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A perfectly linear (front-end + back-end) system delivers a signal (
)
in counts which is defined as
|
(2) |
where
-
is the signal delivered when the (front-end + back-end)
system is blind;
-
is the (front-end + back-end) system gain;
-
the noise added by the receiver and the optics, usually
called receiver temperature;
-
the signal seen by the receiver, usually called antenna
temperature.
As heterodyne receivers mix input signal from two different frequencies,
can be written as
|
(3) |
where
-
and
are respectively the signal in the signal
and image bands;
- and
is the ratio of the receiver gain in the signal
(
) and image
bands (at least 0.1 for
(sub-)millimeter receivers at IRAM).
is a combination of losses
and of the sum of the
atmospheric emission (
) and astronomical emission (
)
attenuated by the atmospheric absorption
|
(4) |
where
The losses come from the imperfect coupling of the receiver to the sky. It is here important
to note that
comes from two sources: 1) the (sub-)millimeter
emission inside the cabin, picked up by the receiver optic and proportional to
the cabin temperature
and 2) the (sub-)millimeter emission of the
ground, picked up by the antenna and proportional to the outside ambient
temperature
. Thus
|
(6) |
with being a largely unknown coupling factor.
It is usually supposed that
and
are identical for both
bands of an heterodyne receiver. This maybe should be revisited with the
large IF of today receivers.
Next: Description of the calibration
Up: Single-dish calibration in a
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Gildas manager
2014-07-01