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Explain me a residual signal on an oscilloscope that's connected to silicon detector

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watertreader

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Hi,

Some explanation needed.

I have connected a silicon photodetector to an fiber optic output... which should exhibit a signal of frequency of 160Mhz... The output to the sillicon detector is observed via an oscilloscope.

Whenever I have powered on the detector, the osciloscope will exhibit a 30Mhz signal which is not related at all to the light output. This signal could not be cleared by switching on and off the detector except via changing the osciloscope coupling and turning off the detector

Does this suggest that there is some capacitative effect in my detector circuit? What could be the reason behind such effect?

Thanks
 

residual signal wiki

1.- What is the frequency response of the silicon photodetector ?
2.- If the fiber optic is disconnected while reading 30 MHz on the oscilloscope, does the trace collapses ?
3.- Is the variable time knob on the oscilloscope on 'cal' position?

Miguel
 

residual signal

Hi,

1) The detector has a small signal frequency response of 150MHz... i know i am undersampling...but probably that's the better detector i have...

2) yes.. the fiber optic is disconnected...the signal does not collapse

3) no...
 

50 ohm oscilloscope terminator

watertreader said:
Hi,

Some explanation needed.

I have connected a silicon photodetector to an fiber optic output... which should exhibit a signal of frequency of 160Mhz... The output to the sillicon detector is observed via an oscilloscope.

Whenever I have powered on the detector, the osciloscope will exhibit a 30Mhz signal which is not related at all to the light output. This signal could not be cleared by switching on and off the detector except via changing the osciloscope coupling and turning off the detector

Does this suggest that there is some capacitative effect in my detector circuit? What could be the reason behind such effect?

Thanks
Click to expand...
There're many reasons may effect :
1. The detector needs a low impedance load (50 Ohm ?) but your oscilloscope in 10M Ohm input mode.
2. No bypass filter (1uF tantalum capacitor) at power pin and ground of detector
3. Optical pulse power is over the dynamic range of detector=> Need optical attenuator or lightly split the optical connector to a little distance!
You'd better send datasheet of this detector. Is this PIN photodiode or APD photodiode or receiver module included pre-amplifier?
 

residual signal

I suppose its noise thats responsible for the spurious trace.. incorporating a decoupling capacitor in the detector circuit wud help.
 

Re: residual signal

hi,

this is a PIN detector i suppose...
 

Re: residual signal

watertreader said:
hi,

this is a PIN detector i suppose...
Click to expand...
That's clear.
Following Data sheet recommendation, to view signal waveform in the output connector, you need a RG-58U (50 Ohm) cable with BNC connector in one cable end and a 50 Ohm terminator-resistor in the other end. High impedance Oscilloscope probe can be clamped to the terminal resistor.
Best regards
Thanhlongbin
 

Re: residual signal

hmmmm...actually i observe the same signal even in 50ohm DC coupling in the oscilloscope with the cable RG-58U......
 

residual signal

Please try to reduce input optical power under -10dBm ( 0.1mW) as I said in my 1-st reply. Your detector has max input intensity =1mW/mm2, so max input power should <1mW (0dBm).
 

Re: residual signal

Hi,

thanks for the help. I think I might have forgotten to add in that the noise exist even without inputting any form of light input to the detector area(have blocked it out)....

It seem to me that the detector behave like an oscillator... maybe i will try to put some capacitor to remove the noise in the input supply

thanks
 

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