In message <l87u7c+ebvu@...>, dated Tue, 10 Dec 2013, "skleiser@..." <skleiser@...> writes:
Given (in this case) a square wave of fixed frequency, fast but non-zero rising & falling edges, 50% duty cycle, and an exact integer number of cycles (etc.), what's the narrowest possible harmonic bandwidth that ought to be expected? In other words, when is the simulation as optimized as it can be?
harmonic bandwidth doesn't depend on those things. Embedded in the Help on the B source is:
"In LTspice, the impulse response is found from the FFT of a discrete set points in frequency domain response. This process is prone to the usual artifacts of FFT's such as spectral leakage and picket fencing that is common to discrete FFT's. LTspice uses a proprietary algorithm that exploits that it has an exact analytical expression for the frequency domain response and chooses points and windows to cause such artifacts to diffract precisely to zero. However, LTspice must guess an appropriate frequency range and resolution. It is recommended that the LTspice first be allowed to make a guess at this. The length of the window and number of FFT data points used will be reported in the .log file. You can then adjust the algorithm's choices by explicitly setting nfft and window length. The reciprocal of the value of the window is the frequency resolution. The value of nfft times this resolution is the highest frequency considered."
The significant words are:
"The reciprocal of the value of the window is the frequency resolution."
Frequency resolution is the same as the observed 'harmonic bandwidth'.
On a more general note, assuming artifacts of a non-ideal simulation have been minimized, does the harmonic bandwidth provide useful information, or do only the peak values matter?
The harmonic energy is averaged over the bandwidth, so widening the bandwidth tends to reduce the observed amplitude.
--
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
