The Filter is a library containing subcircuits of analog and digital filters. There are more details inside the manual but, in a few short, descriptive words, it holds the following: -3dB/octave and Bessel (no Thiran), Butterworth, Chebyshev, inverse Chebyshev and Cauer/Elliptic (analog and IIR), plus windowed FIR filters such as (among others): rectangular, Hann, Hamming, Blackman, Kaiser, Dolph-Chebyshev, but also pulse-shaping Gaussian, raised cosine and root-raised cosine, differentiator and Hilbert transformer. Almost all support low-pass, high-pass, band-pass and band-stop, some also all-pass.
Since LTspice is a dedicated analog circuit simulator, the digital filters are, of course, built in an analog way, limited by the "engine": no N bits quantizing and transients may suffer from spikes or sample distortions. Of course, digital also means speed penalty for time domain so high orders will mean lots of transmission lines, which limit the timestep. This means that the time-domain will be getting slower by the order; how much, it depends on your hardware. Still, the Filter supports both time- and frequency-domain analysis and the usage should come in handy for anyone.
However, I'm only human, so mistakes are possible. If you find this useful and happen to stumble across inconsistencies, please let me know, so I can correct them, by writing a mail at imbvlad at google's. I'll respond as quickly as I see the mail.
The manual can be freely downloaded in a single archive as either a zip or a 7-zip file. It can also be found on the LTspice Yahoo Groups, in the file section. The symbol, Filter.asy, and the subcircuit file, filter.sub, are inside the archive, in
./html/files/, along with all the examples, or separate (the online version doesn't have the files). Any updates brought to the Filter are usually added to the manual, as well, so when a new major change appears, the archives will be updated, as well; no need for separate downloads. If minor changes occur, though, an archived version of the stand-alone files will be uploaded.
has some digital models,
[Filt]has mostly signal processing related models,
[Math]has LTspice native functions, plus some home-brew ones, available as one- or two-input symbols, built with usability in mind, and, finally,
[Pwr]holds some power electronics related blocks, mostly three-phase. Almost all of them, if not all, are behavioural approaches, to make simulation at block level possible without the speed penalty, or sacrificing too much precision. It also allows for replacing a specific ideal block with a real-life case schematic, while not having such an impact on performance. I hope they are as useful to you as they are to me.