X-GM-THRID: 1234880640472300788 X-Gmail-Labels: rsgb lf Delivered-To: daveyxm@gmail.com Received: by 10.35.22.5 with SMTP id z5cs316279pyi; Fri, 27 Apr 2007 04:39:11 -0700 (PDT) Received: by 10.67.22.14 with SMTP id z14mr2976088ugi.1177673951417; Fri, 27 Apr 2007 04:39:11 -0700 (PDT) Return-Path: Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by mx.google.com with ESMTP id 53si6860364ugn.2007.04.27.04.39.05; Fri, 27 Apr 2007 04:39:11 -0700 (PDT) Received-SPF: neutral (google.com: 193.82.116.20 is neither permitted nor denied by best guess record for domain of owner-rsgb_lf_group@blacksheep.org) DomainKey-Status: bad (test mode) Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1HhOjg-0004Zj-59 for rs_out_1@blacksheep.org; Fri, 27 Apr 2007 12:35:36 +0100 Received: from [83.244.159.144] (helo=relay3.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1HhOjf-0004Za-Le for rsgb_lf_group@blacksheep.org; Fri, 27 Apr 2007 12:35:35 +0100 Received: from wx-out-0506.google.com ([66.249.82.226]) by relay3.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1HhOje-00005H-2R for rsgb_lf_group@blacksheep.org; Fri, 27 Apr 2007 12:35:35 +0100 Received: by wx-out-0506.google.com with SMTP id t12so892145wxc for ; Fri, 27 Apr 2007 04:35:33 -0700 (PDT) DKIM-Signature: a=rsa-sha1; c=relaxed/relaxed; d=googlemail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:to:subject:mime-version:content-type; b=pNiWqH9dDuaHlXDBcPi169SyrwlEP62KRMjfxjyIh0wBavY47F6S6bzcELd3eGFRhFVPouoNzHcKVcn49+16j+GDOFwvkQCdGR5uFeQop4IjlYARCld24P/wxU/cxrKkKMA6YnfEdKUtmQHGmUGt1evTVodJfdQ1qQDFY7fIIUg= DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=beta; h=received:message-id:date:from:to:subject:mime-version:content-type; b=MatkBOJ2vtOroBtzXFy9fWh/GX69TOw9i4wswZAETmbkpQ9I3KsQsNeyRq2ixRnDBsNmuzygC7cllVVpZYCYHAmMqDsEALkVvK1ZRc7hIfnoapaJt80WexsYG7+zFtvGAnmcW4BHuVHrVx4tUph0bchg4uY1nRg0H3x0tastvjA= Received: by 10.90.78.9 with SMTP id a9mr3227396agb.1177673733081; Fri, 27 Apr 2007 04:35:33 -0700 (PDT) Received: by 10.90.50.7 with HTTP; Fri, 27 Apr 2007 04:35:33 -0700 (PDT) Message-ID: Date: Fri, 27 Apr 2007 12:35:33 +0100 From: "Andy Talbot" To: ukmicrowaves@yahoogroups.com, rsgb_lf_group@blacksheep.org, SDR-IQ@yahoogroups.com MIME-Version: 1.0 DomainKey-Status: good (testing) X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,HTML_MESSAGE=0.001 Subject: LF: Filter expert needed - Mystery of the lost real zero. Content-Type: multipart/alternative; boundary="----=_Part_6757_14306870.1177673733043" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.0 required=5.0 tests=HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false Status: O X-Status: X-Keywords: X-UID: 1970 ------=_Part_6757_14306870.1177673733043 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline Is there anyone monitoring these groups/reflector who knows the deep insides of filter design theory? I've just synthesized a straightforward 50- 75MHz 5th order Top-C-coupled bandpass filter to form part of a frequency converter that will feed an SDR-IQ digital receiver. While playing with the filter design using a circuit analysis package, and recalling satellite microwave cavity filter designs from the dim-and-distant past, I remembered that by arranging cross-coupling between non adjacent resonators, it was possible to form pairs of real zeros in the upper and lower stop bands respectively. These convert the filter to an elliptic type response, and considerably improve the cut-off. I also recall it was a very critical thing to set up. So, I tried adding an additional cap between resonators 2 and 4, got a wonderful 10 - 20dB improvement in the low end cut-off response around 40MHz exactly where it was needed, BUT ONLY THE ONE REAL-ZERO COULD BE FOUND !!! There is absolutely no sign af any second null above the passband. Where's it gone? Shouldn't they always come in pairs? This is really bugging me, I can't sleep at night, can anyone explain? Could it be because a top-coupled - C BPF is non-symmetrical? Full circuit details can be found at http://www.scrbg.org/g4jnt/50-75bpf.pdf Andy G4JNT www.scrbg.org/g4jnt ------=_Part_6757_14306870.1177673733043 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline
Is there anyone monitoring these groups/reflector who knows the deep insides of filter design theory?
 
I've just synthesized a straightforward 50- 75MHz 5th order Top-C-coupled  bandpass filter to form part of a frequency converter that will feed an SDR-IQ digital receiver.   While playing with the filter design using a circuit analysis package, and recalling satellite microwave cavity filter designs from the dim-and-distant past, I remembered that by arranging cross-coupling between non adjacent resonators, it was possible to form pairs of real zeros in the upper and lower stop bands respectively.  These convert the filter to an elliptic type response, and considerably improve the cut-off.  I also recall it was a very critical thing to set up.  
 
So, I tried adding an additional cap between resonators 2 and 4, got a wonderful 10 - 20dB improvement in the low end cut-off response around 40MHz exactly where it was needed,  BUT ONLY THE ONE REAL-ZERO COULD BE FOUND !!!  There is absolutely no sign af any second null above the passband.
 
Where's it gone?  Shouldn't they always come in pairs?  This is really bugging me, I can't sleep at night, can anyone explain? 
Could it be because a top-coupled - C  BPF is non-symmetrical?
 
Full circuit details can be found at    http://www.scrbg.org/g4jnt/50-75bpf.pdf
 
Andy  G4JNT
 
 
 
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