Fig 3 in that doc is the sort of txline model I mentioned, an LT Spice ideal txline, with infinite common-mode rejection, and an added inductor to simulate the common-mode impedance of a coax shield.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
OK I've only done it with a big spool (~100') of coax. The great thing about hitting it with a nice sharp edge is you get all the frequencies at one shot. (The hard part, (I'm guessing) is figuring out what it means.) But looking for difference's is easy enough.
George H.
TDR is approximately a graph of impedance vs distance. The vertical scale is actually reflection coefficient, from 0 ohms to infinite ohms over the span of 0 volts to Vgen.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Yeah I get it "in theory", I've never done it "in practice". It would be a nice technique to put in my bag of tricks. Is it typically done with steps or pulses?
Silly question, Doesn't zero ohms, give -Vgen, 50 ohms = 0 and open = +Vgen?
George H.
+Vgen?
Yes, but sitting on top of the step, so it's 0, +Vgen, and +2Vgen.
Cheers
Phil Hobbs
= +Vgen?
Got it, a step then... I should make one. (someday.)
Does the 74AC series have the fastest edges? (Without going ECL crazy.)
George H.
Electrical is usually steps, optical is usually a narrow pulse.
Given a 50 ohm step generator, +1 volt unloaded, and the hi-z scope at the generator end, shorting the rig makes zero volts, and open circuit makes +1 volt. A 50 ohm coax (or resistor) loads the gen to 0.5 volts.
I have an ancient Tek 11802 with the SD24 TDR head, about $1000 or so total on ebay. It makes a very clean ~~30 ps TDR step, so it can measure the impedance of short cables or PCB traces. It's dual channel, so it can also terminate and measure the signal from the far end of the DUT, the "TDT" signal.
Like this: upper trace is TDR, lower is TDT.
There is some HF loss in that trace, which trashes the risetime.
This is a fundamental tool for any sort of time-domain high frequency design.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
AC is usually sub-ns. Some of the Tiny Logic parts get to around 600 ps. Parallel all three sections of a NL37WZ16 for a brutal step generator.
But ECL crazy is better, if you have a fast scope.
I have a pcb layout for a fast, cheap, maybe 60 ps TDR, but I haven't had time to build it.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I recently got a chance to try out an OTDR, for an expert witness case. They're fun. The main difference from electrical is that you get a pretty strong Rayleigh backscatter signal that allows a direct measurement of the fibre loss.
(Also just put in an offer on a _brand new_ 2012 Mustang convertible with a 6-speed manual. They're even more fun.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Electrically it is done with either. For example...
The 1975 vintage Tek 1502 uses a step ~200mV step in a 50ohm line; the system's reflected edge risetime is 140ps, 0.6" resolution.
The similar vintage Tek 1503 uses a 10/100/1000ns half-sine pulse, 5V in 50ohm line. Much longer range than the 1502, unsurprisingly.
CRTs, battery operated, operable with cover off while being splashed with water, non-operating can be submerged to 3ft. CRT mechanically isolated from chassis, so withstands 12" drops onto concrete. No wonder the military liked them.
Nice. Can you resolve the silkscreen lettering on the meander line? ;)
Reminds me of my misspent youth, when I did a bunch of microwave filter tuning with an emery board and silver paint.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
= +Vgen?
Well just sorta thinking of a product to sell for student use. Into a 'scope.. (say 100-200 Mhz) So ~1-2ns edges would be fast enough.
Maybe put together sections (say ~6') of 50 ohm and 75 ohm coax and measure the different lengths. (As well as changing the termination.)
AoE3 has a nice appendix on all this stuff... I was just reading it the other day. And yeah, I'd want to do both TDR and TDT.
George H.
My wife drives a '14 ruby red convertible, though it's an automatic
6-cyl. The '15s and '16s are better cars, though I think they look too much like a Beamer.
A little box with a few connectors and a wall-wart could be a nice 1 ns TDR/TDT. Maybe $5 worth of electronic components. I could work on that with you, just for fun.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Mo and I rented a red convertible Mustang in Massechusetts a few years back. We hated it. It was an immense struggle to get the top up or down. The instruments were basically invisible. I thought everything felt tacky.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I don't like the look of the new body style, though, and the IoT-style spying is all over the new ones. Plus the dealer has had this one sitting around for five years and is, shall we say, motivated. ;)
Now to get it painted electric blue.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
You mean, like read the text?
As far as I can tell, solder mask or silk doesn't show on TDR. I think I can sometimes see the fiberglas weave.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I don't much like the Beamer look, either. Of all the body tweaks since '05, I like the '13-'14 body style the best.
Not sure I'd like it sitting around for five years, though.
Nice color. What color is it? Top?
The old ones were horrible, I agree, especially the V6. However, they changed it completely for the 2011 model year, and it's pretty nice, actually. The V6 is faster than the 5-litre 2010 GT. It also doesn't have the GT's super-fascist traction control software--in those things you have to have the wheel pointed dead straight or it won't give you full power. All the budding Mario Andrettis with mid-life crises have to be protected from themselves, I suppose.
It's amusing to drive a 5-year-old car with 150 miles on the odo.
Cheers
Phil "Too old for a mid-life crisis" Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Currently white with a black top and black leather interior. The exact colour scheme of my Mum's old Dodge Dart.
Gotta get the dickey spoiler off the trunk lid, too.
Soft tops last better in the NE than down your way, I expect, and for the price difference, I can buy a lot of soft tops.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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