In other words, the level of the harmonic content is it compared to what?
Hi Atferrari,
Actually, there are two ways to get that type of data from an analysis. One is the FFT plot and the other is from a spice directive that can be view in the spice error log.
Regarding the FFT, 2V P-P in is 0db. But let's say you have a 50Ω input Z and a 50Ω source impedence rather than 0Ω source Z, the input level will be -3db, for obvious reasons. You know how to pull up the FFT plot, obviously, but the other way to look at the harmonic content is by selecting the Spice Error Log rather than FFT from the same menu selection point AFTER you place the necessary Spice Directive on the schematic.
To do that, click on the .op button on the tool bar and type in the directive with the following basic format:
.four <fundamental frequency> <number of harmonics> <label to be evaluated>
The number of harmonics can be omitted and default number of 9 harmonics will be evaluated and presented along with the total harmonic distortion. If you have multiple points to be evaluated, such as a cascaded amplifier, just add their labels to the list; a handy tool to tweek I/O loading and Z.
Here is an example directive for a 3 stage amp sampling the output each stage with a 10 MHz input, 5 harmonics to be evaluated for total harmonic distortion and sampling the outputs of each stage. In this case, it points right to the stage(s) with a problem if any.
.four 10e6 5 Out1 Out2 Out3
or .four <10MHz> <evaluate the first 5 hamonics> <evaluate 1st output> <evaluate 2nd output> <evaluate 3rd output>
I'll include an FFT and a spice error log of a 10MHZ passive tripler I designed with both the FFT and spice error log with the fourier evaluation.
Hope this is clear and will help.