Rfi has been reported
near 1375 MHz since sep09. An azimuth swing was done on 07oct09 to see
if it was outside the dome. High resolution data was taken on 06nov09
to check the rfi stability.
Resolution:
The 1374.988 Mhz birdie was coming from a STRIDE 100
Mbit 5 port ethernet switch that had been installed on the top chord of
the platform (by the weather station). It had been mounted inside a
plastic box. on 20nov09 it was turned off.
Setups:
- Az swing:
- lbw. za=18 degrees, 270->630 and back at .35 degrees/sec.
Linear pols
- mock spectrometer .1 second dumps, 20 KHz resolution.
- High res spectra
- lbw. az=60,za=18. Linear pols.
- mock spectrometer, 2 300 second scans sitting. 1.024
second dumps with 20 hz resolution.
Az swings:
The plots show
the
results from the azimuth swings (.ps) (
.pdf):
The 20 KHz channel centered at 1374.9897 is plotted
vs azimuth. The data is plotted vs Tsys. Black is the clockwise az
swing
while red is the counter clock wise spin.
- Page1: Entire az swing
- TOP: PolA az swings.
- BOTTOM: PolB az swings.
- The clockwise and counter clockwise swings overlap exactly in
azimuth.
- The signal is coming from outside the dome.
- The peaks in az, za are happening at very different azimuths.
- PolB peak: az=60.79
- polA peak: az=117.90
- Page 2: blowup az=50 to 130 degrees.
Hi resolution spectra:
The plots shows
the high resolution spectra and
channel power vs time (.ps) (
.pdf):
- Top: rfi from 1st and last 1 second spectra:
- Black is a portion of the spectra from the first spectra
- Purple is the same spectra 629 seconds later.
- The rfi has moved by about 1 channel (20 hz).
- Start: 1374.988926 MHz
- End: 1374.988946 MHz
- The drift rate of the rfi is:
- 20Hz/620 secs = .032 Hz/sec
- Oscillator stability for the rfi:
- Bottom: Amplitude stability.
- Black is polA, red is polB. The units are Tsys in a 20Hz freq
bin.
- az=60 is a null for polB and a peak for polA in their
respective azimuthal dependence.
- 3 channels (centered at 1374.988926 were summed and then
plotted vs time.
- PolA is wandering from 300 to 160 then back to 300.
- PolA wonders from 200 to 100 and back.
Summary:
- The 1374.988930 rfi is coming from outside the dome.
- The birdie has been seen in lbw and in alfa.
- The birdie is not resolved in 20 hz channels
- The birdie drifts by .032 hz/sec giving an oscillator stability
of 2.3 part 10E11.
- It is not tied to our hydrogen maser.
- PolA and polB peak at very different azimuths.
- The amplitudes drift by factors of 2 (20 Hz channels) in
5 minutes.
- I had previously thought that 1375/2=687.5 is close to the video
carrier of analog channel 50 (687.25).
- this channel is new. It had been at channel 62.
- The problem is that chan 50 is a digital channel and no longer
has a video carrier so this is probably not the culprit.
processing: x101/091007/azswing.pro,
x101/091106/rfi1375.pro