| Electronics > Beginners |
| Bandpass Filter Design |
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| Wimberleytech:
--- Quote from: Rigolon on May 02, 2018, 05:07:44 pm --- --- Quote from: Benta on May 02, 2018, 04:59:39 pm ---his is where Bode plots and asymptotes come in. Your example would (assuming second order filters) result in a filter with a cutoff at 10 kHz with 40 dB/dec roll off, the roll off increasing to 80 dB/dec at 50 kHz. --- End quote --- So if I want a better roll-off isn't better to use the same fc? So I get 80 db/dec roll off? Or are there any advantages or reasons to use different fc? --- Quote from: Benta on May 02, 2018, 04:59:39 pm ---I'm suspicious of your Sallen-Key filter, R3 seems way too high. Shouldn't it be 100 ohms? --- End quote --- I'm using the values that are on the actual circuit, I have an old post that I explain a little how I got to this circuit. Not sure if the author was knowing what they were doing, because there are a lot of mistakes on the circuit, as resistors and capacitors grounded on both pins. There are capacitors on the output of each opamp going to ground, which for me is weird since it decreases the slew-rate of the opamp. Not sure if there any motive to use capacitors like that on the output of the opamps. I will post the whole circuit some time, it's just that I'm taking it by parts to focus and learn in depth about it and electronic in general. ^-^ --- End quote --- I ran your circuit using LTSpice and a LT1058 (close enough) and got the same -3db frequency you got. I think Benta is right. Changing R3 to 100 ohms gives a -3dB of 1kHz |
| Yansi:
Replace it with 0ohm. That is what should be there. (Sallen key 2nd order, bessel Q=0.5 fc=1.5kHz, R1=R2=10k, C1=C2=10n) :-// |
| Rigolon:
Yansi I actually don't intend on using this kind of filter, it's just that it got me curious and since my main objective is to learn, and thanks to you guys I already learned a lot more than I did in my classes few years ago. Just so that it won't get forgotten in all the information and no one anwsers I will copy my last question: So if I want a better roll-off isn't better to use the same fc? So I get 80 db/dec roll off? Or are there any advantages or reasons to use different fc? |
| Wimberleytech:
--- Quote from: Rigolon on May 02, 2018, 05:50:38 pm ---Yansi I actually don't intend on using this kind of filter, it's just that it got me curious and since my main objective is to learn, and thanks to you guys I already learned a lot more than I did in my classes few years ago. Just so that it won't get forgotten in all the information and no one anwsers I will copy my last question: So if I want a better roll-off isn't better to use the same fc? So I get 80 db/dec roll off? Or are there any advantages or reasons to use different fc? --- End quote --- If you have four poles, it doesn't matter where they are...once you get a decade beyond the highest frequency pole, you will be rolling off at 80dB/dec. The advantage of using a different fc for each stage is that you can achieve one of the classic filter approximations (butterworth, chebyshev, etc). |
| Yansi:
Combining the same filter twice means -6dB at the cutoff frequency, meaning a different -3dB cuttoff point. Simply put: Combining two same filters with a -3dB cutoff fc will result in a different -3dB cutoff frequency. Higher order filters (bessel, cebysev, butterworth) require different pole positions, can not be approximated with a repeated block. Try some online calculators to see the results. Do not attempt more than 3rd order sallen key filter, the component precision required will become impractical. Higher orders need more sallen key stages. |
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