Fast, efficient IR LEDs?

The IRDA application is why they need speed specs, and nobody does IRDA in
the visible.

How about a 40V shunt regulator that varies less than 2V from 25C down
to -70C (or less)?
Is that COOL or what?

On Fri, 05 Feb 2010 21:57:27 -0800, Robert Baer
robertbaer_at_localnet.com> wrote:

John Larkin wrote:
On Sat, 6 Feb 2010 05:18:59 +0100, "Ban" <bansuri_at_web.de> wrote:

"Jim Thompson" <To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> schrieb im
Newsbeitrag news:uk6pm5dent8tsdv0642e3id6pqvor0f8d9_at_4ax.com...
On Fri, 05 Feb 2010 11:43:30 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:

On Fri, 5 Feb 2010 11:13:02 -0800 (PST), "miso_at_sushi.com"
miso_at_sushi.com> wrote:

[snip]
Isn't low current and high speed mutually exclusive, like intelligence
and Republicans?
Ah, politics!

I wonder why lefties always assume that conservatives are stupid.
That's the "what's wrong with Kansas?" dilemma, the lament that the
working class won't toe the party line and follow the lead of their
betters.

Conservatives generally assume that lefties are both stupid and evil.

John
...lefties are both stupid and evil _and_ pathological liars.

To me it seems both leftist and rightists are apparently mainly bored and
they appear boring to others as well. Nobody here is interested in political
drivel of any colour and it is a pity this group has turned into 90% crap.
ciao Ban

Design any cool electronics lately?

John

How about a 40V shunt regulator that varies less than 2V from 25C
down to -70C (or less)?
Is that COOL or what?

No. It should be easy to hold that to around 0.25V without even
trying.

...Jim Thompson
Sorry; miss-typed; should have said 400V shunt regulator.

On Fri, 5 Feb 2010 11:13:02 -0800 (PST), "miso_at_sushi.com"
miso_at_sushi.com> wrote:

On Feb 5, 8:56 am, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz, < 50 pF) for an optical feedback gizmo.
  I have some Stanley DN310s, but they've been discontinued.  Other
possibilities are:

Vishay    TSFF5410   -- 870 nm, 0.% W/A typ  15 ns rise/fall, 125 pF typ
Vishay    VSLB3940   -- 940 nm, 0.4 W/A typ  15 ns rise/fall,  70 pF typ
Panasonic LNA4905L   -- 880 nm, 0.3 W/A min  30 MHz typ, no other specs
Osram     SFH4550    -- 850 nm, 0.5 W/A typ  12 ns rise/fall,  no C spec

It would be really nice to find something with a flat front facet and
(especially) lower capacitance, because it has to work at quite low
currents (5-10 uA).

Any suggestions?

Thanks

Phil Hobbs

PS:  Amazing how we're actually talking about electronics at the moment!

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

Isn't low current and high speed mutually exclusive, like intelligence
and Republicans?

Ah, politics!

I wonder why lefties always assume that conservatives are stupid.
That's the "what's wrong with Kansas?" dilemma, the lament that the
working class won't toe the party line and follow the lead of their
betters.

Conservatives generally assume that lefties are both stupid and evil.

John

On 2/6/2010 3:13 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/6/2010 12:41 PM, Joerg wrote:
miso_at_sushi.com wrote:
On Feb 5, 2:20 pm, Joerg <inva...@invalid.invalid> wrote:
Phil Hobbs wrote:
On 2/5/2010 2:13 PM, m...@sushi.com wrote:
On Feb 5, 8:56 am, Phil
Hobbs<pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz,< 50 pF) for an optical feedback
gizmo.
I have some Stanley DN310s, but they've been discontinued. Other
possibilities are:
Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125 pF typ
Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70 pF typ
Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other
specs
Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec
It would be really nice to find something with a flat front
facet and
(especially) lower capacitance, because it has to work at quite low
currents (5-10 uA).
Any suggestions?
Thanks
Phil Hobbs
Isn't low current and high speed mutually exclusive, like
intelligence
and Republicans?
Nice try, Soup. I enjoy talking electronics too much though.
His "email address" always makes me crave miso soup and sushi. So, we
just went to the Japanese restaurant in town ...

But this time I had pork teriyaki after the miso soup. We usually have
sushi in the evenings, I can't really work after that, it's so good
that
I tend to eat too much of it.

--
Regards, Joerg

http://www.analogconsultants.com/

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

Sushi doesn't make for an evening meal. Just too light, even when
supplemented with miso soup. It's better for lunch.


4-5 rolls (servings) with 4 people and I am really stuffed afterwards.
But it's delicious.


Getting back to electronics., this is really an i=c*dv/dt issue. It
seems to me all you can do is reduce C if current is limited.


If Phil has to switch constantly, yes. Otherwise there could be a
"spiking circuit" that swings the C with gusto.


It's inside a feedback loop that needs to have >1 MHz bandwidth, so
it's more a loop stability vs bandwidth issue. There are alternatives,
but they're all more complicated than what we've got. I'd probably
prefer to trade off efficiency rather than speed and capacitance.


Then you'll probably be in the market for the good stuff. This is an
example of one I've used in an optical feedback situation. I only needed
a little under 100MHz BW but it could have given me a lot more:

http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf


Hopefully not--that's megahertz, not gigahertz. But I do want to be
able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs

On Sat, 06 Feb 2010 15:51:29 -0500, Phil Hobbs<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/6/2010 3:13 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/6/2010 12:41 PM, Joerg wrote:
miso_at_sushi.com wrote:
On Feb 5, 2:20 pm, Joerg<inva...@invalid.invalid> wrote:
Phil Hobbs wrote:
On 2/5/2010 2:13 PM, m...@sushi.com wrote:
On Feb 5, 8:56 am, Phil
Hobbs<pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz,< 50 pF) for an optical feedback
gizmo.
I have some Stanley DN310s, but they've been discontinued. Other
possibilities are:
Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125 pF typ
Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70 pF typ
Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other
specs
Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec
It would be really nice to find something with a flat front
facet and
(especially) lower capacitance, because it has to work at quite low
currents (5-10 uA).
Any suggestions?
Thanks
Phil Hobbs
Isn't low current and high speed mutually exclusive, like
intelligence
and Republicans?
Nice try, Soup. I enjoy talking electronics too much though.
His "email address" always makes me crave miso soup and sushi. So, we
just went to the Japanese restaurant in town ...

But this time I had pork teriyaki after the miso soup. We usually have
sushi in the evenings, I can't really work after that, it's so good
that
I tend to eat too much of it.

--
Regards, Joerg

http://www.analogconsultants.com/

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

Sushi doesn't make for an evening meal. Just too light, even when
supplemented with miso soup. It's better for lunch.


4-5 rolls (servings) with 4 people and I am really stuffed afterwards.
But it's delicious.


Getting back to electronics., this is really an i=c*dv/dt issue. It
seems to me all you can do is reduce C if current is limited.


If Phil has to switch constantly, yes. Otherwise there could be a
"spiking circuit" that swings the C with gusto.


It's inside a feedback loop that needs to have>1 MHz bandwidth, so
it's more a loop stability vs bandwidth issue. There are alternatives,
but they're all more complicated than what we've got. I'd probably
prefer to trade off efficiency rather than speed and capacitance.


Then you'll probably be in the market for the good stuff. This is an
example of one I've used in an optical feedback situation. I only needed
a little under 100MHz BW but it could have given me a lot more:

http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf


Hopefully not--that's megahertz, not gigahertz. But I do want to be
able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs


You may wish to consider a laser diode operating below critical current.

On 2/7/2010 12:29 PM, JosephKK wrote:
On Sat, 06 Feb 2010 15:51:29 -0500, Phil
Hobbs<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/6/2010 3:13 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/6/2010 12:41 PM, Joerg wrote:
miso_at_sushi.com wrote:
On Feb 5, 2:20 pm, Joerg<inva...@invalid.invalid> wrote:
Phil Hobbs wrote:
On 2/5/2010 2:13 PM, m...@sushi.com wrote:
On Feb 5, 8:56 am, Phil
Hobbs<pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz,< 50 pF) for an optical
feedback
gizmo.
I have some Stanley DN310s, but they've been discontinued. Other
possibilities are:
Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125
pF typ
Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70 pF
typ
Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other
specs
Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec
It would be really nice to find something with a flat front
facet and
(especially) lower capacitance, because it has to work at
quite low
currents (5-10 uA).
Any suggestions?
Thanks
Phil Hobbs
Isn't low current and high speed mutually exclusive, like
intelligence
and Republicans?
Nice try, Soup. I enjoy talking electronics too much though.
His "email address" always makes me crave miso soup and sushi.
So, we
just went to the Japanese restaurant in town ...

But this time I had pork teriyaki after the miso soup. We
usually have
sushi in the evenings, I can't really work after that, it's so good
that
I tend to eat too much of it.

--
Regards, Joerg

http://www.analogconsultants.com/

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

Sushi doesn't make for an evening meal. Just too light, even when
supplemented with miso soup. It's better for lunch.


4-5 rolls (servings) with 4 people and I am really stuffed
afterwards.
But it's delicious.


Getting back to electronics., this is really an i=c*dv/dt issue. It
seems to me all you can do is reduce C if current is limited.


If Phil has to switch constantly, yes. Otherwise there could be a
"spiking circuit" that swings the C with gusto.


It's inside a feedback loop that needs to have>1 MHz bandwidth, so
it's more a loop stability vs bandwidth issue. There are alternatives,
but they're all more complicated than what we've got. I'd probably
prefer to trade off efficiency rather than speed and capacitance.


Then you'll probably be in the market for the good stuff. This is an
example of one I've used in an optical feedback situation. I only
needed
a little under 100MHz BW but it could have given me a lot more:

http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf


Hopefully not--that's megahertz, not gigahertz. But I do want to be
able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs


You may wish to consider a laser diode operating below critical current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to
be way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.

Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:
On Sat, 06 Feb 2010 15:51:29 -0500, Phil
Hobbs<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/6/2010 3:13 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/6/2010 12:41 PM, Joerg wrote:
miso_at_sushi.com wrote:
On Feb 5, 2:20 pm, Joerg<inva...@invalid.invalid> wrote:
Phil Hobbs wrote:
On 2/5/2010 2:13 PM, m...@sushi.com wrote:
On Feb 5, 8:56 am, Phil
Hobbs<pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz,< 50 pF) for an optical feedback
gizmo.
I have some Stanley DN310s, but they've been discontinued.
Other
possibilities are:
Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125
pF typ
Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70
pF typ
Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other
specs
Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec
It would be really nice to find something with a flat front
facet and
(especially) lower capacitance, because it has to work at
quite low
currents (5-10 uA).
Any suggestions?
Thanks
Phil Hobbs
Isn't low current and high speed mutually exclusive, like
intelligence
and Republicans?
Nice try, Soup. I enjoy talking electronics too much though.
His "email address" always makes me crave miso soup and sushi.
So, we
just went to the Japanese restaurant in town ...

But this time I had pork teriyaki after the miso soup. We
usually have
sushi in the evenings, I can't really work after that, it's so
good
that
I tend to eat too much of it.

--
Regards, Joerg

http://www.analogconsultants.com/

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

Sushi doesn't make for an evening meal. Just too light, even when
supplemented with miso soup. It's better for lunch.


4-5 rolls (servings) with 4 people and I am really stuffed
afterwards.
But it's delicious.


Getting back to electronics., this is really an i=c*dv/dt issue. It
seems to me all you can do is reduce C if current is limited.


If Phil has to switch constantly, yes. Otherwise there could be a
"spiking circuit" that swings the C with gusto.


It's inside a feedback loop that needs to have>1 MHz bandwidth, so
it's more a loop stability vs bandwidth issue. There are
alternatives,
but they're all more complicated than what we've got. I'd probably
prefer to trade off efficiency rather than speed and capacitance.


Then you'll probably be in the market for the good stuff. This is an
example of one I've used in an optical feedback situation. I only
needed
a little under 100MHz BW but it could have given me a lot more:

http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf


Hopefully not--that's megahertz, not gigahertz. But I do want to be
able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs


You may wish to consider a laser diode operating below critical current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.

On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:

You may wish to consider a laser diode operating below critical
current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.


This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine parlour
trick.

The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent. That's
a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.

Two photodiodes in series have the same photocurrent but *half the shot
noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage to
keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA which
produces the output. It's a really sweet solution overall, with the one
disadvantage that it needs two tweaks.

Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:

[...]

You may wish to consider a laser diode operating below critical
current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.


This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine parlour
trick.


Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent. That's
a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.


Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.


Two photodiodes in series have the same photocurrent but *half the shot
noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage to
keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA which
produces the output. It's a really sweet solution overall, with the one
disadvantage that it needs two tweaks.


I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?

Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:

[...]

You may wish to consider a laser diode operating below critical
current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.


This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine
parlour trick.


Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent.
That's a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.


Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.

Two photodiodes in series have the same photocurrent but *half the
shot noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage
to keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA
which produces the output. It's a really sweet solution overall, with
the one disadvantage that it needs two tweaks.


I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?

On 2/7/2010 12:29 PM, JosephKK wrote:
On Sat, 06 Feb 2010 15:51:29 -0500, Phil Hobbs<pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/6/2010 3:13 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/6/2010 12:41 PM, Joerg wrote:
miso_at_sushi.com wrote:
On Feb 5, 2:20 pm, Joerg<inva...@invalid.invalid> wrote:
Phil Hobbs wrote:
On 2/5/2010 2:13 PM, m...@sushi.com wrote:
On Feb 5, 8:56 am, Phil
Hobbs<pcdhSpamMeSensel...@electrooptical.net
wrote:
I need a fast IR LED (> 20 MHz,< 50 pF) for an optical feedback
gizmo.
I have some Stanley DN310s, but they've been discontinued. Other
possibilities are:
Vishay TSFF5410 -- 870 nm, 0.% W/A typ 15 ns rise/fall, 125 pF typ
Vishay VSLB3940 -- 940 nm, 0.4 W/A typ 15 ns rise/fall, 70 pF typ
Panasonic LNA4905L -- 880 nm, 0.3 W/A min 30 MHz typ, no other
specs
Osram SFH4550 -- 850 nm, 0.5 W/A typ 12 ns rise/fall, no C spec
It would be really nice to find something with a flat front
facet and
(especially) lower capacitance, because it has to work at quite low
currents (5-10 uA).
Any suggestions?
Thanks
Phil Hobbs
Isn't low current and high speed mutually exclusive, like
intelligence
and Republicans?
Nice try, Soup. I enjoy talking electronics too much though.
His "email address" always makes me crave miso soup and sushi. So, we
just went to the Japanese restaurant in town ...

But this time I had pork teriyaki after the miso soup. We usually have
sushi in the evenings, I can't really work after that, it's so good
that
I tend to eat too much of it.

--
Regards, Joerg

http://www.analogconsultants.com/

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

Sushi doesn't make for an evening meal. Just too light, even when
supplemented with miso soup. It's better for lunch.


4-5 rolls (servings) with 4 people and I am really stuffed afterwards.
But it's delicious.


Getting back to electronics., this is really an i=c*dv/dt issue. It
seems to me all you can do is reduce C if current is limited.


If Phil has to switch constantly, yes. Otherwise there could be a
"spiking circuit" that swings the C with gusto.


It's inside a feedback loop that needs to have>1 MHz bandwidth, so
it's more a loop stability vs bandwidth issue. There are alternatives,
but they're all more complicated than what we've got. I'd probably
prefer to trade off efficiency rather than speed and capacitance.


Then you'll probably be in the market for the good stuff. This is an
example of one I've used in an optical feedback situation. I only needed
a little under 100MHz BW but it could have given me a lot more:

http://www.excelight.com/pdf/device/PD/SPT2400-x(revC).pdf


Hopefully not--that's megahertz, not gigahertz. But I do want to be
able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs


You may wish to consider a laser diode operating below critical current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to
be way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Cheers

Phil Hobbs

On Sun, 07 Feb 2010 14:10:48 -0800, Joerg <invalid_at_invalid.invalid
wrote:

Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:
[...]

You may wish to consider a laser diode operating below critical
current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.

Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.

This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine parlour
trick.

Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent. That's
a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.

Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.


Two photodiodes in series have the same photocurrent but *half the shot
noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage to
keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA which
produces the output. It's a really sweet solution overall, with the one
disadvantage that it needs two tweaks.

I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?

Sounds fascinating! More info please ;-)

On 2/7/2010 5:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 4:10 PM, Joerg wrote:
Phil Hobbs wrote:
On 2/7/2010 12:29 PM, JosephKK wrote:

[...]

You may wish to consider a laser diode operating below critical
current.


Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be
way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity
gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Or have an offset in there where the LED (or LD below lasing threshold
as Joseph suggested) runs at a regulated base power level. BTDT, but in
my case that was in order to remain above lasing threshold.


This gizmo is an advanced photoreceiver that maintains
shot-noise-limited performance (2 dB above shot noise) from ~10 nA to
100 uA, with an honest 1 MHz bandwidth over (almost) the whole range.
Doing that down near the minimum photocurrent is a real genuine
parlour trick.


Luckily I never had to do that. BW was always tens of MHz but they gave
me plenty of amplitude to work with. However, up there on that pedestal
it had to be super low noise because we had to extract modulation.


The ones uses two photodiodes wired in series (!) to get a
sub-Poissonian photocurrent to null out the primary photocurrent.
That's a trick I've never seen before, so I might have invented it. It
obviously requires some careful feedback to keep the currents in
balance, but the result is a nice linear photoreceiver with almost no
additional input capacitance.


Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what
they do is trigger patent trolls who then bog down whole businesses.


I can patent it for the next year, at least in the USA. I might do
that, we'll see.

Two photodiodes in series have the same photocurrent but *half the
shot noise*, so the cancellation current is actually quieter than the
photocurrent, without needing resistive degeneration. (I also manage
to keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of
the LED light goes to another photodiode, driving an ordinary TIA
which produces the output. It's a really sweet solution overall, with
the one disadvantage that it needs two tweaks.


I assume you mean the balancing of the two PDs in series. Is there no
way to servo that? Maybe by occasionally interrupting the optical path?


There's a bias feedback loop that looks after that. It doesn't have to
be that accurate since the PDs run at 14V of reverse bias--keeping the
junction of the two PDs reasonably still is all that's required.

The tweaks are for making sure that the two photocurrents are reasonably
close to begin with, and to govern the poorly specified efficiency of
the LEDs. (IR LEDs have output power specs that are almost as loose as
the V_T spec of your average JFET.)

You should be able to buy them in a couple of months, if all goes well.
(No home should be without one, after all.) ;)

On Sun, 07 Feb 2010 12:56:22 -0500, Phil Hobbs <pcdhSpamMeSenseless_at_electrooptical.net> wrote:

On 2/7/2010 12:29 PM, JosephKK wrote:

You may wish to consider a laser diode operating below critical current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power
range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to
be way more than enough at the high end, and the problem is to keep the
feedback poles from crossing at a frequency where there's over-unity gain.

There are other approaches possible that require different approaches,
but they require more tweaking--e.g. two ranges with two LEDs using
different optical coupling fractions.


Cheers

Phil Hobbs

Perhaps accurately controllable optical attenuation can be done?
Don't know myself but it sounds possible to me.