675dB,0°)
f_3db: freq=(142.323dB,0°) at 1.30662e+007

The results for vout_max and vout_3db seem correct, they agree with the plot to at least 2 decimal places.

I'm very puzzled by the results for f_3db however. It appears to call V(vout) crossing the -3dB point at 13.06MHz, where the plot (screenshot below) has it at 19.41MHz.

screenshot of the plot window showing cursors measuring the -3dB point at 19.405 megahertz

  • Why are these numbers so different?
  • Have I written these .MEAS commands wrong?

I expected these to agree to at least several decimal places given they're both calculated from the same simulation run.

Best Answer

AC analysis produces complex numbers with 'real' and 'imaginary' parts that carry the magnitude and phase information in a vector. The function MAG() is used to extract the magnitude.

You have done this for vout_max, but not for V(vout), so the comparison will give unexpected results. Change V(vout) to MAG(V(vout)) in the last .MEAS command and it should find the correct frequency.

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