[Laser] Re : Strange effect

Sun Mar 18 12:00:35 EDT 2012

Hi Yves,

And I say to all of us on the "Laser" list. And thank you, Yves, for your input answers to Tim's question. I have several of these Pyro units I picked up on ebay some time back and was wondering, based on your comments, how to test them for frequency response or does it really matter? I have a Wavetek generator that goes down to 0.4Hz (or 2.5 seconds per cycle), so I can get real slow.

Mike C.
sandbar w/fl

--- On Sat, 3/17/12, Garnier Yves <f1avy at yahoo.fr> wrote:

From: Garnier Yves <f1avy at yahoo.fr>
Subject: [Laser] Re :  Strange effect
To: "Free Space LASER Communications" <laser at mailman.qth.net>
Date: Saturday, March 17, 2012, 8:34 AM

Hello Tim
The pyro has two limiting speed response parameters
Its own cristal thermal inertia and the electrostatic pyroelectric loading and unloading speed.
The cristal
 thermal inertia can be from 1 second to 1/100 second duration range versus its size and its thickness.
The electrostatic pyroelectric voltage changing speed depends highly the cristal loading resistor value and the cristal capacitance. 
The lowest this resistor the highest the speed but the lowest the sensitivity and the NEP...
In normal use as a motion detector, the internal FET is mounted as a source follower design that give gigantic impedance but a very slow response
With a charge amplifier design, the cristal sees an artificial grounded input via the feedback capacitor and has this way a low dynamic load.
The charge is high speed transfered in the feed back capacitor but the price to pay after an optical pulse is a slow recovery slope from this capacitor.
The K3PGP RX gives the same effect with a pyro or a photodiode.
A very short optical pulse is transfered in the Miller drain-gate capacitance of the FET and the output signal is followed by a slow recovery
 slope.
The pulse can  be recovered in the following stage by a signal differentiator to remove the slow slope and to keep the high speed transiant rising step.
In my schematic I use a 100 K adjustable resistor with 10 MF decoupling for the source of the internal FET and a 100 K resistor to the drain.
The lowest the current in the FET integrated into the pyroelectric detector case, the lowest its spurious thermal effect.   
Have a look on these datas :
http://www.lasercomponents.com/de/?embedded=1&file=fileadmin/user_upload/home/Datasheets/pyreos/py-ftir-01.pdf&no_cache=1
http://eltecinstruments.com/PDF/Dn/ELTECdata%20100.pdf
73

Yves F1AVY
http://f1avyopto.wifeo.com

________________________________
 De : Tim Toast <toasty256 at yahoo.com>
À : laser at mailman.qth.net 
Envoyé le : Samedi 17 mars 2012 11h12
Objet : Re: [Laser] Strange effect

Thanks for the info Yves. 
Well, i see that it would be difficult to detect then with both 
of those effects conspiring. 
But, so this means you can detect fast pulses at 10 microns, just 
like the PGP works with regular silicon detectors? Nice! 
I had read the pyrodetectors where meant for relatively slow, 
>1 Hz pulse rates or changes in motion
 detectors, but maybe that
doesnt apply here. Pyros are not even diodes, are they? Are you
biasing it or working at zero bias? 
Since the pyro is not a diode, i'm guessing it can produce a true 
AC signal to the FET gate without a DC blocking capacitor.

-toast

------------------------------

Message: 2
Date: Fri, 16 Mar 2012 12:28:31 +0000 (GMT)
From: Garnier Yves <f1avy at yahoo.fr>
Subject: [Laser] Re :  Re : Strange effect
To: Free Space LASER Communications <laser at mailman.qth.net>
Message-ID: <1331900911.4269.YahooMailNeo at web29504.mail.ird.yahoo.com>
Content-Type: text/plain;
 charset=iso-8859-1

Hi Tim

A pyroelectric detector is not able to d?tect a very slow optical decreasing and the silicon layer of the Pin photodiode used as LED has a rather small thermal inertia.

My pyroelectric detector with two reverse side cristals for thermal compensating (one of them is absorbing) is mounted as a classical K3PGP optical receiver.

A K3PGP receiver also works as a charge amplifier with the FET drain to gate capacitor as a Miller capacitance.

This enhances greatly the rising or decreasing slope of the output signal !

73

Yves F1AVY
http://f1avyopto.wifeo.com

________________________________
De?: Tim Toast <toasty256 at yahoo.com>
??: laser mailinglist <laser at mailman.qth.net> 
Envoy? le : Vendredi 16 mars 2012 12h38
Objet?: [Laser] Re : Strange effect

Dave,
That is probably the reason. Without looking it up, that seems to ring a bell somewhere 
about silicon emitting in the deep IR. I hadn't fully realized till now that all LEDs?might not?
have the near same detect + emit wavelengths. It makes me wonder how some of the 
other LED compositions? wavelengths are arranged too. I think i've read, in the 
UKnanowaves group, they had used green and blue LEDs in their (single led type) 
transceivers with good results. Those both may use gallium arsenide though, i don't 
know. I was thinking the blue LEDs used a nitride based construction.
?
Yves, 
You may be able to tell whether the pyrodetector is seeing the actual 1.1 micron 
radiation or the 10 micron heat, by
 turning the silicon detector on and off rapidly. 
If the
 pyrodetector is just seeing the heat generated, then the silicon detector should 
continue to "glow" for a short time after it is turned off (as it cools off) rather than a 
crisp instant?on/off responce.. It may be sensitive to a combination of both even.
?
-toast
?
?

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