The 174Khz ripple in pixel 4A

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History: (top)

29jun05: cleaned the fiber rcvr input to pixel 4a. The ripple got better but the next day it was bad again.

30jun05: We swapped the fiber inputs 4a<->4b and the problem stayed in 4a so the problem is not in the fiber, it is in the fiber receiver box.

01jul05: opened the fiber receiver box. There is a feed barrel where the fiber enters. We cleaned the fiber on the inside of the box. This did not improve the ripple.

01jul05: We noticed that plugging the fiber into the downstairs fiber receiver is extremely sensitive. We disconnected the fiber and then reconnected it (to look inside the fiber receiver box). After reconnecting it, there was no fail light on the fiber receiver front panel but there was also no signal in the control room. We unplugged and then reinserted the fiber into the receiver feed through connector and then the signal came back.  This same problem occurred on 29jun05. Alfa has large gain variations with temperature. It may be that the problem is in these feed through connectors upstairs and downstairs.

05jul05: we cleaned the upstairs fiber connection (that plugs into the fiber txmter). The ripple in pixel 4a got worse after we did this.

08jul05: we replaced the fiber receiver for pixel 4a and the ripple went away. pixel 2a still has ripple (.2% of Tsys).

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What we know about the ripple:  (top)

• The ripple is in both the wapps and galfa so it is not a back end problem.
• A 174 Khz ripple is a 5.747 usec delay. This is 862 meters in air. The index of refraction for the fiber cables (platform to clock room)  is about 1.462 (accuracy??). This gives a  distance in the fiber cable of 590 meters. Eddie castro thought there were 549 meters of fiber cable. The discrepancy is about 7% (so maybe the index of refraction was not that accurate).  The ripple distance points at the fiber as the problem.
• For a standing wave to occur in the fiber there needs to be a reflection at the bottom and and a reflection at the top end. Removing either of these will make the standing wave go away.
• The fiber cables for the non alfa iflo are very lossy (I seem to remember 18 db or more although i don't know whether this is all in the fiber). The alfa fibers have about a 3db loss.  Since the standing wave in the fiber has to pass through the fiber attenuation twice, alfa fibers will have a larger standing wave than the older fibers. This also shows that putting a 3db  pad in the fiber will reduce the regular signal by 3 db and reduce the ripple by 6 db.
• The IF outputs of the downstairs fiber receiver were switched pixel 4a<->2a. The ripple moved from 4a to 2a so the problem is before the output of the downstairs fiber optic receiver.
• We switched the fiber inputs to the downstairs fiber rcvr (4a<-->4b). The problem stayed in 4a. This says that the problem is in the fiber optic receiver box downstairs (this also says that we are getting a reflection for both 4a and 4b upstairs...).
• The ripple is also seen in other pixels: 0a,1a (1b has been dead so not sure about this one). 2A, 3A,3B. The strength of the ripple in these other pixels is at least 3db weaker than pixel 4a.
• The strength of the ripple is not constant day to day.

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Processing the data to see the ripple:  (top)

1. Compute the average spectrum using 200 seconds of data.
2. Do a robust fit of a 7th order harmonic and a linear polynomial to each baseline   divide the average bandpass by the fit and then subtract 1. The data is now normalized to Tsys
3. For display purposes, smooth the spectra by 19 channels. the ripple of interest has a period of  39 channels.
4. Compute the transform of #2  y=abs(fft(spc))*2. Channel 39 will be the delay for the 174 khz ripple. The amplitude will be in units of Tsys.

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Tests that show that the ripple is in pixel4a fiber receiver.  (top)

1. Normal configuration. Pixel4A xmter goes to pixel4A receiver.  Test 1 has the ripple  in pixel4A (.ps) (.pdf). This data was taken on 050630.
2. Pix4a xmter goes to barrel4B and then to pix4B receiver. Test 2  still has the ripple in pixel4A (.ps) (.pdf)..  This means that the problem is in pix4A rcvr or the pix4A barrel. It also tells us that there is no reflection in pix4Brcvr (since we know from 1 that pix4Atx has a reflection). This data was taken on 050630.
3. Pix4a xmter goes to barrel4A and then to pix4B receiver. For test3 the ripple remained in pixel4A (.ps) (.pdf). This rules out pix4A barrel. So the problem is in the pix4A fiber optic receiver (or the small cable that goes from the barrel to the rcvr). This data was taken on 050705.

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Ripple variation with time.  (top)

• Page1: These are the spectra for polA pixel 0a through pixel 6a.  The vertical axis units are Tsys. The high frequency ripple in 4a is the 174 khz ripple. It is about .5% of Tsys. You also see a little of the ripple  in pixel 3a. The slower ripple is the 1 Mhz ripple from the dish (and the residuals from the 7th order harmonic fit).
• Page 2: Same plots for polB.

    The variation of the ripple strength is seen in the a2011 data taken 20may05 through 29jun05. For each day that they observed, 1 file was processed and the amplitude of the 174Khz ripple was computed. The plots show the ripple strength versus time (.ps) (.pdf).

• Page 1 : The amplitude of the bandpass ripples  (abs(fft(spc))*2) for ripple number 0 through 100 (the 174 Khz ripple falls at 39). The strongest ripple if beam 4a. You also see ripples in beams 0,1,2,3 .
• Page 2:  The strength of ripple number 39 (the 174Khz ripple) versus day. The top plot is polA and the bottom plot is polB. Each color is a different pixel.

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08jul05: Pixel 4a Ripple goes away after we replace the pixel4a fiber receiver:  (top)

• The  strength of the bandpass ripples are plotted versus the number of cycles (of the ripple) that fit in the bandpass (i've limited it to 100 cycles max). Black in polA and red is polB. There are 19 200 seconds integrations over plotted on each frame. The strength is in units of Tsys. Pixel 4a no longer has any measurable ripple (it had been  .005 Tsys). Pixel 2A still has ripple at the .002 Tsys level. We will try to get the other spare fiber receiver working and see if we can get rid of it.