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LC notch filter not working!

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!!
 
A

Anthony Fremont

Jan 1, 1970
0
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).

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?
 
M

Michael Black

Jan 1, 1970
0
Anthony Fremont" ([email protected]) said:
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.

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
 
A

Anthony Fremont

Jan 1, 1970
0
Michael said:
"Anthony Fremont" ([email protected]) writes:
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.
 
J

John Fields

Jan 1, 1970
0
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
|
[L]
|
[C]
|
GND>-----+---->GND
 
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