Re: Spice models of laser diodes?

Phil Hobbs wrote in news:FcadnUCm-

During the transitions, perhaps, but in steady state operation there seems to be a remarkable electrical similarity between a dead diode and a live one. What matters is the way the diode responds to hard electrical changes on the input, as that's what makes the ringing and damaging overshoots. This is true with NO consideration of optical nature, and just modelling that alone, realistically for real laser diodes, is a lot more than we currently have. And likely not that big an ask, it's just not been done much, it seems.

Yes, it should be feasible to place it in a subcircuit. However, since you said you just started out with LTSpice I suggest not to do that (yet).

I use DesignCAD 3D for that. Ten bucks at a liquidator. Mostly because it can read in AutoCAD files.

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

Ah, but Ghost lets me retain a known working OS config, recalled at will, and an INTENSELY useful side effect of this is infinitely renewable demo periods. :)

That's the easy part :-)

BTW, I do not understand why EDN placed the SPICE list as a graphic. Somehow that doesn't leave a very professional taste.

The ones I dealt with last were about $1200 a pop. Lots of them. The client would have been very p....d if I had blown some.

Often you can still use them as LEDs :-)

What killed them?

:-)

I don't do that. IMHO it's quite borderline from an ethics POV to fool demo SW into a fresh time period. Then I either buy it or move on.

Ringing? I never had that. Don't drive them through a built-in inductor :-)

LDs are usually current driven. The prudent way is to impose a constant DC current, very well stabilized and equipped with belts, suspenders, cushions, airbags. Then the fast signals are fed in via a current "robbing" shunt circuit to ground. That pretty much makes sure you can't fry it.

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

Precisely. I hope. If a decent electrical model isn't enough then we all have to think further, but most of us haven't even got that much yet. We'd save a LOT of resources if we did.

I wondered about that too. They let registered users on their site download a text list for free though. I think maybe they just want to lead the horse to enough water to improve the chances it will drink theirs. Anyway, I copied it out by hand and posted it earlier..

I bet it would still hurt them less than the ten diodes I bought for £520 when my weekly income was about £75, when I killed four of them just trying to prove to people the existence of death by retroflection (and not by ESD as was claimed) when the sellers and makers refused to accept what is now generally KNOWN. And no, they did NOT reimburse!

Exactly. That weird fact is why I think it's valid to consider meaningful separation between their full behaviour and their electrical behaviour.

Mostly retroreflection as described above, and also deliberate brinksmanship when trying to find the optimum compromise between a short blazing fun life, and a long boring stable one. :)

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

Not intentional.. I'm using an LM317. Yes, I know it's not the approved way. It WORKS, far better than expected, and most people who explore weird tricks with an LM317 are mightily impressed with it, from the guy who built a class A headphone amp, or the radio ham who built a transmitter round one...

My physical circuit build, AND the later spice modelling, both indicate that an LM317 for a cheap way to get up to an amp-amd-a-half of DC coupled proportional laser drive at up to 500 KHz is very likely to work well. Maybe the inductance is in the LM317 model, I don't know where else it can be in my simple circuit models. What's crucial is that it is the SAME overshoot I saw in the real circuit so spice is already telling me good things, and I've already improved the driver on the strength of that spice model.

Definitely true. Shunting is my favourite method, and I might return to it with this current indea. But half the fun is trying to see how well I can push this LM317 idea. I've seen circuits that are 'better' that don't seem either much better or worse than mine, which also happens to be very polite at startup, no spikes at that moment at all..

Joerg wrote in news: snipped-for-privacy@mid.individual.net:

That's what I usually do (and I make a poing of buying from small firms or good individual programmers rather than big firms, with special emphasis on paying for tools written by coders who value writing for ALL of Win32).

The only reason I don't use the freeware Sketchup version 5 is it won't run on W9X like the old demo v4 does. I really dislike Windows XP.

Seconded. A nice RF transistor in shunt is the ticket. (Anti-snivet resistors required.)

Cheers

Phil Hobbs

Ohhhhhh..... You're using a voltage driver, and trying to trick it into being a current driver. Bad, bad, bad, bad news.

Due to the rolloff in the loop gain, the output of a voltage regulator appears inductive, which will reliably give you a big noise peak if the output cap is too big, and some ringing if it's too small. As Joerg said, you're way better off using current drive. It isn't difficult, just an op amp and a Darlington. Use the Darlington's collector as the output, and sense the current in its emitter. Adding an outboard current limit is easy then too.

Cheers,

Phil Hobbs

I tried to use an LM317 as a constant current driver for a regular old LED (a high-power white LED -- needed a few watts; this was >5 years ago when white LEDs were still new and spendy) and it just sat there and oscillated on me.

You do see people suggesting using LM317s (or similar) as constant current sources all the time, though... when can it work? It sounds like you're saying it's almost always a bad idea?

Compared to an LM317 though, isn't the main difference that the LM317 essentially has Power->drive transistor->LED->sense resistor whereas Joerg's approach is Power->LED->drive transistor->sense resistor? ...so you're essentially isolating the current sensing from the load itself, to some degree?

---Joel

Phil Hobbs wrote in news:xoidnU2- iIbBSu3XnZ2dnUVZ snipped-for-privacy@supernews.com:

I thought if constant current config for an LM317 is good enough for the standard data sheet then it's good enough for me. :) (And if you look at the LaserFAQ closely you'll also see that it's good enough for Winfield Hill, though he wasn't trying to modulate it..) Like I said, part of the fun is in making the LM317 do weird and wonderful things. It's fun to know that wherever you have some, you can do some amazing things normally done with other parts, usually exotic, more costly, with MUCH more complex board layouts, etc..

That fits what I see. I've managed to tame it to something respectful. I think an 'overshoot' that results in a minimum-to-maximum deviation of about 3 mA along the 'flat' top of a 500 KHz square wave at 160 mA isn't bad. People who know a lot more than I do have been content with worse.

That's what I intend to try too, though I'll try a MOSFET rather than a darlington. I take it the darlington is to avoid the gate capacitance of a MOSFET? I can see that it will work because its total Vf will be less than the laser diode's own. (A quirk my own circuit is exploiting, in a different way).

Phil Hobbs wrote in news:58CdnWEqNIuvS-3XnZ2dnUVZ snipped-for-privacy@supernews.com:

"Joel Koltner" wrote in news:_W4cm.439882$ snipped-for-privacy@en-nntp-04.dc.easynews.com:

Never seen that, though I know it HAS been seen, the guy who built a transmitter round an LM317 did so to explore it. I use an LED lamp fopr my main room light when I'm at the compiter, it uses an LM317 with a voltage reference and a pot to dupe it to work as a dimmer. It's not very efficient but it doesn't flicker either like most power converters do, and it's much cheaper. Solid as a rock.

I tend to trust manufacturers data sheets. It beats assuming they're useless. :)

The high speed LD drivers I've seen used the differential amp topology. The LD was the load in one of the shoulders; the max. current was set at the tail. For the faster switching, when the LD is in the "off" state, it was kept at the bias current little below the threshold of lasing. This type of circuit can easily modulate the LD at the rates of up to several GHz; the limits are set by the RF parasitics.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Vladimir Vassilevsky wrote in news:IMWdnYD_roXLQ-

But what would you do if you were asked to do 'analog' proportional DC coupled modulations that tracked a continuously varying signal, instead of high speed switched signalling? And how fast would it go then?

The LDs are VERY nonlinear (light vs current), so I would use voltage to frequency conversion, PWM, delta sigma or some other pulse modulation technique. A lot depends on your requirements, however tracking at the speed of the 1/10 of the pulse rate ballpark should not be a big problem.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Vladimir Vassilevsky wrote in news:8_2dnS4nlv14f-3XnZ2dnUVZ snipped-for-privacy@giganews.com:

I like that idea. I've considered it but I think it could be more complex than the simple modulator based on an LM317. I like the real visual linearity that could result from very fast PWM though. It's another idea I want to try once I get this LM317 lark worked out of my system.