Question about ltspice .measure

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Svenn Are Bjerkem - 29 Jul 2007 20:31 GMT

Hi,

I simulate a passive high-pass filter with a 1 pF capacitor and a 50
Ohm resistor.
I simulate reom 10 to 100 GHz
With the waveform tool I measure the -3 dB frequency to be 3.19 GHz
and then I tried to measure with a .measure statement from a file. The
measure statement I took from the manual:

.MEAS AC tmp max mag(V(out)); find the peak response
.MEAS AC meas_3db trig mag(V(out))=tmp/sqrt(2)

The measure report windos tell me:
tmp: MAX(V(out))=(-0.0043981dB,1.82317°) FROM 10 TO 1e+011
meas_3db=9.68205e+010 FROM 3.17955e+009 TO 1e+011

What is the number directly behind the =? (9.68205e+010)

--
Svenn

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Helmut Sennewald - 29 Jul 2007 21:04 GMT

> Hi,

> I simulate a passive high-pass filter with a 1 pF capacitor and a 50
> Ohm resistor.
> I simulate reom 10 to 100 GHz
> With the waveform tool I measure the -3 dB frequency to be 3.19 GHz
> and then I tried to measure with a .measure statement from a file. The
> measure statement I took from the manual:

> .MEAS AC tmp max mag(V(out)); find the peak response
> .MEAS AC meas_3db trig mag(V(out))=tmp/sqrt(2)

> The measure report windos tell me:
> tmp: MAX(V(out))=(-0.0043981dB,1.82317°) FROM 10 TO 1e+011
> meas_3db=9.68205e+010 FROM 3.17955e+009 TO 1e+011

> What is the number directly behind the =? (9.68205e+010)

Maybe the last before last frequency point in the AC-simulation.

> Svenn#

Hello Svenn,

The following command will do what you want.

.ac dec 100 10k 100g

.MEAS AC tmp max mag(V(out)); find the peak response
.MEAS AC meas_3db when mag(V(out))=(tmp/sqrt(2))

--- output in the .log file
tmp: MAX(mag(v(out)))=(-0.0043981dB,0°) FROM 10 TO 1e+011
meas_3db: mag(v(out))=(tmp/sqrt(2)) AT 3.18011e+009

Best regards,
Helmut

LTspice user group:
http://tech.groups.yahoo.com/group/LTspice/

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Electronics Forum / CAD / July 2007

Question about ltspice .measure