Log entry time 19:26:16 on June28,2004

Entry number 127276

I'm worried about our bcms.  Which one should we use to normalize?

bcm1   Upstream 1X (1MHz)
bcm2   Downstream 1X (1MHz)
bcm3   Downstream 3X (1MHz)
bcm7   Upstream 3X (10kHz)
bcm8   Downstream 3X (10kHz)

bcm1 and bcm2 agree pretty well, but are not gained properly for this
current (around 30 uA) and so have relatively poor resolution.
bcm3 is gained properly, but has no similar bcm to check against.

bcm7 and bcm8 have a bit worse of a mean in double difference, 
and possibly have a slightly worse double-difference tail problem.
They do have high resolution.  

BCMs 1, 2 and 3 are read out in one ADC, BCMs 7 and 8 in another. 

BCMs 7 and 8 show evidence of picking up the pairsynch and helicity
control signals.  This is generally bad news, as it makes them 
unreliable. (They've been reported to have 1.5 ppm helicity pickup
during in-time tests!)

Double-differences between the sets of bcms are terrible: approx 2ppm
means are not unusual!  So, is the problem just bcms 7 and 8?  Can we
be sure that bcms 1,2 and 3 are ok, or do they just agree so
well because they are read out in the same ADC?

I will feel more comfortable if I can read out two bcms from two 
different ADCs.  I propose moving bcm3 to adc22, and bcm7 to adc 2,
in order to test this.  We probably will need to re-calibrate 
pedestals after the switch: possibly this can be done with an 
extremely brief current ramp.

Used runs from this morning to check if bcm double difference problem
was still a problem.  (There was a rumor that Bryan had fixed some 
termination problem last night.)  
The figure below includes data from runs: 2486 2487 2488 2489 2491 2492 2493.