Weird (to me) 60 Hz pickup

George Herold · 2017-01-17T19:15:02+00:00

So this is still a thermal circuit. I posted a simple schematic and 'scope shots here. (Let me know if that doesn't work. I linked to a folder and not individual images.) I'm looking at the error signal out of the second instrument amp. There's a lot of pickup from the room lights and such, but when I get right near zero in the error signal I get these big spikey things with alternating polarity at 60 Hz. (AC line.) The second 'scope shot shows the same pickup, but I've now added a 1uF filter cap across the first int. amp input. (about a 2ms TC.. impedance of diode about 2.5k ohm.) As the error signal moves away from zero the spikes shift in time such that there is always 16.6 ms between the negative spikes, with the position of the positive spikes moving around. (Like some zero crossing spike.) Any ideas of where I should look? Or what is going on? The diode is a diode connected transistor. When I increased the diode current to 100 uA the spikes became much smaller.

Jan 17, 2017 21 Replies

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Phil Hobbs 9 years ago

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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George Herold 9 years ago

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.

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