[sdiy] Digital Bandpass Filters

[jhaible at debitel.net]

> I try to do this with Matlab, or using National Instruments tools, 

Labview ?!


and
> programming the rest in Visual Basic. Though since I have very little
> experience with digital filters, I have some problems designing these.
> 
> For the lowest bands in the analyser, I will need bandpassfilters with a
> very small passband.
> For example the utter lowest one needs a low Cutoff of 0,712 Hz and High
> cutoff of 0,898 Hz.
> That means a Passband of only 0,186 Hz! And I still have to maintain only a
> low attenuation in the passband following the IEC Norm.
> 
> While checking out some filters from National Instruments, I found that they
> are quite nice, but when I try to design these lowest bandfilters, I have to
> make them small and I have to crank up the Order to keep the curve steep.
> The problem is only, that even if I increase the order, it's still not
> enough, and the curve doesn't make it to the top of the band before it's
> going down again (the high cutoff).
> Not only that, if I increase the filter order to over 50, the filter seems
> to go crazy, and that's no option either ;)

I've played with higer order filters in Labview recently, and they went crazy
indeed. But I didn't try more than a few days, and I'm in no way a Labview 
expert. But I can confirm that with very high orders they don't do what would 
be expected.

Anyway: you don't need high order filters for a 1/3 octave analyzer.
6th order filters should be well enough.

As for your extremely low (absolute) bandwidth on the lowest frequency bands, 
don't worry: The required filter order depends (among other factors) on the 
_relative_ bandwidth, i.e. bandwidth / center frequency. Doesn't look that bad 
anymore, does it? Expect long settling times for such a low center frequency, 
however.


JH.


-------------------------------------------------
debitel.net Webmail