resetting a filter

You really need a bunch of DPDT switches to swap segments in and out, or else a bunch of hi-z taps into a multiplexer. It really gets messy, electrically. Really.

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nd back to Q looks like a forward biased c-b current path?

No, it's because you are a sloppy tinkerer who rarely thinks about what he's doing.

More often than not, and you won't tell us about it if it doesn't.

Yes, it does bring on waves of nostalgia. Maybe I'll be able to get some work in Sydney, after we mover there in October ... my wife thinks that I'm nuts to even think about it, but she hasn't read the Sydney electronics jobs listings on the web, and wouldn't have been able to make much sense of them if she had.

So why didn't you plug it in? My "guess" was taken from the Spice model of the Wurth 742 792 092 ferrite bead which is what I use for lower frequency work. 3.6uH would be a bit big for your purposes - I wasn't excited enough to check a real 330nH inductor. Now that you have raised the ante I spent a minute finding one

and this one does come out at 0.25pF.

-- Bill Sloman, Nijmegen

If he objects to an RM core former, he's going to see two SMA connectors as huge. There are smaller coax connectors, but they are even more ruinously expensive, and can difficult to get hold of in small quantities. SMB and SMC are a bit cheaper and a bit smaller, and probably good enough for his frequencies, but not small enough nor cheap enough for this kind of application.

-- Bill Sloman, Nijmegen

yeah, fine at low frequencies, but that shorted segment is just a big capacitor hanging on there unless you also switch it out...

back to Q looks like a forward biased c-b current path?

You guessed 1 pF, which is off by about 5:1. 500% error is well within my definition of "sloppy."

Because it wouldn't affect things enough to matter. I didn't include the PCB parasitics, the capacitor ESR, the speed of light, or any of that stuff either. They wouldn't matter. I included the things that matter.

My "guess" was taken from the Spice

Why use a ferrite bead - which is complex - when high-Q real inductors are cheap? And why guess at all? Sloppy, sloppy.

I'll be using an 0603, with a bit higher SRF.

The SMAs will cost a couple of dollars each. The cable will cost $10 or so. The surface and volume requirements will be enormous. The surfmount parts will cost about 35 cents, if I use the expensive super-low-capacitance low-barrier Skyworks diode.

A bit of an acquired taste, I think, like appreciating the beauty of a flood in a candy factory.

Cheers

Phil Hobbs

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c and back to Q looks like a forward biased c-b current path?

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I was making a point about your idea of what constitutes a simulation, rather than saying anything about the circuit - I'd have used the numbers from a real part (if I could find them) if I were actually interested in the circuit.

The whole point about simulation to to cross-check your ideas of what things matter; leaving out the parallel capacitance of an inductor is never a good idea. Tagging stray capacitances to ground onto critical lines can be a good idea, but if you don't even bother plugging in inductor capacitance, it seems unlikely you'd ever be careful enough to work out the stray capacitance of a trace (actually microstrip line, but that's probably too high-falutin for you).

Because - to echo your comment - the exact value wasn't relevant to the point I was making, so a guess served the purpose. The choice of a ferrite bead over a high-Q real inductor reflects the fact that a high- Q resonance can be a real nuisance if it's at a frequency where the circuit has gain - I've spent too much time shoe-horning damping resistors into circuit to tame high frequency resonances (and inconvenient peaking) to be inclined to use a high-Q inductor unless I actually need to.

Whatever.

-- Bill Sloman, Nijmegen

back to Q looks like a forward biased c-b current path?

Did you factor in the special charge of the near by electron accelerator?

Jamie

Four wire-wrap posts and a few feet of twisted 30 gage wire makes more sense than coaxial (if you have true/inverted signals available, it won't even have much capacitive coupling to shield against). Volume requirement is an issue, and that's where the shlockmeisters start trying to sell you lumped-constant 'delay line' modules at high prices with long leadtimes.

back to Q looks like a forward biased c-b current path?

Yeah, what John said. You see, over here, Obama hasn't taken our freedom of selecting the type of circuit and components we want to use, yet.

Jamie.

saturate

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and back to Q looks like a forward biased c-b current path?

It's fine to leave it out when I know that it doesn't matter.

Tagging stray capacitances to ground onto critical

Moron. I design picosecond electronics that works. Microstrips, striplines, CPW, angles, vias, serpentines, pours. The vast majority of it works first time, rev A board, shippable and billable.

I recently posted my ATLC electromagnetic sims of an edge-launch SMA connector transitioning to microstrip, but I suppose you weren't interested enough to notice. The result was a nearly perfect TDR match, and gorgeous 42 ps output edges. People are buying the box now.

You've putzed around with one low frequency LC oscillator for years, and it still doesn't work. Your sim didn't seem to "tag stray capacitances to ground" anywhere.

So, now you sanction guessing. Thing is, you guessed wrong.

back to Q looks like a forward biased c-b current path?

The San Francisco Ballet School is just the other side of my wall. I can hear them practising. All that spinning around does frame-drag my more sensitive measurements.

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Until it turns out to matter in a slightly - but critically - different situation.

And I was doing that back in 1990. The stuff that didn't work first time didn't work because the drafting office "knew" that the order of the inner layers didn't matter, despite my having told them quite explicitly that it did in the release notes. We didn't get to ship the stuff, but it wasn't because it didn't work.

Back in 1990 we were pushing the state of the art. Twenty years later, you are doing what every Tom, Dick and Harry should be able to do, if they are moderately careful. Because you've now iterated through five fairly similar designs, you are confident that you can afford to cut corners, which isn't wise.

We were producing a 500psec wide beam blanking pulse back in 1985, when edge-launched SMA connectors were obscure items in manufacturer's catalogues which you couldn't buy because of some paranoid US export rule that basically said that if you weren't part of the US radar community you were a Russian stooge trying to buy stuff to put into Russian radars.

For a 16kHz oscillator?

Since I was pointing out that you had effectively guessed zero, which has to be wrong by an infinitely larger margin, you are hoist by your own petard.

-- Bill Sloman, Nijmegen

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Why 30 gauge? 42 gauge (SWG) enamelled transformer wire makes equally good (or bad) twisted pair, and is a whole lot less bulky - even with thick enamel each wire is less than 0.15mm thick. Anything thinner and I tend to break it when winding it, but professional transformer winders can do a lot better.

-- Bill Sloman, Nijmegen

The high frequency response will be ghastly.

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You really think so? Why? And define "high frequency".

-- Bill Sloman, Nijmegen

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c and back to Q looks like a forward biased c-b current path?

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Scarcely the point at issue. If you want to model an inductor, model something that looks vaguely like a real inductor, not something with zero parallel capacitance.

Nobody wants to prescribe which inductor John models, but he'd be better off if his models were a little more realistic - this is probably a slightly more subtle idea than you can process, but some of the lurkers may get the message.

-- Bill Sloman, Nijmegen

She did not come up with it. She *may have* introduced its use in the industry, however. But the remark itself makes it sound as if it was already in use, even at that point.