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Entry  Mon May 10 16:59:35 2010, Alberto, Update, 40m Upgrading, Unexpected oscilaltionin the POY11 PD 2010-05-10_POY11_CalibratedOpticalResponse0-500MHz.pngPOX11.png
    Reply  Mon May 10 18:56:53 2010, rana, Update, Electronics, Unexpected oscilaltionin the POY11 PD 
       Reply  Mon May 10 19:09:45 2010, rana, Update, Electronics, Unexpected oscilaltionin the POY11 PD 
Message ID: 2902     Entry time: Mon May 10 16:59:35 2010     Reply to this: 2904
Author: Alberto 
Type: Update 
Category: 40m Upgrading 
Subject: Unexpected oscilaltionin the POY11 PD 

The measured transimpedance of the latest POY11 PD matches my model very well up to 100 MHz. But at about ~216MHz I have a resonance that I can't really explain.



 The following is a simplified illustration of the resonant circuit:



Perhaps my model misses that resonance because it doesn't include stray capacitances.

While I was tinkering with it, i noticed a couple of things:

- the frequency of that  oscillation changes by grasping with finger the last inductor of the circuit (the 55n above); that is adding inductance

- the RF probe of the scope clearly shows me the oscillation only after the 0.1u series capacitor

- adding a small capacitor in parallel to the feedback resistor of the output amplifier increases the frequency of the oscilaltion

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