Checking lbw stability, rfi sep10
PolB instabilities were reported in lband wide
(lbw) in jul10. In sep10
while warming the dewar, the window blew out.
The receiver was brought down to the lab and the window was replaced.
The receiver was reinstalled 25sep10 and cooled down.
Tests were done to check the stability of the
receiver (and also to look at the rfi):
- 27sep10: 300
second test covering all of lband using the mock spectrometer
4200 second test using the interim correlator and the setup of a2335
(who originally reported the problem).
second integration over entire band:
A 300 second test was done with the receiver
on 27sep10. The setup was:
- The mock spectrometer was used to cover the entire band.
- 4 bands of 172 MHz bandwidth and 8192 channels (20KHz resolution)
- The cfr for the bands were: 1200,1350,1500,and 1650 MHz.
- 3000 .1 second integrations were taken.
- The telescope sat at az=285, za=9 degrees.
- The data was taken around 17:00 ast. There was some
lightening in the distance.
Processing the data:
- dynamic spectra were made for each band (polB)
- the average spectra was computed for each band.
- the rms/mean by channel was computed for each band.
- An rfi mask was generated by finding all rms/mean channels larger
than 3 sigma
- The rfi mask was used to compute the total power for each band.
- PolB images of the 3 bands
- each spectra has been normalized to the median spectrum to
flatten the image.
- Since the telescope was stationary, continuum sources drift
through the beam.
- The vertical scale is seconds (with .1 sec resolution)
- I've labeled some of the more obvious rfi.
- 5 minute average spectra
and rms/mean (.ps) (.pdf)
- top: 5 minute average
- bottom: rms/mean by channel
- The green line is the
expected rms/mean from the radiometer equation.
- The rms/mean is greater
than the expected value because of the continuum sources.
- The dc channel birdie is
not stable (since it's rms is larger).
- Some of the rms/mean rfi
is polarized.. 1302,1303,
- total power vs time (.ps) (.pdf)
- the rfi was excluded from
the total power accumulation.
- top: total power vs time
- bottom: TotalPwrA/totalPwrB
- You can see the compression
every 12 seconds from the radars.
- There is no large drifting
as seen back in jul10.
second test around 1385 MHz.
On 29sep10 a 4200 second test was done. The setup
- lband wide, linear mode, 1280 hipass filter and 1230-1470 filter
- telescope sitting at 272 and 8 degrees za
- interim correlator 9 level sampling, 1 second spectra, 25 Mhz bw,
1024, centered at 1385.
- take data for 4200 seconds.
Processing the data:
The plots show the
total power and its spectrum (.ps) (.pdf):
- rms/mean computed over the 4200 seconds and then a mask was
created using a linear fit to rms/mean.
- total power was computed using the above mask.
- the spectrum of the total power was also computed.
- page 1 top: total power vs RA.
- black is polb, red is polb.
- the horizontal scale shows how the sky drifted by for the 4200
- page 1 bottom: ratio pola/polb vs time
- horizontal scale is not in secondsl
- You see variations in the ratio at the beginning.
- the variations are probably coming from the sky.
- The telescope was pointing close to the north polar spur
which is polarized.
- Page 2 top: spectrum of total power.
- black is polA, red is polB. polB has been offset for display.
- The max freq is .5 hz (from the 1 second sampling).
- polA seems to have more variation in the .1 to .3 hz region
- the green line at .2 Hz would be the alias of the 1.2 hz
crosshead frequency. nothing is seen.
- Page 2 bottom: blowup of horizontal scale.
- On this scale both pols look the same.
- no large drifts or jumps are seen.
- polA looks looks a little noiser in the .1 to .3 hz range.
- there is no power variation at the crosshead frequency