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sinara-hw / Pounder

PDH/phase lock signal generator for Stabilizer
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IF amp oscillating #79

Closed jordens closed 3 years ago

jordens commented 4 years ago

@gkasprow you mentioned the THS4631 is weird. It oscillates when the IF gets to -0.4 V (at the output, G=3). A couple pF negative feedback fix it.

image

gkasprow commented 4 years ago

The THS4631 input is weird. You cannot make classical integrator out of it for example due to strange non-lineairty.

gkasprow commented 3 years ago

I added 4.7pF mounted by default

gkasprow commented 3 years ago

There is new fast JFET opamp which behaaves as expected - it's OPA818

jordens commented 3 years ago

THS4631DGN is still in the schematic v1.1rc1. don't you want to use the opa818?

gkasprow commented 3 years ago

It has a completely incompatible footprint. I can add it as an assembly option and populate it by default.

jordens commented 3 years ago

Maybe there are footprint compatible alternatives? The oscillation is really a functional problem. Or can we change the feedback to reliably prevent it?

gkasprow commented 3 years ago

OPA657, AD8065

jordens commented 3 years ago

AD8065 for lower power. I don't need offset stability or robustness against RF rectification. Can you add 100 or 1k series protection to ADC_[01]_P? The 50R termination would otherwise exceed abs max rating.

jordens commented 3 years ago

Let's go for the AD8065.

gkasprow commented 3 years ago

ok, done added 50R resistors and replaced the opamp