Sallen-Key input Z

All you need to do to convince me is to show another active filter topology that will maintain DC accuracy as well as a unity-gain S-K, while avoiding the oh-so-terrible 2 or 3 dB of gain peaking that we're actually talking about here.

I really am all ears--if there's actually a better way to do this, that meets the DC specs without wasting money on ultraprecise resistors, not only will I use it, I'll give you and miso the credit in print.

So, how about it?

Cheers

Phil Hobbs

In the early 1970's, I was working for Alpha Electronics in Stanton CA making CTCSS tone encoder/decoder boards that used a hybrid twin-T filter. 50ppm/C frequency stability at 100Hz from about 0C to 70C. We started with SS White abrasive air blasters and ended up with home made laser trimmers. Each had their own nighmares. However, it was cheap, moderate volume, and much better than hand selected 1% components. Later, we used the technology to make some of the first LED wrist watch hybrids and some odd telco stuff. Hybrids had their problems, but they were the right answer in the 1970's.

Ummm... slide rule accuracy perhaps? Nope. Most of the tables are 5 and 6 significant figures.

Yep. About 1/4th of the book is on LC filters. Chapter 8 is all on crystal filters. There's also a small section on microwave interdigital and cavity filters. Lots of different filters are covered.

The inside cover price marker shows that I paid $52 in 1969. That would be about $315 in todays inflated dollars. Ouch.

Conservation of energy, and not much else.

Hey, my HP35 cost $400 in about 1972, more than a modest motorcycle.

with

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Right now I can't solder any of this because of exactly that problem. Serious cold, lots of *ACHOOO* ... courtesy of flight UA1015 where almost everyone around me had a hacking cough.

Hybrids are easy >:-} ...Jim Thompson

DC was thrown into the mix when it wasn't on-topic. It's a red herring. If you want to use S-K, go ahead, but making higher-order filters is going to requires trimming.. ...Jim Thompson

But it certainly _is_ good engineering practice. I posted a version here that inherently has no internal peaking... but it apparently was too complex for anyone to analyze >:-} ...Jim Thompson

Sure, but not on a shoe string budget. Everything was surplus or home built. Of course, nobody knew what they were doing. The best part was that the building was adjacent to a railroad track. Whenever the train rolled past, the building would shake. I have some hybrids somewhere with have a perfect sine wave trim cut from the vibration. The solution was simple. We posted the train schedule and everyone would take a break until it passed. Technically, the most difficult part was getting the ink vendors to cease playing with the formulation. No sooner did we have something that worked, they would make some changes. Silvered mirrors on the CO2 laser were another mistake. The carbonic acid did a nice job of etching the mirrors. Plenty of other screwups such as storing volatiles in the same room as the oven. If there was a wrong way to do anything, we tried that first. In retrospect, hybrids were probably easier than monolithic, but when we were knee deep in problems, things didn't look quite so easy.

Hmmm... looks like LED watches are making a comeback:

"Good engineering practice" is invariably a euphemism for someone's irrational prejudices.

Dickson Electronics wasn't on a shoe-string budget. We were MIL-qualified and I was churning out bucket-loads of stuff for TOW, etc.

I discovered a similar problem... my assembly floor was pre-stressed concrete hung by the edges :-( As I tried to do finer and finer laser trims I had to move the laser to the basement... the assembly floor was constantly swaying :-(

Several nice ones there. I'm tiring of my expensive Omega that requires winding daily, or it stops. (I only "jewel-up" when I leave the office.) Time for a new Timex ;-)

Can anyone recall my long-ago post describing my easy, and very accurate way of trimming pole locations on hybrid filters? ...Jim Thompson

Nice to know you could prioritise ;)

Grant.

Things go easier with government money to lubricate the system.

Another of the fun problems with using a laser to trim resistors is the width of the kerf is rather narrow. Looking at a cross section, it looks like two vertical walls with very little space in between. Use too much power, and the top edges melt and short across the kerf. With square top edges, if we were sloppy in handling, the edges would crumble, fall into the kerf, and short across the kerf. When I widened the kerf, it took more laser power to ablate the resistor material, which sometimes caused the substrate to shatter. Of course, the resistor changes value as it cools down, which required slowing down the trim rate near the end of the cut.

I think I had a nightmare last night with a laser trimmer playing a key role.

Also, due to security clearance issues, I don't do MIL work.

For our twin-T hybrid filter, it rather simple. The twin-T consisted of four resistors, two of which were in parallel. Same with the corresponding capacitors. We bin sorted the capacitors to be as identical as possible. Using the bin capacitance value, we calculated what the corresponding resistance had to be in order to be on frequency. The laser trimmer was set for that value, and all four resistors were trimmed to the same value. The resistors were huge (about 5mm x 5mm) so we had plenty of room to play.

I had an HP9100, an HP35, a PDP-11, two motorcycles, and an Austin Healey Sprite. I guess I wasn't doing badly for a kid still in school.

It's no fun. FBI (and whomever else they decide) talks to all your neighbors, people you barely remember from your past, family, you name it. And it takes a while, too. Long while. Special access required (SAR) is painful.

Then at work you can walk in with stuff into a Faraday caged room, but you can't walk out with it. They smash anything electronic (and other things they feel might carry info) on the way out. No exceptions. Scary, too, gun-toting folks; places you are supposed to know not to walk around, yet no signs so, easy to make mistakes that could end your life in a moment. I made a few innocent mistakes during a lunch break, for example, and got shaken up plenty from it.

It paid well, but I won't do it again.

Jon

Warning: Topic Drift...

After college, I lusted for a calculator that could do square roots. I eventually bought a TI SR-10 for $100. Big mistake. Within 6 months, the list price was half. I still have it buried somewhere:

However, before buying the above mistake, I decided to build my own vision of where I thought computing was going. It was a slide rule emulated by an analog computah. It consisted of two 10 turn Helipots, a Philbrick(?) multiplier, several discrete log amps, several op amps, and a gigantic mirrored panel meter. There was also a rather awful AC power supply. Everything was crammed into a large brief case. It worked, but was not very accurate. The last mutation used Veeder-Root counters in place of the geared Helipots for additional accuracy. At the time, my vision of a digital computer was a mainframe, which I never suspected could be miniaturized to pocket size. Analog was the future, or so I thought.

Roll forward slighting into the early 1970's, and I found myself in the land mobile radio biz. At the time, the high tech mobile telephone occupied about half the trunk with a rather large control head. I decided that the world needed a portable mobile phone. I threw together my incantation of Secode selector and tube walkie talkie into yet another brief case and offered it to my customers, most of which were sales types. The problems were soon apparent. Mostly, they wanted to make calls while at their customers location. Why bother doing battle with the mobile operator, when they could just pickup the customers phone and make the call cheaper and easier? After several embarrassing failures, I gave up on that idea. (Many years later, the idea was raised from the dead in the form of the bag phone. It had the same problem. Few bag phones were ever used with the bag, but instead were installed in the vehicle).

Roll forward a few more years and I'm designing radios. Personal computahs had just arrived and I purchased a Don Lancaster TV Typewriter kit. I eventually made it work and decided that the world needed a portable data terminal. Once again, I built it in a brief case, and once again, nobody wanted it. The terminal worked well enough (at 16 lines of 64 chars), but nobody wanted to drag around a TV set in order to use it. I added a video modulator, but no takers. Offices just didn't want TV's on people's desks. Add the modem, and it was no longer very portable.

Looking for other opportunities, I decided that the world needed a portable 2-30Mhz marine SSB radio. At the time, anything decent was about the size of a laser printer and weighed about the same. I managed to cram something functional into a brief case and again ran into problems. The marine radio customers were accustomed to solid and heavy duty radios. A light weight brief case just wouldn't survive. It also didn't perform very well when wet. Worse, the antenna was never anywhere close to where a portable SSB radio was going to be operated (i.e. anywhere except the radio room).

There are many lessons to be learned from all this. However, the important one is to not build anything into a brief case. It won't sell.

I had an HP45 and a wife. At the time I thought the calculator was expensive. ;-)

Good grief! I've never had any hassles. But then I'm already "known by the FBI" ;-) ...Jim Thompson