LTSpice FFT simulation settings and inconsistent results.

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The simplest multi-tone analysis is just two sine waves, frequency shifted so they heterodyne to make a signal with a sine-shaped envelope. This is basically the same thing as a sine-burst signal. Here is a amp test rig I made set up for CCIR.

If you want to do multi-tone analysis you must really remember the "noise floor" in SPICE doesn't mean anything, and it has to be low enough before you will get meaningful results. A high noise floor in simulation indicates the simulator is not set up optimally. This rig attempts to correct that.
 

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The simplest multi-tone analysis is just two sine waves, frequency shifted so they heterodyne to make a signal with a sine-shaped envelope. This is basically the same thing as a sine-burst signal. Here is a amp test rig I made set up for CCIR.

If you want to do multi-tone analysis you must really remember the "noise floor" in SPICE doesn't mean anything, and it has to be low enough before you will get meaningful results. A high noise floor in simulation indicates the simulator is not set up optimally. This rig attempts to correct that.

Thanks, this is very useful information.
 
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