19nov09 oscillations from bias box
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
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:
The plots show the sine wave and
the oscillation frequency mixing in (.ps) (.pdf):
- 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)
- 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