In analog filter design, you get to choose between good phase performance and good amplitude performance. You can't have both. If you want a flat passband and steep skirts, you'll get wonky phase. The Chebyshev filter is a good example of controlling amplitude response. If you want good phase performance, you'll get a gentle rolloff from the center frequency and long, shallow skirts. Bessel filters are a good example of that.
None of these are completely linear in phase response, but the Bessel filters are better. The usual approach is to choose a filter with steep skirts that is wider than you need, so you can use the middle of the passband where the phase response is not so bad. Then do additional filtering in DSP, where you can have good amplitude and phase performance at the cost of delay. Better filters, more delay. There is no free lunch, it's physics. My analog filter design textbook was by Van Valkenburg, though it is fairly math-heavy and dang! textbooks are expensive these days. wunder K6WRU BSEE Rice University 1981 On Apr 23, 2014, at 10:13 PM, Larry Lopez <lawlop...@gmail.com> wrote: > Some of the wider filters in that paper are pretty good. > The narrowest filters are not so good. > The flat part of the passband has constant group delay. > The skirts are awful > > For narrow filters it starts going weird. > > Larry Lopez [via Elecraft] wrote: >> It turns out that the Elecraft 2.7 KHz 5 pole filter is the winner >> according to: >> >> http://www.qsl.net/gm3woj/k3filtertestssn2203.pdf >> >> which makes me want to make my own filter. >> Still, I'm going to keep the 400Hz 8 pole filter which >> is coming with my S/N 27xx Elecraft Radio !!! >> >> >> ------------------------------------------------------------------------ >> If you reply to this email, your message will be added to the >> discussion below: >> http://elecraft.365791.n2.nabble.com/What-crystal-filters-for-the-K3-are-linear-phase-delay-constant-group-delay-tp7587633p7587774.html >> >> >> To start a new topic under [K3], email >> ml-node+s365791n365792...@n2.nabble.com >> To unsubscribe from Elecraft, click here >> <http://elecraft.365791.n2.nabble.com/template/NamlServlet.jtp?macro=unsubscribe_by_code&node=365791&code=bGF3bG9wZXozQGdtYWlsLmNvbXwzNjU3OTF8MjU5MTAzNDE2>. >> NAML >> <http://elecraft.365791.n2.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> >> >> > > > > > > -- > View this message in context: > http://elecraft.365791.n2.nabble.com/What-crystal-filters-for-the-K3-are-linear-phase-delay-constant-group-delay-tp7587633p7587779.html > Sent from the Elecraft mailing list archive at Nabble.com. > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to wun...@wunderwood.org -- Walter Underwood wun...@wunderwood.org ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
