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.
rfi measurements
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