Today I was taking transfer functions through the newly repaired North TTFSS in the usual test suite: Excite Test Point 1 before the Common path op-amp, and continue through the PZT and EOM paths. I thought I was exciting at -30 dBm, but I was in fact exciting at 0 dBm. When I reached Test Point 16 on the PZT path the transfer function began returning garbage, which prompted me to start looking at the voltage levels of the test points. Test point 16 was railed at 14.6 V, very close to the 15 V voltage regulators on board the TTFSS. We went backwards through the PZT path, eventually finding that Test Point 5, the output of the common path, already has a -600 mV offset, which is just amplified through the PZT path op-amps until it saturates at DC.
We removed the excitation and checked the voltage levels of the later PZT stages, and found they were still saturated. We found that the common path output was also still heavily offset to -600 mV. We messed around with the offset dial on the TTFSS control panel, to no avail. We turned off and on the power supplies, still saturated. We checked the South path output, it is NOT similarly saturated along the PZT path. We still are not sure why the North common path output is so heavily offset from 0, and suspect this is why the PZT path has little range since it is operating so close to the positive power rail.
We decided to put a function generator signal at -35 dBm and 123 Hz through Test Point 1 and measure it through the PZT and EOM paths. The signal made it through the PZT path just fine, as the excitation was small enough to not rail the signal. However, the EOM path output at Test Point 13 showed significant railing (Picture 1).
We began to turn up the frequency. The EOM path output stopped railing at 7 kHz (Pictures 5 and 6). We turned down the frequency, and the EOM path stopped railing at around 70 Hz (not pictured).
The passive high pass filter between the common path output and the EOM path input is made up of a capacitor C23 and resistor R22. C23 is supposed to be 1µF and R22 is supposed to by 1.1kΩ. R22 is in fact 1.1kΩ but C23 is measured to be 2.1µF, making our high pass filter "weaker", aka moving down in frequency. The corner of the high pass filter from these values is 69 Hz, so it seems that we have nailed the railing issue, but "strengthening" our high pass filter back up to a capacitor with 1µF would only get our corner frequency back up to 144 Hz, and we were seeing railing up to 7 kHz!
Unclear what our next move should be, maybe we can change the capacitor back to 1µF and bring the resistance down from 1.1kΩ to around 20Ω? I don't know much about electronics but this seems like a bad idea. Maybe a second stage HF filter with smaller capacitance and high resistance? How do we avoid railing our EOM path?