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Weird (to me) 60 Hz pickup

Not so. A forward biased diode has nonlinearities of all orders due to the exponential. And the slowness isn't too relevant since the only thing leaving the die is the rectified part. The RF swing across the diode will be reduced by the capacitance, of course.

Yup.

Well, there's the well known IF noise blanker approach, beloved of QST fans since probably the early '60s. You just turn off the signal till the pulse is over. Another approach is a boxcar integrator with just the right sampling interval.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
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Phil Hobbs
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OK I found the problem. It's my FET heater drive... even though the power is not on to the resistor load the gate is still connected to my drive electronics. When the error signal gets close to zero, the gate drive signal changes from off (-15V) to on +15V. (I updated my drop box link with schematic of heater and 'scope shot of the gate swinging +/- 15V.) The gate drive wire runs next to the diode wire and probably has ~10-100 pF of capacitance. So the small 60 Hz pickup gets amplified and then capacitivly coupled back.

So now how do I slow it down? Bigger gate resistor? Bigger feedback cap?

George H.

Reply to
George Herold

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