Brushing up on theory: Butterworth LCR filter design?

Be aware that there are a lot of useful transforms you can use (tee to and from pi and Norton series/shunt being the most common ones) to "tame" unreasonable component values in filters. In fact, knowledge of these transforms is really the main difference between what you learn in college and what you need to know for many real-world applications... although the usual problem is that the component values get too small (e.g., 0.2pF or 0.8nH) than too large.

The whole topic of circuit synthesis (starting with poles and zeroes or similar) used to be a hot topic that was thoroughly covered in college some

30+ years ago, but as far as I can tell it pretty much became a completely "solved" problem -- at least for passive circuitry -- and therefore these days is largely ignored. The guys who pioneered the field were pretty sharp cookies -- guys like Guillemin and Van Valkenburg, and (for filters in particular) Zvevrev, etc.

Around here The Phantom seems to have rather more of the knowledge of these things than most people.

---Joel

of

pass

ting

and I

on H(s)

alues.

e some

othing

here,

o with

To be accurate here, I've only used those tables for SCF leapfrog designs. Since you can get phase inversion pretty easily in SCF, the leapfrog design works well. Designing them is really a great exercise in signal flow graph theory, especially if the filter has transmission zeros. Daniel Senderowich was consulting at Exar at the time, which is where I learned the techniques.

One of the funnier lines Dan used was "it is hard to move a ladder." The translation is ladder filters are very insensitive to component tolerance. The line he would use is "As I told you once before...". And yes, he did tell you once before, but I probably didn't get it at the time.

Yeah, but they're also challenging to debug if they're of high order, since everything is all coupled together... one of the things Gabor Temes always told us was, "Don't use a leapfrog topology for your first post-graduation filter design assignment in industry."

Of course, he was thinking of the kind you built out of discrete op-amps and R's and C's like he had done back working for the telephone industry in the '60s/'70s; these days if it works in SPICE the fab will guarantee it'll work on-chip, rgiht? :-)

ce

s n

and

he

ork

There isn't much SCF being done these days, but Switcap was what we used.Spice doesn't conserve charge.

I really don't see the logic in Temes statement. In SCF, it was an opamp per pole regardless of topology. If you can design a ladder, so much the better. The "localized Qs", a term I never liked, are lower with a ladder. That is, the response at individual nodes is less peaked in a ladder. This kept the component spread (cap ratio) low. We also used filtor (sic) out of University of Toronto.

What you were selling in the day was a very tightly controlled filter shape. The customer put up with THD and clock feedthrough. You had to have some selling advantage, and repeatability was what you sold.

One of the more annoying things was in the dark ages when Bell 212 filters were being sold for $25 a pop in NMOS technology, the THD plus noise was more significant than what came through the filter. However, for marketing purposes, you had to deliver the Bell 212 pole/zeros, even if you could do better. When we finally integrated the entire modem, i.e. nobody could see what the filters looked like, I yanked at least a third of the filters out. It was truly a waste of silicon. The lower Q meant less group delay compensation, so fewer allpass stages.

Often less is more if you look at the big picture.

of

bandpass

connecting

and I

function H(s)

values.

some

nothing

here,

with

Yep, me too. even have the TTL cookbook around. Don's a fine man.. He's got a few crazy and twisted ideas, but he's OK in my book (shelf). It wouldn't be quite as CooL of an electronics industry without him.

boB

of

bandpass

connecting

and I

function H(s)

values.

are some

nothing

here,

do with

I also have "a few crazy and twisted ideas", but no one cuts me any slack :-(

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

That's a startling concept. Can you elaborate?

John

Well crap, it is generally accepted that spice doesn't conserve charge, but this is an area of electronics where I have to plead ignorance on details. But you can set up examples and show errors in charge conservation. Charge a cap, slap it against another cap with an ideal switch, and watch the voltage settle. Spice cranks until tolerance parameters are met, so you never reach the ideal value. That is, charge transfer will always be lossy.

There is a blurb in Jim Williams book about this:

Switcap was designed using cutsets on the capacitive network, much like you create by hand when deriving network equations, then computing the ideal value after charge has transferred.

This is an area where my knowledge is just cursory. That is, I don't know much about circuit simulators beyond using them. Circuit simulators are a discipline all to themselves, as is device modeling. It is good to know the gotchas, but I'm not sure a designer has to know what is under the hood.

Another bug in spice is charge injection of mosfets. Apparently charge injection isn't modeled perfectly This has been the subject of many pappers. They either use new models that match reality better, or subcircuits that augment basic mosfets. Papers on cyclic converters often were obsessive about charge injection modeling error.

Didn't we address this years ago, USING MATH?

IIRC it is ALWAYS lossy...

Do the math, with a real Rsw, then let Rsw -> 0.

Yep ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

Hard-paralleling different-voltage caps is a classic circuit puzzler. Most newbies get it wrong, so it's sort of fitting that Spice gets it wrong, too. The problem is trying to conserve both charge and energy.

John

=A0 =A0 ...Jim Thompson

=A0 | =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

At least I admit my limitations.

The point is there are simulators that do conserve charge, but of source have other limitations. Spice is a tool with limitations. All software is riddled with bugs, but as long as you know what the bugs are, you can use the programs appropriately. For instance, the spice time step isn't really the time step unless you use the extra parameter on the .tran line. If you don't force a time step, spice lets events pick the time step for simulation, but then interpolates the the "display" time step. This often screws up THD analysis.

Oh, all chips have bugs too.

I think Berkeley spice started conserving charge in one of the later versions. I think Spectrum Software's Microcap VIII conserves charge also.

LTspice seems to conserve charge. Here is part of a post from msg 24309:

Files -> Temp -> varicap -> varicap.* shows that LTSpice variable capacitances DO conserve charge as the capacitance is changed. This is a rather important subtlety.

This might be worth investigating to confirm it is true. But in this day and age, I would be surprised if LTspice doesn't conserve charge. Mike is a very competent engineer.

Mike

A circuit with just ideal caps and switches is physically impossible, since closing a switch between different-voltage caps creates a singularity. Spice should throw an error and quit simulating, as it does for some other conditions.

I suspect that adding a little series resistance will make Spice come out awfully close.

John

[...]

The LTspice switch defaults to 1 ohm and zero inductance.

You can change these to suit.

Mike

I can't imagine how any simulator could be accurate and not conserve charge. It needn't handle physically impossible situations.

John

"it is generally accepted that spice doesn't conserve charge"...

only by leftist weenies ;-)

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
 I love to cook with wine     Sometimes I even put it in the food

It appears BJT's, diodes, and everything else conserved charge around version 3e5. MOSFETS are a problem. There are many versions of BSIM, for example. Some have no relation to the physics, others are accurate in one region but not another. Some have discontinuities as you cross boundaries between regions. Some are too slow. Some work with ony one vendor's spice.

MOSFET's are a real mess.

Mike