passive rc-bandpass with 0dB attenuation in the passband
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From: cassiope on 4 Aug 2010 13:32 On Aug 4, 2:20 am, nukeymusic <nukeymu...(a)gmail.com> wrote: > On Aug 3, 8:54 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: > > > > > On 08/03/2010 11:19 AM, Jim Thompson wrote: > > > > On Tue, 03 Aug 2010 10:35:41 -0700, Tim Wescott<t...(a)seemywebsite.com> > > > wrote: > > > >> On 08/03/2010 10:23 AM, cassiope wrote: > > >>> On Aug 2, 1:35 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: > > >>>> On 08/02/2010 01:06 PM, nukeymusic wrote: > > > >>>>> On Aug 2, 7:04 pm, cassiope<f...(a)u.washington.edu> wrote: > > >>>>>> On Aug 2, 7:50 am, nukeymusic<nukeymu...(a)gmail.com> wrote: > > > >>>>>>> Is it possible to make a passive rc-bandpass filter which has 0dB > > >>>>>>> attenuation in the passband with only 4 components? > > > >>>>>>> nukey > > > >>>>>> How close to 0dB? What sort of Q (bw/f0)? What's the load? > > > >>>>> exactly 0dB, unloaded, Q to be determined from the other specifications > > > >>>> Q is almost meaningless in this case -- any passive RC bandpass filter > > >>>> is going to have a damping ratio greater than 1, and the various > > >>>> definitions of Q only converge for damping ratios much less than one. > > > >>>> -- > > > >>>> Tim Wescott > > >>>> Wescott Design Serviceshttp://www.wescottdesign.com > > > >>>> Do you need to implement control loops in software? > > >>>> "Applied Control Theory for Embedded Systems" was written for you. > > >>>> See details athttp://www.wescottdesign.com/actfes/actfes.html > > > >>> Ok, so I have used a more generalized definition of Q (bw/f0). > > >>> Fortunately, > > >>> definitions rarely have convergence problems. Applying them in > > >>> specific instances > > >>> is another matter ;) > > > >>> The simpleminded 4-component RC filter (the obvious serial-parallel > > >>> arrangement) > > >>> won't get to exactly 0dB... would only approach it for truly wide > > >>> bandwidths, even with > > >>> no load. > > > >> Yes, I cited that circuit to contradict Jim's statement that "you can't > > >> make a passive RC bandpass circuit", not to answer the OP's question.. > > > > With horrible skirts. "Band-pass" usually implies skirt-rate relative > > > to bandwidth. > > > Yes. It's more of a "scholar's bandpass" than anything you might want > > to use in real life. It's not a bad mental tool to use when cooking up > > an active filter, because an active bandpass can be made by "sharpening > > up" a passive one, ditto with an active lowpass and (with due respect > > for stability) an active highpass. > > > -- > > > Tim Wescott > > Wescott Design Serviceshttp://www.wescottdesign.com > > > Do you need to implement control loops in software? > > "Applied Control Theory for Embedded Systems" was written for you. > > See details athttp://www.wescottdesign.com/actfes/actfes.html > > Dear Tim, > You probably did not see my question in the other message, therefore > I'd like to ask it here once more: > Can you prove the following statement or point to a proof for it: > > any passive RC bandpass filter is going to have a damping ratio > greater than 1, > > regards, > nukey While the standard form of a twin-T filter isn't a band-pass, it's a band-reject, it can have a very high Q. I'm (almost) sure some clever person can find the right combination of terminals to yield a bandpass. Now if you limit your design to ladder networks, the proof might be possible. I have a vague recollection of a proof that you can't have coincident poles in a passive RC ladder network (but I could be wrong about that, it's been a long time...).
From: Tim Wescott on 4 Aug 2010 14:15 On 08/04/2010 10:32 AM, cassiope wrote: > On Aug 4, 2:20 am, nukeymusic<nukeymu...(a)gmail.com> wrote: >> On Aug 3, 8:54 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >> >> >> >>> On 08/03/2010 11:19 AM, Jim Thompson wrote: >> >>>> On Tue, 03 Aug 2010 10:35:41 -0700, Tim Wescott<t...(a)seemywebsite.com> >>>> wrote: >> >>>>> On 08/03/2010 10:23 AM, cassiope wrote: >>>>>> On Aug 2, 1:35 pm, Tim Wescott<t...(a)seemywebsite.com> wrote: >>>>>>> On 08/02/2010 01:06 PM, nukeymusic wrote: >> >>>>>>>> On Aug 2, 7:04 pm, cassiope<f...(a)u.washington.edu> wrote: >>>>>>>>> On Aug 2, 7:50 am, nukeymusic<nukeymu...(a)gmail.com> wrote: >> >>>>>>>>>> Is it possible to make a passive rc-bandpass filter which has 0dB >>>>>>>>>> attenuation in the passband with only 4 components? >> >>>>>>>>>> nukey >> >>>>>>>>> How close to 0dB? What sort of Q (bw/f0)? What's the load? >> >>>>>>>> exactly 0dB, unloaded, Q to be determined from the other specifications >> >>>>>>> Q is almost meaningless in this case -- any passive RC bandpass filter >>>>>>> is going to have a damping ratio greater than 1, and the various >>>>>>> definitions of Q only converge for damping ratios much less than one. >> >>>>>>> -- >> >>>>>>> Tim Wescott >>>>>>> Wescott Design Serviceshttp://www.wescottdesign.com >> >>>>>>> Do you need to implement control loops in software? >>>>>>> "Applied Control Theory for Embedded Systems" was written for you. >>>>>>> See details athttp://www.wescottdesign.com/actfes/actfes.html >> >>>>>> Ok, so I have used a more generalized definition of Q (bw/f0). >>>>>> Fortunately, >>>>>> definitions rarely have convergence problems. Applying them in >>>>>> specific instances >>>>>> is another matter ;) >> >>>>>> The simpleminded 4-component RC filter (the obvious serial-parallel >>>>>> arrangement) >>>>>> won't get to exactly 0dB... would only approach it for truly wide >>>>>> bandwidths, even with >>>>>> no load. >> >>>>> Yes, I cited that circuit to contradict Jim's statement that "you can't >>>>> make a passive RC bandpass circuit", not to answer the OP's question. >> >>>> With horrible skirts. "Band-pass" usually implies skirt-rate relative >>>> to bandwidth. >> >>> Yes. It's more of a "scholar's bandpass" than anything you might want >>> to use in real life. It's not a bad mental tool to use when cooking up >>> an active filter, because an active bandpass can be made by "sharpening >>> up" a passive one, ditto with an active lowpass and (with due respect >>> for stability) an active highpass. >> >>> -- >> >>> Tim Wescott >>> Wescott Design Serviceshttp://www.wescottdesign.com >> >>> Do you need to implement control loops in software? >>> "Applied Control Theory for Embedded Systems" was written for you. >>> See details athttp://www.wescottdesign.com/actfes/actfes.html >> >> Dear Tim, >> You probably did not see my question in the other message, therefore >> I'd like to ask it here once more: >> Can you prove the following statement or point to a proof for it: >> >> any passive RC bandpass filter is going to have a damping ratio >> greater than 1, >> >> regards, >> nukey > > While the standard form of a twin-T filter isn't a band-pass, it's a > band-reject, it can have a very high Q. > I'm (almost) sure some clever person can find the right combination of > terminals to yield a bandpass. > Now if you limit your design to ladder networks, the proof might be > possible. I have a vague recollection > of a proof that you can't have coincident poles in a passive RC ladder > network (but I could be wrong about that, > it's been a long time...). It can have a very deep null, but as a passive network it isn't resonant. If you want a twin-T notch filter to be resonant then you need to use it as part of an active filter. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: JosephKK on 8 Aug 2010 18:54
On Tue, 3 Aug 2010 01:14:44 -0700 (PDT), nukeymusic <nukeymusic(a)gmail.com> wrote: >On Aug 3, 7:24�am, Mark Freeman <a4533...(a)bofthew.com> wrote: >> Tim Wescott <t...(a)seemywebsite.com> wrote innews:SPednY_R0pRPY8vRnZ2dnUVZ_oednZ2d(a)web-ster.com: >> >> >> >> > On 08/02/2010 10:12 AM, Jim Thompson wrote: >> >> On Mon, 2 Aug 2010 10:04:29 -0700 (PDT), cassiope >> >> <f...(a)u.washington.edu> �wrote: >> >> >>> passive rc-bandpass filter >> >> >> Key words: passive rc-bandpass filter... an oxymoron, statement and >> >> poster :-) >> >> > It depends on how loose your definition is of "bandpass filter". >> >> > � � � � � � � || � � � ___ >> > � �Vin �o-----||---o--|___|---o-----o � Vout >> > � � � � � � � || � | � � � � �| >> > � � � � � � � � � �| � � � � �| >> > � � � � � � � � � .-. � � � �--- >> > � � � � � � � � � | | � � � �--- >> > � � � � � � � � � | | � � � � | >> > � � � � � � � � � '-' � � � � | >> > � � � � � � � � � �| � � � � �| >> > � � � � � � � � � �| � � � � �| >> > � � � � � � � � � === � � � �=== >> > � � � � � � � � � GND � � � �GND >> > (created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de) >> >> > There's a passive RC bandpass filter for you. �It's not a _resonant_ >> > bandpass filter, by any means, but it has a magnitude response that's >> > zero at f = 0, rises to some maximum, then falls to zero as the >> > frequency approaches infinity. >> >> LTSpice fans, here are 4 RC's with a peak "gain" of 1.57dB at 113Hz: >> >> Version 4 >> SHEET 1 884 680 >> WIRE 96 224 -48 224 >> WIRE 208 224 176 224 >> WIRE 320 224 208 224 >> WIRE 208 240 208 224 >> WIRE -48 320 -48 224 >> WIRE 96 320 -48 320 >> WIRE 208 320 208 304 >> WIRE 208 320 176 320 >> WIRE -48 336 -48 320 >> WIRE 208 336 208 320 >> WIRE -48 432 -48 416 >> WIRE 208 432 208 400 >> FLAG -48 432 0 >> FLAG 208 432 0 >> FLAG 320 224 Output >> SYMBOL res 192 208 R90 >> WINDOW 0 0 56 VBottom 0 >> WINDOW 3 32 56 VTop 0 >> SYMATTR InstName R1 >> SYMATTR Value 10k >> SYMBOL res 192 304 R90 >> WINDOW 0 0 56 VBottom 0 >> WINDOW 3 32 56 VTop 0 >> SYMATTR InstName R2 >> SYMATTR Value 1k >> SYMBOL cap 192 240 R0 >> SYMATTR InstName C1 >> SYMATTR Value 0.1� >> SYMBOL cap 192 336 R0 >> SYMATTR InstName C2 >> SYMATTR Value 1� >> SYMBOL voltage -48 320 R0 >> WINDOW 123 24 132 Left 0 >> WINDOW 39 0 0 Left 0 >> SYMATTR Value2 AC 1 >> SYMATTR InstName V1 >> SYMATTR Value SINE(0 1 261) >> TEXT -24 480 Left 0 !.ac oct 10 10 1k >> TEXT 256 336 Left 0 ;Voltage Gain > 1\nFrom Epstein, "Synthesis of >> Passive Networks\nWith Gains Greater than Unity," Proc. IRE,\nJuly 1951 >> >> Mark Freeman > >That's the low pass version of the delayed-recovery filter patented by >G.A. Philbrick, I wouldn't call this a bandpass filter >Do you think this can be transformed into a bandpass-filter? (you will >probably end with more than 4 components?) > >thanks for sharing your time > >nukey So you are just a troll. |