lbw polB cal jumps 04dec05

    The cal value for lbw polB changed on 04dec05. The value of the system temperature changed as well as the gain (which is measured using the cal value). The sefd (which is independent of the cal) remained unchanged.  The Tsys jump in polB occurred with both diode 1 and diode 2 so the problem was after the switch that selects which diode to send to polB.

Looking at cal on/offs taken by users:

• Top: The frequency of the cal on/offs versus date. Many asterisks are over plotted on top of each other. The date tick marks are for 12 noon utc on the date.
• Center: Tsys  versus date. The black * are polA and the red * are polB. Part of the spread in Tsys is coming from the frequency variation.  PolA  stays close to the same level for the entire period. The polB Tsys  begins to rise around 13 Hrs (utc) 04dec05 (project a2121 data).
• Bottom: The ratio TsysA/TsysB is plotted versus date. The dashed vertical lines are a2121 data taken on 03dec05, 04dec05, and 05dec05. Tsys polB was ok on 13 hr 03dec05, it had increased by 10% on 04dec05, and on 05dec05 it  stabilized at a value 20% larger than the 03dec05 value. For this to happen the cal power getting to the amps had to  decrease by about 20%.  Note that the data was sampled with 24 hour resolution so we can't pinpoint exact times.

The daily tsys measurements taken at 1400/1405 Mhz.

• Top: Tsys polA vs date. The black * are from diode 1. The red * are from diode 2. The dashed lines mark where things occurred:
• 01jun05 (dashed black). Dewar warmed up and then cooled down. After the cool down, a resonance appeared at 1400 Mhz in polA. It had not been there previously.
• 10/13jun05 (dashed purple): cal values remeasured.
• 21jul05 (dashed red) : diode 1 breaks. All tsys measurements switched to diode 2.
• 23nov05 (dashed green): new diodes installed.
• 23/25nov05 (dashed purple): cal values remeasured.
• 06dec05 (dashed blue): tsys measurements moved 1400Mhz->1405Mhz to get off of the resonance in polA at 1400Mhz.
The PolA Tsys wandered around between jun05 (when  the polA resonance was created) until   06dec05 (when the measurements were moved to 1405 Mhz). This wandering  occurred with both diodes.
• 2nd: Tsys polB vs date. It was relatively stable except for the jump on 01jun05 and on 04dec05.
• 3rd/4th: These are blowups of the 01nov05 to 01jan06 time range. PolB jumped up prior to the 06dec05 change of frequencies.

Tsys, gain from the calibration scans:

    The plots show lbw calibration scans jul05 to feb06 at 1415 Mhz (.ps) (.pdf):

• Top: TsysA and TsysB were each divided by their average value over this period to show their variation with time. The black polA jumped down after the frequency was moved from 1400 to 1405 Mhz. PolB red jumped up after 04dec05.
• 2nd: Ratio of TsysA/TsysB. The major jump occurred around 04dec05.
• 3rd: The ratio of TsrcA/TsrcB. This is the ratio of the measured source strength in polA and polB (polarized sourced were excluded). This also showed a jump in early december. These measurements include the cal value.
• bottom: The ratio (TsysA/GainA)/(TsysB/gainB) is constant over the entire period. This value does not use the cal. If TsysPolB had actually jumped then this data would not be flat. So the problem is in the cal value.

Conclusions:

• TsysPolB jumped on 4/5 dec05. It took about 2 days to change. It changed by about 20%.
• The change was caused by the value of the cal changing. The actual tsys value did not increase.
• The increase was independent of the diode used to measure it. So the problem is after the switch that chooses the diode to use (the diodes are ok).
• This is not the first time that the polB cal value has jumped. It also jumped in apr04.
• The cal values were remeasured on 23feb06 and then back dated to be valid from 05dec05.

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