Links:

Intro:

     When alfa was reinstalled in nov09, oscillations were seen in the output signal. These were traced to the bias box. Part of the "work" done on alfa was to move the capacitors in the bias lines from inside the dewar to outside. This may have affected the bias circuit causing the oscillations (or maybe they were there before at a lower level).

    On 19nov09 we transmitted a sine wave from the control room to see if the 2.5 MHz oscillation frequency would mix with our sine wave.  The setup was:

• dome at az=0,za=17.23, alfa_ang=19.
• sine wave 1430MHz 10dbm connected to  1420 helical antenna inside the control room pointed out the window.
• mock spectrometer with 1 second dumps and 20 KHz resolution.

• Top: The strength of the sine wave (in a 20 KHz channel) for each beam. Vertical scale is db above Tsys. Black is polA and red is polB.
• Middle:Spectrum of sine wave and side lobes. Bottom to top is bm0.. beam6 (with offsets for display). The oscillation is flagged (+/- 2.37 MHz from 1430).
• There is some side lobes in: bm0,bm4,bm5  but it is less than .1 db above tsys.
• Bottom: Spectrum of sine wave and side lobes for polB.
  
• oscillations are strong in 5b (many Db above Tsys) and 4b (about .5 db above Tsys)

Conclusions:

• The bias oscillation frequency is 2.37 MHz
• It is strongest in bm5b (many db above tsys) and 4b (.5 db above tsys). It is weaker (<.1db) in bm0a,4a,5a