Project p2030 reported a time variation in
the total power of the winking cal while using alfa.
Winking cal on continuum source:
on 21dec13 p2030 took 25 Hz winking cal data
while tracking 3C138 (10 Jy at 1400 MHz).
Their setup was:
- alfa receiver, 172 MHz bw, cfr 1450
- hardware winking cal at 25Hz.
- 65usec sampling, 512 lags.
- Total power computed for the 1450 MHz band (172MHz)
- PolA and polB were averaged in the mock spectrometer.
- Data sampled for about 10 seconds on 3C138.
- Data then folded at the 40 millisecond period of the
shows the folded cal data (.ps)
- Top: 10 seconds of winking cal data folded to .04 seconds
- The vertical scale is degK. It is high since 3C138 is
about 10 Jy at 1400 MHz.
- You can see the slope of the signal.
- Bottom: remove the median value from the calon and caloff
- This shows the differential change for during the calon
and cal off
- The change is about .28 K or .21%
Winking cal with the alfa cover in place:
On 14jan14 25 Hz winking cal data was
taken while the alfa cover was in place.
The setup was:
- 172MHz bw centered at 1450 MHz.
- 1 millisecond sampling, 2048 channels
- start on 1 sec tick so the samples are synchronized with
- 40 samples per cal period
- Pol A and PolB were processed separately
- Data was analyzed for all 7 beams.
show the winking cal with the alfa cover in place (.ps)
- There are 7 pages (1 page per alfa beam).
- Top: cal on,off folded for 25 seconds.
- + shows polA
- * is polB
- The left half of the plot is cal on, the right half is
- You can see the drift in the cal value.
- Bottom: remove the median value from the cal On and calOff
- The cal transition is occurring during the first samples
of the transition (so they were ignored.).
- PolA and polB show the same variation
- The change is about .06K,-.2K this is about .3 %
- a time variation was seen in the total power of the
- The variation is the same for all 7 beams of alfa
- The variations are also the same for polA and polB
- this argues that it is not an alfa amplifier problem
since you would expect them to vary a little
- it take up to 10 milliseconds for the cal to reach a fixed
value when the cal is turned on.
- It also takes about 10 milliseconds for the cal to reach a
constant value when the cal is turned off.
- The variation with time is about .28% of Tsys (with Tsys
varying by a factor of 10).
- This argues that the problem is a gain and not a noise
- At first i thought it might be an amplifier saturation
problem where it took a long time for the amp to recover
(like we had with the amps and the radars before the leds
were installed on the amps).
- It's not this, since there is no saturation possible
with the cover on.
- If the problem was a cal diode problem, then it would be a
constant degK difference and the ratios to Tsys with 120K
and 12K should be way different
- But fraction drift was the same for Tsys's that varied
by a factor of 10.
- So how would the cal know what the system temperature
was going to be...