Ripple in pixel 4A seen in alfalfa (wapp) data
Alfa pixel 4 polA has a ripple in the bandpass with
a period of 174 kHz. It was discovered while debugging the galfa spectrometer
back in sep04 (see pixel4aripple
Project a2010 (alfalfa) does 600 second drift
scans dumping once a second using 100 Mhz centered at 1385 Mhz. A single
drift from 21may05 was examined and shows that the ripple is still present
in pixel4a. The band passes for pixel4 with and without bandpass correction
were examined to see the strength of the ripple and how stable it was over
time. The plots show the
band passes and the ripple for the 600 second drift (.ps) (.pdf)
fig 1 No bandpass correction:
Top: The bandpass averaged over the 600 seconds. The edges of the bandpass
have been removed to exclude the 1350 radar. Black is polA, red is polB.
The black trace looks noisier than the red one.
Center: A 10 Mhz blowup of the average band passes. You can clearly see
the ripple in polA.
Bottom: Each spectra was fft'd and then the magnitude computed. The average
was then computed over all of these spectra. The spike at 5.78 useconds
is the ripple. It has an amplitude of .0033 Tsys or about .09K. The corresponding
ripple frequency is 173 Khz.
Fig 2 With bandpass correction: The median bandpass was computed
and then divided into each 1 second spectra.
Top: This is the average of the bandpass corrected spectra. The spike at
1380 Mhz is a harmonic of the 1350 radar folding n.
Middle: A 10Mhz blowup of the average bandpass. You do not see the ripple
Bottom: The spectra were fft'd and then then magnitude were computed and
then averaged over. The ripple spike is still present at 5.78 Usecs although
it is not much weaker : .0002 Tsys or .007K.
The 174Khz spike is still present in alfa pixel4a. It is not seen in any
The amplitude of the ripple is about .09 Kelvins.
The strength of the ripple can be reduced by about 11 db if you divide
each spectra by the median. This was computed over a 600 second time period.
5.78 usecs is a distance of 150m*5.78=867M in air. If the reflection is
in the fiber optic cable then the distance would be about 580 meters.