The 174Khz ripple in pixel 4A



what we know about the ripple
Processing the data to see the ripple.
Tests show that ripple is in pixel4A fiber receiver.
How the ripple changes over time.
Pixel 4a Ripple goes away after we replace the pixel4a fiber receiver.

Other pages related to 174Khz ripple:

pixel4a ripple: first seen sep04
pixel4a ripple: reported by alfalfa may05

History: (top)

    A ripple in beam 4a of alfa was found back in sep04 while working on the galfa spectrometer (pixel4a ripple sep04). It looked like it was coming from the fiber optics cable. Electronics replaced the fiber connectors (some time in oct04??). This did not make the ripple go away.
    The ripple was reported again by a2010 alfalfa group (pixel4aripple may05). They were seeing it in the wapp data.
    On Jun05 galfa group a2011 again complained about this ripple.
    A timeline of when we did the cleaning/tests:
  • 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).

  • What we know about the ripple:  (top)

    Processing the data to see the ripple:  (top)

        To monitor the ripple strength  i did the following processing:
    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.

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

        We did a series of tests with the fiber connections downstairs to see where the ripple was coming from. The picture shows the 3 tests that we performed.
    The fiber signals from the platform enter on the left and arrive at the fiber rcvr chassis. There is a bulkhead barrel connector that mates the fiber from the platform with the fiber that goes to the receiver.
        The 3 tests were:
    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.
     To perform the tests we had to connect and disconnect the fiber cables into the chassis and the fibers inside the chassis. Some of the change in amplitude could be from these changes. The important result is that there was never a standing wave in pixel 4B. So the problem is definitely associated with pixel4A receiver and/or interior fiber.

    Ripple variation with time.  (top)

    The first set of plots shows the ripple on 28jun05 (.ps) (.pdf) (galfa a2011 file .0002).

        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).

    You can see that there are large changes in the ripple strength.

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

        On 08jul05 we replaced the fiber receiver of pixel 4a with a spare fiber receiver. The ripple went away. We then ran 19 200 second integrations and computed the strength of the ripples for all of the pixels. The plots show the  bandpass ripples after pixel 4a fiber receiver was replaced (.ps) (.pdf) :
    processing: x101/050630/,, 050708/