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'[EE]: Lasers again'
2001\02\03@222812 by James Newton

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But as Matt says, the rise and fall times in the transducers just kills
it...

...everybody on this thread seem to be ignoring my post on Optical
Ranging...

...Its simple, its cheap, its PICable, its the way to go...

Just see:
www.piclist.com/io/sensor/pos/opticalranging.htm
and
http://www.cyberg8t.com/pendragn/actlite.htm

for a complete description. Am I missing something?

James Newton, PICList Admin #3
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{Original Message removed}

2001\02\03@225933 by Sean H. Breheny

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Hi James,

I must really be missing something here. AFAIK, LED rise times are usually
a few ns, maybe tens but not 100s. As for the receiving end, it is really
governed by the amplifier you hook to the photodiode (and how you bias it,
etc.)

As I said, I have sent 1 MHz signals over cheap LEDs (they were actually
metal can, high power IR types, but still only $3 or so) and cheap
photodiodes. We aren't talking about pulsed signals here, right? Just
comparing phase.

I saw your page on optical ranging ,and it looks interesting, I am just
curious about why people seem to insist that LEDs can't handle 1MHz
signals. I would also recommend that people check out the triangulation
method, which is the way many autofocus cameras do it. For example, see

http://www.hobbyrobot.com/info/gp2d02/index.html

Sean



At 07:08 PM 2/3/01 -0800, James Newton wrote:
{Quote hidden}

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2001\02\04@001639 by Matt Bennett

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"Sean H. Breheny" wrote:
>
> Hi Matt,
>
> Aren't you talking about modulating with a sine wave? If so, why do you
> need such fast rise times? I have done >1MHz with cheap photodiodes and
> LEDs (although, I wasn't doing ranging, I was experimenting with high speed
> data transmission).

For the XOR phase detector to work right, it needs to be full on/ full
off, I'm not sure how a traditional mixer based phase detector works
with two very different amplitude signals, I'm guessing that you need
run an AGC, which adds some complexity that I'd rather not have.  I'd
rather just use the XOR gate and look at the edges, and not worry about
the amplitude.

For really high speed data (gigabit ethernet, for example) it is really
just an on/off modulation- there are no intermediate steps, so I know it
can be done, and the XOR method is nice and simple.

Matt

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2001\02\04@002905 by Matt Bennett

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> I must really be missing something here. AFAIK, LED rise times are usually
> a few ns, maybe tens but not 100s. As for the receiving end, it is really
> governed by the amplifier you hook to the photodiode (and how you bias it,
> etc.)

From my experimentation, the rise/fall times in LEDs (especially the
high power LEDs)
are pretty poor, they're engineered for optical power, not low
capacitance.  The normal photodiodes I've used have way too much
capacitance- more than a few pF, and you just can't get good sharp
rise/fall times.  I'd like to get a 10MHz signal, 50% duty cycle, with
under 5ns rise/fall times, which, if I remember the equations, is about
a 280 MHz bandwidth. (which is still probably easier than a 500 MHz
counter)

{Quote hidden}

James, I'm not ignoring it, I'm just more interested in the phase
detection method.  Using triangulation, the error largely dependent upon
range, and the distance between the transmitter and sensor.  I'm not
trying to put out a product- I'm trying to make something that intrigues
me.  If I absolutely had to get something out, I'd be much more
interested in triangulation.

Matt

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2001\02\04@005116 by Randy Glenn

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-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

I was skeptical about the simple image subtraction thing, but after
trying it...

Wow.

I used my crappy little QuickCam Express and took a photo of my
whiteboard. Then I drew a line on the whiteboard, and took another
photo. Imported both as layers into Paint Shop Pro, and set the blend
mode to difference. White background, text, drawings - gone. Only the
line remains.

If you could get a laser line to show up on a black and white imager,
it would be sooo easy to do with a PIC - scan one frame into some
storage, kick in the laser, scan in another frame - same storage
space, just subtract and take the absolute value. Simple math from
there on in.

- -Randy Glenn

This coming from the guy with a system tray 7 icons wide... by 2
tall...
=================================================
Randy_Glenn-at-tvo.org - PICxpert-at-picxpert.com
PICxpert-at-yahoo.com - PICxpert-at-home.com
           http://www.picxpert.com/
=================================================

- {Original Message removed}

2001\02\04@120510 by Olin Lathrop

face picon face
> I was skeptical about the simple image subtraction thing, but after
> trying it...
>
> Wow.
>
> I used my crappy little QuickCam Express and took a photo of my
> whiteboard. Then I drew a line on the whiteboard, and took another
> photo. Imported both as layers into Paint Shop Pro, and set the blend
> mode to difference. White background, text, drawings - gone. Only the
> line remains.

That sounded like such a good idea I had to try it myself.  I agree with
Randy.  Wow!  I would have included the results in this email message, but I
figured that the images would exceed the PIClist line length limit.  I
therefore set up a web page at http://www.embedinc.com/temp for those of you
that would like to see the results.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\02\04@202741 by rottosen

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With respect to risetime verses bandwidth:
An approximation is Risetime = 1/Frequency * 0.35.
This gives 70MHz bandwidth for a 5ns risetime. Not trivial but a lot
easier than 280MHz. There are a lot of voltage and current feedback
opamps available for these frequencies and risetimes. Not terribly
expensive either.

-- Rich


Matt Bennett wrote:
{Quote hidden}

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2001\02\04@222558 by Matt Bennett

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Richard Ottosen wrote:
>
> With respect to risetime verses bandwidth:
> An approximation is Risetime = 1/Frequency * 0.35.
> This gives 70MHz bandwidth for a 5ns risetime. Not trivial but a lot
> easier than 280MHz. There are a lot of voltage and current feedback
> opamps available for these frequencies and risetimes. Not terribly
> expensive either.
>


Thanks- I guess I did remember the equation wrong.  This also brings the
frequency well within what I would expect from some fast TTL... now the
big question is, how do I modulate the LED and recieve it that fast?  I
remember reading in a scope manual about how two consecutive rise times
affect each other- something like a 10ns rise time into an amplifier
with a 10ns rise time gives about a 14ns rise time? (I seem to remember
a sqrt(2) in there).  I guess the LED needs about a 100MHz BW from the
LED to get the 5ns rise time out of a 70MHz reciever.

So, how about it... anyone have circuits for transmitting/recieving
light modulated at about 100MHz?  Gigabit Ethernet does it at over 1GHz,
and OC192 approaches 10 GHz optically, so there should be a way.

Matt

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2001\02\05@052452 by Michael Rigby-Jones

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{Quote hidden}

The likes of Maxim, Analog Devices make laser drivers, limiting amps, AGC
amps and CDR IC's.  The company I work for even make their own, although
they are not marketed externaly AFAIK.  HP make a fair range of high speed
amps suitable for PIN diodes.

Direct modulated 10GHz is amazingly tricky.  Most 10GHz systems use a CW
laser and some kind of external modulator, e.g. mach zhender or elctro
absorbtion.

However, getting the driver is only half of the battle.  PCB design is also
pretty important with such fast rise times, to avoid ringing closing the
optical eye.  You *may* be able to get an eval board from one of the above
with some smooth talking, although don't bank on it.

Mike

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2001\02\05@065222 by Roman Black

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Hi Matt, so you really want to do this? Why 100MHz?
I would use a lower frequency, the commercial units
do. Maybe 5MHz?

What distance and resolution did you want to achieve?
:o)
-Roman



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2001\02\05@154015 by Peter L. Peres

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>For the XOR phase detector to work right, it needs to be full on/ full
>off, I'm not sure how a traditional mixer based phase detector works
>with two very different amplitude signals, I'm guessing that you need
>run an AGC, which adds some complexity that I'd rather not have.  I'd
>rather just use the XOR gate and look at the edges, and not worry about
>the amplitude.

A traditional phase detector deals just fine with signals of very
different amplitude, because it is a multiplier so Uo = K * Ui1 * Ui2. Of
course you need to achieve amplitude calibration, but instead of AGC I'd
suggest a limiter amplifier designed for FM broadcast service. This will
pass the phase information undisturbed and provide plenty of gain (and
even AGC). It also contains a multiplier (product detector) that could be
hijacked to do the detection. A MC3189 comes to mind, or older TBA810(??),
or something much newer but more difficult to obtain.

Peter

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2001\02\05@184050 by Olin Lathrop

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> A traditional phase detector deals just fine with signals of very
> different amplitude, because it is a multiplier so Uo = K * Ui1 * Ui2. Of
> course you need to achieve amplitude calibration, but instead of AGC I'd
> suggest a limiter amplifier designed for FM broadcast service. This will
> pass the phase information undisturbed and provide plenty of gain (and
> even AGC). It also contains a multiplier (product detector) that could be
> hijacked to do the detection. A MC3189 comes to mind, or older TBA810(??),
> or something much newer but more difficult to obtain.

Even easier would be to create digital square waves from both signals from
their zero crossings, then go back to the XOR method.  You might want to run
each raw square wave thru a divide by two counter to make sure they are
really square.  In other words, only use one of the two zero crossing per
cycle from each input signal.  That's all you need to measure phase angle.


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(978) 772-3129, RemoveMEolinspam_OUTspamKILLspamembedinc.com, http://www.embedinc.com

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2001\02\06@142328 by hard Prosser

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Just to add a note toward the end ? of the thread:-
We used this method to measure the elongation of fibre optic cables during
laying. A mirror was formed at the far end of the fibre and at the near
(stationary) end a laser source was fitted, modulated at 200MHz . To
improve resolution a number of fibres were spliced though so the actual
fibre length was about 6 times the cable length (total signal path length
of 12km/cable km).  An optical directional coupler fed the signal reflected
back from the far end of the cable to a detector (apd?) , preamp and an HP
network anaylser. The other channel of the network anaylser was fed
directly from the 200MHz source. By precalibrating the fibre on a tensile
tester and using phase measurement we were able to resolve to about 5mm of
elongation on a 1km length of fibre.  This was used to calculate residual
stress on the fibre and get an estimate of its likely lifetime - along with
producing a direct comparison of different cable laying techniques and
equipment.

Richard P




> A traditional phase detector deals just fine with signals of very
> different amplitude, because it is a multiplier so Uo = K * Ui1 * Ui2. Of
> course you need to achieve amplitude calibration, but instead of AGC I'd
> suggest a limiter amplifier designed for FM broadcast service. This will
> pass the phase information undisturbed and provide plenty of gain (and
> even AGC). It also contains a multiplier (product detector) that could be
> hijacked to do the detection. A MC3189 comes to mind, or older TBA810
(??),
> or something much newer but more difficult to obtain.

Even easier would be to create digital square waves from both signals from
their zero crossings, then go back to the XOR method.  You might want to
run
each raw square wave thru a divide by two counter to make sure they are
really square.  In other words, only use one of the two zero crossing per
cycle from each input signal.  That's all you need to measure phase angle.


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(978) 772-3129, EraseMEolinspamspamspamBeGoneembedinc.com, http://www.embedinc.com

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2001\02\06@183020 by Peter L. Peres
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>Even easier would be to create digital square waves from both signals from
>their zero crossings, then go back to the XOR method.  You might want to
>run each raw square wave thru a divide by two counter to make sure they
>are really square.  In other words, only use one of the two zero crossing
>per cycle from each input signal.  That's all you need to measure phase
>angle.

I think that I would not pick the zero crossings of a noised-over signal
for anything, especially not for extracting phase. I would pick the areas
with the maximum amplitude which contain most energy (and best S/N). A
multiplier does precisely this, but there are other ways. To obtain
squarewave signals from a low amplitude noised over signal one usually
uses a high gain low noise amplifier with limiting in the last stages. The
FM receiver/demodulator systems do just this and have a good S/N figure.
I'd be very tempted to modify a FM radio from the junkbox and use a PIN
photodiode connected to the 10.7MHz IF directly (with a polarization
circuit), and a laser modulated by a 10.7MHz oscillator (using a ceramic
filter probably). I'd also be tempted to transmit 38400 Bauds digital data
through this system (I have done experiments with 455kHz AM and 9600
Bauds using IR LEDs before).

Since I have some experience with such circuits, I say that it will be
very hard to keep the transmitted signal's RF from entering the receiver
and obliterating any received signal. The separation must be 120dB or
better even for small distances. This is *very* hard to do in my
experience, especially if the two units share a power supply or the same
box (and if the box must be smaller than a Chubb safe).

Peter

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