lbw rfi from cross head
13may09

    Progect a2335 (GASS) reported wideband rfi while using lband wide and the interrim correlator (with 1 second dumps) on 04may05. While investigating the problem, the rfi was seen to have a 1.2 Hz periodicty. This pointed to the cross head which has a 1.2 hz period.

    Data was taken with lbw and the mock spectrometers on 05may09 to investigate the problem with higher time resolution and bandwidth. The setup was:

• Telescope sitting at 270,10.
• 6 172 Mhz bands were taken centered at: 1200,1300,1400,1500,1600, and 1700 Mhz.
• 8192 channels (20Khz resolution) was used for the low time resolution (.1 second dumps).
• 4096 channels (40Khz resolution) was used for the high time resolution (.001 second dumps).

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.1 second time resolution:

    Data was recorded for about an hour. One of the worst 60 second spans of data was then selected to show how the interference appeared across the lbw band of 1100 to 1750 Mhz.

    Dynamic spectra

• 1115-1285 Mhz (.gif)
• 1215-1385 Mhz (.gif)
• 1315-1485 Mhz (.gif)
• 1415-1585 Mhz (.gif)
• 1515-1685 Mhz (.gif)
• 1615-1785 Mhz (.gif)
• 600 .1 second spectra where used.
• The images were normalized to the median spectra and scaled so that full scale was +/- 4 sigma
• 1215 and 1398 Mhz had some of the strongest rfi.

• The total power was computed for each 172 Mhz band and then plotted vs time (.1 second resolution).
  
• The total power was  transformed and the magnitude plotted vs frequency to check on the time periodicity of the rfi.
•  Each page has 2 frequency bands.
• Each frequency bands shows the total power vs time followed by the spectrum of the total power.
• For some band the rfi in the 172 Mhz band was greater than 5*tsys.
• The spectra show a peak at 1.2 hz and 2.4 hz.
  

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.001 second time resolution.

    1 millisecond by 4096 channel, 172 Mhz data was also take for about 60 seconds. The rfi was not as strong during this data set.

    Dynamic spectra of 1115-1285 Mhz Band

• The data were averaged to .1 second and then dynamic spectra were made to show the frequency extent of the rfi in the 1115-1285 Mhz region where the total power was computed.
  

    Total power vs time and periodicty of rfi (.ps) (.pdf):

• Page 1: total power vs time:
• Black is polA and red is polB
• The dashed purple lines are the section used for the following plot.
  
• Top:  The full 60 seconds with .1 second time resolution.
• It was strongest during the time 45 to 55 seconds.
• Middle: 45 to 55 seconds with .01 second time resolution.
• You start to see regions where the rfi goes away completely.
• Bottom: 48.5 to 49.5 seconds with .001 second time resolution.
• The rfi turned off for about .15 seconds at 49.1 seconds.
• Page 2: spectrum of total power for 1211 to 1225 Mhz.
• Top: 500 hz .08 Hz resolution
• 2nd: 125 to 135 Hz .016 Hz resolution showing spikes at 128.5 and 129.5 hz.
• 3rd : 232 to 242 Hz .016 Hz resolution showing spike at 237.8 hz.
• 4th : 0 to 10 hz showing 1.2 hz cross head frequency and it's harmonics.

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Summary:

• Wide band rfi is coming from the cross head or at least being caused by the crosshead motion.
  
• It was first seen in the x111 data around the first of May. It was not there on the 20th of april.
  
• the rfi showed the following frequency dependence:
•  1.2 hz and it's harmonics. This is the frequency of the cross head.
  
• 128.5, 129.5, 237.8 hz. These might be higher frequencies that were aliased by the 500 hz max frequency.
• When the cross head driver cable was removed, the rfi went away. When the cross head driver cable was plugged back in, the rfi started again in about 10 to 30 seconds.
• Switching cross head drivers from the nrao driver  to another one made no difference. The rfi remained. This also switched the cross head driver cable.
  
• If the rfi was not present, disconnecting and reconnecting the driver cable could make the rfi appear again.
• Banging on the lbw can had little affect on the rfi so it is probably not something loose inside the dewar. Banging close to where the cross head cable plugged into the cross head did make the rfi go away.
• We covered the mouth of the lbw horn with a mesh. This did not affect the strength of the rfi. So the rfi was not coming in through the horn.
• High frequency spikes were seen on top of the 1.2 hz signal of the cross head. When these spikes went away, the wideband rfi disappeared.
• High  time resolution (.001 secs) showed that the rfi would go away completely for up to 100 milliseconds.
  
• 07may09 afternoon...Replacing the cross head fixed the problem:
• There was still a 1.2 hz variation in the total power.
• The high frequency spikes were gone and the wide band rfi went away.

processing: x101/090505/masdata.pro, masdata_hr.pro