# Magnitudes of harmonic content at different points

Question asked by ABUELMAATTI on Aug 23, 2006
Hi All

Hope somebody can help me with this as it is really vital for me.

I am designing some amplifier circuits using a nonlinear transistor model and hence of course I get a nonlinear amplifier. I am trying to investigate the paths the fundamental components as well as the harmonic and Intermodulation components take around the circuit. This would be in terms of where in the circuit the Harmonic and Intermodulation components are produced where do they go or get fed back and what adds to what and so on, i.e. before the transistor, after the transistor, on top of the inductor, on the drain of transistor and so on. For this I need to use something like a probe which if I but anywhere in the circuit gives me the harmonic content at that point.

The way I tried to do this was as follows, I give the node a name, say v1 or v2 and then in the output display window I use the dBm function in the sense â€˜spectrum_v1=dBm(v1)â€™ to give me the frequency content at that point. As explained in the help file of the dBm function, this uses the impedance at this point to convert the voltage to power and produces it in dBm. If the impedance is not specified to the function, it assumes its 50 Ohm. So at the output that is no problem because the termination is 50 Ohm any way and I get accurate result. But at any other point in the circuit, the values produced are wrong. The reason for this is the impedance at that point is not the default 50 that the function assumes and at the same time I canâ€™t specify any other impedance to the function because I donâ€™t have any thing like an impedance probe to give the impedance at the point I am interested in on the circuit.

So how do I find the magnitudes of the harmonic content at any point on the circuit and produce accurate results?

Many Thanks in advance.
Abuelmaatti