Back in '98, Electronic Design had a design tip on a homemade time domain reflectometer. I'd sort of like to build one up, but this one used the 74AC series of logic and the best it would do is a rise time of 5 ns. I'm only needing something to go 50 feet (15m) or so, but would like glitch resolution to a couple of inches. Is there a logic family readily available that can do 500ps to 1ns or so?
Jim
Hello Jim,
Sure, for example the ONSemi MC10EL89. It is an ECL coax driver and its transition time is under 400psec. Of course, they aren't cheap ($3-4).
You could also roll your own with ordinary BFS17A. Those can go down to
700-800psec or so.-- Regards, Joerg http://www.analogconsultants.com
DS90C402, 650 ps maybe, fairly clean step. I could post a pic to abse.
MC10EP89 is a lot faster but the ecl levels are a bit of a nuisance.
What sort of scope do you have?
John
Why stop at the 2.8Ghz BFS17? Farnell's range of low power wideband transistors goes up to the 9GHz BFR40 which I've never used, but I did use the 5GHz NPN BFR93 and the 5GHz PNP BFT93. Provided that you remembered to put a "base-stopper" resistor (22R to 33R) in series with the base, and close up against it, they were pretty well behaved, and good for risetimes around 500psec.
-- Bill Sloman, Nijmegen
Hello Bill,
Well, yes, you can get 40GHz+ versions at Infineon. But a BFS17A is something that can often be found right in the lab part bins and it's fast enough for this job.
-- Regards, Joerg http://www.analogconsultants.com
Well duh, John, I hadn't gotten that far yet in my thinking (sometimes my desires get ahead of my brain). Seems the best I can do is 50 nS/div, so getting picosecond times with nanosecond oscilloscopes isn't going to buy me a thing. Thanks for the reality check.
BTW, Joerg, ordered the TI little USB uC development kit this morning.
Jim
I'm doing some design work for AZMicrotek and just complained that I'd need to increase the power to get a good sine wave at 600MHz.
The owner replied...
ECL = Energy Consumption Logic.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
My experience is that bipolars, even the 40 GHz SiGe parts, don't switch very fast, even with absurd drive. Figure that if Ft = 40G and beta=200, the -3 dB point on current gain is merely 200 MHz. It's easier to use a fast CMOS gate or some ECL.
(and Farnell yet again?!)
John
You may be thinking of "Build Your Own Cable Radar TDR Multivibrator", by Gunnar Englund,
A simplified circuit is shown in Fig. 1 of
But here's two gotchas:
Regards,
Mike Monett
Hello John,
At those speeds I use "semi-absurd" drive, expecting no more than
10-15dB per stage. The good thing is they are really cheap. I had the BFS17A do well under a nsec.Of course ECL is a lot easier if it isn't for a mass product and cost doesn't matter much.
-- Regards, Joerg http://www.analogconsultants.com
Hello Jim,
Plus things get toasty rather fast. Those are the reasons why I prefer two, three discretes for a single pulse application like this.
-- Regards, Joerg http://www.analogconsultants.com
Hello Jim,
Guess then you'll have to build a little "single event correlator" that sees the return pulse. Or a "walking sample gate" that looks for it and would show you the time directly. Sampling at sub-nsec windows is not for the faint of heart. You'd have to use diode bridges and RF transformers or use something from Mini-Circuits.
Great! You can actually go ahead and load the latest IAR Kickstart from TI's web site. Then you don't have to wait.
-- Regards, Joerg http://www.analogconsultants.com
Hello Mike,
Sometimes it's easier to just look for resonances that are inconsistent with cable length. That's how I once found a bubble in a coax cable. Being in college that didn't help much as I did not have the funds for a new cable run. Or let's say those funds had been invested in cerveza. So I kept ham radio going, hoping it'll last just one more month. Kaboom!
-- Regards, Joerg http://www.analogconsultants.com
Sometimes it's easier to just look at the cable visually and feel it with your fingers. You can spot dings, cuts, crimps, pinches, staples, nails, flat spots, broken and cut shielding, some types of corrosion, and many other kinds of defects that affect performance.
The nice thing is you already have the instrumentation. As long as you don't have too much cerveza:)
Regards,
Mike Monett
Hello Mike,
True. However, when the cable is behind paneling and you'd have to tear that off for an inspection that's another story. In my case it was in kind of a concrete raceway, hard to pull out (I'd have had to pull all of them out for that).
The "instrumentation" consisted of a grid dip meter. Heathkit, $80 or so as a kit. Still have it.
-- Regards, Joerg http://www.analogconsultants.com
You'll want to download it anyway, the version on the CD is not as new. Interesting that mine was marked "Made in Germany". Two solder bridges between pins on the large MSP controller chip. 8-(
Best regards, Spehro Pefhany
-- "it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Yeah, I like that part... it makes a good current-mode NIM driver, and it's fast but not so fast that it screams too much. But a 40 GHz transistor isn't 20x as fast as a 2 GHz one. Gaasfets and especially phemts are really fast.
We use some GigaLogic parts. A NOR gate costs $35.
John
Your experience is a little unspecific. Mine was - as I posted - that we got about 500psec for 10% to 90% transition times with 5GHz parts.
So who is your favourite broad-line distributor? Farnell owns Newark, and you can get their range in the USA even if your own tame distributor doesn't stock them.
I know about the Infineon parts, but I'm less sure about being able to buy them - if Farnell stocked them I could have been reasonably confident that they were readily available
-- Bill Sloman, Nijmegen
Is that Gigabit Logic, or some descendant? We were using their parts back in 1988 and the prices were much the same back then. ECLinPS is a lot cheaper and easier to get hold of, and almost as fast.
-- Bill Sloman, Nijmegen
My God, you too? I've got an original Eico tube dip meter and one of the Heathkit tunnel dippers. Still use the Heath; was going to convert the Eico to solid state and life got in the way.
Jim
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