Laser Pulse discriminator

[mr_byte31]

Hi All,

I am working on implementing a laser pulse discriminator so I can use it for laser rangefinder or reconstruct a digital signal.

I found this design in IEEE paper which was looking very interesting since it is a differentiator without any feedback components.
The author suggested to use RC=RL for a reason that I didn't understand.
The Rt is used to adjust the voltage threshold.



for the time being I would like to use this circuit to find digital laser signal (red color) with max frequency 1 MHz so I decided to use a phototransistor instead of photodiode since phototransistors have higher gain. I selected SFH-3410 from OSRAM.

For the comparator, I planned to use MAX913 since it is ultra-fast. I selected the reamaining parameters as shown in figure below.



The output of the OP-AMP will be connected to microcontroller(RP2040) to give rising edge as indication of laser pulse.

I have few questions,

1- when I connect a resistor (ex. 10K) to output (pin-7) of the OP-AMP, the output signal completely disappear!, any reason for this?
2- the laser pulse would generate a spike signal on the output of the OP-AMP, this signal is very very narrow in time domain, would the microcontroller be able to sense this signal ?
3- what is the benefit of the coil and the resistor ?

[dietert1]

50 Ohm times 33 pF gives 1.5 nsec, but that comparator needs at least 3 to 5 nsec. The cicuit works as a high pass. The upper branch with the inductor takes the DC current of the photo diode. In general photo diodes are used in reverse mode in order to capture as much charge as possible, maybe even with some gain (avalanche).

Regards, Dieter

[mr_byte31]

Thx for the hint.

I modified my circuit to match the 1us pulse.



What is not clear to me is the Threshold voltage that is represented by V2 in my figure. I keep tuning this voltage but I don't see any change to the output.

any hints?

[dietert1]

What is the reason for choosing a photo transistor? As far as i remember photo transistors are pretty slow in comparison to a diode in reverse mode where photo charge collection gets accelerated by DC bias.
In order to estimate the signal amplitude one needs a linear amplifier for the photo signal so that it can be measured by a scope. We also have a linear optical receiver (HP 81519A, 400 MHz bandwidth).
As far as i remember in our pulse oximeters the photometer "gain" is only 0.0001, i mean the photo current is so much smaller than the LED current. One uses a transimpedance amplifier with several MOhm.
Don't know what this means when using a laser as a light source, but i guess the photo signal will be difficult to detect fast. Probably one needs some modulation scheme to filter out ambient light using synchronous detection (lock-in method). The TOF determination becomes a phase measurement.

Regards, Dieter

[JohnG]

Disclaimer: I am only peripherally familiar with these lidar systems.

The first question: How accurate do you wish your rangefinder to be? To first order, system timing accurate of 1 ns translates to a distance accuracy of 15 cm, assuming that the laser and detector are located close to each other relative to the target. There are a lot of ways to get there. If you are looking at pulse edges, it places strong demands on absolute timing accuracy.

While there may be exceptions, phototransistors are sensitive, but relatively slow. For photodiodes, speed is inversely proportional to capacitance (small diodes are faster) and sensitivity is proportional to capacitance (small diodes are less sensitive). Once you make the move to photodiodes, you can choose, in order of increasing sensitivity, PIN, Avalanche, and Geiger (the last also know as silicon photomultiplier avalanche diodes, or SPADs). The detector circuitry grows in complexity as you pick the more sensitive diode types, particularly SPADs.

John