xband RFI measurements.

last updated 23nov04

21may13: lidar xband radar interference
21may13: azswing 9-10  ghz with lidar radar on
23nov04: birdie at 9186.2 Mhz with 85 Khz sidebands

23nov04: birdie at 9186.2 Mhz with 85 Khz sidebands.   (top)

    A strong birdie was reported on 20nov04 at 9186.24 Mhz in the xband system. It was about 5000 Tsys in polB. The width was lt 25 Khz and it had side bands spaced 85 Khz away. The fractional occurrence of rfi by month showed that this birdie has been around for a long time (more than  a year).
    On 23nov04  the xband receiver was tuned to  9186.0 Mhz. The 260 Mhz IF was displayed on the spectrum analyzer with a 1 Mhz span. The birdie was 20 to 30 db above the noise floor. The IF signal was plugged into the radio. Using fm demodulation you could easily hear the birdie. It sound just like the dewar refrigerators going through their cycle !. The time for the period was .8 seconds (which is equal to the refrigerator cycle). Closing the horn shutters made the birdie go away. We then cycled off each refrigerator one at a time. The birdie did not go away. A metal short was placed over the horn. This would block the incoming radiation and leave tsys low (the shutter has some absorber on it). This also made the birdie go away.
    (this is where we got lucky...)
    While ganesh was on the service platform the shutters were closed again.  The birdie went away immediately, even before the xband shutter had moved very far. There are two kinds of shutters in use. On is mechanical (slow) and the other is compressed air (fast). The sbw and cband shutters were closing immediately while the others took a long time. We then powered off the sbw bias box and the birdie disappeared. Ganesh looked at the output of the sbw dewar and saw an oscillation at 9186 Mhz with the 85 Khz side bands (only in one channel of sbw). By adjusting the bias voltages he was able to get rid of the birdie. So the xband birdie at 9186 Mhz is now gone.

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