Winking cal phase offset in galfacts data.
cal phase offset in galfacts data.
Galfacts takes data using the pdev
spectrometer setup. The setup is:
A phase offset was seen in some of the data taken on
14nov08. Data on 13,15,16,17nov08 did not have the problem (see galfacts page)
- 172.032 MHz bw, 16 bit sampling, 1 millisecond dumps, and 4096
- 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 plots show the first
calcyles of 2 data files (.ps) (.pdf):
Some properties of the problem are:
- 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
- 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
The oscilloscope was also used to measure the time from the 1 second
tick of the start to the first cal on transition.
- 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
- 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.
- 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.