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Entry  Wed May 12 23:31:17 2010, Alberto, Update, 40m Upgrading, 216 MHz resonance in the POY11 PD killed POX11.png2010-05-12_POY11_CalibratedOpticalResponse0-500MHz.png
    Reply  Thu May 13 05:06:43 2010, rana, Update, 40m Upgrading, 216 MHz resonance in the POY11 PD killed 
Message ID: 2925     Entry time: Wed May 12 23:31:17 2010     Reply to this: 2926
Author: Alberto 
Type: Update 
Category: 40m Upgrading 
Subject: 216 MHz resonance in the POY11 PD killed 
It turned out that the resonance at 216 MHz in the 11MHz PD that I showed in the elog entry 2902 was casued by an instability of the of the MAX4107 opamap' feedback loop.
As the datasheet of the opamp shows, the close-loop gain has a peak at about 200-250MHz, in presence of even small capacitive loads.
In my case, perhaps either the capacitance of the BNC cables plugged to the RF output of the PD box, or the shunt capacitance of the circuit parts after the opamap (traces and resistors) might have introduced capacitance at the output of the amplifier.
LISO had failed in predicting the resonance because it has only ideal transfer functions of the opamps. In particular the open-loop gain of the opamaps in the library is just a function with a simple pole.
At RF frequencies the output impedances of the opamp starts having a non-negligible inductance that interacts with the load capacitance, generating a typical LC-circuit resonance.
In cases like this, such effect can be mitigated by introducing an "isolating" resistor at the output of the opamp.
So I did that and modified the circuit as in this simplified schematic here:

The choice of 100 Ohm for the isolating resistor was mainly empirical. I started with 10, then 20 and 50 until I got a sufficient suppression of the resonance. Even just 10Ohm suppressed the resonance by several tens of dB.



In that way the gain of the loop didn't change. Before that, I was also able to kill the resonance by just increasing the loop gain from 10 to 17.  But, I didn't want to increase the closed-loop gain.

One thing that I tried, on Koji's suggestion, was to try to connect the RF output of the PD box to an RF amplifier to see whether shielding the output from the cable capacitance would make the resonance disappear: It did not work.

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