[sdiy] Ladder filters and gain drop, that old chestnut
Richie Burnett
rburnett at richieburnett.co.uk
Mon Aug 31 15:25:07 CEST 2015
> Also my point is that dB to me seems like an arbitrary unit, it makes
> more sense to me to see on a plot a drop of 1 unit per octave
> (incidentally which is another power of 2) for a 1 pole filter, or a
> rise of 1 unit per octave for a 1 pole high pass filter, 1 pole = 1
> doubling / halving per octave. It just makes things easier to count in
> my head instead of having to multiply and divide by 6 all the time.
I totally agree, Andy. As an EE I have to deal with things in dB all the
time, so have just gotten used to it. But for students learning I get them
to plot their bode-plots on log-log paper where the y-axis is amplitude in
volts, and the x-axis is frequency in Hertz. Then we talk about 1st order
(1-pole) low-pass filters having a stop-band slope of minus one, 2nd order
low-pass having a slope of minus two, high-pass filters having a positive
stop-band slope, etc...
As you said, one factor of 2 change in amplitude for a factor of 2 change in
frequency for a 1-pole filter, or two factors of 2 change in amplitude for a
factor of 2 change in frequency for a 2-pole filter. It is more intuitive
than saying -6dB/octave or -12dB/octave, but that's what they will encounter
in EE circles so they need to know about that too. And then to confuse them
even more there's slopes quoted as 20dB/decade, etc...
-Richie,
PS. Tim Stinchcombe those 3D animated root-locus plots are cool. I only
just realised you could click on them and make them come to life!
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