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Calculating photodiode bandwidth

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mik3ca

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I'm trying to figure out how to calculate the maximum bandwidth receivable based on this circuit. I think R1 has a large factor in that role and I think R2 can't be smaller than R1/hFe.

I set my R1 to 100K and R2 to 10K. With the rest of the components the same as shown, will I be able to achieve a clean signal at a 9600bps speed?

Also, if I removed R2 and the transistor and instead connect R1 and the photodiode cathode directly to the input of an 74HC14 schmitt trigger, would R1 need to be the same to achieve the 9600bps speed or do I need to modify R1 based on a value listed in the IC datasheet?
circuit.png
 

dknguyen

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You don't amplify photo diodes that way. Photodiodes are not voltage sources. Treat them as current sources with quirks (these quirks are important. Do not ignore them)

edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf

http://www.ti.com/lit/pdf/tidu535
 

mik3ca

Member
But I made numerous circuits this way and they do respond to light so I don't want to redo everything just because these diodes aren't connected a "typical" way.
 

dknguyen

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But I made numerous circuits this way and they do respond to light so I don't want to redo everything just because these diodes aren't connected a "typical" way.
Simply responding to light isn't quite the same thing as actually predictably, accurately, and linearly responding to light in a useful way. Not to mention really slow, as you seem to have realized. To get a larger signal you need large resistors but that makes the circuit really slow. This is already a big enough issue in the "typical" circuits.

Maybe save up for an oscillscope. Even a cheap USB like one from Parallax. Not having one means you have no idea what's going on in a circuit, especially analog circuits so you can't see how poorly it performs.

The datasheet for that photodiode of yours doens't even list the junction capacitance though so there's no way to calculate the bandwidth.
 
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Nigel Goodwin

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I set my R1 to 100K and R2 to 10K. With the rest of the components the same as shown, will I be able to achieve a clean signal at a 9600bps speed?
No, you can't just connect RS2s2 to a laser diode at one end, and a photodiode at the other, RS232 doesn't work like that.

As already mentioned elsewhere it's usual to use Manchester coding (or similar) to make such schemes work.
 

gophert

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I'm trying to figure out how to calculate the maximum bandwidth receivable based on this circuit. I think R1 has a large factor in that role and I think R2 can't be smaller than R1/hFe.

I set my R1 to 100K and R2 to 10K. With the rest of the components the same as shown, will I be able to achieve a clean signal at a 9600bps speed?

Also, if I removed R2 and the transistor and instead connect R1 and the photodiode cathode directly to the input of an 74HC14 schmitt trigger, would R1 need to be the same to achieve the 9600bps speed or do I need to modify R1 based on a value listed in the IC datasheet?
View attachment 113829

If you use a reasonably modern comparator, you should be able to do up to well over 100x of your desired 9600 baud.
 

rjenkinsgb

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One of the most basic things is that the input circuit needs to be AC coupled so changes in ambient light (and the DC state of the photodiode) do not prevent the data slicer working.

An RS422 line receiver works well as a data slicer. We use half a DS9637 or DS8921 as we used a load of those in some other gear.
The have a schmitt trigger input with ~35 or 200 mV hysteresis respectively so don't need a massive signal and they are rated up to 10MHz data rate.

Connect one input directly to the output of whatever preamp circuit you use, with a resistor to the other input and a capacitor to ground from that second input.

The R-C values need setting so the second input follows the first but slowly enough that it takes just a few bit times to get within the hysteresis range of the RX.
(Off the top of my head, 4k7 and 0.1uF should work).

That gives a fast settling time during the data run-in and good immunity to low speed changes like 100 / 120Hz lighting flicker.

As others say, you really need a 'scope to _see_ the signals for fine tuning things like that.


For the input amp, see the PDF link dknguyen posted.


Of course, if you use the little three-pin IR receivers I mentioned in the other thread, each of those already has all this built in to it:
 
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