EMRFD Message Archive 10733

Message	Date	From	Subject
10733	2015-01-18 11:37:12	Chris Howard w0ep	mystery of filters
Hi, Thanks to all for their replies as I was fooling around with the nichrome wire inductor. I was doing some more thought experiments about filters and have a couple of questions: 1 - how is it that two narrow filter sections in series can turn into a wider filter? 2 - all of the filters I see in radio schematics are filter sections in series. Is there any example of filter sections in parallel? (sections probably isn't the right word, but I'm too lazy to go look it up. I am talking about the combinations of L-C-L or C-L-C which are joined together.)
10735	2015-01-18 13:55:33	Ephemeral	Re: mystery of filters
>I was doing some more thought experiments about filters >and have a couple of questions: Some very good questions. >1 - how is it that two narrow filter sections in series >can turn into a wider filter? I don't think they do. I think what is happening is that the adjacent sections load each other. A loaded section has a wider bandwidth.   If filters are connected directly together the loading effects significantly alter the combined response.   If coupling is not desired the filters can be connected by unidirectional buffers with purely resistive input and output impedances. Then the combined response will be the product of the responses of the sections. >2 - all of the filters I see in radio schematics are >filter sections in series. Is there any example >of filter sections in parallel?   Filter sections are connected in series to refine the selectivity, somewhat analogously to set intersection, because the signal must be passed by ALL sections. With such arrangement one can adjust the coupling to achieve a flat pass band with steep skirts, or maybe linear phase, whichever is desired.   Whereas if sections were connected in parallel they would tend to increase the range of frequencies accepted somewhat analogously to set union, because the signal must be passed by ANY section. I am not sure this is likely to lead to anything useful unless perhaps we wanted a filter with two disjoint pass bands.
10736	2015-01-18 14:16:37	Thomas S. Knutsen	Re: mystery of filters
2015-01-18 20:37 GMT+01:00 Chris Howard w0ep w0ep@w0ep.us [emrfd] <emrfd@yahoogroups.com>: 1 - how is it that two narrow filter sections in series can turn into a wider filter? Its about the resonator Q. When coupling resonators together the total product of the Q will be less.  This is the same as connecting resistor in parallel. We use the same equation.