Harmonics in the alfa/wapp system
An idl routine was written to compute the possible
harmonics in the alfa/wapp system (alfawappharm).
The user specifies a sky center frequency as well as a list of possible
input rfi frequencies (e.g. faa radar, aerostat radar, etc..). The program
then steps through each section of the receiver chain creating harmonics
of these frequencies and then removing any that get filtered out.
The result is a set of interfering spikes that end up in the final bandpass
(the routine includes the fundamentals of any input rfi frequencies).
The routine was run for the setup of a2010 (alfalfa
with center frequency at 1385 Mhz). The plots show the
harmonics created by the standard set of radars (.ps) (.pdf):
A 600 second drift scan take on 24feb06 was analyzed
to demonstrate the typical birdies in the band (cfr=1385 Mhz). The rms/mean
by channel shows the biridies in the band (.ps) (.pdf):
Top: This includes the 1350 Mhz radar. The horizontal axis is the frequency
where the harmonics appear. The y axis is the harmonic that created this
output frequency (0=1 is the fundamental). The color is the Sky frequency
of the original rfi (1350 in this case).
Bottom: There is a 100 Mhz bandpass filter centered at 250 MHz in the first
IF. The top plot assumed that this filter was infinitely steep. The bottom
plot assumes that the filter is 110 Mhz wide rather than 100 Mhz. So any
harmonics close to the edge of the filter can fold in. You see that the
1330 FAA radar (green) now appears with it's harmonics.
The black plot is polA, the red plot is polB. This data is from pixel 0.
The birdies seen are:
1350: the fundamental of the 1350 radar.
1380: the 3rd harmonic of the 1350 radar.
1405: the 2nd harmonic of the 1350 radar.
1410: the 5th harmonic of the 1350 radar.
1430: this does not match any of the harmonics up to the 5th order.