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Дата изменения: Thu Feb 5 21:24:19 2009
Дата индексирования: Sat Oct 17 13:59:03 2009
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Large birdies near 419 Mhz

13dec06

The plots:
The characteristics of the 419.2 Mhz Birdie (.ps) (.pdf):
The 418.625 Mhz birdie in the 1 second averaged spectra (.ps) (.pdf):
The dynamic spectra of 10 seconds worth of data at 418.625 (.gif):
 When the 418.625 Mhz modulation was on and when it was off (.ps) (.pdf):

Intro:

    430 aeronomy runs have seen a large birdie that saturates the receiver chain with a frequency near 419 Mhz. Data was taken with the 430 dome and the interim correlator on 01dec06 and 06dec06 to check this out. 100 Khz base band data was also taken on 01dec06 near 418.625 Mhz using the RI.

06dec06: large birdie at 419.2 Mhz

    The birdie was seen intermittently between 12:30 and 14:30 AST on 06dec06. The telescope was sitting at az=312.2, za=10. This was the only time it was seen. It was not seen during a few hours of monitoring on 01dec06 or 30nov06. It was also not seen in any of the x111 data taken during nov06,dec06 (about 11 1 minute integrations spread over midnight to 10 am).
    Data was taken with the interim correlator using 25 Mhz bandwidth and 2048 channels (24 Khz resolution after hanning smoothing). The spectra were s dumped once a second.
    The plots show the characteristics of the 419.2 Mhz Birdie (.ps) (.pdf):
  • Page 1 Top 5 minute averages:  30 5 minute spectral averages are over plotted.  Black is polA, red is polB (the feed is circular). The birdie was not resolved in the 24 Khz channel widths (after hanning smoothing). The amplitude of the birdie averaged over 5 minutes got up to about 12000 times Tsys. Since the birdie was not on all the time, the actual strength was greater (see below).
  • Page 1 Bottom. the strength of the birdie: the single birdie channel (averaged over 5 minutes) is plotted vs time.  It reached a peak during the 5 minute average at 14.36 (14:21 ast).
  • Page 2 top. birdie channel strength with 1 sec resolution: This plots the 24 Khz channel at 419.2 Mhz vs time with 1 second resolution. The vertical scale is now db's above Tsys. The  constant value of 50 db probably means that the system was saturated. The slow rise of Tsys around the green dashed line is the galaxy passing through the beam.
  • Page 2 bottom how long did the birdie stay on. This plot shows how long the birdie stayed on (using the 1 second sampling). The most common duration was 3 seconds. At 14.3 hours it stayed on for 25 seconds at a time.

    • 01dec06: large birdie at 418.625 Mhz.

        The 418.625 Mhz birdie was seen for about 1 hour between 11:30 and 12:30 AST on 01dec06 (after this time the frequency was changed so we have no data). 1 second spectra were taken as well has base band data with 100 Khz bandwidth. The 1 second spectra have 48 Khz resolution (after hanning smoothing).
        The plots show the 418.625 Mhz birdie in the 1 second averaged spectra (.ps) (.pdf):
  • Top a 1 second spectra. Black in polA and red is polB. The dashed green line is at 418.625 Mhz. The birdie has not been resolved in the 48 Khz channel. For some reason polA is  a lot stronger than polB (even though the main beam of the feed is circular).
  • Middle the time variation of the birdie. This shows the 48 Khz channel at 418.625 Mhz vs time (with 1 second resolution). The birdie gets up to about 7000 times Tsys.
  • Bottom how long the birdie stayed on. This shows how long the birdie stayed on vs time. The most common duration was 3 seconds.
    •     Base band data with 100 Khz bandwidth was taken after the 1 second data above. The spectra were computed using 100 Hz resolution (1K transforms). This gave a new spectra every 10 milliseconds.
        The image shows the dynamic spectra of 10 seconds worth of data (.gif):
  • The birdie turned on twice in the 10 seconds. The ghost at 418.75 is probably instrumental. The birdie has a carrier and then a modulation phase. Some of the properties are:
    •     The final plot shows the birdie spectra averaged over a single pulse when the modulation was on and when it was off (.ps) (.pdf):
  • The black trace is averaged over 50 milliseconds when the  modulation was on. The red trace was averaged over 67 milliseconds when the modulation was off. The vertical scale is db's above Tsys. It got up to about 50 Db (being on during the entire time).  It is about 2.4 Khz when the modulation is off and 9 Khz when the modulation is on. This data was probably not saturated.

    • Summary:

  • A birdie at 419.2 Mhz was seen on 06dec06.
  • A birdie at 418.625 Mhz was seen on 01dec06. Data was taken with 1 second and 10 usecond resolution.
  • It may  be that these are the same birdie. It would be nice to get some base band data of the 419.2 Mhz birdie to compare it with the 418.625 birdie.
  • Neither of these birdies have shown up in the x111 rfi monitoring data. It uses the 422-442 Mhz filter, but we can still see other birdies in this region.
    • processing: x101/061201/chk430.pro riraw430.pro, x101/061206/chkrfi.pro

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