xband RFI measurements.
last updated 23nov04
lidar xband radar interference
azswing 9-10 ghz with lidar radar on
23nov04: birdie at 9186.2 Mhz
85 Khz sidebands
23nov04: birdie at
with 85 Khz sidebands. (top)
A strong birdie was reported on 20nov04 at
in the xband system. It was about 5000 Tsys in polB. The width was
Khz and it had side bands spaced 85 Khz away. The fractional
of rfi by month showed that this birdie has been around for a long
(more than a year).
On 23nov04 the xband receiver was tuned
9186.0 Mhz. The 260 Mhz IF was displayed on the spectrum analyzer
a 1 Mhz span. The birdie was 20 to 30 db above the noise floor. The
signal was plugged into the radio. Using fm demodulation you could
hear the birdie. It sound just like the dewar refrigerators going
their cycle !. The time for the period was .8 seconds (which is
the refrigerator cycle). Closing the horn shutters made the birdie
We then cycled off each refrigerator one at a time. The birdie did
go away. A metal short was placed over the horn. This would block
radiation and leave tsys low (the shutter has some absorber on it).
also made the birdie go away.
(this is where we got lucky...)
While ganesh was on the service platform the
were closed again. The birdie went away immediately, even
the xband shutter had moved very far. There are two kinds of
use. On is mechanical (slow) and the other is compressed air (fast).
sbw and cband shutters were closing immediately while the others
long time. We then powered off the sbw bias box and the birdie
Ganesh looked at the output of the sbw dewar and saw an oscillation
9186 Mhz with the 85 Khz side bands (only in one channel of sbw). By
the bias voltages he was able to get rid of the birdie. So the xband
at 9186 Mhz is now gone.