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I am sorry for this as its not really a "question" per se. I am keen to learn analog filter design. So what prerequisites am I looking at before I start self studying myself about analog filter design? I have covered most of Network Analysis.. Network Synthesis is something which wasn't covered in our syllabus.. and I have been studying about op amps for quite a while. Thanks!

EDIT: For the "design" purposes.. Do I need an extensive knowledge of Network Synthesis? I was working through a synthesis text and its gonna take quite a while if I have to grasp all of it.. I dnt wanna delve deep into synthesis if I dnt have to fr my purposes.

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    \$\begingroup\$ Did you have anything on "signals and systems"? That's the subject which a) helps you understand what you're filtering and b) how a filter acts and which properties it needs to do that. \$\endgroup\$ Commented Jan 3, 2020 at 19:03
  • \$\begingroup\$ Yes i hv read it.. We didn't hv it as a subject so i had to self teach.. I understand abt different kinds of filters nd their responses.. Bode plots nd stuffs \$\endgroup\$
    – nn08
    Commented Jan 3, 2020 at 19:06
  • \$\begingroup\$ yeah, typically quite a bit more than just bode plots, though. \$\endgroup\$ Commented Jan 3, 2020 at 19:07
  • \$\begingroup\$ Yeah actually thts why i wanted to delve deeper bt i didn't know if i cud go on with my present knowledge or else wht do i need to learn before getting to design some basic filters by myself.. \$\endgroup\$
    – nn08
    Commented Jan 3, 2020 at 19:10
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    \$\begingroup\$ you'll mainly need to understand signals and systems on a serious level. It's not clear what you're specifically referring to with Network Synthesis – that's a gigantic field, and in the end, yes, designing an analog filter is network synthesis. \$\endgroup\$ Commented Jan 3, 2020 at 19:20

2 Answers 2

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There are math intensive ways, or matrix methods or easy online tools. But you ought to understand how they work.

Filters are generally defined by ;

  • center fo , -3dB BW, bandpass ripple , Band reject fBR, Attenuation @ fBR which affects order N
  • then band shape offers many choices : Butterworth Q=0.7, Chebychev with X dB BP ripple., Gaussian, Bessel (lowest Q,), Linear Phase, Raised Cosine ( null ISI) etc.
  • even order , odd order, BPF + Notch , Tolerance and Sensitivity analysis (better with low Q)

  • Filters are a result of complex impedance attenuation ratios

  • matched impedance for RLC filters
  • Active includes ; Gyrators , Transimpedance, Sallen-Keys and Multi-feedback Active (positive FB and negative FB gain )

There are a bunch of free web filter design tools, that get a design quickly that may required impedances to be raised or transposed or balanced. (TI (reg. email), Falstad.com > Circuits > Pass,Active ...)

For Falstad> Options> show phase... adjust slider options.

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    \$\begingroup\$ Tony...you have listed several points which are relevant for a good understanding of the various filter functions (bandpass only) and circuits - however, I like to add something: For my opinion, it is very important to understand the concept of poles and zeroes in the complex s-plane - and to know the definitions for "pole frequency" and "pole-Q". As you know, THESE two parameters are listed in all the tables which are available for the different approximations (Butterworth,Chebyshev,...) \$\endgroup\$
    – LvW
    Commented Jan 4, 2020 at 10:55
  • \$\begingroup\$ I agree @LvW and Falstad’s active filter designer link has the > options > view poles for the builtin filters. it is interesting to see how Chebychev poles when 0dB ripple is chosen and filter shape is similar to Butterworth, just poles are shifted. Removing the Load reveals the Q of each pole. Changing the load value then the slider recomputes for the new load, interactively. So one can get a feel of Poles vs shape of filter much faster. \$\endgroup\$
    – D.A.S.
    Commented Jan 4, 2020 at 17:08
  • \$\begingroup\$ although in the early 80’s we did not have these tools and Raised Cosine was a bit of a mystery yet essential for data filters. So we had to read the red guru book by famous Russian author who’s name I forget. \$\endgroup\$
    – D.A.S.
    Commented Jan 4, 2020 at 17:16
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Intro

If you want to understand filters in a more intuitive sense, the process is to first understand the response curves you might expect to see from one-, two-, and three-section cascaded filters. Once you have an idea about the response curves, you will have developed some intuition about how poles work and then you can reverse the process (as one person said to me, "turn things inside out") and learn how to start with a bandwidth or needed rejection, figure out the number of poles you'll need, determine the Q, the center frequency, and pole separation numbers.

In short, you start out by first understanding response curves. Once you get enough of that, you will be in a better position to figure out how to back your way into those from the usual specifications you are more likely to have on hand.

Bandpass Discussion

A one-pole bandpass filter section (also known as a second-order bandpass filter) lets you control only the center frequency \$f_{_0}\$ and the \$Q\$ (or, its inverse, the damping factor \$\zeta\$.)

With two such sections, assuming you treat them as separate sections and not as a single section with two poles, you can control \$f_{_0}\$ and the \$Q\$ of each section, individually. In this case, it's usually better to keep the \$Q\$ the same for both sections and to generate the bandpass shape you want by moving the two center frequencies, one to the left and one to the right of the middle of your bandpass region, in symmetrical fashion (geometric, of course.)

And for three such sections, where you can control \$f_{_0}\$ and the \$Q\$ of all three sections, you'd probably place one section's \$f_{_0}\$ right in the middle and the other two at either end of your bandpass region. Here, you will probably want the \$Q\$ of the centered section to be one-half the \$Q\$ of the two outer sections so that its contribution to the bandwidth is twice that of the two outer sections.

Before I get too deeply into this (and I probably will stop soon, as the point is likely made sooner than later), please note that in the two- and three-section cases, the further we separate the \$f_{_0}\$ from each other, there is an increasing "dip" between each bandpass region. I'm sure this makes intuitive sense to you, yes? As you move the center frequencies of two sections further apart from each other, then you will expect there will be some "voltage sag" in between their center frequencies. If you keep them closer together, then perhaps no sag at all. Or nothing much, anyway.

This is the whole idea of adding more sections. The more you add, the closer the separation is (or the wider the bandwidth), and the dips in between (usually given as flat, or else \$1\:\text{dB}\$, \$2\:\text{dB}\$, or \$3\:\text{dB}\$) will have a lot to do with the design choices you make.

Think of this like "poles" or "sticks" upon which are drapped some sheets of cloth. The closer the spacing of the poles, the less "droop" of the cloth in between them. And if you space them further apart, then there will be more sag of the cloth in between the poles. It's kind of like that.

Summary

I think you should start by just playing with a single pole bandpass filter transfer function. The simplest one you can find:

$$G_s=K\:\left[\frac{2\zeta\,\omega_{_0}\,s}{s^2+2\zeta\,\omega_{_0}\,s+\omega_{_0}^2}\right]$$

Just set \$K=1\$ here to simplify it further. If you want some math to work with, see here for more details.

Once you follow more how filters are shaped, then you will be in a much better position to understand how to start with a shape you want and work backwards to the number of poles you'll require and have a better idea how to cascade them.

This brings up another point. Cascading sections and the spans between them (the dips you allow) relate to insertion loss. You may need to add gain at some point to compensate for the insertion loss of your filter system. For example, the center frequency attenuation will follow this table I just cobbled up:

$$\begin{array}{c|rr} \text{Insertion Loss} & \\ & \text{2-Pole} & \text{3-Pole}\\ \hline \text{maximally peaked} & 0\:\text{dB} & 0\:\text{dB}\\ \text{maximally flat} & 7\:\text{dB} & 18\:\text{dB}\\ 1\:\text{dB dip} & 11.2\:\text{dB} & 24.2\:\text{dB}\\ 2\:\text{dB dip} & 14\:\text{dB} & 28\:\text{dB}\\ 3\:\text{dB dip} & 17\:\text{dB} & 31.2\:\text{dB} \end{array}$$

Anyway, that's about all I want to say for now. It's actually not terribly hard. It's just that most books get overly-algebraic about things and forget to be more graphical and intuitive. But just go with it. Spend some time to understand the impact of moving the center frequencies closer or further apart, or playing with the value of \$Q\$ for each filter section. It won't take long to develop some basic intuitions. Once you have those, you'll find the mathematics a lot easier to interpret and the development of a custom filter design a lot easier to develop on your own.

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