22jan16: alfa bm6b instabilities

• 08dec15,17dec15, and 26dec15.

I finally got around to looking into this using the 20jan16 galfacts data:

• 1 millisecond sampling, winking cal (20ms on, 20ms off)
• 128 channel spectra, full stokes (after decimating in freq).
• i used the 172 mhz channel centered at 1450 Mhz.
  
• driving za from 2 to 19.5 degrees za (basket weaving).

• a nights run lasted for about 1 hour.
• i read in the data, averaged the 172Mhz 1450cfr band giving 1 total power sample per millisecond
• I then average over the 40millisec cal cycle (throwing out the transition points) giving 1 total power sample each 40 milliseconds.

The first set of plots shows the total power vs time for bm0,1, and beam 6 (.ps) (.pdf)

• black in pola, red is polb
• the data sampling ends up being 40 millisecs/point
• top frame: beam 0
• middle frame: beam1
• bottom frame: beam6
• you can see the system temperature change with za
• beam 6 has stability  problems when it is at low zenith angle.
  

The 2nd set of plots, shows beam 6 vs za and time (.ps) (.pdf)

• black is polA, red is polB
• Top frame: total power vs za for then 1 hour run (2.5 times up/down in za)
• polB is going unstable below about za=6
• middle frame: blowup show the za vs time  for the first pass through za=2
• bottom frame: this is the corresponding total power vs time for the za track in the middle frame.
• the dashed green lines enclose when the za was below 3 deg.
• coming down the instabilities started around 3deg za. On the way up they continues until around 4.5 deg za.

The za dependence looks like there is something loose.

• The receivers are level at za=10,  tilted -10deg at za=0,and tilted +10deg at za=20deg.

On 22jan16 we looked at the bias voltages and saw that beam 6a stage 1 biases were jumping around

• This was happening at za's above the 4.2 degrees of the plots above.

We monitor the alfa bias's (usually once every 5 minutes). But.. during pulsar experiments, the bias monitoring has to be turned off so it doesn't affect the observations (reading the bias's causes glitches in the bias's output).

The plots below show the measured Drain Current Id for stage 1of the  alfa amplifiers for 01jan-22jan16 (.ps) (.pdf)

• Top frame: PolA stage 1 Id drain currents for amps.
• Each beam is color coded. beam 6 is light blue
• The dropouts are when the monitoring was turned off (8jan-15jan)
  
• Bottom frame: polB stage 1 Id drain current
• b, 6b Id jumped up to 17 milliamps on 17jan and 20jan16 (and stayed there till end of plot.. 22jan16).
  
• The 20jan16 galfacts data was taken while Id was at 17 milliamps

I went back and looked at the alfa bias monitoring during jason's runs on 08,17,26 dec15. Unfortunately the bias monitoring was turned off during this time period (the operators turn off the monitoring, but it doesn't always get turned back on when alfa observing is done).

Summary:

• The instabilities in beam 6 is coming from 6b. PolA is ok
• The problems are coming from the bias values jumping around
• The original za dependence  from 20jan16 data points to something loose (because of the changing tilt with za)
• but on  22jan16 it looked like it was happening on other za's outside the 0-4.2za range.
• We need to find out if the problem is inside or outside the dewar.