25jan16:327 hires azimuth swings
Processing the data
Dynamic spectra of the azimuth swings
Average/PeakHold spectra for each az swing:
The azimuth dependence of all the birdies
birdies with no az dependence (coming from
birdies with common az dependence
The rfi at 327 MHz has been getting worse with
time. Spectral line observers stopped using it over a year ago
because of the rfi. Recently some of the pulsar observers have
abandoned their observations (single pulse) because of the rfi
present in the band.
On 25jan16 two hi frequency resolutino azimuth
swings were done using the 327 MHz reciever.
- 327 Mhz receiver. 750Mhz IF
- azswings 270 to 630 degs and back at .35 degs/sec
- mock spectrometers:
- 7 spectrometers cover 300 to 350 Mhz.
- Spectrometer cfr, 304,311,318,325,332,339,346 Mhz
- + 5 MHz added in iflo menu, -5 Mhz added in mock digital
mixer.. to move DC spikes out of band
- 8 Mhz bandwidth, 8192 channels; 976 hz resolution, 1
second sampling (about 1 sec)
- The alfa motor controller and the dome cameras were turned off
for the azimuth swings.
Processing the data: (top)
- For each azimuth swing a bandpass was constructed from the
1027 spectra. A fit to a bandpass was needed rather than an
average since some rfi are constant and would remain in the
bandpass average. The bandpass was created by:
- Compute a median bandpass from the 1027 spectra.
- Fit a 20th order harmonic and a linear polynomial to this
median bandpass using robfit_polyfft(). This iterates the fit
throwing out outliers.
- Divide each of the 1027 spectra by this fitted bandpass.
This should remove most of the bandpass.
- For each spectra compute the median total power and subtract
it. This should remove the source contribution.
- PolA and polB were then averaged.
Dynamic spectra of the azimuth
Dynamic spectra were made for each of the
azimuth swings showing spectra density plotted against azimuth and
frequency. The plots were aligned so that azimuth always increased
bottom to top (so the swings from 270 to -90 were flipped). At
the bottom of each image is a peak hold spectra (maximum in each
frequency channel) for the entire spin.
- Some of the birdies have an azimuth dependence. This means
that they are coming from outside the dome
- A number of these came from 3rd harmonics of 102.9,104.1,
107.3, and intermods between 102.9 and 104.1 (these towers are
located within 100 meters of each other). (more info
- Other biridies have no azimuth dependence. This tell us that
they are probably coming from inside the gregorian dome.
The first plots show the average and peak hold for
each azimuth swing (.ps) (.pdf)
The second set of plots has the azimuth dependence of 140
different rfi frequencies (.ps ..12Mbytes) (.pdf 4.6Mbytes)
- all 7 bands were combined into a single spectra (976 Hz
- The top two frames show the average of each azimuth swing:
- black is CW azswing, red is the CCW swing.
- This average over 1027 seconds.
- Topframe: vertical scale 0 to 10*Tsys
- 2nd frame: vertical scale 0 to 1*Tsys
- The bottom two frames are a peak hold for each swing
- frame 3: vertical scale 0 to 10*Tsys
- frame 4: verical scal 0 to 1*Tsys.
The third set of plots shows rfi that has little azimuth
dependence (.ps) (.pdf)
- I went through and selected 140 different birdies.
- they had to stick up at least .2*Tsys (in 976 hz channels, 1
- I left out some of the wider rfi (3rd harmonics of fm
stations and their intermods)
- This list is not complete..i know i skipped some of the rfi
- The black trace is the CW swing, red is the CCW swing
- Where the black and red traces overlay each other in azimuth
shows that the rfi is stable in time and coming from a
particular azimuth direction (outside of the dome).
- rfi that does not change much with azimuth is probably
coming from inside the dome.
The 4th set of plots
shows rfi that has a common
azimuth direction (.ps) (.pdf)
- To compute rfi pwr vs azimuth i:
- for each 1 sec spec i took the max value within a few
channels of the peak value for the az swing.
- This list is not complete. I have probably left out some
- Page 1: over plot the 28 rfi frequencies vs azimuth
- I normalized each frequency to its max value for the swing
(to keep them all on the same page)
- I then added a display offset to each spectra (so they
wouldn't sit on top of each other
- Each line is then labeled with its frequency.
- Page 2: plot the average power over the azswing for each rfi
- This shows how strong each rfi was
- The actual values are labeled with
- the strongest rfis were:
- 331.909 3.848*Tsys.. this is a harmonics of little
star inside the turret control box
- 315.000 3.063*Tsys..
- this is a camera frequency.. the dome cameras were
- the sb console showed this on 27jan16 with the anritsu,
but it did not go on,off during the test on 28jan16..
- 308.000 1.791*Tsys.. also seen with the anritsu
27jan16, but did not cycle during the 28jan16 test..
- We may have a 1 MHz comb coming from something..
- Looking through the 140 rfi freq vs azimuth of set 2, i
picked 11 frequencies that had a common azimuth dependence
- The two frames over plots these 11 frequencies (6 in the top
frame, 5 in the bottom frame)
- I've normalized each frequency to its maximum value for the
- the azimuth peaks are at:
- -30deg azimuth .. single peak
- 150 deg azimuth.. multiple peaks
- 330Deg is about the direction of the control room
- This is probably the direct path
- 150 is probably rfi scattering into the dome when it is
180 deg away from the control room
- The multiple peaks are probably from the lengths of the
platform it is scattering off of.
- Looking back at plot set #2. the 320 Mhz biridie is coming
from the computer room air conditioner on top of building 1.
- This has an azimuth peak at 340 deg azimuth..
- using a higher resolution spectrum (about 1khz) allowed us to
see a lot more of the 327 birdies (previously i'd used 6.4Khz)
- hundreds of birdies were seen above the noise floor in
976 hz, 1 second integration (covering 50 MHz).
- Many of the birdies showed no azimuth dependence.. so they are
coming from inside the dome
- see sbconsole, turret controller 327 rfi.
- tens of birdies are coming from the direction of the control