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Thread: Stan_Ham diode driver (analog)

  1. #121
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    Quote Originally Posted by b52productions View Post
    i checked the website but i cant buy lasorbs there. so whats the deal bill?
    You can. We just haven't had a chance to update our order form yet. In the mean time, you can order anything that's not on the form by putting text in "Other Pangolin products or services".

    Bill

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    thanks. i found it in another thread

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    Quote Originally Posted by mixedgas View Post
    ...I too want some lasorbs, and they cannot come soon enough.

    I might flip one on some test gear for a lark, in order better understand the device, but I need the things ASAP to go into something being prototyped for "our" boys and girls in the "stans" and have enough other development projects right now.

    I also suspect it might be useful in " impulse" generation, for another area of my interests.

    I had a strange thought today, and have been hunting for something to anchor it too, this whole field is totally new to me so I don't grasp it well, but this post seems to confirm what might be a very elusive connection (elusive to me, anyway).

    I came across mention of a Grekhov effect in Usenet recently. It's been known since 1981 and there's lots of discussion of it, which I don't really understand, but the basic practise seems to imply that some very ordinary cheap diodes have been found to allow a forward bias of a few tens of volts for a very short time to build up a region of charge at the junction that is immediately switched to a reversal of a few hundred volts. The result is a snap of kilovolts, even hundreds of kilovolts, on timescales like 2ns, or 100ps, mentioned in various pages I found.

    So I thought, what if this works in reverse?! I have no idea if this is what's going on at all, but I know a lot of electronic effects are reversible. Unlike a transzorb or TVS that doesn't clamp low enough or perhaps fast enough to save a laser diode from ESD, if some diode whose Vf was already lower than that of a laser diode could be connected in inverse parallel and would respond to a fast ESD event as a kind of reverse Grekhov effect, I imagine the result is a ns or ps scale conduction that recovers so fast that the laser diode never notices.

    Steve, is your comment on a Lasorb being interesting for impulse generation an indicator that you have thought this same thought?

  4. #124
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    \

    Steve, is your comment on a Lasorb being interesting for impulse generation an indicator that you have thought this same thought?[/QUOTE]

    There is a different mode from Grekhov, easier to get, doesnt require the higher bias, and it generates useful broad harmonics and you can filter out say a chunk in the 10s of gigahertz with a 100 mhz excitation. But since some of us now know Lasorb is not a single diode discrete part, its a different story.

    Since i now work for a R&D company, and may need that mode, but not from a lasorb, I'm going to shut up now, but remember F = 1/T and if T is nanosecond..... That lasorb leading edge can be useful, especially if you were to shock excite a resonant line with it. (last sentence true, but deliberately misleading from my train of thought, read between the lines)

    I was thinking of snap diodes, but not Grekhov, which is very packaging/die dependent, one brand of 1n4148 might do it, but a 1n4148 using the same specs but by a different manufacturer wont.

    you might wish to google parametric amplification and parametric multiplication, neither of which a lasorb will do.

    I was hoping lasorb had a diode structure or a pnpn layer, but it doesnt, acording to someone who smashed one.

    Steve



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  5. #125
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    Thankyou. Interesting answer. And still a reach beyond me for now, I'll have to revisit that point a few times no doubt, to get it. For now I have just one question whose answer may or may not help me in understanding: Was my thought on reversibility meaningful? Right, even? Specifically, can a process in diodes capable of exciting short HV pulses from longer lower V ones be reversed to absorb fast short HV pulses and reduce them to longer surges that are no hazard to sensitive devices? For the record, I'm also strangely reminded of the 'ram pump', as used to raise water from a low heavy flow to a small high one, using pulses. Water is often used to analogise electrical events, but I don't know if that's useful here yet..

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    Given how some people like to tell someone when and exactly how they are wrong, I'll take that as a firm 'yes' because the silence is absolutely deafening now. Never mind the ram pump observation though, that was just a passing thought.

  7. #127
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    Quote Originally Posted by The_Doctor View Post
    Given how some people like to tell someone when and exactly how they are wrong, I'll take that as a firm 'yes' because the silence is absolutely deafening now. Never mind the ram pump observation though, that was just a passing thought.

    No, not really, unless you sling a cap across the diode, and then what you get is a lousy RC integrater with a clamp. Diodes in their slow states act as varacters, the capacitance varies with the applied voltage and decreases as higher V moves the charge carriers apart. So no on the reversability. If its not snapping, it acts, well, like a normal diode, you might find a so called zero bias, or hot carrier microwave diode more to your liking, but considering how hard to get they are, its not for the faint of heart.

    what a poorly matched diode does is usually splatter the incoming pulse into a bunch of harmonics or into thermal adsorption within the diode. or more likely it blows right past the diode looking for a more impedance matched home.

    http://www.thegleam.com/ke5fx/cg.htm

    if you want to have some fun, find yourself a "lambda" diode, made of 2 fets or a fet-pnp pair. but they are hard to make fast due to parasitics from the packages.

    Steve
    Last edited by mixedgas; 08-21-2009 at 17:53.
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  8. #128
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    Wasn't thinking of capacitance at all though, my description of reducing and lengthening of a pulse peak can be misleading, like describing an integrator, I realise that, but I thought Bill Benner early on stated that this thing wasn't about filtering or capacitance at all (though I think he did add that to the circuit?).

    I haven't been looking at this much lately (hurt my back and had other stuff on my mind too) but as far as I could tell, 'snapping' wasn't a capacitance thing at all, even though diodes do have a voltage dependant capacitance in reverse bias, some a large one: the varactors you mentioned.. Basically I'm wondering if snapping is NOT due to capacitance, and if not, then can a reverse biased diode, appropriately chosen, do whatever this thing is in reverse, in such a way as to generate a low peak from an ESD pulse as a result of extremely fast avalanche breakdown, faster than the laser diode could respond to it in forward bias, and dump the energy as heat.

    In short, I know that Bill mentioned a diode that IS reverse biased. He said it was not the important part of the circuit, i.e. it was just a useful part of it to catch reverse biased pulses. What I still can't shake off is an idea that we've been sold a very fine Red Herring. A McGuffin, even... It seems to me that this same diode might actually be the business end also, if it was chosen for a fast avalanche in reverse. If so, no wonder no-one could think of a suitable component in all known types, because he might have had people looking at the wrong place when what we should be examining is a diode we were told was not at all important for forward bias protection for the laser diode. If this is an illusion feel free to dispel it. But as clearly as possible, because I'm lousy at riddles.
    Last edited by The_Doctor; 08-25-2009 at 17:59.

  9. #129
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    Quote Originally Posted by The_Doctor View Post
    ... If this is an illusion feel free to dispel it. But as clearly as possible, because I'm lousy at riddles.

    And here I thought you were telling one...
    Love, peace, and grease,

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    Not intentionally...

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