Links:
The radars were causing a longer recovery time in
the alfa receiver. On sep09 alfa was brought down to the lab to be
worked on. During the lab visit, leds were installed on the amplifiers
to cure the long recovery problem. It was reinstalled on the telescope
on 30oct09.
On 19nov09 tests were done to see how well the
system performed with the leds on. The setup was:
- dome sitting at az=360,za=17.23, alfa_ang=19.
- mock spectrometer used to take the data. It's setup was:
- 2 bands of 172 MHz centered at 1300 and 1450 MHz.
- .1 second sampling for 5 minutes
- adc blanking was active.
- repeat 5 minute scan 3 times.
The data was then processed and the total power at
each .1 second interval was computed. The processing was:
- Process each 300 second scan separately. There were 300*10=3000
spectra.
- Correct for the a/d blanking.
- Remove channels that had rfi:
- compute rms/mean for each channel then linear fit to the
rms/mean. Any channels above 3 sigma were exluded (1450 band usually
excluded 5 %, 1300 MHz radar band excluded about 15%).
- compute the power for each .1 second spectra using the good
channels. Normalize the total power to the median value (calling this
Tsys).
The plots show
the total power vs time for the 6 beams
(.ps) (
.pdf):
- Each page shows a different beam of alfa.
- The colors black,red,green show the 3 different 5 minute scans.
- The top two frames have polA 1300 MHz and 1450 MHz
- the bottom two frames show polB 1300 MHz and 1450 MHz.
- The negative going spikes are compression from the radar
- the positive going spikes are increases in the power from the
radars.
Comments on the plots:
- beam 1a 1450 MHz is wandering around. The bm1a 1300 does not so
the problem is probably not in alfa.
- beam 2a jumped by about 3% during the 3rd scan.
Observers have been complaining about 2a.
- beam 3a and 5a have strong radar power increases. This may be
from the geometry (alfa angle).
- beam 4a is compressing and it takes a while to come back up.
The second
plots are a blowup of
beam3a, 5a showing the recovery (.ps) (
.pdf):
- page 1: beam 3a. each step is .1 seconds. It take about 1 second
to get back to the baseline.
- page 2: beam 5a. This is recovering faster.
Summary:
- beam1a 1450 MHz band is wandering around
- beam2a jumped by 3% once in 15 minutes of data taking.
- beam3a,5a have the strongest power increase from the radar. This
may be because of the geometry.
- beam5a takes up to 1 second to recover completely.
- The measurements do not rule out any other part of the if/lo
chain as being the cause of the problem (but we did single out the
dewar amps prior to the led installation).
- While processing the data, 2.5 Mhz sidelobes are seen around the
1350 Mhz radar. These turned out to be the oscillations in the bias
cards.
processing:x101/091119/chkleds.pro