rfi Alfa data 18-26sep11:
Project A2010 (alfalfa) reported rfi at 1408 MHz on
18sep11. The data setup was:
- alfa receiver.
- 600 second drift scans
- 1 second dumps of 100 MHz by 4096 channels (24KHz resolution)
centered at 1385 MHz
Processing the data:
- I processed one nights data at a time. The observations
went from 20:00 to about 3:00 hours.
- For all files for a night i kept 101 channels about 1408
- For each second, the power at 3 channels (centered at 1408
MHz) was averged and then it was normalized by channels [0:29,
71:100] from the 101 channels saved about 1408 MHz.
- Plots were made of the total power at 1408 MHz (channels) vs time
- Dynamic spectra images were also made for the 101 channels
about 1408 for the entire night.
- The median spectra was removed from each night so constant
values will be removed.
18sep11 50 second average spectra when
1408 birdie on (.ps)
Things to check:
- beam 4a was turned off for these measurements.
- The 1408 pwr vs time shows periodicity in the rfi.
- the pulses are space by 32 minutes drifting down to 15-20
minutes during the 5 hour run.
- When pulsing the rfi stayed on for about 230-240 seconds.
- The dynamic spectra show that sometimes the signal remains on for
long stretches of time. When this occurs, you do not see the 1408 tp
moving up and down.
- The 1408 birdie covers 3 24 khz channels:
- 1408. to 1408.47 with a dip in the middle.
- The high frequency ripple (see 20_21 beam3b) is
probably coming from the fiber optics. A 174 khz ripple is created by
reflections from the two ends of the fiber optics. The dynamic images
only show the ripple of it changes (we divide by the average bandpass).
- Monitor 1408 till you see it turn on.
- Then switch in the 1370-1470 MHz filter. If it goes away, it is
probably an intermod in our system.
- Az swing and see if it changes with az direction. Problem is that
it is also time variable.
29sep11 Opening, closing AC doors on building
An azimuth spin on 28sep11 showed that the 1408 MHz
rfi peaked at az=338.1, za=15. After some snooping with the spectrum
analyzer i found that the rfi was coming from the new AC unit on top of
building I (by the ladder).
On 29sep11 I took some data (using lbw) with the
to the AC unit closed, open, and then closed again.
The setup was
The image shows 590 seconds of data while
the doors were opened and then closed (.gif)
- az=338.1, za=15,lband wide receiver in linear pol mode.
- Mock spectrometer with 8192 channels over 4.4 MHz (538 chan
resolution), 1 second dumps.
- Data collection was started and then:
- went to the roof and opened the doors for about 240 seconds.
- I used my cell phone time to mark when the doors were opened
and closed. Unfortunately it didn't display seconds so I only had the
time to minutes (must do that when it is in airplane mode ???).
- The horizontal axis is frequency (but it is flipped). The
vertical scale is seconds.
- The door was opened around sec=250 (vertical scale).
- The door was closed around 480 secs.
- There was a jump in the strength of the signal around 60
secs (maybe it turned on??). The large jump in strength came around 250
seconds (the image stretch makes the strongest birdie appear saturated).
- When the door is opened, the entire noise bandpass increases.
This was also seen with the spectrum analyzer on the roof of blding I.
The plots show the strength of the
- Page 1: the average spectra while the doors were open. Top pola,
- You can see the 50 KHz comb.
- The strongest peak is 1408.0143 then 1408.0638.
- Page 2: 1408.0143 and 1408.0638 birdies for the 590 seconds
- Top PolA, bottom polB
- Green line shows where the door was opened.
- Blue line shows where the door was closed.
- Pol B is much stronger than polA at this az,za.
- the 1408.0143 channel gets up to 14 times Tsys. It was not
resolved in a 538 Hz channel.
- The 1408 MHz rfi is coming from the AC unit atop building I (the
- I looked at the AC unit on top of blding II with the spectrum
analyzer but didn't see anything at 1408. It may not have been running
at the time.
- The rfi is strongest at 1408.0143 MHz.
- It is not resolved in 538 Hz. channels.
- Using the spectrum analyzer on the roof, the entire noise band
increases when the AC unit doors are opened.
- The unit has electronic modules as well as printed circuit boards
inside the enclosure on the roof.
- The 1408 MHz frequency is the one that pops up. I'm sure there
are other birdies at different frequencies.