Ripple in pixel 4A


    Alfa pixel 4 polA has a ripple in the bandpass with a period of 174 kHz. It was discovered while debugging the galfa spectrometer. The problem was initially blamed on a bad board in the galfa spectrometer.  The same ripple was seen using the wapp spectrometer so the galfa boards are fine (or at least they don't create this ripple...). The plots show data taken on 03sep04 (proj a1946 drift scan) using the wapps and the galfa spectrometer. The wapp took data for 15 minutes. galfa took data for 5 minutes starting 48 seconds after the wapp start. The ripple is seen in both spectrometers.

The ripples seen by the wapps (ps)  (pdf)

The ripples seen by the galfa spectrometer (ps)  (pdf)     On 08sep04 we switched the IF input cables to the distribution amplifiers for pixel 2a and pixel 4a. After doing this the ripple moved from correlator board 5 (pix4) to correlator board 3 (pix2). So the oscillation is occurring before the IF distribution amplifiers in the receiver room.

    A ripple of 174 Khz corresponds to a one way distance of (300/2 meters * 1./.174 usecs) = 862 meters in air.  The index of refraction of the analog fibers that bring the signal down from alfa is 1.462. A 174 Khz ripple in this type of material would have a distance of  (862m/1.462)=590 Meters. Eddie castro thinks there is about 1800 feet (549 meters). The difference (590-549) is about a 7% error. This could be in the measurement of the period, distance, or index of refraction.


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