How to determine the center frequency of a photodiode

frequencyphotodiode

I'm using IR LEDs to blast away at a bunch of photodiodes, which I then latch into a shift-register and read into a PIC.

While reading/browsing about the topic I found an article that stated that the best way to ensure the conduction in the photodiodes would be to blast them with the LED radiation generated by a PWM signal at the center frequency of the modulated light sensor.

What I take from this is already what I intended to do, but the frequency aspects got me thinking.

Given that the PDs I'm using have an untraceable datasheet (I know that the reference is SWPD3330-16, from Sunwell), does anyone have any info that might help me figure out how to calculate the best frequency? As an alternative, is there any rule of thumb for this kind of problem? What would be a good starting point/estimate for the PWM frequency?

EDIT: After reading the comments, I would like to add the following:

  • I am using a photodiode. It looks like a LED and has two leads. The only thing I know about it is the aforementioned part number: SWPD3330-16.
  • The article does not specifically relate to my question, but on page 5 (2169), in the paragraph starting with "Step 2", you'll find what I quoted. LINK

EDIT2:

  • I found in a folder here at work the datasheet (in chinese) for this photodiode. It only covers mechanical aspects and (unfortunately) a few electrical specs (in parenthesis are the test conditions):
    • forward current: 100mA
    • pulse forward current (@1KHz, DC 10%): 1A
    • reverse voltage: 30V
    • power consumption: 150 mW
    • (some stuff about temperature)
    • light current (VR=5V, H=5mW/cm2): 64uA
    • dark current (VR=10V, H=0mW/cm2): 30nA
    • open voltage (H=5mW/cm2): 350mV
    • short current (same): 65uA
    • total capacity (5V; 5mW/cm2): 20pF
    • rise time (VR=10V; RL=1Kohm): 10ns
    • fall time (VR=10V; RL=1Kohm): 10ns
    • peak sensistivity wavelength: 940nm
    • sensitivity wavelength: 750~1100nm
    • radiation angle: 20 deg

With the hightlighted spec, is it possible to calculate/infer a educated guess for the PWM frequency I should use?

Best Answer

The peak sensitivity wavelength of the photodiodes is completely unrelated to the PWM frequency that might be used to modulate the IR signal. The system you have assembled does not use modulated IR and cannot benefit from applying PWM to the emitters.

What you can do is verify that the peak emission from your IR LEDs is close to the wavelength of peak sensitivity for the photodiodes. You could also consider using an IR filter that would block visible light from getting to your photodiodes. They may not be very sensitive to visible light but there is a lot of visible light that competes with your IR signal.

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