The Larkin front end

Hi John Just a few questions from the ABSE post

what are the typical values of L4 and L5, I guess 100uH? do you ever use a common mode i/p filter?

martin

Reply to
martin griffith
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They are surface-mount ferrite beads, I think rated 300 ohms (at 100 MHz, which how they usually rate beads.) Beads are nice because the Q is so low, they pretty much never resonate.

I haven't ever use a common-mode filter for stuff like this. The intent was to just squash any RF, common mode or differential. Nowadays, cell phones and things are everywhere.

One of my guys has an older, low-band cell phone, and whenever he walks into my office my PC speakers make horrible raspberry noises.

John

Reply to
John Larkin

If only that were true. I had a ferrite bead in one of my designs that had an inductance of 1.6uH, and resonated nicely with a 100nF capacitor at 400kHz, with a Q of about 5.

I ended up having to add a wound 100uH SMD inductor in series, with 22R of damping resistance. Dead embarassing. The quick and dirty fix was

4u7F tantalum capacitor in parallel with the 100nF ceramic - the ESR of that tantalum did all the damping required, but the exta capacitance chewed up current whenever we intermittently activated the low-power system involved.

In my day it was the electron beam microfabricator that wrote the wrong stuff on a $2,000 mask if the security guy went down the corridor with his two-way radio. Fixable, eventually.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

Hi Bill

do you mean the 22R was in parallel to the inductor?

martin

Reply to
martin griffith

I didn't specify, but it was in series with both the inductors (who were also in series).

The whole thing was intended to keep the switching spikes from a comparator out of the the op amp that was providing one of the inputs to the comparator - always worth doing, and particularly so in this case.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

Hello John,

Might not be a problem with most SMT versions but it is with thru-hole: They can exhibit have mechanical resonance. Or in more popular speak a "rattle".

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Hey, there's something that'd never occurred to me - if the core is loose, it will actually become like a little motor, right? :-)

Thanks! Rich

Reply to
Rich Grise

Some years ago, the head of R&D for a big NMR company called me and asked if we wanted to make sample temperature controllers for them. They'd been using units from Oxford Instruments for years, and were having problems, one of which was astounding EMI sensitivity (the other problem being that they had apparently lost the Z80 source code, and couldn't fix bugs!)

I got one of the Oxfords, and found that I could shut it down from across the room with one of those old GR unit oscillators driving a banana lead antenna. There were multiple, very narrow bands in the

150-400 MHz range where it was extraordinarily sensitive. Turns out the sensitivity was hugely enhanced by internal wiring and PCB resonances. This is the range where beads work well.

We built a new controller with much better layout and bypassing, and it was about 20:1 less sensitive. When we ran the internal thermocouple leads (t/c connector to pcb) through a double-hole ferrite bead, we got another 10:1, presumable because it blocked entry and killed the Q of that path. We've shipped close to 3000 units by now.

John

Reply to
John Larkin

I wouldn't be surprised if, even with all this, your build cost is still less than The Big Guy's.

Congratulations! :-)

Reply to
Joel Kolstad

Actually, that is a minimal balun, and gives you good differential rejection at frequencies where it has a significant impedance.

If you want more turns, you can use a small ferrite torroid - if you confine the wires to a single layer and space them at about their own diameter, you are supposed get the maximum availalbe resonant frequency. I've not had occasion to try it yet, but the theory seemed to make sense.

Nice work.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

John Any good articles re EMI suppresion you can point us to? Which authors do you feel do a good job explaining the problem and solutions? (I'll have to remember ferrite beads next time I come across a narrow band sensitivity. Thanks for the tip) M Walter

Reply to
mark

are

previously

I don't understand this "matching" thing. If you lowpass filter both sides effectively, you're way better off, even if the lowpass filtering isn't exactly the same on both sides. The transistors in the front-end of an opamp are essentially square-law RF rectifiers. If you have 100 mv of RF, they will rectify it big-time. If you have 1 mv, they barely will at all. 100^2 = 10000, and 10000:1 is usually considered to be a pretty big improvement.

The really insidious RF is in the 100+ MHz range, where amp internal feedback and bias mechanisms can't track, and things go nonlinear... every junction drives stray capacitance to form a peak detector. Ferrite beads attenuate the RF and kill resonant Q's.

Wound inductors can be useful, too, but you've got to make sure they don't resonate anywhere dangerous, make sure their parasitic capacitance doesn't let hf stuff blow through, and make sure they're not hum detectors.

John

Reply to
John Larkin

Hello Rich,

Never seen one spin :-)

But often, upon diagnosing some EMI problem at a client, I found shards in the base of the system. It's amazing what a rough flight or a truck barreling along a washboard road can do to electronics. Goes back to the first law of Murphy: If there is anything that can break it will.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I did some testing on Kodak's shake "table" in Rochester. The had a simulation profile called "Truck".

We were warned not to stand on the "Table", it would shake your kidneys loose :-(

Particularly disturbing to me was watching the necks of CRT's with a strobe, and seeing them FLEXING!

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Hello Jim,

There are various grades when simulating "truck". There is the smooth glide along Interstate 80, then there is the not so smooth rumble from Shannon Airport to Ballinsloe, and then there is the driver who tries to find the "speed of least shaking" on a sastrugi track through the Sahara desert.

After finishing a hi-rel module they showed me the impact tables where it'll all be tested. Now I knew where that regular ka-bummmmph sound was coming from. They raised the unit several feet and then let it drop onto a concrete pad. Then again. And again. Every once in a while someone would pour another layer. The concrete slowly became a pillar that drove itself into the ground.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

I was told at Ford 40 years ago that our packaging had to stand a "boxcar" test.

Huh?

It was explained to me: Shove the shipping container off of a boxcar deck height onto the pavement below ;-) Talk about "ka-bummmmph"!

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

I'm going to have some 7,000G shock testing done real soon. That should be really interesting to watch. Not often does one get to play with explosive charges. (I'm not doing it: a contract lab so should be safe)

M Walter

Reply to
mark

Hello Mark,

Do you remember that ad in an electronics magazine (in the days when they still cut trees to be able to distribute them...) with a missile test gone wrong? Big fireball, stuff flying, arrow pointing to a little speck in all that flying debris: This is our XYZ supply, now involuntarily airborne on its own. Then a totally mangled piece of metal on a test bench: See, it still works!

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

We did some big engine-room control consoles for the Navy LHA ships. We had to send them up to a quarry pond in Vermont where they bolted them into a barge and then set off charges in the water. The high-speed film was impressive... indicator lenses flying off into the air and such. They used some sort of magnesium flares to illuminate the scene for the fast cameras, and that caught some of our stuff on fire, but they didn't count that as a failure.

John

Reply to
John Larkin

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