[Laser] Reasonable Sensitivity Checks ?

[KY1K]

Hi Steve,

I think your receiver is quite deaf.

Even though you have a capacitively coupled output, your photodiode 
should be saturating, and you should not be able to hear anything in 
the daylight! If you measure your output before the blocking cap, it 
should show saturation with any ambient light during the daytime. Of 
course, you can't measure across the photodiode directly:>: An ac 
coupled photodiode is a bit atypical.

Your photodiode appears to be ac coupled, which in general is a bad 
idea....except for wideband aps where the output frequencies are in 
MHz or higher. For narrowband aps such as most of us use, avoid the capacitor.

Strongly suggest you ignore using your ears and listening to noise as 
an indicator and test in darkness only. This eliminates the 
'daylight' factor, which clouds the issue.

A standard intensity light source (for receiver sensitivity 
quantification) is easier said than done. I've used all kinds of 
sources and light attenuating schemes. All are relative and none are 
repeatable.

Even the dark nighttime sky is not a good standard because it's 
widely variable depending on location and weather.

Can you send a schematic of your system????

I suspect very strongly that your problem might be a combination of 
electrical and mechanical. Perhaps there is no electrical problem at 
all though. The photodioes from cd players are quadrant types with 
very very very small active area. This is necessary to allow for the 
reception of high speed pulses present in the data stream of a cd.

With 100 uM photodiodes used by most cd players, it is very unlikey 
that you can capture all the light that falls on the lens due to the 
small active area on the photodiode. This could be a cause of your 
receiver appearing to be deaf.

You will need to shield the electronics with any kind of high 
sensitivity narrowband receiver.

With a 4 inch lens, I can detect a low power LED at over 1000 feet.

Ultimately I don't think you will be happy with that hardware. It's 
more appropriate for high speed data links or video.

Burr-Brown makes an OPT-101 which includes a .09 inch by .09 inch 
photodiode and amp in the same package and has a built in amp. All 
for $6. They can be bough for much less though. They are 2 dollars 
each on ebay right now, but you have to buy 5. In an 8 pin DIP package! See:

http://cgi.ebay.com/Burr-Brown-Monolithic-Photodiode-OPT101P-OPT101-5_W0QQitemZ7561982142QQcategoryZ7287QQrdZ1QQcmdZViewItem

There is a link to the spec sheet.

There are some even nicer opt-202's there in a 5 lead clear plastic 
package for $1.25 each. See:

http://cgi.ebay.com/Burr-Brown-Photodiode-OPT202-4_W0QQitemZ7563477131QQcategoryZ4663QQrdZ1QQcmdZViewItem

Can you send the schematic or post it to the web somewhere?

GL.

Art


At 10:52 AM 11/16/2005, you wrote:

>I finally got back to working on my laser gear and now
>have a basic receiver going (60mm lens, unbiased
>CD-player detector capacitor-coupled to high-impedance
>MPF102-2N5088 preamp, followed by half a TL072 and an
>LM380N-8 feeding headphones).  Audio bandwidth is
>about 1 kHz at -3 dB points with peak response near
>500 Hz (down nearly 10 dB from the peak response at
>120 Hz, in theory).
>
>Are there any simple tests to check for reasonable
>sensitivity ?  I am not, at this stage, looking for
>the ultimate in performance...just something to get
>started with.
>
>So far I have observed that I can tell the difference
>in noise level between pointing out the window at a
>cloudy daytime sky and pointing into the room (room
>lights off).  However, this difference is not huge,
>and the receiver does not saturate on the daytime
>cloudy sky noise (keep in mind that due to the
>capacitive coupling it can not saturate due to DC
>levels alone).  I have been able to detect
>streetlights at night at up to about 500m distance and
>a radio tower strobe light in the daytime at around 5
>km (gives just a popping sound in the headphones).
>
>Can anyone comment from their own experience on
>whether this sounds reasonable ?
>
>Steve VE3SMA
>
>
>
>
>
>
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