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For a class I have to filter out two signals separately and demodulate them (AM) before processing them any further. The only topology we have seen so far for active filters is the versatile Sallen-Key. However this one is way too instable with real components for a filter with a high Q factor. So I am looking into other topologies(come accross variable state so far) and I realized how hard it is to get good references on this topic and our course is not very well sctructured so I don't feel like it's going to come from them

Any good references on the internet or textbooks I could ''acquire''

SolipsistElvis
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  • First question; What kind of filter (lowpass, bandpass)? Second question: Which oder? What about multi-feedback-topology (MFB)? – LvW Mar 10 '15 at 21:18
  • Another question: What means "high-Q"? Q=5 or Q=20? And the most important question: Which frequency range (pole frequency)? All these questions need to be answered for a good advice. – LvW Mar 10 '15 at 21:28
  • Band pass filter with a Q factor around 50. Like I said I am not asking for help in the design so much as a general reference tool for the different available topologies. For example now I know about MFB since you mentionned it but no traces of it in my notes and it is difficult topic to google... – SolipsistElvis Mar 10 '15 at 23:07

2 Answers2

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1) Yes - you are right, Sallen-Key topology is not well suited for high-Q filters;

2.) MFB topology is better, however - it also is not the best solution for high selectivity (by the way: You didn`t give the center frequency!)

3.) You have mentioned "state-variable filters"; I would not recommend KHN or Tow-Thomas because they suffer from phase errors (3 opamps in series); more than that, they are primarily used if all three outputs are needed;

4.) For my opinion, the best choice is a GIC-based solution (also called "Fliege filter"). However, you need two opamps. I recommend to google for "GIC filter" or "Fliege filter". One link is given below:

http://www.google.de/imgres?imgurl=http://www.schematica.com/active_filter_resources/images/fliege_bp_dual_supply_red.jpg&imgrefurl=http://www.schematica.com/active_filter_resources/op_amp_virtual_ground_circuits.html&h=299&w=395&tbnid=UkfQKtYPZXcwFM:&zoom=1&tbnh=98&tbnw=130&usg=__RMmOGMvqAIIbh83lmN3MS_k_6N0=&docid=C6RXKsxEe7_qdM&sa=X&ei=bPv_VJXjE4LmUtXHgIgE&ved=0CCMQ9QEwAA

Remark: All GIC-based filters are easy to design and have very good sensitivity (active) properties. Which means: They don`t react sensitive to opamp phase deviations because both opamps (if on the same chip) compensate each other in this respect.

GIC: Generalized Impedance Converter.

LvW
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A good basic reference is the book Active Filter Design by Carson Chen (Hayden 1982). Wikipedia articles on Sallen key and State variable filter give the design equations for the filter types you mention. Both types give bandpass outputs and identical bandpass filters can be chained to get higher Q.

nidhin
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cuddlyable3
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