I just ran onto a situation where a modulated laser impinging upon a photodiode had such a high light level that the photodiode was "saturated"... no more available carriers.
Thus "clipping" of the modulation was occurring in the photodiode itself, creating harmonics.
And lowering the DC bias on the laser raised the AC signal level... really strange lab results to ponder when you're tired ;-)
Any ideas how one might *model* the photodiode in Spice?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
How can it run out of carriers? Photons make h-e pairs, and more photons should make more, right? Does high pair density increase recombination before the charges can be collected? Is it possible that the photocurrent is limited by ohmic effects? The DC effect does need pondering.
One of the makers of GaAs photodiodes told us that their devices would limit above about a milliwatt optical input, so we got access to a multi-watt pulsed fiber source, and it turns out they were wrong.
John
I don't, didn't have to use SPICE on that section in my current design. But the optical folks have attenuators for that case.
This had me puzzled as well. The common DFB modules these days pump out
5mW-20mW but photodiodes tend to saturate around 1mW-2mW. So we'll attenuate in order to be able to run the DFB at reasonable current levels. I guess it's all for the telco market where they assume a mile or more of fiber.If you come across a DFB that is really, really low in phase noise let me know ;-)
-- Regards, Joerg http://www.analogconsultants.com
Hell! I don't know, I'm a circuit guy, not a physicist ;-)
10V reverse bias.
Problem cured by going to a larger photodiode.
This data is all remote provided... lab in Atlanta, me in Phoenix.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I was told that besides non-linearity there comes a point beyond which there would be irreversable damage. Also, the datasheet for the Sumitomo diode I am using here states 2mA Ir as the abs max limit.
BTW I found this to be a pretty good paper:
-- Regards, Joerg http://www.analogconsultants.com
That can reduce your selection. Some are spec'd at 5V max, so I went from 12V to 5V.
Paper said it was 22F at night. Ask them to scrape the ice off the PD's front and measure again :-)))
-- Regards, Joerg http://www.analogconsultants.com
Jim,
The model would be a light controlled current source. PD's have about
0.8A/W conversion ratio. Double the light POWER and double the current output. his is a log convertsion ratio. so it is never really "linear" but always log. This continues till the voltage acrros the diode (the signal) is about 1/10 the reverse bias and then you get somne cross prodcut distortion. adign more power and then you saturate the amplifer front end, add more power and there is a power density/max current which is athe peak power limit.If you send the schematic Ill give it a lokk
Marc Popek
I'm not saturating the amplifier... that was the first thoughts, given peculiarities in a SwitCap filter output (harmonics) that followed the TIA.
But something is happening in carrier generation... reducing light, or going to a different PD (larger) solves the issue.
I'm just looking for some direction on how running out of carriers might be modeled. It's sort of like channel saturation in a MOSFET or maybe current-crowding in a bipolar device.
Can't do that... proprietary... start-up with VC money ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...
You'll have to give us your take on whether or not the company will make it. :-)
I have a friend who worked for a number of years as a consultant (these days he's an FAE at a big distributor) who said that of all the consulting jobs he worked one, although in the end the widget always worked and the schedules were generally close to being on time, only something like 1 in 10 ever ended up with a profitable product!
This is high-end medical. I think it has a very good chance of success. After all, they hired me to convert their concept into working schematics ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Hmm, that's very different in med electronics. It's pretty much 100% to production there, with many of the designs being produced longer than many car models are.
-- Regards, Joerg http://www.analogconsultants.com
I often wonder why you don't start a manufacturing company. With your skills and background, you should do very well, especially since you already have the experience running that kind of operation.
You are probably doing very well consulting, but boy, you could make a mint making and selling your own product line. For you, the entire field is wide open. There's always the Chinese to think about. But you could start in niche areas that require an excellent technical background for support, and where sales are largely word-of-mouth. By the time they catch on, you could dominate the market and it would be difficult or impossible for them to break into it.
It may take a year or so, but I'd bet you would be a great success.
Regards,
Mike Monett
Antiviral, Antibacterial Silver Solution:
Thanks for the kudos. Well, what can I say, we live in California and when you do production or sales of OEM'd goods the bureaucrats can get in the way really fast.
I did run a division of a company for a few years until we were bought. This included manufacturing and it was great fun. But we had our bouts with strange rules being imposed (that was before the previous governor was recalled). Example: The whopper was a new law that required overtime pay after 8hrs/day instead of after 40hrs/week. Keeping 10hr shifts suddenly became a real financial problem. But we had people who needed that so they could take 4day/week turns and care for frail relatives. I had people burst into tears in my office. That might have all made sense to some union boss but it sure didn't to the workers.
Much of the stuff I designed is being produced in China. Works great.
Maybe in another location :-)
-- Regards, Joerg http://www.analogconsultants.com
You'd love Arizona. I can even point you to a location very similar in weather to where you live now ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Oh, I do love it there. Spent quite some time in the PHX area and north of it in the 90's. But when you said something about "being californicated" a while ago that had we worried.
-- Regards, Joerg http://www.analogconsultants.com
There you go. You already have your manufacturing facilities and necessary contacts. You know who to deal with and who to avoid. That information is priceless.
Just send them your new designs and get them to put your name on it. It's handed to you on a platter - you couldn't ask for a better opportunity.
Make sure you tell your current customers you would never go into competition with them. Then keep that promise until after you make your first billion:)
Regards,
Mike Monett
Antiviral, Antibacterial Silver Solution:
Just the cost of housing in the PHX metro area.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I'm not a big Spice guy--I use it about twice a year--but the physics of what's going on is probably local forward bias caused by big lateral voltage drops in the (very thin) epi layer. That's usually the situation with modulated CW beams. Femtoseond beams can do truly evil things to photodiodes, messing up the populations etc, but for reasonable duty cycles the diode generally melts first.
Your first line of defense is a big fat reverse bias.
Cheers,
Phil Hobbs
Hi Phil,
I have 10V reverse bias, but it's behaving as if the PD is running out of carriers.
A larger PD behaves itself.
Modulation is ~2KHz
If running out of inducible carriers is a possibility I can probably figure out how to behaviorally model it. I'm just trying to match theory to lab.
Pretty durn close except for this anomaly ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
What color laser?
I'm sure the photodiode is sensitive to a certain wavelength. This is probably part of the problem. Increasing the die size is masking it.
I think I may be familiar with a few companines that might be involved in this. Were they at this years RSNA?
Cheers
"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.