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Designing LC Ladders

So I need to do some wideband low RF filtering (flat to 0.5 dB from

40-100 MHz, fairly sharp cut off on the high end, low end can cut off or not as convenient), and have found that my lossless LC ladders are in fact anything but. Who knew that several ohms of resistance in my 0603 inductors would add up, other of course than anyone who had stopped to think about it.

I've managed to massage the filters back into shape through trial and error in RFSim99, but in the process have beaten my nice little Butterworth and Chebyshev filters into unrecognizable scattered heaps of very sore poles, making irrelevant decades of filter theory. It strikes me that there must have been an easier way of doing it.

So, for future reference, how should one go about doing wideband RF filtering? I've got books here with plenty of suggestions if I felt like blowing ten thousand dollars designing an analog ASIC, but I'd like a solution with a few less zeros, not to mention not having to learn analog ASIC design.

-- Rob

Reply to
Rob Gaddi
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I think you were on the right track. Karl Willy Wagner dealt with some of the same issues in his very classic paper "Kettenleiter und Wellensiebe" in 1919 (_Arkiv für Elektroteknik_ was the journal that I recall, though I should verify). There are also some good solid-state monolithic filters today with some flexibility.

Reply to
Max

My previous reply on this will teach me not to rely on memory. Wagner's 1919 paper is "Spulen und Kondensatorleitungen." _Arkiv für Elektrotechnik,_ Band 8, Heft 2 u. 3 (July 1919). His paper "Kettenleiter und Wellensiebe" appeared in _Elektrische Nachrichten-Technik,_ Band 5, Heft 1 (January 1928).

Reply to
Max

I read in sci.electronics.design that Rob Gaddi wrote (in ) about 'Designing LC Ladders', on Wed, 5 Jan 2005:

Are you using separate low-pass and high-pass filters with a buffer between them, rather than a band-pass? There are formulae for filters with lossy inductors in the classic books, Zverev and Williams et al.

I suggest you stick to Butterworth; Chebs are much more sensitive to component values.

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Reply to
John Woodgate

Besides that, component variations will kill your filter as wel. I recently designed a low pass filter in about the same frequency region using an active sallen-key second order filter. To make it roll off more sharply I added an LC series snubber.

The topic is more or less covered on page 3 of this document:

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Reply to
Nico Coesel

Hi Rob,

It depends on what the words "fairly sharp" mean. If you have to be down to -30dB or more at 110MHz it'll be quite a challenge. Then those tiny

0603 inductors won't cut it. I'd start with 'real' inductors.

Also, depending on band rejection outside your range you may have to make compartments or at least spread it out a bit. Another question would be whether this is a one-off or something for mass production and what the cost and other constraints are.

One trick that may seem a bit obscure but has worked for me in the past is to up-convert the whole chebang to above a GHz where you then can design a regular filter, abandoning the lowpass-highpass combo and using resonant filtering instead. Then just convert back down with the same LO. Make sure to suppress the unwanted sideband. Who knows, maybe you could even convert to some range where you can buy commercial filters that happen to fit a 60MHz bandwidth. If you want to scope out this route check Murata's 'Gigafil' series of dielectric filters. I believe there was one that has 60MHz bandwidth. In case you go for it be prepared for some really suspicious looks from the other engineers in the design review.

With respect to mixer chips for such a task that has become much easier these days, thanks to the bargain pricing going on in the cell phone industry. Same for filters.

Regards, Joerg

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Reply to
Joerg

In this particular application, you could just cascade simple op-amp filters. You could use a spreadsheet as a design aid to let you try different R and C values (including tolerance) and different numbers of stages. The more stages you use, the farther up you have to move the 3 dB point to satisfy your 0.5 dB at 100 MHz spec.

If I remember my schooling correctly, with 4 stages, you get 80 dB per decade of rolloff. Of course, the op-amp is probably running out of steam at that frequency, anyway, so you may get even more rolloff.

--Mac

Reply to
Mac

I use ELSIE for filter synthesis. The limited free version is OK for my purposes, but the full version isn't expensive.

Leon

Reply to
Leon Heller

Depending on which set of components I have soldered down at any given time, I either have just strict low-pass 6th order Butterworth or a low-pass 5th order Chebyshev with a couple big DC blocking caps that put a zero at about 5 MHz. What goes on on the low end is fairly unimportant, I'm just trying to knock out the D/A harmonics from the direct digital synthesis chips generating the RF.

Went to the local university library, blew the dust off of Zverev. Lots of good information on Q pre-distortion. Useful stuff to know about; makes me wish my undergraduate years hadn't been spent after the prematurely announced death of analog.

Reply to
Rob Gaddi

Did some poking around on Coilcraft's site. Yep, lo and behold a little less stinginess on the footprints could have doubled or better my sorry Q of 40. Yeah, fairly sharp in that I'm trying to keep that whole band while putting at least 20dB of hurt on the 3rd order harmonics that start at 120 MHz. Hence I was using 6 pole filters, and the parasitics added up.

Ideas like that make me wish I had budget enough to design with honest-to-god impedance controlled boards.

Reply to
Rob Gaddi

Tony (remove the "_" to reply by email)

Reply to
Tony

Hi Rob,

As an experiment, try to start with a not so aggressive filter, more in the direction of Butterworth rather than Chebychev. Then place a few notches (resonant circuits) above your passband to steepen the slope there so you get to the 20dB.

Actually, the final circuitry was remarkably cheap. Much cheaper than low tolerance inductors and all that labor involved with tuning. What really blew the others away was that it also ended up being smaller in real estate.

With respect to impedance control you can get that with plain old FR4. Maybe not like on a hybrid or a teflon board but often it will be good enough.

Regards, Joerg

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Reply to
Joerg

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