Measuring the cband cal values.
16may03
Links to sections:
The hcorcal
measured values:
The
spectra of (calon/caloff -1).
The
power levels during the measurements.
Links to Sections:
Measuring the hcorcal
The ratio of the other
cals to the hcorcal:
Intro:
New diodes were installed on the cband system on 22apr03.
On 25apr03 the cals were measured using absorber and tracking blank sky.
A ripple in polA cal pointed to a loose cable in the cal box. The cable
was tightened on 05may03 and the cals were remeasured on 16may03.
The correlator was configured as 25 Mhz by 1024
lags by 4 sbc. 4000 to 6000 MHz was covered in 100 Mhz chunks
(20 steps). At each step a 3 second integration with the hcorcal on was
followed by a 3 second integration with the cal off. The frequency range
4000 to 6000 was covered 6 times on absorber and 6 times on the sky.
After this the "other cals" were measured relative to the hcorcal
while tracking blank sky. See
measuring cals on the telescope using sky and absorber for a more detailed
description.
Measuring the hcorcal:
(..top)
The temperatures used for cband were:
Temperatures Used
| Trcvr |
from amplifiers |
8 K |
| Tomt |
orthmode transducer |
4 K |
| Tabs |
absorber temperature |
301 K |
| Tsky |
sky + scattered ground radiation |
3 + 15=18K |
| Trcvr |
Tamp+Tomt |
8 + 4 = 12K |
This shows the results of the measurements.
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Figure 1 plots the cal Values measured the 3 different ways:
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Black is cal polA, polB measured relative to the absorber.
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Red is cal polA,polB measured relative to the sky.
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Blue is cal polA, polb measured using the absorber,sky ratio.
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Fig 2 fits a high order polynomial (red line 13th order) to the absorber
and skyAbsorber measurements. The green crosses were used for the fit.
The blue line is a cumfiltered version of each 25Mhz set. The cal values
installed are computed from the red line.
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Fig 3,4 shows the individual measurements : absorber, sky, skyAbsorber
ratio.
The spectra for the cal (calOn/caloff -1)
for the data taken on the sky is shown in the plots. Each color is a different
pass through the frequency range 4000 to 6000 Mhz (6 in total). Offsets
have been added to each pass for plotting purposes.
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Fig 1. The entire frequency range 4000 to 6000.
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Fig 2-4. The same data in 500 Mhz chunks.
The power levels during the measurements are shown.
Black is pola and red is polB.
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Fig 1. Top the power level measured at the fiber optic transmitter (the
actual power is +20db more since a 20db coupler is used).
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Fig 1.Bottom the power at the downstairs power meter (actual power is +20db
more).
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Fig 2. The 0Lag power (25 Mhz) normalized to the median value for all of
the measurements. The figures are sbc1 thru 4. The large variation at the
beginning and end is on the sky (where the cal value is a large fraction
of tsys). The center part is on the absorber.
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Fig 3. The same as figure 2, but the power counter data (50 Mhz that goes
to the digitizer).
All three methods give consistent values for
4300 through 5000 Mhz (of course I twiddled the value of Tsky for this
to happen!). Outside this range the Cal values measured from Tsky do not
agree with those from Tabs or the ratios. This disagreement comes
from using a constant Trcvr of 12K. calAbsorber, calRatio is not sensitive
to the Trcvr while calSky is. When the red line drops below the other two,
Trcvr in that region is higher than the 12K that was used for the computation.
processing: x101/030516/hcorcal/cbcal.pro
Measuring the
ratio of the "other" cals to the hcorcal. (..top)
On 16may03blank sky was tracked with the cb receiver.
A sequence of 10 cal on/off measurements were done using the correlator
configured as 25 MHz x 4 boards. The sequence was (hcorcal, hcal, hxcal,
h90cal, hcorcal, lcorcal, lcal, lxcal, l90cal, hcorcal). This sequence
was repeated covering 4000 to 6000 Mhz. The hcorcal was measured using
the sky and absorber (see above). The ratio of the "other" cals was then
computed relative to the hcorcal (see
measuring cals on the telescope using sky and absorber ).
For each cal in the sequence, the cal on/off spectra
was computed and then cumfiltered to remove rfi. The ratio with the hcorcal
was then computed by interpolating between the 3 hcorcal measurements.
The interpolation was used since each cal is measured relative to
the calOff (or tsys) and tsys varies as we track blank sky. A 13th order
polynomial was then fit to each curve to generate the cal values.
The
ratio of the other cals to the hcorcal is shown in the plots:
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Fig 1 top shows the high cals measured relative to tsys versus frequency.
The * are pol B. The vertical lines show the 100 Mhz steps. There were
4 25 Mhz measurements at each frequency setting. Some of the cal sizes
track each other since they come from the same diode (e.g.. polA hcorcal:hcal,
hxcal:h90cal, polB hcorcal,h90cal). The bottom plot has the low cals.
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Fig 2 top plots the ratio of the cal/hcorcal for the high cals.
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The horizontal blue, blacks line have diode1->polA and diode 1->polB. This
is the same configuration for the hcorcal so the ratio is close to 1.
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The red lines have diode2->polB through the 90 deg leg of the hybrid.
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The purple,green lines have diode2->polA through the 0 deg leg of the 90
deg hybrid.
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The black lines overlaying each data curve is a 13th order polynomial fit
the to the data. The * are the frequencies that were used for the fit.
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Fig 2 bottom is the same plots for the ratio of the low cals to the hcorcal.
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Fig 3 shows how the hcorcal measurement varied for the 3 samples taken
at each frequency. The 2nd (red) and 3rd (blue) hcorcal measurement were
divided by the first hcorcal measurement (and then 1 was subtracted). Some
of the slope is the change in Tsys with za (decreases at the start then
increases at the end).
processing: x101/030516/othercals/cbcal.pro
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