18nov08:winking cal phase offset in galfacts data.

• 172.032 MHz bw, 16 bit sampling, 1 millisecond dumps, and 4096 channels.
• The winking cal is used with:
• cal off 20 milliseconds followed by cal on 20 milliseconds.
• The cal is set to transition 500 usecs into millisecond 20 of the off and the on.

• The top window shows the first 40 milliseconds of the file. The bottom window shows the first 60 milliseconds.
• With each window the top 3 plots have the 1st cal off 1 millisecond short. The bottom plots are from 17nov08 when the cal was correct.
• The color coding is:
  
• black: total power polA
• red: total power polB
• green: The pdev cal status bit that comes with each 1 millisecond spectra. It will be 1 if the ttl cal input was high for any portion of the 1 millisecond integration.
• blue dashed lines: the expected cal transition
• red dashed lines: the cal transition that is 1 millisecond early.

• The problem with the starting phase (it is 1 too large). The cycle length remains correct.
• Once the scan starts, the phase remains constant for the entire scan.
• All spectrometers show the same cal phase.
  

• I took the cal multiplexer output, the 1 second tick, and the detected rf signal and put them into the scope.
  
• I started the spectrometer 35 times:
• 7 scans had a 1 millisecond offset, 28 scans were correct so it is off about 20% of the time.
• The cal on transitions after the 1 second tick start are:
• Cal correct: 19.75 milliseconds
• Cal off: 18.75 milliseconds.
• We expect the cal transition at 19.5 milliseconds. The extra time is probably measurement error and propagation.

Conclusions:

• The pdev spectrometer starts the winking call off by 1 accumulation 20 % of the time.
• The offset remains constant for a scan.
• The error is coming from the spectrometer. It is probably a timing problem in the fpga setup of the start of the calcyle.