1.2 hz oscillation in lbw gain (something loose in the dewar)
Links to plots:
jumps and oscillation in the a2006 total power data (.ps) (.pdf)
tapping the dewar causes the gain to change (.ps) (.pdf)
the periodicity's in the time series for 53 data files taken in early apr05
Project A2013 looked at a flare star with lbw. The setup was:
Jumps, 1.2 hz oscillation seen 16,17may05.
When looking at dynamic spectra and total power
versus time, the power level was unstable. There was a 1.2 hz periodicity
as well as discreet jumps. When the polB power level jumped, the polA level
would always jump in the opposite direction. The plots show the jumps
and oscillation in the a2006 total power data (.ps) (.pdf):
100 Mhz bands centered at 1170,1370,1470,1570 Mhz with 128 lags
10 millisecond dump time, full stokes.
The dewar crosshead has a 1.2 hz period. On 17may05 we did some tests:
Fig 1 top: PolA total power versus time.
Fig 1 2nd: PolB total power versus time.
The 3 colors are the 1170,1370, and 1470 Mhz bands. The power jumps
are at 70 seconds and again at 270 seconds. The dashed lines are blown
up in the 3rd and 4th plots. The polB jumps up to 8% of Tsys. Pol A is
a little less.
Fig 1 3rd: PolA blowup in time.
Fig 1 4th: PolB blowup in time.
In the blown up data, you can see the 1.2 Hz oscillation. This is coming
from the dewar crosshead.
Fig 2 top: Periodicity's in the polA time series.
Fig 2 Bot: Periodicity's in the polB time series.
The power time series was transformed and then the magnitude was taken.
The 3 colors are the 1170 (black), 1370 (red), and 1470 (green) 100
Mhz bands. The spectral resolution is 1.6 millihz. The strongest period
is 1.2 hz spikes. It is strongest in polB where it gets to .28% of Tsys.
(the red comb is the 12/2 sec rotation period of the faa radar).
continuum data was taken (5 Mhz centered at 1420 Mhz
with a 1ms time constant) for 120 seconds. During this period we
tapped the lband wide dewar. The plots show how tapping
the dewar causes the gain to change (.ps) (.pdf):
Turning off the crosshead caused the 1.2 hz ripple to go away.
Tapping the lbw dewar caused jumps in the gain. Tapping the sbw dewar made
The postamp chassis are mounted with the dewar. To see if the variation
was in the postamp chassis we did:
The polA input to the postamp chassis was switched to a 30db amp.
The dewar was tapped again. PolB jumped but polA was rock solid. This says
the problem is in the dewar.
Fig 1 top: This shows the total power versus time. Black is polA
while red is polB. We started tapping the dewar around 62 seconds and continued
tapping for about 10 seconds. The power level for polB jumped up by 5%
while the power level for polA decreased by about 2%. The power levels
did not return to their previous values until 30 seconds after the tapping
Fig 1 bottom: This is the magnitude(fft(tp)) for the 120 seconds.
It shows the 1.2 hz periodicity of the crosshead.
Fig 2 Top: PolA mag(fft(tp)) taken in 20 second sections.
Fig 2 bottom: PolB mag(fft(tp)) taken in 20 second sections.
These plots show how the periodicity varied during the 120 seconds.
Each strip comes from 20 seconds of data. The 1.2 Hz oscillation is present
in polB from the start. The 4th strip (blue) started 60 seconds from the
start when the tapping commenced. The 1.2 hz oscillation increased
because of the tapping.
Project A2013 looked at a flare star on 08apr05 thru
10apr05. 4 100 Mhz bands were taken centered at 1170,1370,1470, and 1570
Mhz with 128 lags and a 1 millisecond dump time. I looked to see
if the 1.2 hz oscillation is present in this data by:
The 1.2 hz oscillation was present but weak in early april05
The plot shows the
periodicity's in the time series for 53 data files taken in early apr05
Compute the total power versus time for the 1470 Mhz band using the
first 300 seconds of each file. Do this for 53 data files spanning the
Compute the magnitude(fft(timeseries)) to see the periodicity's in
Two of the strips have power at the 1.2 Hz crosshead
oscillation frequency. It is stronger in polB than in polA (as is the may
data). The amplitude of the oscillation is a factor of 10 less than the
16may05 data. The occurrence of the oscillation is also much less in early
The magnitude of the spectra of the total power
time series is plotted. Each color is a separate 5 minute data set.
The horizontal scale is limited to 1 to 2.5 hz to see the first and 2nd
harmonic of the 1.2 hz oscillation. The vertical scale is the amplitude
of the oscillation in units of Tsys. The comb (.083 hz spacing) comes
from the 12 second rotation period of the faa radar.
There is something loose in the lbw dewar that causes the gain (or attenuation)
PolA and polB gain change 180 degrees out of phase with each other.
Tapping the dewar causes the gain to jump. It is then more susceptible
to the crosshead vibration. The tapping must jar something loose and then
the crosshead continues to shake it.
Normal motion/vibration of the dewar by telescope motion is probably causing
the gain jumps seen in a2006 of up to 8% of Tsys.
The 1.2 hz oscillation was present in early april05 but it was 10db weaker
and occurred much less frequently.