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Calc freqs of triple tuned bandpass filters?

Started by BobG May 27, 2010
Whats the rule of thumb for how far to detune the two outside filters
for third octave bandwidth? Is the middle filter supposed to have a
little less gain to flatten the passband out? Is the Q of all 3
filters the same? Do the skirts roll off at 18dB per octave? (I
searched for 'triple tuned' but no hits, so I hope I dont get flamed
for asking)
On 05/27/2010 11:14 AM, BobG wrote:
> Whats the rule of thumb for how far to detune the two outside filters > for third octave bandwidth? Is the middle filter supposed to have a > little less gain to flatten the passband out? Is the Q of all 3 > filters the same? Do the skirts roll off at 18dB per octave? (I > searched for 'triple tuned' but no hits, so I hope I dont get flamed > for asking)
Well, not a flame perhaps, but: It sounds like you're trying to make a filter in 2010 using design rules from 1940. There's nothing inherently bad in this, but there are certainly faster ways. Back up a bit and tell us what you're really trying to do. These days -- at least if this is a DSP project -- one would normally think of this in terms of a 6th-order bandpass filter, one would specify it by passband ripple, 3dB (or 6dB) bandwidth, shape factor, and ultimate attenuation. Do that, and most of your questions become moot, because the answer to all of them is "whatever the filter design coughs up". Generally with filter chains like this the filters are cascaded, not paralleled. So the middle filter gain neither makes the center of the passband rise or fall -- the Q of the middle filter does. Depending on your filter design the Q of the filters may or may not be the same, but I _think_ that for a 'normal' Butterworth or Chebychev filter you can expect the 'end' filters to have higher Q. The skirts will roll off at 18dB per octave far from the passband, but close to the passband the more ripple you accept the sharper the skirts will be. Someone who lives and breaths filter design would be able to quote you figures at this point, all I can say is that it happens. Try searching on "bandpass filter" and "IIR". In Altavista the syntax for the search would be +"bandpass filter" +IIR -- I don't know how you could coerce Google to find what you want. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On 5/27/2010 2:14 PM, BobG wrote:
> Whats the rule of thumb for how far to detune the two outside filters > for third octave bandwidth? Is the middle filter supposed to have a > little less gain to flatten the passband out? Is the Q of all 3 > filters the same? Do the skirts roll off at 18dB per octave? (I > searched for 'triple tuned' but no hits, so I hope I dont get flamed > for asking)
Do you perhaps have analog filters in mind? The design procedures for digital filters don't usually go in that direction. I remember doing the math for stagger-tuned IF strips in the early 1050s. It wasn't all that hard, but I don't remember off hand. There's an old paper in the Proceedings of the IRE. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
On 05/27/2010 12:25 PM, Jerry Avins wrote:
> On 5/27/2010 2:14 PM, BobG wrote: >> Whats the rule of thumb for how far to detune the two outside filters >> for third octave bandwidth? Is the middle filter supposed to have a >> little less gain to flatten the passband out? Is the Q of all 3 >> filters the same? Do the skirts roll off at 18dB per octave? (I >> searched for 'triple tuned' but no hits, so I hope I dont get flamed >> for asking) > > Do you perhaps have analog filters in mind? The design procedures for > digital filters don't usually go in that direction. > > I remember doing the math for stagger-tuned IF strips in the early > 1050s. It wasn't all that hard, but I don't remember off hand. There's > an old paper in the Proceedings of the IRE.
Whoa! Jerry, I knew you were old, but I didn't realize you were _that_ old. For that matter, I didn't realize that they were building IF strips in the middle ages. Were you in England for the Battle of Hastings? What was it like? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On 27 Mai, 21:25, Jerry Avins <j...@ieee.org> wrote:

> I remember doing the math for stagger-tuned IF strips in the early > 1050s.
Er...? Rune
Tim Wescott <tim@seemywebsite.now> wrote:
> On 05/27/2010 11:14 AM, BobG wrote: >> Whats the rule of thumb for how far to detune the two outside filters >> for third octave bandwidth?
(snip)
> It sounds like you're trying to make a filter in 2010 using design rules > from 1940. There's nothing inherently bad in this, but there are > certainly faster ways.
When I bought my first FM tuner 30 years ago (which I still have, though don't use so much), I was told that I would have to retune the IF every some number of years. Of course I never did that, though I did buy the service manual for it. Does anyone tune the IF section of FM tuners to correct for any drift? (Especially after 30 years.) -- glen
Jerry Avins <jya@ieee.org> wrote:
(snip)
 
> Do you perhaps have analog filters in mind? The design procedures for > digital filters don't usually go in that direction.
There not being a comp.asp newsgroup, it seems that analog questions end up here. When I was in high school I had my grandfather's Heathkit vacuum tube amplifier and tuner to play with. I have written about them before, the preamp that is powered by the amplifier through an octal socket connector. Among other features, there are about four choices for phono equalization, though I was reminded that RIAA was always the right one. (I never tried to find any really old records.) -- glen
glen herrmannsfeldt  <gah@ugcs.caltech.edu> wrote:

>Does anyone tune the IF section of FM tuners to correct >for any drift? (Especially after 30 years.)
Sure. Tune precisely to a strong RF signal, and trim the little IF filters until you see a maximum at their output on a scope. I haven't done this for maybe 40 years but that doesn't mean it isn't still done.... S.
On 5/27/2010 3:57 PM, Steve Pope wrote:
> glen herrmannsfeldt<gah@ugcs.caltech.edu> wrote: > >> Does anyone tune the IF section of FM tuners to correct >> for any drift? (Especially after 30 years.) > > Sure. Tune precisely to a strong RF signal, and trim the > little IF filters until you see a maximum at their output > on a scope.
That gets you maximum sensitivity at the expense of distortion at higher volumes. Low distortion is achieved by stagger tuning to broaden the response. This is best done with a sweep generator, but if one is good at it (I was once) it can be done fairly well by ear.
> I haven't done this for maybe 40 years but that doesn't > mean it isn't still done....
It's been longer than that for me. Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
On 5/27/2010 3:29 PM, Tim Wescott wrote:
> On 05/27/2010 12:25 PM, Jerry Avins wrote: >> On 5/27/2010 2:14 PM, BobG wrote: >>> Whats the rule of thumb for how far to detune the two outside filters >>> for third octave bandwidth? Is the middle filter supposed to have a >>> little less gain to flatten the passband out? Is the Q of all 3 >>> filters the same? Do the skirts roll off at 18dB per octave? (I >>> searched for 'triple tuned' but no hits, so I hope I dont get flamed >>> for asking) >> >> Do you perhaps have analog filters in mind? The design procedures for >> digital filters don't usually go in that direction. >> >> I remember doing the math for stagger-tuned IF strips in the early >> 1050s. It wasn't all that hard, but I don't remember off hand. There's >> an old paper in the Proceedings of the IRE. > > Whoa! Jerry, I knew you were old, but I didn't realize you were _that_ > old. For that matter, I didn't realize that they were building IF strips > in the middle ages. > > Were you in England for the Battle of Hastings? What was it like?
Oops! Jerry -- Engineering is the art of making what you want from things you can get. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;