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)

• Figure 1. This is the average of 15 minutes of data taken with 100 Mhz bandwidth and 4096 channels (25 khz resolution).  The first lo was set to 1635 Mhz. The red is polA and the green is polB. The plots are labeled B1 thru B7 this corresponds to Pixel0 thru pixel6. The ripple is in pixel 4A (B5 red trace). You can not see anything on this scale.
• Fig 2. I fit a 3 order polynomial and a 12th order sin/cosine to the bandpass. The fit was subtracted from the data and then this difference was divided by the  mean value of the fit (over the "good" channels). This put the data in units of Tsys.  The horizontal scale was expanded to 10 Mhz. You can now see the ripples in pix4a (B5 red trace).
• Fig3. Pixel4a was plotted by itself using 3 Mhz of data. The oscillation has a period of 174 Khz. The amplitude is about .005 Tsys (a peak to peak of about .01 Tsys).

• Fig 1. This is the average of 5 minutes of data. It started 48 seconds after the start of the wapp data above. The horizontal scale is in galfa channels (872 Hz/channel). The amplitude is in galfa counts. The bad pixel is in B5 red. You can see a little of the oscillation on this scale.
• Fig 2. This is pixel 4 (B5). A baseline has been removed (2nd order polynomial and 6th order harmonic) and then smoothed to 38 Khz resolution (about the same resolution as the wapp data). The baseline was removed and then the difference spectrum was divided by the mean of the good channels to put the difference spectrum in Tsys units. Three Mhz of data was then plotted. The frequency scale was computed from the 1st Lo (recorded in the wapp data) and assuming the 2nd lo was 256.25*2 Mhz. I tried lining up the 60 MHz drifting harmonics in both data sets. They were off by about 2 Mhz (or 62 Mhz) so my assumption about the 2nd lo may have been wrong). The amplitude of the ripple is .04 Tsys (.08 Tsys peak to peak). This is 8 times stronger than the wapp data. The only difference is that the wapp data was a 15 minute average while the galfa data was 5 minutes (or that the frequencies plotted did not overlap).

    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.

Summary:

• There is a 174 Khz ripple in pixel 4a of the alfa data.
• It is seen in both the wapp and galfa spectrometer.
• It has been there since at least 21aug04 (that is the first data I had to look at).
• It is occurring before the distribution amplifiers in the control/receiver room.
• The most likely culprit is a reflection in the fiber optic cable for pol4a. We need to clean the ends of the fiber an make sure that the connection is good.