Log entry time 04:53:06 on October 03, 2010

Entry number 333462

This entry is a followup to: 333459

We did some (limited) BPM troubleshooting tonight. We had signals of 4 
wires in the scope and played around with beam current (mostly), raster 
and beam position. Below there is a runlist for reference. Here are some 
conclusions

- Without beam, BPM signal read about 100mV after the attenuators. With 
beam they vary between 100 and 300mV, roughly proportional to the beam 
current as far as we can see by eye on the scope.

- Wires 3 and 4 of both BMP continue to saturate in normal (reasonably 
high current) conditions (>1 uA)

- We reduced the current, until BMP signal stop saturating. For BPM A, we 
need to go down to 0.5uA, for BPMB even lower: 0.3uA.

- We tried to measure raster size, but at the current where both BMP 
don't saturate, they don't seem to see any raster either. I only got one 
combination of current (0.5uA) where BMPA don't saturate and do measure a 
raster, but BMPB saturates so I cannot see the raster size at the target.

- Concerning the ADC values corresponding to the BPM signals, without 
beam we see values between 2200 and 3200 ADC channels depending on the 
wire. With beam, they vary depending on the current up to 4600 (mean of 
the distribution). After that, they saturate (but they shouldn').

An expert should look at the system.

In the meanwhile, if we want to measure the raster size, the only thing 
we can do is to reduce the ADC gate even more (it is now at 40ns).

Halog contains a few screenshots of currents in the scope.


Runlist
========
6918: No beam
6919-27: 10uA
6928: 20uA
6929: 1uA
6930: 0.5uA
6931: 0.1uA
6932: 0.1uA, 3x3 raster
6933: 0.1uA, 7x7 raster
6934: 0.5uA
6935: 0.5uA, beam at (+4,+3)
6936: 0.5uA, beam at (+4,+3), 7x7 raster
6937: 0.3uA, (+4,+3), 7x7