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We test here the AVERAGE command behavior and the noise
properties on a frequency switch dataset. Here we use the 3 frequency
switch tracked spectra observed on 03-jun-2013, scans 100, 101, and
102, on the Vespa 4 backend part (30ME2VLI-V04).
Table 1:
Status of the raw spectra w/t their theoretical and
measured noise in all CLASS versions.
is the integration
time,
is the frequency resolution. The theoretical noise
is computed from
,
, and
(in Hz):
. The measured noise is computed
by the BASELINE command, excluding the windows with signal.
Scan |
Tsys |
t |
 |
Theoretical noise |
Measured noise |
Status |
|
(K) |
(s) |
(MHz) |
(K) |
(K) |
|
100 |
224.66 |
56.61 |
0.0195 |
0.214 |
0.219 |
OK |
101 |
224.53 |
56.61 |
0.0195 |
0.214 |
0.220 |
OK |
102 |
224.40 |
56.61 |
0.0195 |
0.213 |
0.217 |
OK |
Table 2:
Same as Table 1, for the averaged spectra
produced by the command AVERAGE in CLASS version feb21. We
combine the 3 spectra either in a single call (100+101+102), or in
2 calls with an intermediate average (100+101, then
(100+101)+102). This CLASS version gives unexpected results (in
red). 1) After a single average, the system temperature, and thus the
theoretical noise, is underestimated by a factor
, but the
measured noise is correct as the weighted average used proper weights.
2) when averaging in several calls, the error on the system temperature
propagates also to the weighted average; we can show that the measured
noise is divided by
instead of
(
overestimate) after the 2-average sequence.
Average |
Tsys |
t |
 |
Theoretical noise |
Measured noise |
Status |
100+101+102 |
red158.77 |
169.84 |
0.0195 |
red0.087 |
0.127 |
Unexpected |
100+101 |
red158.81 |
113.23 |
0.0195 |
red0.107 |
0.154 |
Unexpected |
(100+101)+102 |
red122.99 |
169.84 |
0.0195 |
red0.068 |
red0.131 |
Unexpected |
Table 3:
Same as Table 1, for the averaged spectra
produced by the command AVERAGE in CLASS version mar21.
Average |
Tsys |
t |
 |
Theoretical noise |
Measured noise |
Status |
100+101+102 |
224.53 |
169.84 |
0.0195 |
0.123 |
0.127 |
OK |
100+101 |
224.59 |
113.23 |
0.0195 |
0.151 |
0.154 |
OK |
(100+101)+102 |
224.53 |
169.84 |
0.0195 |
0.123 |
0.127 |
OK |
Next: About this document ...
Up: class-weighting
Previous: Consequences
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Gildas manager
2023-06-01