Driver to drive?

[Joerg]

Lostgallifreyan wrote:

Joerg <invalid@invalid.invalid> wrote in
news:7dbvjkF2bdsc0U3@mid.individual.net:

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


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.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

 

[Joerg]

Lostgallifreyan wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in news:FcadnUCm-
5ZRI-3XnZ2dnUVZ_vKdnZ2d@supernews.com:

I rather suspect you're barking up the wrong tree with a SPICE model of
a diode laser...it isn't the terminal voltage you care about, it's the
light output, and that depends on a whole lot of optical and thermal
things that SPICE is never, ever going to get right. They have widely
differing timescales, for one thing, which SPICE is horrible at, and for
another thing, small amounts of optical feedback have a _huge_ effect on
DL performance, including feeding back to the terminal voltage.


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.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

 

[Lostgallifreyan]

Joerg <invalid@invalid.invalid> wrote in
news:7dbvr6F2717n6U1@mid.individual.net:

Again, a laser diode is not a linear device when you look at lasing
output. Not at all.

No, but I'm looking at electrical INput..


That's the easy part

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.

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

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..

Let them disagree. Will they pay for your dead laser diodes?


No, but they DO take my point when I tell them that SpiceMod shouldn't
cost a personal user more than a very few of them.


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.

I bet it would still hurt them less than the ten diodes I bought for Ł520
when my weekly income was about Ł75, 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!

From what I've seen of my dead diodes, they are still electrically very
similar to live ones so I'm not concerned with advanced optical
modelling.


Often you can still use them as LEDs

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

What killed them?

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.

 

[Lostgallifreyan]

Joerg <invalid@invalid.invalid> wrote in
news:7dc0a0F2aeembU1@mid.individual.net:

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

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.

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.

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..

 

[Lostgallifreyan]

Joerg <invalid@invalid.invalid> wrote in
news:7dc01uF2717n6U2@mid.individual.net:

Lostgallifreyan wrote:
Joerg <invalid@invalid.invalid> wrote in
news:7dbvjkF2bdsc0U3@mid.individual.net:

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


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.





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.

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.

 

[Phil Hobbs]

Joerg wrote:

Lostgallifreyan wrote:
Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:FcadnUCm-
5ZRI-3XnZ2dnUVZ_vKdnZ2d@supernews.com:

I rather suspect you're barking up the wrong tree with a SPICE model
of a diode laser...it isn't the terminal voltage you care about, it's
the light output, and that depends on a whole lot of optical and
thermal things that SPICE is never, ever going to get right. They
have widely differing timescales, for one thing, which SPICE is
horrible at, and for another thing, small amounts of optical feedback
have a _huge_ effect on DL performance, including feeding back to the
terminal voltage.


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.


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.

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

resistors required.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

 

[Phil Hobbs]

Lostgallifreyan wrote:

Joerg <invalid@invalid.invalid> wrote in
news:7dc0a0F2aeembU1@mid.individual.net:

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


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.

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

 

[Joel Koltner]

"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message
news:xoidnU2-iIbBSu3XnZ2dnUVZ_r6dnZ2d@supernews.com...

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

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?

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.

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

 

[Lostgallifreyan]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in news:xoidnU2-
iIbBSu3XnZ2dnUVZ_r6dnZ2d@supernews.com:

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

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..

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.

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.

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.

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).

 

[Lostgallifreyan]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:58CdnWEqNIuvS-3XnZ2dnUVZ_vBi4p2d@supernews.com:

snivet

żQué?

 

[Lostgallifreyan]

"Joel Koltner" <zapwireDASHgroups@yahoo.com> wrote in
news:_W4cm.439882$Lo1.373342@en-nntp-04.dc1.easynews.com:

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.

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.

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?

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

 

[Vladimir Vassilevsky]

Phil Hobbs wrote:

Joerg wrote:

Lostgallifreyan wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote in
news:FcadnUCm-
5ZRI-3XnZ2dnUVZ_vKdnZ2d@supernews.com:

I rather suspect you're barking up the wrong tree with a SPICE model
of a diode laser...it isn't the terminal voltage you care about,
it's the light output, and that depends on a whole lot of optical
and thermal things that SPICE is never, ever going to get right.
They have widely differing timescales, for one thing, which SPICE is
horrible at, and for another thing, small amounts of optical
feedback have a _huge_ effect on DL performance, including feeding
back to the terminal voltage.


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.



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.

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

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
http://www.abvolt.com

 

[Lostgallifreyan]

Vladimir Vassilevsky <nospam@nowhere.com> wrote in news:IMWdnYD_roXLQ-
3XnZ2dnUVZ_rqdnZ2d@giganews.com:

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.

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?

 

[Vladimir Vassilevsky]

Lostgallifreyan wrote:

Vladimir Vassilevsky <nospam@nowhere.com> wrote in news:IMWdnYD_roXLQ-
3XnZ2dnUVZ_rqdnZ2d@giganews.com:


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.


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
http://www.abvolt.com

 

[Lostgallifreyan]

Vladimir Vassilevsky <nospam@nowhere.com> wrote in
news:8_2dnS4nlv14f-3XnZ2dnUVZ_g2dnZ2d@giganews.com:

Lostgallifreyan wrote:

Vladimir Vassilevsky <nospam@nowhere.com> wrote in news:IMWdnYD_roXLQ-
3XnZ2dnUVZ_rqdnZ2d@giganews.com:


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.


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.

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.

 

[George Herold]

On Jul 29, 6:20 pm, Joerg <inva...@invalid.invalid> wrote:

Lostgallifreyan wrote:
Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote in news:FcadnUCm-
5ZRI-3XnZ2dnUVZ_vKdn...@supernews.com:

I rather suspect you're barking up the wrong tree with a SPICE model of
a diode laser...it isn't the terminal voltage you care about, it's the
light output, and that depends on a whole lot of optical and thermal
things that SPICE is never, ever going to get right.  They have widely
differing timescales, for one thing, which SPICE is horrible at, and for
another thing, small amounts of optical feedback have a _huge_ effect on
DL performance, including feeding back to the terminal voltage.

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.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.- Hide quoted text -

- Show quoted text -

I love laser diode's.. K. Libbrecht and Jan Hall's RSI article circa
'93? is the classic current robbing circuit. I'd been trying to
modulate a LD current driver at high speed. I read the abstract to
the above article and said.. "no way". Then I read it and learned
something.

But I'm mostly interested in how the current modualtes the wavlength.
Which as Phil H. said has all sorts of different time scales. The 785
nm LD's I use have a relaxation oscillation frequency of 6+GHz!

I have no idea about spice models.

George H.

 

[Lostgallifreyan]

George Herold <ggherold@gmail.com> wrote in news:379c1689-21a2-456e-9d3b-
bf5874b7feb6@2g2000prl.googlegroups.com:

I love laser diode's.. K. Libbrecht and Jan Hall's RSI article circa
'93? is the classic current robbing circuit. I'd been trying to
modulate a LD current driver at high speed. I read the abstract to
the above article and said.. "no way". Then I read it and learned
something.

Here's something to consider: I think many posts here are assuming a
switching modulator wheras I am after a continuously proportional and DC
coupled type. I can hybridise with a PWM control, and eventually will go to
that, probably, but for now, what do you do if you want linear control? Never
mind that diode light output isn't at all linear, we might as well start with
what we CAN control... So do we really use a Darlington? I understand that
those are made to switch fast, they're not chosen for a long linear operating
region... So that leaves some fast single stage transistor, and a sense
resistor, and a controller, likely an op-amp.....


Is this getting through? The LM317 in data sheet approved constant
current mode IS just that, though not a shunt mod. But it does the opamp, the
sense resistor, and the hefty transistor to drive up to 1.5A. It's not as
daft as it looks, and it performs better than anyone ever told me it could.

Anyway, assuming I do shunt part of a constant current from the diode, what
might be the simplest way to do it? As far as I know, something like Robin
Bowden's 'Die4drive' circuit might have a basis for this with its MOSFET,
opamp amd sense resistor, but that's not a shunt mod either...


Getting back to the topic, whatever we do, it would REALLY help if we all had
access to some reasonable semblance (electrically) of a single mode laser
diode in spice. It's long overdue. It would save students and schools and
hobbyists a lot of money if it was there. Linear Technology have given us
LTspice to use for free, now we need things like this to put it to use.

In other news, I seem to have dropped alt.lasers from the cross-post list.
Annoying.

 

[Lostgallifreyan]

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9C581C5A32400zoodlewurdle@216.196.109.145:

In other news, I seem to have dropped alt.lasers from the cross-post list.
Annoying.

By which I mean, it should have been there too.
*sleeps*

 

[Charlie E.]

On Wed, 29 Jul 2009 14:32:27 -0700, Joerg <invalid@invalid.invalid>
wrote:

Lostgallifreyan wrote:
Joerg <invalid@invalid.invalid> wrote in
news:7dbrk7F2bch8pU1@mid.individual.net:

Thanks, by now I've seen it. But the model presented ain't much to write
home about, IMHO.



I read somewhere that it was a better version of an earlier one in a Pspice
library. How good does it have to be? ...


That depends on you. If you want the sims to model all the way to
failure modes I'd really be surprised if PSPICE had something in a
library that would even come close. My PSPICE license is too old so I
wouldn't know. But that would be one tough job.

Again, a laser diode is not a linear device when you look at lasing
output. Not at all.


... No point in modelling more detail than
needed as a starting point. We don't even HAVE that much, any of us hobbyists
and small scale designers. If experts raise their hands in horror saying
there's no point in simple modelling rather than too much detail (with which
Intusoft, who really know this stuff, would solidly disagree), ...


Let them disagree. Will they pay for your dead laser diodes?


... then we'll all
continue to have nothing to go on except dead laser diodes! Surely someone
who knows how might change this?


All I am saying is that I believe it is impossible to correctly model a
laser diode by trying to find electrical equivalents for all its
behaviors. You need a behavioral model in addition. All it takes for a
LD to die is optical overload inside the cavity. A brief wiggle of a
fiber connector, a spike of a few usec ... poof. The EDN model isn't
helping you with that.

I can pretty much assure you that PSpice doesn't include a laser diode
model, at least not that I recall. LEDs and photodiodes was about as
far as they went. Laser diodes would require a lot of
characterization and experimentation (boom!) to get all the details
correct.


Charlie

 

[Phil Hobbs]

Joel Koltner wrote:

"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message
news:xoidnU2-iIbBSu3XnZ2dnUVZ_r6dnZ2d@supernews.com...
Ohhhhhh..... You're using a voltage driver, and trying to trick it into
being a current driver. Bad, bad, bad, bad news.

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?

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.

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?

The LM317 has an inductive output characteristic, which resonates with
the bypass cap. The reason for the inductive characteristic is that
there's a feedback loop inside. The output of the 317 is an emitter,
which is naturally low impedance--it's a good voltage source, and the
feedback just makes it a better one. Outside the feedback bandwidth,
it's still a low impedance because it's an emitter.

Collectors make good current sources naturally--a bit of feedback makes
them stiffer and more accurate, but outside the loop bandwidth they're
still current sources. Using an emitter as a current source gives you
problems, because the feedback is fighting the natural tendency of the
device--the output crosses over from a high impedance at low frequency
where feedback dominates, to a low impedance at high frequency where the
feedback is unimportant.

It's a bit like a LDO, where a collector is forced to act like a voltage
source, by wrapping feedback around it, leading to similar sorts of
stability problems.

The usual 317 fixed current source circuit uses a resistor in series
with the output lead, and the feedback lead connected to the other end
of the resistor. That works great for fixed current applications, where
there's nothing to excite the resonance, but if you're using the 317 as
a modulator, you're bound to have trouble with the resonant peak.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net