| Electronics > Beginners |
| Bandpass Filter Design |
| << < (3/10) > >> |
| Rigolon:
I see, thx. Have a lot of thing to google for now. :scared: hahahaha |
| Mr. Scram:
--- Quote from: Wimberleytech on April 27, 2018, 01:10:00 pm ---Go here: http://www.analog.com/designtools/en/filterwizard/ When I was first introduced to filter design, it was all tables. Active filters of anything other than 2nd order were implemented with gyrators or leap frog. Now, somebody has done the work for you. --- End quote --- Thanks, that's actually helpful. |
| Rigolon:
So I've been having fun with some filters and trying to understand the math and all. I tried with a Sallen-key that i never saw before. @Wimberleytech actually you were the one that helped me see that it was a Sallen-key, from a old post of mine. The only thing that I found about something like that was this: http://www.ti.com/lit/an/sbaa237/sbaa237.pdf But in my case the resistor wasn't to compensate the GBW. Circuit Attached. And I got down to the transfer function (I guess :D): H(s)=(s*R3*C1 + 1)/[s2*R1*R2*C1*C2 + s*(R1*C1+R2*C1+R3*C1) +1] I got stuck on this point, how do I find the corner frequency of this function? I never saw a LPF with a zero. I simulated and plotted and it's in fact a LPF, and R3 has influence on fc. It's just that I don't see how to go on with the math. From what I found on my old notes from my classes i am using this function as general function for 2nd order filters: H(s) = N(s)/(s2/w2n + s/Qwn + 1] where N(s) determines the type of filter: N(s) = k; is a LPF N(s) = k*s2/w2n; is a HPF N(s) = k*s/Qwn; is a BDF N(s) = k*(1 - s2/w2n); is a notch filter |
| Wimberleytech:
The zero in the modified SK compensates for the roll off of the op amp. For an ideal opamp, the filter is still a LPF but with some peaking (depending on where the zero lies) and a first-order roll off (-20dB/decade). |
| Rigolon:
--- Quote from: Wimberleytech on April 30, 2018, 08:42:57 pm ---The zero in the modified SK compensates for the roll off of the op amp. --- End quote --- I'm having trouble to understand this, the roll off shouldn't impact on the corner frequency right? If I want a better roll off I'll have to use a higher order filter, correct? For example, if I use two identical Sallen-key LPF I will have a 4th order filter (I know that it's better to not use identical filters) and have a better roll off than just 1 SK LPF. But this resistor is actually impacting on my corner frequency, I just can't find the equation for the corner frequency. All equations that I find/calculate doesn't include that resistor. |
| Navigation |
| Message Index |
| Next page |
| Previous page |