Just an additional thought - the ringing looks like it is coming from the pigtail connections to the scope. The long leads form an underdamped low- pass filter that is shock-excited by the fast step. So you are not seeing the actual step, but the effects of the filter. A similar effect is shown in the Binary Sampler Description, where a short length of unterminated trace was used to connect the sampler to the 50-ohm output of the step generator. You can see the overshoot in Fig. 3. The unterminated trace was only 1/2 inch long:
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The Motorola EclinPs design guide recommends for fast steps like this any unterminated traces should be less than 3/8 inch long. The twisted pair wires shown in your photo are much longer. This attenuates the fast step.
So the moral of the story is to use a groundplane and very short leads!
breadboard is ok for generating the low frequency squarewave but if you measure the capacitance between the terminal rows its pretty high, a bit too high for comfort for anything much over 10mhz, also the large inductor wich has some parasitic capacitance accros it is conected to the point where you expect to see the pulse, most reverse recovery circuits ive seen have the low frequency drive on one side and the pulse is seen at the other side of the diode where there is only a small inductor. A 10volt 1ns pulse accross
10pf equates to .1amp, the reverse recovery current of most diodes falls off quite a lot before it snaps.
I use thinner coax and find the center conection pushes into the breadboard quite well, the shield needs a bit of wire soldered to it, it might be best if you can put the breadboard right up agaisnt the scope so the twisted cable shown there can reach and do away with the coax cable altogether. theres a linear app note wich gives all sorts of usefull high frequency prototype construction tips and things to avoid Il try and find it.
These days I find it easiest to prototype by cutting small tracks on a double sided copper clad board using a dremmel like tool (or scalpel), just make rectangular pads big enough to solder an ic and .2mmm ec wire to go from pad to pad, I find it works like a charm for surface mount you dont have to drill holes or glue ics on their backs or bend legs the wrong way up, plus if you do one part of the circuit at a time and test it its easy to add any modifications before you add the next bit.
you can also make small circuits this way on small bits of pcb then solder the whole thing onto a larger pcb, you can then move them about as you like.
I used those as well, also CK722, CK760, and some SBT thingies I can't remember the number of now... long silver-colored tube packaging with flying leads.
...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.
--
| 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.
I kept pretty much all those Ge devices from my childhood. A few weeks ago that saved our bacon. One Minolta dead, waiting for an SMT cap from Mouser to get the shutter to work again. The other's battery was flat.
1.35V Mercury, now banned. Plucked an OA91 from my old box and bingo, from now on it is happy with regular 1.55V cells and we could take some nice photos of our foxes.
CK722. Was VERY disappointed to find out how easy they die. At least, if you plug a 6AU6 into a 12V auto radio, the heater glows much brighter. If you are quick, you might be able to save the tube. Similarly, you can burn holes in the plate of a 6C4 running with no bias and it still works fine. Too much voltage on a 6146? No problem. It glows purple to let you know something is wrong.
But transistors? Bah. No future in these things. Make a simple mistake and they die without warning. Nobody will ever use them. Regards,
I've fried quite a few components in my day... it's the way I learn best ;-)
...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 first job interview, I told the guy I preferred tubes because transistors were too easy to blow out. He sniffed, literally, and said "that won't do" and kicked me out. My next interview, I said the same thing, the guy laughed, and hired me. I designed about $100 million worth of stuff for the second guy.
You picked the right boss. One with a sense of humor. Invaluable when he has to sign the bill for parts:)
Same thing happened to me. When DeGaulle kicked the Americans out of France, the Canadians with NATO were offered a chance to go to Belgium and work as a spy, or take early retirement. The cold war was was on, and spies were shot, so I went to Boston to see if I could get into MIT.
No money, a wife, and a new baby, so my chances were slim to none. Then I saw an ad. Tech needed to work in graduate school. Professor Campbell Searle interviewed me with a few other professors. They asked if I was familiar with transistors. I replied that I had burned out quite a few. They laughed and hired me on the spot. I took a few classes, but learned it would take 16 years to graduate. Having a new and growing family made it clear I had to find work that made more money. I went to Memorex in Santa Clara and designed a new magnetic recording analysis system that eventally saved the industry hundreds of millions, if not billions of dollars. And made me a few shekels also.
But MIT was the most interesting work I've ever done. I'd go back in a heartbeat.
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