Check lbw polB cal cables.

 
    The polB cal value for lbw jumped on  23apr04 and again on   04dec05.   (by 20%).  In both cases the jump occurred for both diode 1 and diode 2 feeding polB. This shows that the problem was after the relay in the post amp chassis that selected diode 1 or diode 2 (i guess it could still be that relay). 

    On 06mar06 we tapped/wiggled the polB cable inside the post amp chassis as well as the  polB cables that goes from the post amp chassis to the dewar. While the cables were moved, the hcorcal 
was being turned on/off at a  1 second rate. The interim correlator was set for 4 25 Mhz bands centered at 1385, 1405,1425,a nd 1445 Mhz. The receiver was set for linear polarization's.

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Data reduction.

• The cal deflection was computed for each 2 second calon,caloff.
• The total power of the cal deflection was computed by taking the median value of each  Caldeflection spectra.
• For each 25 Mhz band the calDeflection total power was normalized to the median value for the entire 400 seconds of data. This emphasized the changes in the cal deflection. Since the cal deflection is only 30% of Tsys, the noise on the measurements will be that of (Tsys + cal).

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The data

• The top 4 traces are polB, the bottom 4 traces are polA. Each trace is color coded by frequency (black 1385, red 1405, green 1425, blue 1445 Mhz). The polA, polB traces have been offsets for display purposes. The vertical scale is  in units of the median cal deflection.
• The dashed vertical lines delimit when something was moved:
• black: We tapped on the dewar
• red:  tapped on the polB cal cable where it connected to the dewar.
• green: wiggled the dewar side of the  polB cal cable that goes from the dewar to the post amp chassis.
• blue: tap the connector where the polB cal cable enters the post amp chassis.
• purple: wiggle the post amp chassis side of the polB cal cable that runs between the dewar and the post amp chassis.
• dark red: tap the polB cal cable inside the post amp chassis that goes from the bulkhead to the diode select relay.

     

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What changed:

• When the dewar was tapped, the polB cal deflection for the 1385 Mhz band started to move around. It continued to wander after the tapping stopped. This band includes the 1394 Mhz resonance in polB. If the resonance is coming from the omt, it is a bit strange that the cal deflection (calOn-calOff) would be affected since the cal is injected after the omt.
• The red dashed lines where we tapped the polB cal cable as it entered the dewar shows a change for all 4 polB bands. The cal deflection decreased by about 1%.  Unfortunately it also looks like the polA cal deflection (bottom traces) also dropped a little during this same time period.
• There is definitely no 20% change in the cal caused by our moving the cal cables that are external to the dewar. So maybe the problem is in the dewar.

 home_~phil