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Study of Cross-talk on the MAPMT-to-Hybrid cable

                                 

Overview

As HV is increased beyond ~600V, we start to see capacitive
coupling between channels.
Initial studies indicated it was due (mostly) to the
flat cable connecting the baseboard to hybrid.

Possible Short-Term Solutions

Use the same cable (~3) but with spacing between
the wires.
Use a very short cable of 0.5 (approximated a direct
connection from baseboard to hybrid)
Study each alternative cable as a function of HV and
Threshold.

Schematics

Optical fiber

                                                MAPMT

LED

Plots

Old cable (3) at 600V and threshold 117                 
New cable (3) at 600V and threshold 117

Old cable (3) at 700V and threshold 117               
New cable (3) at 700V and threshold 117

Old cable (3) at 700V and threshold 105                
New cable (3) at 700V and threshold 105

Plots

Old cable (3) at 750V and threshold 105                 
New cable (3) at 750V and threshold 105

Old cable (3) at 750V and threshold 90                      
New cable (3) at 750V and threshold 90

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

Old cable (3) at 800V and threshold 90                      
New cable (3) at 800V and threshold 90

Plots

Old cable (3) at 700V and threshold 105              
Old cable (3) at 800V and threshold 90

New cable (3) at 700V and threshold 105              
New cable (3) at 800V and threshold 90

Short cable (0.5) at 700V and threshold 105        
Short cable (0.5) at 800V and threshold 90

Summary

Both the split cable and short cable reduce the interwire
coupling, but neither eliminates it.
Possible other sources/tests.

Make a cable with 2 wires only at positions 1 & 32 

and check if coupling still persists.
Connectors?
Traces on hybrid, baseboard?
Possibly try injecting charge directly into the pre-amp through a
10 pF cap (nominally, 62Ke-/mV)