If one conducts an FFT on a pure sinewave in theory there should be a single spike of infinite height and zero width."
It will have finite height, not infinite. ?The finite height is related to the amplitude of the sine wave.
Zero width isn't quite true either because the FFT is discrete in the frequency domain. ?Ideally (when done properly) it will "fill" only one "slot" in the frequency domain and not the adjacent slots. ?In that sense, your intent is probably correct; but it technically isn't zero width.
"What I have seen in LTPSPICE is far off and essentially useless"
Then perhaps you should try Mike Engelhardt's sinewave FFT example in LTspice's Help pages. ?Start Help, go to:
? Waveform Viewer > Waveform Arithmetic
and scroll down to the bottom. ?Notice that there is one narrow spike in the FFT's spectrum. ?He uses a SPICE Netlist, but it's almost a no-brainer to replicate that as a schematic. ?Or you could type it in as a Netlist (LTspice accepts those too).
