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Are quasi-elliptical or pseudo-elliptical and Chebyshev Type 2 filters the same filter response?

Quasi-elliptical or pseudo-elliptical filters appear to be defined as "... filters with finite transmission zeros." The terms "quasi" and "pseudo" appear to be interchangeable, and most probably refer to the same type in the references I've seen. One reference tantalisingly close to what I'm looking for that I've seen referring to QE/PE filters is here, in Chapter 6, para 1 and 2, however the References section at the end of the chapter is not included in the excerpt. I've found the reference in a local uni library, but won't get there for a few days to check it out.

If they are not, what do their gain, phase, return loss and delay responses look like in the wild? I know the responses for the Cheb type 2 and elliptical filters.

I've looked all over the internet, but the answers appear to be either proprietary or behind paywalls.

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A Chebyshev filter is a degenerate case of an Elliptic filter: it's an Elliptic filter that zero ripple in either the stop band (Chebyshev Type I) or the pass band (Chebyshev type II). The term "elliptic" is typically reserved for filters that do not meet the special-case restrictions of Chebyshev/Butterworth etc filters.

A quasi-elliptic filter is an elliptic filter (Chebyshev, Butterworth, other or not), that has been distorted so that it is not an elliptic filter. Apart from accidents of realization, the reason for distorting the response is to get a different response.

The typical response that is desired in a quasi-eliptic filter is one with better sharpness in the transition region, at the expense of worse response far out in a stop band. You typically get this by adding extra transmission-zeros near the transition region.

The filter is quasi-elliptic because it's got ripple in both pass and stop bands (so it's not a special case like Chebyshev or Butterworth), and a sharp cut-off edge, and some of the poles/zeros are close to where they would be placed by an "elliptic" design, but some of them aren't.

You could call it a "fully-custom-design", but at least for a while, the design was done by starting with an Elliptic design, and tweaking it.

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  • \$\begingroup\$ So you're saying that the main advantage of the QE filters is in the shape of gain response, \$ S_{21} \$. I take it then that the response of the other filter parameters is not much different to that of the elliptical filter, maybe not as wild. Which may be a pity, as I was hoping that the group delay response may be a bit more tamer than that of the Cheb type 2 \$\endgroup\$ Commented Aug 18, 2015 at 17:05
  • \$\begingroup\$ Maybe you want a quasi-Bessel design instead :) You get the extra zeros in a quasi-elliptic design by allowing leakage between the stages of your elliptic design - something that is easy to do in practical analog designs. It might be possible to use the same techniques to improve phase response, but I totally do not know how hard that is. \$\endgroup\$
    – david
    Commented Aug 20, 2015 at 3:27
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This explans eliptical vs chebyshev. The phrases "quasi" and "pseudo" both sound to me like something that approximates an elliptical filter in some way. I do not think they are a whole separate class of responses in the sense that Butterworth, Chebyshev, and Elliptical filters.

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  • \$\begingroup\$ Please see my edit updates. \$\endgroup\$ Commented Aug 18, 2015 at 8:19
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I think I am pretty familiar with filter theory, however, I never have - up to now - heard or seen in a textbook the term "quasi-/pseudo elliptical". For my opinion, if somebody is using this expression he should give at the same time an explanation (in words or as a graph). My first guess is that this term could refer to the Chebyshev-Type-2 response.

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  • \$\begingroup\$ Please see my edit updates. \$\endgroup\$ Commented Aug 18, 2015 at 8:18

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