LC ladder filter questions

[John Larkin]

On Thu, 08 Apr 2004 09:40:38 -0500, Roy McCammon <rbmccammon@mmm.com>
wrote:

Tom Bruhns wrote:
Hi all,

I have an LC filter, ladder topology. I wish to change the source
resistance from which it is driven from the original finite value to a
different finite value, but keep the poles and zeros in the same
places by changing the values of the ladder components appropriately.
The load resistance at the output is different from either the
origianl or the new driving-source resistance. Is there some
mechanical way to arrive at the required component value changes?

there is another easy way, get your employer to buy
a copy of FILSYN from ALK engineering

http://www.alkeng.com/

Ooh, dongles. I hate dongles.

John

 

[Roy McCammon]

John Larkin wrote:

On Thu, 08 Apr 2004 09:40:38 -0500, Roy McCammon <rbmccammon@mmm.com
wrote:


Tom Bruhns wrote:

Hi all,

I have an LC filter, ladder topology. I wish to change the source
resistance from which it is driven from the original finite value to a
different finite value, but keep the poles and zeros in the same
places by changing the values of the ladder components appropriately.
The load resistance at the output is different from either the
origianl or the new driving-source resistance. Is there some
mechanical way to arrive at the required component value changes?

there is another easy way, get your employer to buy
a copy of FILSYN from ALK engineering

http://www.alkeng.com/



Ooh, dongles. I hate dongles.

John

I hate dongles too.

But I've used FILSYN and find that it works well.

--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

On Wed, 07 Apr 2004 15:38:28 -0500, Roy McCammon <rbmccammon@mmm.com> wrote:

Tom Bruhns wrote:
Hi all,

I have an LC filter, ladder topology. I wish to change the source
resistance from which it is driven from the original finite value to a
different finite value, but keep the poles and zeros in the same
places by changing the values of the ladder components appropriately.
The load resistance at the output is different from either the
origianl or the new driving-source resistance. Is there some
mechanical way to arrive at the required component value changes?

I seem to recall there is an easy method if the filter is
symmetric about the middle and the termination's are equal,
you can use Bartlett's Bisection Theorem. If you can't
find it, I can fax the info. A little to complex to
describe here, but easy once you see it.

Yep. Bartlett's Bisection works ok. Tutorial is here:

http://members.tripod.com/michaelgellis/bartlett.html

 

[Peter O. Brackett]

John:

[snip]
\> OK, we need at least one goofy idea per day:

An active filter is easy to design, since you can just plop down the
poles and zeroes. LC filters are nasty, because all the sections
interact. So how about chopping up an LC filter into managable chunks,
and isolate them with buffers? That would bridge the gap between
active filter territory (10s of MHz maybe) and the range where the
buffers get wimpy, 1 GHz maybe.

You could call it a half-passive filter.

John
[snip]

Besides the high frequency limitations of active RC filters....

When you do what you suggest with active filters the sensitivities to the
passive elements and to the active gain
elements go *way* up.

Your approach is fine for hand tuned hobby filters or for low precision
non-demanding
applications.

But for precision high volume applications it just won't work very well...
yeilds will
kill you.

Nothing has been found yet that can beat passive LC ladders for low
sensitivity high frequency filters!

--
Peter
Freelance Consultant
Signal Processing and Analog Electronics
Indialantic By-the-Sea, FL

 

[Peter O. Brackett]

Roy et al:

[snip]

But I've used FILSYN and find that it works well.

[snip]

Good recommendation.

FILSYN was written by "one of the best:, George Szentirmai, after he left
Bell Labs.

FILSYN is probably "the best" merchant market filter design program
available today. It is
industrial strength and aimed at "big gun" problems, for professional users.

There are quite a few "down market" programs available, both free and for
$$$ which lack versatility.

You might try to obtain a copy of FilTor from the University of Toronto
which is a professional quality
filter design code in the same rank with FILSYN which was available for
free. FilTor was developed by
myself and an army of graduate students over several years and used to teach
graduate level filter
design courses at University of Toronto and several other Universities.

There are lots of example problems in:

Adel S. Sedra and Peter O. Brackett, "Filter Theory and Design", Matrix
Publishers, Champaign, IL 1978

that are solved using FilTor. Some of the internal workings of FilTor are
outlined in that book as well.

There are a few even better and more versatile codes out there but they are
in private hands [consultants]
and not available for general use by the public.

--
Peter
Freelance Professional Consultant
Signal Processing and Analog Electronics
Indialantic By-the-Sea, FL

 

[John Larkin]

On Fri, 09 Apr 2004 01:57:17 GMT, "Peter O. Brackett"
<no_such_address@ix.netcom.com> wrote:

John:

[snip]
\> OK, we need at least one goofy idea per day:

An active filter is easy to design, since you can just plop down the
poles and zeroes. LC filters are nasty, because all the sections
interact. So how about chopping up an LC filter into managable chunks,
and isolate them with buffers? That would bridge the gap between
active filter territory (10s of MHz maybe) and the range where the
buffers get wimpy, 1 GHz maybe.

You could call it a half-passive filter.

John
[snip]

Besides the high frequency limitations of active RC filters....

When you do what you suggest with active filters the sensitivities to the
passive elements and to the active gain
elements go *way* up.

Agreed so far. But I'm not really proposing an "active" filter as
such.

Your approach is fine for hand tuned hobby filters or for low precision
non-demanding
applications.

Why? I can get an opamp that has beautifully clean response as a
buffer, out to 2.4 GHz.

(And I'm not a hobbyist: I'm in the high-precision picosecond
business.)

But for precision high volume applications it just won't work very well...
yeilds will
kill you.

Why? The filter response will be independent of buffer behavior as
long as we stay safely below the buffer rolloff frequency.

Nothing has been found yet that can beat passive LC ladders for low
sensitivity high frequency filters!

Saws can!

I am suggesting using LCs as the filter elements, but with a few
essentially transparent buffers breaking up sections (and providing
distributed gain, if we want it.) Once sections are unilateralized,
the filter design becomes much simpler, and I'd guess that
sensitivities would be much lower. A multistage LC filter often has
awkward component ratios, but the sectionalized thing I'm suggesting
would have much reduced interaction, so each little clump can be
whatever impedance that's handy, and use a more practical set of
values.

John

 

[Tom Bruhns]

Roy McCammon <rbmccammon@mmm.com> wrote in message news:<407563E4.70001@mmm.com>...
....

there is another easy way, get your employer to buy
a copy of FILSYN from ALK engineering

http://www.alkeng.com/

Thanks again to Roy, Peter, and the many others who have posted
suggestions.

I'm not sure exactly what you mean by "easy way"--I'd a lot rather
work the whole thing out symbolically on paper than try to get that
much money right now. ;-)

What I've found is that I can indeed use Mathcad to find the pole and
zero locations of the original filter from its specific part values,
and I do have design software to synthesize the needed filter from
that. I also found that although I can quite easily enter symbolic
node equations for the system, including coil Q modelling, into
Mathcad, it chokes on about the third iteration of substituting from
one back into the next on the way to getting the output in terms of
only the input--there are just too many terms for it to deal with.
There are several other numerical (generally iterative) methods that
occur to me that I could also use.

I've been surprised how much discussion this has received. I hope
others have learned from it, as I have. I can at least be pretty sure
that there's not a trivial way to do what I wanted.

Something I think some readers missed from my original posting that
I'd like to clarify: I have an existing circuit in which the overall
filter function is divided between two sections, separated by a buffer
amplifier. That overall filter is (probably) a standard elliptic
design, but its poles and zeros are split between the two sections.
My task has been to redesign the buffer stage because of a parts
obsolsecence, and I've found that I get significantly better
performance in that stage if I run the output amplifier at a lower
gain into a lower load resistance. That's why I want to change the
filter input resistance. I do NOT want to muck about with the output
load resistance--if anything, I'd want to raise it a bit to pick up
some gain, but the best would be to leave it alone. Anyway, scaling
the whole filter to a different impedance would be trivial for me to
do, and I'd never have posted a question about it. And designing a
new filter, either a standard elliptical or one with custom pole and
zero locations, with my desired source and load impedances, would not
have been much of a problem.

Cheers,
Tom

 

[Tom Bruhns]

maxfoo <maxfooHeadFromButt@punkass.com> wrote in message news:<dl69701qd4e8remob0nqte19oekcjpmvi1@4ax.com>...

On 7 Apr 2004 10:26:23 -0700, k7itm@aol.com (Tom Bruhns) wrote:

Yep. It's just messy. I was hoping the mess I can get in a
straightforward but tedious way would reduce to some elegant form
that's well-known to all but me. Seems to not be the case. It's
probably time for me stop avoiding it and just dive in and do it.

Cheers,
Tom

Do a search for the free proggy rfsim99.exe, has everything you need and then
some.

Well, not quite, at least not easily. RFSim99 is one I've used for
some time now, and it's nice and I often recommend it to others. But
it doesn't have any tools to really help doing just what I want to do.

In fact, in trying some things with this filter in RFSim99 before I
posted my original question, I discovered that the transformer
component assumes both non-dotted ends are gounded...not very nice for
trying to create a center-tapped transformer! It's probably worth
another thread to start listing all the things in it that don't work
quite right. There are definitely more. And I say that because I
think it's a program worth having...it's just worth more if you know
its quirks before you stub your toes on them.

Cheers,
Tom

 

[Tony Williams]

In article <3200347.0404082316.3fa08d7a@posting.google.com>,
Tom Bruhns <k7itm@aol.com> wrote:

I've been surprised how much discussion this has received. I
hope others have learned from it, as I have.

Good thread to read, and learn something about a
totally unfamiliar area. Thanks.

--
Tony Williams.

 

[Roger Gt]

X-No-Archive: yes
"Tony Williams" wrote
: Tom Bruhns wrote
:
: > I've been surprised how much discussion this has received. I
: > hope others have learned from it, as I have.
:
: Good thread to read, and learn something about a
: totally unfamiliar area. Thanks.
: Tony Williams.

So have they stopped teaching about Filters in the schools?

Never heard of a Carr-Carr, elliptic function, Bessel, Chebyshev,
or any of the many other types? There are so many, both active and
passive. Mechanical filters, Comb filters, Notch filters, Roofing
filters, IF filters, Ladder filters, or the DCA specification?
The list goes on.

WoW!

I guess a phase corrector or delay equalizer would be really be
confusing, right?

 

[John Larkin]

On Fri, 09 Apr 2004 15:18:54 GMT, "Roger Gt" <not@here.net> wrote:

X-No-Archive: yes
"Tony Williams" wrote
: Tom Bruhns wrote
:
: > I've been surprised how much discussion this has received. I
: > hope others have learned from it, as I have.
:
: Good thread to read, and learn something about a
: totally unfamiliar area. Thanks.
: Tony Williams.

So have they stopped teaching about Filters in the schools?

I hate to shock you [1], but lots of EE degree programs now offer
electromagnetics as an elective.

John

[1] pun potential[1] noted

 

[Roy McCammon]

Tom Bruhns wrote:

Roy McCammon <rbmccammon@mmm.com> wrote in message news:<407563E4.70001@mmm.com>...
...

there is another easy way, get your employer to buy
a copy of FILSYN from ALK engineering

http://www.alkeng.com/


Thanks again to Roy, Peter, and the many others who have posted
suggestions.

so, will Bartlett's Bisection method work for you?



--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

[Roy McCammon]

Peter O. Brackett wrote:

Roy et al:

[snip]

But I've used FILSYN and find that it works well.


[snip]

Good recommendation.

FILSYN was written by "one of the best:, George Szentirmai, after he left
Bell Labs.

FILSYN is probably "the best" merchant market filter design program
available today. It is
industrial strength and aimed at "big gun" problems, for professional users.

FYI

S/FILSYN (Includes Passive / Microwave, Digital and Active synthesis)
First key ........... $1795.00
Each additional key . $1495.00



--
local optimization seldom leads to global optimization

my e-mail address is: <my first name> <my last name> AT mmm DOT com

 

[Roger Gt]

X-No-Archive: yes
"John Larkin" wrote
: "Roger Gt" wrote:
: >"Tony Williams" wrote
: >: Tom Bruhns wrote
: >:
: >: > I've been surprised how much discussion this has received.
I
: >: > hope others have learned from it, as I have.
: >:
: >: Good thread to read, and learn something about a
: >: totally unfamiliar area. Thanks.
: >: Tony Williams.
: >
: >So have they stopped teaching about Filters in the schools?
:
: I hate to shock you [1], but lots of EE degree programs now
offer
: electromagnetics as an elective.
: John
: [1] pun potential[1] noted

The possibility that Engineering schools have dumbed down that
much is NOT Berry Puny at all!
:

 

[Roger Gt]

X-No-Archive: yes
"John Larkin" wrote
: "Roger Gt" wrote
: >"John Larkin" wrote
: >: "Roger Gt" wrote
: >: >"Tony Williams" wrote
: >: >: Tom Bruhns wrote
: >: >:
: >: >: > I've been surprised how much discussion this has
received.
: >: >: > hope others have learned from it, as I have.
: >: >: Good thread to read, and learn something about a
: >: >: totally unfamiliar area. Thanks.
: >: >: Tony Williams.
: >: >So have they stopped teaching about Filters in the schools?
: >: I hate to shock you [1], but lots of EE degree programs now
: >offer
: >: electromagnetics as an elective. John
: >: [1] pun potential[1] noted
: >
: >The possibility that Engineering schools have dumbed down that
: >much is NOT Berry Puny at all!
:
: Sounds like you must be very insulated from what's really
current.

Graduated about 40 years ago! Wondered about the dumb bells I
assigned to me for work on many projects. Didn't go to the
schools to check it out. Latest was an EE who couldn't bread
board or test an App-note circuit and got the grounding all wrong
so it oscillated, before that a guy with a MS in telephony didn't
know the difference between single ended and balanced circuits,
and couldn't calculate a voltage divider to have a required source
impedance. Didn't understand why we put low pass filters in our
cable drivers, wanting to save the 0.32 per line in favor of many
pounds of shielding (on a plane) But I just caned him and got an
old timer to do the work so I didn't have to teach him. Hell, I
only wish I had had the time and opportunity to get a MSEE! I
just keep studying! I learned how and now I can't get out of the
habit!

Back to my study's See ya!

 

[Peter O. Brackett]

John:

[snip]

the filter design becomes much simpler, and I'd guess that
sensitivities would be much lower. A multistage LC filter often has
awkward component ratios, but the sectionalized thing I'm suggesting
would have much reduced interaction, so each little clump can be
whatever impedance that's handy, and use a more practical set of
values.

John
[snip]

Your guess would be incorrect!



--
Peter

 

[Tony Williams]

In article <y9zdc.34938$ML1.33829@newssvr29.news.prodigy.com>,
Roger Gt <not@here.net> wrote:

So have they stopped teaching about Filters in the schools?

Never heard of a Carr-Carr, elliptic function, Bessel, Chebyshev,
or any of the many other types? There are so many, both active and
passive. Mechanical filters, Comb filters, Notch filters, Roofing
filters, IF filters, Ladder filters, or the DCA specification?
The list goes on.

WoW!

I guess a phase corrector or delay equalizer would be really be
confusing, right?

Probably.......If usenet has done anything it has
shown me that I have a lack of knowlege and little
experience in a number of areas of electronics.

Which is why threads such as this (with expertise
being shown) are so interesting to follow.

--
Tony Williams.

 

[Russell Shaw]

Peter O. Brackett wrote:

Tom:

Well in all of the "textbook" literaturethat I know of, including Zverev
and most others [BTW... I have most of of the best filter references in
my personal library.] the only one that I ever found who discussed, let
alone tabulated, pre-distorted design was/is Louis Weinberg.
...

Zverev has predistorted filter tables.

 

[John Woodgate]

I read in sci.electronics.design that John Larkin <jjlarkin@highlandSNIP
techTHISnologyPLEASE.com> wrote (in <931g701pd0f6un9t33qgh9l6bjjgs0lqeu@
4ax.com&gt about 'LC ladder filter questions', on Sat, 10 Apr 2004:

So why does the presence of the buffer increase component sensitivity?
If you can explain why it does, I'll be suitably embarassed. [1]


John

[1] actually not embarassed, just better informed. If every idea gets
slapped down for being imperfect, pretty soon people will stop having
ideas. Ideas are toys, fun to play with but fragile.

I can say why it *might*, but I don't think it's a certainty.

If component values in the section after the buffer vary, that variation
has its full effect on the filter response. But in the unbuffered case,
those variations affect the responses of the preceding sections, which
*might* provide a compensating effect.

Maybe it can be shown that it always does have such a welcome effect.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Larkin]

On Sat, 10 Apr 2004 17:58:30 GMT, maxfoo
<maxfooHeadFromButt@punkass.com> wrote:

On Sat, 10 Apr 2004 07:42:40 -0700, John Larkin
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:


OK, we have a low-order, third maybe, LC filter, which has nice
reasonable L and C values. We terminate it and run the signal through
an opamp buffer. The buffer has an input capacitance of 1 pF, an
output impedance of a few ohms, and it's flat to over 2 GHz. After the
buffer, we have another low-order LC filter. So we've made a
higher-order LC filter out of simpler sections, just like we do when
we make an active filter out of 2nd-order unilateral sections. Since
interactions are reduced, we have another degree of freedom in the
design.

So why does the presence of the buffer increase component sensitivity?
If you can explain why it does, I'll be suitably embarassed. [1]


John

What's that part number on the buffer op amp that has a flat response from
dc-2400mhz? I want to buy out the stock....

thanks,

THS4302.

Welcome.

John