[Gb-ccb] further phase switch tests & ccb levels

[Brian Mason]

This morning Galen disconnected the snubber circuit and we took more 
dump-mode profiles of the phase switch transition in the ka band receiver 
using the ccb. Attached are results from these tests

a) unfixed-- only two of the 4 phase switch controls were "fixed" ie 
snubber circuits disconnected.  Note the decay of just over 50 counts on 
8000 over the course of 30 usec or so.

b) port 11, switch a-- was fixed.  By eye this is quite a notable 
improvement! I would say a 10 usec blanking would suffice (perhaps even 8) 
although quantitative analysis is called for.

c) zoom in on that

So to recap this differs from our original profile by: having the monitor 
circuit disconnected, and having the twisted pair cable. This is great 
progress & I'm pretty hopeful that with the faster op-amps we can really 
get the blanking down to something reasonable.

  -=-=-

Randy & Galen also checked the RF levels into the detectors, which looked 
sensible, and the voltages going into the CCB, which didn't, but did 
correspond roughly to the counts seen by the CCB. A difference to earlier 
lab measurements is that the detector circuits have been thermally bonded 
to the temperature-stabilized baseplate.

To work on this further it will be necessary to pull both the receiver & 
the CCB off the telescope for investigations in the lab. We have also 
found some problems in the spectral line side during commissioning 
exercises, the investigation of which will be greatly accelerated by 
pulling the receiver back to the lab as well.

In order to pursue all of these issues we have decided to pull the Ka 
receiver & CCB off the GBT, which will happen over the course of this 
afternoon and tommorrow morning. We aim to have them back up for further 
GBT observations as soon as possible.

  -Brian

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Brian Mason                           | office: +1(304)456-2338
Associate Scientist                   | fax:    +1(304)456-2229
National Radio Astronomy Observatory  | mail:   PO Box 2 
bmason at gb.nrao.edu                    |         Green Bank, WV 24944
http://www.gb.nrao.edu/~bmason/       |
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