Intro:

     Beam 2a,2b of alfa has looked noisier that the other beams for about a year. These two beams had been switched to an open loop bias card back some time in dec09. On 15feb11 it was found that the total power variation of these beams had a periodicity of about 5.22 seconds. This was later traced to the monitoring of the bias voltages and currents that occurs approximately every 5 seconds.

    To get a better idea of the problem and when it occurred, data was taken:

• 08,14feb11,16feb11  with the mock spectrometers at 10hz. The telescope was sitting still
• 08feb11: 172 Mhz bw centered at 1470 Mhz
• 15feb11: 172 Mhz bw centered at 1450 with 100 Mhz rf filter installed to get rid of rfi
• 16feb11:172 Mhb bw centered at 1450 Mhz.
• open loop card now in beam 1
• also take data with dewar monitoring turned off.
  
• a2010 alfalfa data from the last year was used to get an idea when the problem started.

Mock data at  10hz.

• 08feb11:
  
• 10hz mock data cfr=1470 MHz (.ps) (.pdf)
• Page 1: total power vs time for 900 seconds.
• top polA, bottom, polB. Each colored trace is a different beam
• Page 2: blowup of total power vs time (100 seconds).
• The vertical deflection of the periodicity gets up to 1% of Tsys.
• The duration of the periodicity is 5.22 secs. The duration when it is high is about 1 second.
  
• Page 3: spectra of the total power. top polA, bottom polB
• You can see the .1915 hz periodicity in polA and polB (green lines).
  
• 15feb11:
  
• 10Hz mock date cfr=1450MHz, 100MHz filter in (.ps) (.pdf)
• To make sure that the problem wasn't related to a radar in the band the setup used was:
• 100 MHz rfi filter (1390-1490) installed. This filtered out the radars (after the dewar).
  
• 10 hz sampling, 172 MHz bw centered at 1450 MHz.
• Pages 1-3 have the same details as the 08feb11 data.
• You can see the oscillations clearly in beam2a,b
• for some reason beam3a had a large variation in gain (trouble with the filter switch??). It later stabilized.
  
• dynamic spectra beam 2a for 70 seconds of data (.gif)
• spectra vs time for 70 seconds of data
• You can see the 5.2 second periodicity in the dynamic spectra.
• The oscillation does not show much frequency dependence.
  
• 16feb11:
• The open loop bias card has been moved to beam 1
  
• Dewar nonitoring On, 10 hz mock data. Open loop bias card now beam 1 (.ps) (.pdf)
• the .1915 Hz signal is present in beam 1.
  
• Dewar nonitoring Off, 10 hz mock data. Open loop bias card now beam 1 (.ps) (.pdf)
• the .1915 Hz signal is no long present when the dewar monitoring is off.
  

A2010 data with 1 second sampling.

• Using the wapps, a 100 MHz bandwidth centered at 1380 MHz is sampled for 600 seconds at 1 hz sampling.
• The telescope is stationary.
  
• Page 1 has 4 frames each showing a 600 second total power data set.
  
• The beams are color coded.
• PolA and polB are over plotted.
• the vertical scale is degrees K (offsets have been added for plotting).
• Page2 has the spectra of the total power from page 1.
• the Nyquist frequency is .5 hz.
• Vertical dotted lines have been placed at multiples of .1915 hz (5.22 seconds).

• 14feb11 (.ps) (.pdf)
• beam 2 shows oscillation
• beam 5 also has problems. A 2.5 MHz oscillation was seen at the dewar bias voltages when a spectrum analyzer was attached at the dewar.
  
• 21jan11 (.ps) (.pdf)
• beam 2 shows oscillation
• 30jan11 (.ps) (.pdf)
• beam 2 shows oscillation
• 13oct10 (.ps) (.pdf)
• beam 2 shows oscillation
• 07sep10 (.ps) (.pdf)
• beam 2 shows oscillation
• 18aug10 (.ps) (.pdf)
• beam 2 shows oscillation
• 02may10 (.ps) (.pdf)
• beam 2 shows oscillation
• 06apr10 (.ps) (.pdf)
• beam 2 shows oscillation
• 17mar10 (.ps) (.pdf)
• beam 2 shows oscillation
• 11jan10 (.ps) (.pdf)
• beam 2 shows oscillation
• 16dec09 (.ps) (.pdf)
• beam2 no oscillation
  
• 05nov09 (.ps) (.pdf)
• beam2 no oscillation
• 09sep09 (.ps) (.pdf)
• beam2 no oscillation

Summary:

• A 5.22 sec periodicity was seen in beam 2 of alfa.
  
• If was not present on 16dec09. It was present on 11jan10. So it appeared somewhere between these two dates.
• the amplitude of the oscillation is up to 1% of Tsys (typically .25 to .5 % of Tsys).
  
• Turning the dewar bias monitoring off caused the oscillation to disappear.
• The dewar monitoring reads all of the info from the bias box, waits 4 seconds, and then repeats the procedure.
• it must take about 1 second to read the information. This is when the gain increases on the measured total power.
  
• When beam2 open loop bias card was moved to beam 1, the 5.2 second oscillation moved with the open loop card to beam 1.
  
• Changing the open loop bias card back to close loop will decrease this problem.