[Laser] differential detector

[TWOSIG at aol.com]

Hmmmmmmmm.   Going to have to put on my thimking, errr thinking, Cap.   When 
I was trying to come up with a way to test a differential photo sensor, I had 
in the back of my mind the local astronomy club, of which I am a member, has a 
10 inch Dobsonian telescope which I can reserve.  If the sensors are mounted 
in a "dummy" 1.25 inch eyepiece, they could be used with many good quality 
scopes.  I have a 4 inch to start my testing.


Comments on number (1):
Separation of color by the differing focal length reminds me of an article 
many years ago in CQ.  I will need to look up the reference.  Anyway, it doesn't 
happen with mirror lenses, so it applies to refractor telescopes, most of 
which try to prevent it.  Assume that the red focuses to a minimum size dot 
first, the other light will spill over that dot, which means that the detector 
should be same size as the red dot.  If it was bigger, then it would gather the 
other color photons.  If the sensor is large enough to capture the out of focus 
dot for all colors, then it gains nothing.   Small sensors mean that you have 
to have the good quality refractor optics, that are not color corrected.  My 
guess is that I can get a 12 or 16 inch Newtonian scope (maybe an f/12 
spherical figure, which would be a poor optical scope) for less money than I can get 
an 8 inch glass lens polished to those requirements.  

Comments on (2):
I think that what you are describing would be a photodiode connected from 
ground to the non-inverting input of a low noise high impedance op amp, with a 
(large)feedback resistor from the output to the inverting input and a gain set 
resistor from that junction to a capacitor that is grounded.  The half volt or 
so bias on the diode would be "dribbled" into the capacitor from the output 
through the two resistors, so the capacitance sets the frequency response.  I 
think this has potential.  Probably needs + and - power to the op amp.  I can 
send you a sketch if it didn't make sense.

Would be nice to compare differential and (2) in the same optics --- I have a 
114 mm Newtonian and light receiver "tinker" using a 100 mm magnifier lens.


James
N5GUI






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Hi James,

I just found your message, I had meant to reply long ago. Sorry it got lost 
temporarily.

2 photodiodes, 2 amps, an active mixer IC and extremely high grade light 
gathering optics is a high price to pay for rejection of ambient and 
interfering light. The optics is not a big deal, except that you want a 
large light gathering surface....so, you are going to need a 12 inch glass 
lens/mirror instead of a 12 inch fresnel. When you get into larger surface 
area optics, it is a very expensive proposition to have high quality light 
gathering optics.

I have 2 alternative suggestions, that is I am suggesting 2 countermeasures 
that should be considered before going to the added expense/complexity of 
the hardware you have suggested.

1)      Use small active area photodiodes. This is a common method for 
narrowing the field of view for optical receivers. A small photodiode 
placed at the focal length of the lens will narrow the receivers field of 
vision to the milliradian range (same at the collimated laser transmitter). 
If the focal length of the optics is long, this method allows one to take 
advantage of the fact that different wavelengths of light focus at slightly 
different distances...in effect, creating a nearly loss less optical 
passband filter!

2)      Use a 'leaky integrator' and a single photodiode instead. Make sure 
the time constant of the integrator is 10x slower than the frequency of the 
signal you are trying to receive (the desired signal). Feed the output of 
the leaky integrator back into the inverting input of the photodiode amp 
(at the junction of the photodiode and the op amps inverting input), in 
effect cancelling dc (constant background light level). Feedback must be 
through a resistor to avoid having the feedback take total control. This 
method allows full daylight operation of photodiode based receivers, an 
area that most have not considered to be worthy of experimentation because 
previous attempts at daylight laser communications have been implemented by 
dumbing down the receiver sensitivity to avoid overloading by ambient light.

Burr-Brown has an ap note describing this method, along with an actual 
design example, but I can't locate it in my archives here, it's not on the 
Burr-Brown website anymore and TI's website doesn't list it either (TI 
bought BurrBrown in 1999). This method allows a fully sensitive receiver to 
be used although the feedback electronics does generate a slight bit of 
additional noise. The only drawback is that the integrator takes several 
seconds (or tens of seconds) to cancel the background light level after the 
receiver is powered up.

I'm wondering whether anyone has tried method #2, or has the original 
Burr-Brown ap note describing this method???

Regards,

Art