LC notch filter not working!
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Posted by on April 26, 2007, 5:32 am
Please log in for more thread options Hi, I am trying to filter out a 13.56 MHz signal (and if possible I would want to filter some of its harmonics in succeeding circuits). I have tried a LC parallel resonance circuit put in series with the load. In theory the impedance of the LC parallel circuit becomes infinite at resonance frequency, i.e. the circuit becomes open. I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to get the product L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however, due to +/-20% tolerances in L, I use a trimmer. However, I can turn the trimmer (in the range from 10 to 20pF) as much as I want and I don't see ANY effect at all on my scope. Any hints ? What am I missing ? I have also looked at active notch filters, but this seems to be rather difficult at these high frequencies (see http://focus.ti.com/lit/an/slyt235/slyt235.pdf). Thank you!! | |||||||||||||||||||||||||
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Posted by Anthony Fremont on April 26, 2007, 8:51 am
Please log in for more thread options mrhyde377@googlemail.com wrote: Please learn to cross-post, it makes everyone's life easier when you want to cover the topic in more than one group. What kind of termination do you have on the filter? | |||||||||||||||||||||||||
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Posted by Michael Black on April 26, 2007, 9:51 am
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"Anthony Fremont" (spam-not@nowhere.com) writes: > mrhyde377@googlemail.com wrote:
>> Hi,
>> >> I am trying to filter out a 13.56 MHz signal (and if possible I would >> want to filter some >> of its harmonics in succeeding circuits). >> >> I have tried a LC parallel resonance circuit put in series with the >> load. In theory >> the impedance of the LC parallel circuit becomes infinite at resonance >> frequency, i.e. >> the circuit becomes open. >> >> I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to >> get the product >> L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however, >> due to +/-20% >> tolerances in L, I use a trimmer. >> >> However, I can turn the trimmer (in the range from 10 to 20pF) as much >> as I want and >> I don't see ANY effect at all on my scope. >> >> Any hints ? What am I missing ? >> >> I have also looked at active notch filters, but this seems to be >> rather difficult at these >> high frequencies (see http://focus.ti.com/lit/an/slyt235/slyt235.pdf). >
Well no.
> Please learn to cross-post, it makes everyone's life easier when you want to > cover the topic in more than one group. > Learn to figure out the most appropriate newsgroup, and post there. There is rarely any good reason to cross-post, it is usually the sign of someone too lazy to find the best place, or too clueless. Just because cross-posting is better than multiple posting does not make cross-posting a good thing. It's just better than multiple posting. And multiple posting is just as much a mark of laziness or cluelessness as cross-posting. This is a .basic question, not a .design question (I assume that's where else the guy posted to, you and the rest have made it such a cesspool with your cross-posting and non-beginner questions in there that I have stopped reading it). ANd once again, Mark Zenier's guide to the sci.electronics.* hierarchy explains, including a bit about how cross-posting is better than multiple-posting, but not by much: ftp://ftp.eskimo.com/u/m/mzenier/seguide9706.txt Michael | |||||||||||||||||||||||||
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Posted by Anthony Fremont on April 26, 2007, 11:20 am
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Michael Black wrote: > "Anthony Fremont" (spam-not@nowhere.com) writes:
>> Please learn to cross-post, it makes everyone's life easier when you
>> want to cover the topic in more than one group. >> > Well no.
> > Learn to figure out the most appropriate newsgroup, and post there. > There is rarely any good reason to cross-post, it is usually the sign > of someone too lazy to find the best place, or too clueless. > > Just because cross-posting is better than multiple posting does not > make cross-posting a good thing. It's just better than multiple > posting. > > And multiple posting is just as much a mark of laziness or > cluelessness as cross-posting. Oh gawd here we go again. :-( Are you a person or a bot? Look, I don't care what your opinion on this matter is. I don't know how to make it any more clear Michael, but GFY comes to mind. Do you have anything TECHNICAL *OR* ON TOPIC to add to this thread? > This is a .basic question, not a .design question (I assume that's
> where else the guy posted to, you and the rest have made it such a > cesspool with your cross-posting and non-beginner questions in there > that I have stopped reading it). I really hadn't noticed you missing, but then you don't offer much in the way of technical stuff anyway. The cesspool that exists there now is primarily a result of trolls and netkops like yourself, not from people x-posting electronics questions. So you consider LC filters and reactive impedance a .basics thing huh? If so, why don't you add something since we're here? > ANd once again, Mark Zenier's guide to the sci.electronics.* hierarchy
> explains, including a bit about how cross-posting is better than > multiple-posting, but not by much: > ftp://ftp.eskimo.com/u/m/mzenier/seguide9706.txt Have you actually bothered to read it yet? I certainly have. Building/debugging LC filters *do* constitute design AND x-posting is ok in this context, should I post the appropriate excerpts? After seeing MZ himself tell you to give it a rest, I figured you would stop with this. I guess I figured wrong. You're not going to change my beliefs on this matter, so can't you just PLEASE give it a rest? OTOH if I'm still not making myself clear, I'll be happy to choose verbiage more to the point. | |||||||||||||||||||||||||
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Posted by John Fields on April 26, 2007, 11:39 am
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On 26 Apr 2007 02:32:34 -0700, mrhyde377@googlemail.com wrote: >Hi,
> >I am trying to filter out a 13.56 MHz signal (and if possible I would >want to filter some >of its harmonics in succeeding circuits). > >I have tried a LC parallel resonance circuit put in series with the >load. In theory >the impedance of the LC parallel circuit becomes infinite at resonance >frequency, i.e. >the circuit becomes open. > >I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to >get the product >L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however, >due to +/-20% >tolerances in L, I use a trimmer. > >However, I can turn the trimmer (in the range from 10 to 20pF) as much >as I want and >I don't see ANY effect at all on my scope. > >Any hints ? What am I missing ? Try this: (View in Courier) VIN>-----+---->VOUT
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[L] | [C] | GND>-----+---->GND
-- JF | |||||||||||||||||||||||||
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>
> I am trying to filter out a 13.56 MHz signal (and if possible I would
> want to filter some
> of its harmonics in succeeding circuits).
>
> I have tried a LC parallel resonance circuit put in series with the
> load. In theory
> the impedance of the LC parallel circuit becomes infinite at resonance
> frequency, i.e.
> the circuit becomes open.
>
> I used a fixed inductance L=10uH and a variable C, i.e. a trimmer to
> get the product
> L*C = 1 / (2*pi*13.56MHz)^2 right. C should be approx 14 pF, however,
> due to +/-20%
> tolerances in L, I use a trimmer.
>
> However, I can turn the trimmer (in the range from 10 to 20pF) as much
> as I want and
> I don't see ANY effect at all on my scope.
>
> Any hints ? What am I missing ?
>
> I have also looked at active notch filters, but this seems to be
> rather difficult at these
> high frequencies (see http://focus.ti.com/lit/an/slyt235/slyt235.pdf).