LED Frequency Ranges

[LightBoy]

Hello,

I am working with some "super bright LED's" - so called - and I have
been looking for "specs" on the upper limit of how fast (Frequency) at
which LED's can be modulated. Measureably switched on and off.

I can find nothing doing a Google search on the subject. Can anyone
give me a place where I can go (on the Internet that is - LOL) where
such information might be obtainable? Or information that you may be
aware of on this subject? The place I bought these LED's has lots of
specs - but none relating to this capability.

Thanks so much for any help!

Don H

 

[Larry Brasfield]

"LightBoy" <don@nitestarrs.com> wrote in message
news:1111040989.017475.233240@z14g2000cwz.googlegroups.com...

Hello,
Hi.
I am working with some "super bright LED's" - so called - and I have
been looking for "specs" on the upper limit of how fast (Frequency) at
which LED's can be modulated. Measureably switched on and off.

I can find nothing doing a Google search on the subject. Can anyone
give me a place where I can go (on the Internet that is - LOL) where
such information might be obtainable?

I once tried to find the same kind of data, and found a SLED
(superluminescent diode) with the feature "pulsed operation"
http://www.nolatech.ru/nolatecheng.htm#b6
I wrote to the manufacturer to see if they would part with
some data as to what that meant, and got a nice reply with
some frequency response data. Sorry I cannot lay hands
on it. (It was for a one-time job.) But you should be able
to retrace that path.

Or information that you may be
aware of on this subject? The place I bought these LED's has lots of
specs - but none relating to this capability.

All I remember was that it was much faster than necessary
to be considered flat in the 100 KHz to 10 MHz band.

You might see if you can find anything on the diffusion
capacitance of GaAs diodes. That relates directly to
carrier lifetime which in turn controls emission frequency
response since emission is directly related to carrier
density/population.

Thanks so much for any help!
Good luck. You'll need it as I recall.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[Paul Mathews]

Virtually all LEDs have rise/fall times in the range of 10nsec to
1usec. For 100% modulation, the upper limit for LEDs is in the
neighborhood of 20MHz for types designed to be modulated (e.g.,
communications devices at 850nm wavelength). The upper limit is
usually just a MHz or less for devices optimized for illumination
energy output.
It doesn't take much equipment to make your own measurements. Just be
sure to use a really small photodiode, a small load impedance, and to
keep LED and photodiode far enough apart that you're not looking at
capacitive or magnetic coupling between them. You can check for either
by blocking the light with an opaque non-metallic object: any light
coupling should go away.
Paul Mathews

 

[LightBoy]

Thanks to both of you - Paul and Larry for the information. At least I
now have some clue and something more to go on - which is more than I
had when I originally wrote my question. I just wanted to be sure they
would modulate to at least 500KHz and from what you are telling me -
that should not be a problem.

Best to both of you!

Don H

 

[James Meyer]

On 16 Mar 2005 22:29:49 -0800, "LightBoy" <don@nitestarrs.com> wroth:

Hello,

I am working with some "super bright LED's" - so called - and I have
been looking for "specs" on the upper limit of how fast (Frequency) at
which LED's can be modulated. Measureably switched on and off.

Don H

So far, all the responses have concerned "bare" LEDs. I'll be willing
to bet that "white" LEDs, the ones that use a UV LED to excite a white phosphor,
will have dramatically slower rise and fall times because of the phosphor.

Jim

 

[Tom Becker]

... modulate to at least 500KHz ...

I use conventional T1-3/4 LEDs driven by a stiff square 50% 40mA source at
6MHz. You will have no difficulty at 500KHz.

Tom