I reduced the power falling on the PMC to ensure the high signal level isn't causing this problem. It was not. The problem still persisted.
Then, I did this reproducible step (quoted below)again, but this time I had a small 10 mV signal from SR785 going into FP2TEST and I was taking transfer function to TP2. If the U5A AD602 is switched off by the Blanking pin, the transfer function should remain null. This gave me a way of checking if the AD602 is wrongly getting switched on on its own.
- To start, I kept Engage OFF. This gave a voltage of 4.41 V at pin 4 of U5A AD602. So it should be shut off.
- The PZT voltage was about 97 V at this point.
- The transfer function was flat to about -80 dB from 10Hz to 100Hz.
- Then I started scanning the RAMP voltage of PZT. As the PZT voltage reached near 70V value, the PMC got locked on its own.
- The transfer function value jumped suddenly to between -20 dB to 0 dB (an increase by about 70 dB).
- The gain on AD602 was set to 0dB. So if it got on its own, the transfer function was expected to be 0dB.
- The gate voltage at pin 4 of U5A AD602 was still 4.41 V. So ideally, it should still be off.
- I have attached the data I captured. During swept sine, the ramp was being increased and we see a jump clearly when the PMC got locked on its own.
This is good evidence in my opinion that the AD602 at U5A is faulty. I need comments on this conclusion. If I don't hear otherwise by tomorrow noon, I'll start working on replacing it.
Data
Edit Thu Oct 31 10:26:50 2019
Issue fixed. See CTN:2469.
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- But, when I connected the RFPD back, the problem was not there anymore. At this point, I found the following reproducible issue:
- Engage button is off.
- PZT displacement is following the ramp normally.
- Slowly change the ramp and at the sweet spot, the PMC gets locked. Remember the engage is still off.
- Now, changing the ramp doesn't change the PZT displacement anymore.
- Switching on the engage ON doesn't change anything.
- Switching it back OFF unlocks the PMC and the PZT displacement starts responding to ramp voltage again.
- That's weird right. Since this was reproducible, I did this a few times and found that the problem doesn't necessarily happen at the locking point. It can happen anywhere. And in that case, the 5th step of switching on the engage does show a difference in locked mode. And again, switching it back OFF resolves the issue.
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