In message <l8q4um+1ikvaao@...>, dated Tue, 17 Dec 2013,
ronw6wo@... writes:
I am a simple soul? ( BTW maybe we are both Brits )
I am, I don't know about you. (;-)
If one conducts an FFT on a pure sinewave in theory there should be a
single spike of infinite height and zero width.? The more the spike
departs from those unattainable ideals shows the accuracy of the
process.?
The height is just the voltage; not infinite. The width would be zero if
the sine-wave started at the Big Bang and went on till the Big Crunch. A
sine wave of finite duration doesn't have a precise zero bandwidth.
Having said that, if you do a FFT on exactly a whole number of cycles,
the result is the same as for the truly infinite-duration signal. But
since it is a digital process, the apparent width of the line is limited
by the bit-depth. Similarly, the line width in an analogue Fourier
analysis is limited by the settling time of the analogue filter.
What I have seen in LTPSPICE is far off and essentially useless?
The FFT can give useless results if it's not set up correctly. That's why I spent some time trying to get a clear statement of the process.
I've now filed that for future reference:
LTspice FFT resolution settings
To get x Hz resolution, you should, in practice, simulate for 2/x
seconds. You can simulate for longer to get a clearer spectrum display,
say N/x seconds.
To get a spectrum up to X Hz, you then need more than 2*N*X/x samples,
preferably the next higher power of 2.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
