Junior Member level 3
This is killing me - I am at my wits end with this awful problem - somebody please help! I have some instrumentation that excites a differential coil (one drive, two symmetric pick-ups). The coil is at the end of a 50 ft. cable. There is a preamplifier also at the end of the cable that amplifies the signal from the two pick-up coils and then has a differential line driver to reduce noise back to the main unit. Back at the base station, an instrumentation amplifier amplifies the differential signal then there is a filter, more gain, phase detector, integrator, offset subtractor and amplifier and finally another low-pass filter. The whole thing runs at 10 kHz. Sometimes things are fine for days and then every now and then this very low-frequency oscillation takes hold (under 1 Hz) and can be quite dramatic and completely swamps my signal. Then it goes away and everything is back to normal. I have looked at EVERYTHING. Mostly I thought it was a problem with the driver but I finally resorted to buying a cheap audio power amp from Walmart and I still get the same behavior. Thought it was some kind of motorboating but no amount of rerouting powerlines and adding decoupling seems to help. Maybe some kind of positive feedback in the cable due to coupling between drive and the detector lines?? This is now a quite complicated system so I don't expect anybody to tell me what's wrong but if somebody could give me a hint or idea as to something new to look at..... anything, please!! Thanks.