Mon., Jan. 9th, '12

Plan for today:  Remove the surface-mount chip capacitor C148 (next to chip resistor R91) using a relatively sharp-tipped soldering iron, by going back and forth between the two pads heating them.  OK, we did that.

David and I measured the strength of the pulse on the PMT_3 (TimingSig) internal node at 360-400 mV (equilibrium "high" pulse level, although there is ringing up to about +900 mV with these probe cables).  This is, I think, a bit better than before the capacitor was removed.  But it's still not high enough to allow us to leave the current threshold levels unchanged.  We'll have to modify the firmware to set the last threshold level to about 300 mV (as opposed to the +1.5V where it is currently).  I am a little bit concerned that this 300 mV level is so low that we may see occasional noise pulses if we won't actively filter them out.  We can filter out noise pulses in software, except for the occasional one that is close in time to the expected timing pulses.

We also re-checked the resistance between node PMT_3 and the +2.5VCC node.  It is still only 10 ohms, which is probably still contributing to our problems.  I went through the layout & schematic and checked the nominal resistance of all the resistors connected to the +2.5VCC node - they are all high, and many of them only connect to disconnected paths, so the problem is still unexplained.  I also checked the R between +2.5VCC and GND, it is 365 ohms, so it doesn't account for the 10 ohms.  I'm about ready to give up on trying to track down this problem.  The only way to make progress on it at this point might be to get some kind of IR imaging camera and try to find an unexpected hot-spot on the board.  2.5V across 10 ohms is 250 mA, times 2.5V makes 625 mW; although this is less than a watt, it still is possibly enough to produce some observable local heating.

I took Samad's power distribution board out of the loop for now, because it is too difficult to maintain a reliable power connection to the CTU with it in place.  This problem needs to be addressed sometime.  For now, we will just power the CTU directly from the power supply.

The CTU's Wi-Fi module is not reliably connecting to the server today for some reason.  It did manage to connect once, but it didn't feed through any data.  Need to run some more tests sometime to try to track that problem down.

I had a minor syntax bug in the new appdefs.py file, which was quickly fixed.