LTSpice how to measure THD+N ?

[sameerdhiman]

Hi,

How to measure THD+N of an audio oscillator under LTSpice ?

[Aucosticraft]

LTspice : LTspice/SwitcherCAD III

This group is dedicated for LTspice. will get lot of information here. As well help.

[jcx]

my fundamental null circuit could help for single tone - just take the RMS of the residual

Spice simulation

for an oscillator circuit you will need a very large ncycles number for any high Q oscillator sim to startup, stabilize

set the plot limits to the last cycle and use the <ctrl>, left click the trace lable in the waveform viewer

or add a .meas directive to print the result in the error log


noise in spice requires added sources in a .TRAN sim

[sameerdhiman]

@Aucosticraft : Thanks

@JCX: Very nice null circuit

Do you mean .Tran does not add N-Sources to passive and active components while simulation. Do I need to add N-Sources to each and every component during .TRAN simulation ? What if active component have noise source in their spice model like LME49860 ?

[sameerdhiman]

Originally Posted by jcx

noise in spice requires added sources in a .TRAN sim

Hello JCX,

Please clear my doubt.

[jcx]

I don't use noise analysis in spice, not an expert in that use - the doc claims .NOISE is an AC analysis

for .TRAN I'm pretty certain you have to add your own noise modeling sources

for xtal osc noise sim is going to be "interesting" close in noise requires millions of cycles to evaluate, the measured noise has multiple slopes, unmodeled thermal and acceleration effects become significant

[sameerdhiman]

Thank you so much for confirming.