No .disto in LT Spice?

[Paul Burridge]

Hi all,

Is there a way to run a distortion analysis in LT? I can't see any
option for it off-hand...

Thanks,

p,
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Helmut Sennewald]

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> schrieb im Newsbeitrag
news:jer5e0hbcramhelkgvqt248omtk4td2m5q@4ax.com...

Hi all,

Is there a way to run a distortion analysis in LT? I can't see any
option for it off-hand...

Hello Paul,
if you are only interested in the harmonic contents, then the .FOUR
analysis can do the job.
If you need the intermodulation products, then you could do an FFT
after a .TRAN analysis. Intermodulation menas f2-f1, f1+f2, .... .
Both methods are based on a large signal .TRAN simulation.

The mentioned methods are mathematically not exactly the same as the
..DISTO analysis. When I understand it correctly, then .DISTO is a
small signal distortion and intermodulation analysis.

Accidentally distortion is often more common in large signal
applications. Maybe thats the reason why this command isn't implemented
in LTSPICE. Only Mike will know that.

Are you interested in "real" large signal distortion or in
"theoretical" small signal distortion and/or intermodulation?

Best Regards,
Helmut



Original Text from the LTSPICE help pages.
------------------------------------------

..FOUR -- Compute a Fourier component after a transient analysis

Syntax: .four <frequency> [Number of Harmonics] <data trace1> [<data
trace2> ...]

Example: .four 1kHz V(out)

This command is performed after a transient analysis. It's supplied in
order to be compatible with legacy SPICE simulators. The output from this
command is printed in the .log file. Use the menu item "View=>Spice Error
Log" to see the output. For most purposes, the FFT capability built into
the waveform viewer is more useful.

 

[Paul Burridge]

On Thu, 1 Jul 2004 00:54:49 +0200, "Helmut Sennewald"
<helmutsennewald@t-online.de> wrote:

Hello Paul,

Hi Helmut; another very late night, eh?

if you are only interested in the harmonic contents, then the .FOUR
analysis can do the job.

Indeed.

If you need the intermodulation products, then you could do an FFT
after a .TRAN analysis. Intermodulation menas f2-f1, f1+f2, .... .
Both methods are based on a large signal .TRAN simulation.

The mentioned methods are mathematically not exactly the same as the
.DISTO analysis. When I understand it correctly, then .DISTO is a
small signal distortion and intermodulation analysis.

Accidentally distortion is often more common in large signal
applications. Maybe thats the reason why this command isn't implemented
in LTSPICE. Only Mike will know that.

Are you interested in "real" large signal distortion or in
"theoretical" small signal distortion and/or intermodulation?

The former in this instance. Let's assume it's not implemented in LT,
then.Do you happen to know if there's any way of superimposing an
input trace and an output trace in a single plot pane without scaling
so one can simply compare the difference in waveshapes visually?

Thanks,

p.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

 

[Kevin Aylward]

Paul Burridge wrote:

On Thu, 1 Jul 2004 00:54:49 +0200, "Helmut Sennewald"
helmutsennewald@t-online.de> wrote:

Hello Paul,

Hi Helmut; another very late night, eh?

if you are only interested in the harmonic contents, then the .FOUR
analysis can do the job.

Indeed.

If you need the intermodulation products, then you could do an FFT
after a .TRAN analysis. Intermodulation menas f2-f1, f1+f2, .... .
Both methods are based on a large signal .TRAN simulation.

The mentioned methods are mathematically not exactly the same as the
.DISTO analysis. When I understand it correctly, then .DISTO is a
small signal distortion and intermodulation analysis.

Accidentally distortion is often more common in large signal
applications. Maybe thats the reason why this command isn't
implemented in LTSPICE. Only Mike will know that.

Are you interested in "real" large signal distortion or in
"theoretical" small signal distortion and/or intermodulation?

The former in this instance. Let's assume it's not implemented in LT,
then.Do you happen to know if there's any way of superimposing an
input trace and an output trace in a single plot pane without scaling
so one can simply compare the difference in waveshapes visually?

Well, obviously there *must* be some scaling to do this. Of course, with
SS, you just check an options box to have each wave displayed on in its
own scale on the same graph, so you don't have to manually do it

I do find the .dist quite useful. For the main class A bits of a power
amplifier it saves a lot of time in getting the essentials debugged. If
it shows large distortion of frequency, you know your F*&ked.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.