Coax modelling question

Your boss needs more imagination.

Pulse and step would be easy.

It might be done double-sided, but 4 layer would be much better. Probably 1 ns on 2 layers would work.

OK, let me know if you want to play with this.

I do have a simple deconvolution algorithm that can beautify an ugly step waveform. Suck a waveform out of the scope into a PC and pass it through an adaptive FIR filter to pretty it up. That might make the product more interesting; add some signals+systems math.

When I got the flaming red 156-MPH Audi, I had to explain to Mo that it's just a red car.

Wanna drag ?>:-} ...Jim Thompson

Nah, never was my style except in boats. I'm getting the 6-sec zero-to-sixty model and no the 4-sec model. That's lots fast enough for grins.

Now all I need is a box of ball caps in the back seat, for when the one I'm wearing blows off in the wind. ;)

Cheers

Phil Hobbs

ote:

d open = +Vgen?

n.

razy.)

't

h.

he

Well that's a discussion that would take at least two beers.

That's what I figured, say a 10 -20 ns pulse. I think students would understand the pulse more easily.

Yeah I'll get some 74AC somethings and make some edges.. see what things look like on various 'scopes.

We've got no fast 'scopes (200 MHz max)

Yeah that would be kinda fun too... The big issue is that making little ~$100-200 dollar boxes (even $500) and selling 10 - 20 of them a year just does not make enough money to pay for development and such. We've never sold to much more than physics departments. And I'm not sure how many of them will have any interest.

We could sell only 10 units total!

Anyway don't hold your breathe.

George H.

Kinda plain but not an outrageously ugly combination. It's surprising that it didn't sell.

They're useless but I don't find them objectionable. I wouldn't want to patch holes, just to get rid of it.

We get a lot more use out of them down this way, too. ;-) My wife drives with her top down half the year. ;-)

Well, in SF, it figures you drive a flamer. ;-)

I was going to suggest one with a brim all the way around to protect the ears and neck, too, but then remembered that you live were there is no sun. ;-)

I've wanted to build a TDR for my own amusement. Would you mind giving some more pointers about your circuit or the ECL parts you'd use?

Clifford Heath.

Almost all the cars here are grey or silver or white or black. Same as most cities in the world, according to Street View. Mine is highly visible in a parking lot. I wonder how other people can find their car.

Is that fast? I belong to a club where 12 year olds go 0-40 in 4s and 0-60 in 5s. Then it is climbing with your feet higher than your head. And when they are 14 they can, if they are safe, do it solo.

Visitors usually say something like "Oh my God" when they first see it, and are occasionally heard screaming when they first do it. The last one I heard do that finished with the usual inane grin on her face, and did it all again :)

The faster the edge rate, the better. Some suggest Analog Device's fastest comparators, but I haven't seen any measurements on circuits.

Three parallel (74LVC1G14+130ohms) is sub-nanosecond, but you have to pay attention the decoupling capacitors.

Apart from that you just need a fast enough scope.

ADCMP582 has diff ECL outputs with typ 37 ps edges. It's around $10.

Right. If you want to make a sub-100 ps TDR, the real problem is waveform digitizing.

ote:

d open = +Vgen?

n.

razy.)

't

h.

he

Hi John, I've been wandering around my fields, (with 'cobber' me mum's dog, an Australian kelpie. herding dog... very active!)

Anyway lotsa random thoughts and if anything works for you... there we go.

Well first off I'm totally jazzed about making a tdr, mostly because I know nothing. Second this reminds me SPAD, cool physics, ~few dollar circuit. I thought it should have been sold as a kit. (for ~< $100. $99) We've sold 3... I've helped ~10 people with the circuit.

So my third thought is could we make it a kit? A kit sounds like through hole, to me, at least the primitive version. Two kits? one through hole and another surface mount? Do people do surface mount kits?

Fifth thought, can you do something else with fast edges/pulses. (Add functions) I only have one idea here, (maybe two things) and that is drive it from a random noise source. ~100 MHz noise source and pseudo shot noise generator (with the pulse) Pulses of charge are like big fat electrons and make more noise.

How do you support a kit? I don't know... get someone else (the customers) to do it? Some forum? Adafruit, eevblog, ?

George H.

e:

and open = +Vgen?

gen.

crazy.)

en't

ugh.

n.)

the

Oh, seventh thought, TDR should be marketed first to electronics geeks... there's lot's more of those than there are physics majors.

GH

Thanks, that's the kind of info I was looking for. A good deal faster than I could build a sampler for...

I'd put a sampler on the same board, using a voltage-controlled delay like what Win showed recently using an LVDS receiver, pulsing a pair of microwave diodes. Sweep the delay with enough pulses for averaging and you don't need a fast scope.

Drive the whole thing from an Arduino and you have a geek TDR kit.

Clifford Heath.

No, that's where I'm _from_, not where I live. The weather here suits me very well--much sunnier than Vancouver and nicer than Palo Alto. (I got so sick of _brown_ living in CA. Westchester County is a really nice place.

Cheers

Phil Hobbs

= +Vgen?

If you want bigger-than ECL voltage swings, using ECL to drive broad-band t ransistors - BFR92 is NPN 5GHz, BFT93 PNP 5GHz complement - is pretty stra ight forward.

Both offer peak bandwidth at 14mA, which gives you 0.7V into 50R - +/-0.7V with complementary drive.

There are bigger cheap broad-band transistors, good for perhaps 60mA. We us ed the NPN BFR96. A similar PNP part showed up a bit later, too late for me to use.

Back in the 1980's we used BFR92s to drive HP RF transistors to get 7.5V sw ings into 50R with a roughly 300psec 10% to 90% transition time.

As John points out, transmission lines are do-able on a double-sided board but a four-layer board make the job a lot easier. It might be worth making the outer layers of the board with one of the Rogers microwave substrates

Talk to you printed circuit supplier to see if they keep any of them in sto ck - Rogers seems to sell the stuff in large sheets, which are expensive.

E-mail me if you do decide to go ahead. I've done this kind of stuff (and u sed step-recovery diodes to get fast edges) and I'd be happy to help.

After postponing my mid-life crisis for a couple of decades, I retired my Taurus wagon and went shopping for something that "went like stink". Ended up with a Camaro 1LE - track suspension and brakes, different gearing, fat tires, oil coolers, but an otherwise normal 430HP V8. I was a little concerned that mileage would suck but the rest of the package suited. Turns out that I get 27-30 mpg on a leisurely highway trip and somewhere around 23-24 on a commute into the big city.

I leave the traction control in the "performance mode" - no traction control so it won't fight me if I want to get frisky but it will add a little "torque vectoring" should the direction the car is going not match the direction I'm requesting by a significant amount - I can steer with the throttle up to a point but it'll assert itself when I request turn-in and slipping a bit more than the front wheels can overcome without howling in protest. I'm OK with that.