What does the output of .meas mean?

QSPICE
1

I am using the .meas statement to find the magnitude of a signal at a specified frequency:

.meas baselv_V find V(OUT) when frequency=10e6
.meas baselv_dB find dB(V(OUT)) when frequency=10e6

I get this result:

.meas baselvl_v find v(out) when frequency=10e6:
     (  0.798363, -0.377422)
.meas baselv_db find db(v(out)) when frequency=10e6:
     (     -1.08,  -3.83575)

So I get tuples with two values back. The first value matches the value I see in the magnitude plot in the waveform viewer, but the second… I’m not sure. I can imagine it would be phase, but it doesn’t match the phase plot…
Where can I find any clarification on the .meas output?

2

First result is → (Re, Im)
Second result → (Mag, /)

.meas amp find v(out) when frequency=100k:
( 0.500487, -0.5)
.meas ampdb find db(v(out)) when frequency=100k:
( -3.00607, -6.81765)

image
image790×626 12.3 KB

3

The number in .meas is (real, imag) for .ac directive

.meas baselvl_v find v(out) when frequency=10e6:
     (  0.798363, -0.377422)
.meas baselv_db find db(v(out)) when frequency=10e6:
     (     -1.08,  -3.83575)

in this case, v(out) = 0.798363-j*0.377422
and db(v(out)) = 20*log(v(out)) = -1.0800 - 3.8357i

if you need a magnitude, you can consider abs(v(out)) or dB(abs(v(out)), and imaginary part is 0
Or you need phase, you can phase(v(out)), and real part is phase (in degree) and imaginary part is 0.

Here is matlab calculation if you calcuate dB with imaginary number.
image

4

Many thanks you both for your explanations.
How do you know that the numbers have this meaning? Based on “probably; makes sense”, or is it documented anywhere? I have seen other .MEAS results before, where I also got an additional value that initially didn’t make sense for me, and I’d like to know where look it up…

5

I cannot recall Help has this information. I believe Mike is very busy and may not possible to include everything in help as spice has so many feature and years of history. Sometime, I cross reference to LTspice, LTWiki or other spice manual… but in many cases, run some tests and can identify thing out.

We know .ac calculate phasor, and phasor is complex number, and complex number in general express in real+imag, or magnitude+phase, where phase in degree or radian. With these possiblity, by setting up a measurement, you can figure out meaning of these two number.