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Using an attenuator just after the 1st IF conversion may be more convenient, because it then allows the backend to work at (almost) constant level. Let us assume an attenuation factor $\eta$ (smaller than 1) is used on the chopper. Then
Mean\_load / K = \eta * (Trec+T_{load})
\end{displaymath} (21)

and in TREC mode, the sky emission is given by:
T_{emi} = \frac{\eta * (Trec+T_{load}) * Mean\_atm}{Mean\_load} - Trec
\end{displaymath} (22)

while with a cold load, $Trec$ is given by
Trec = \frac{Mean\_cold * T_{load} - Mean\_load * T_{cold}}
{Mean\_load - \eta * Mean\_cold}
\end{displaymath} (23)

$T_B$ is now given by
T_B = e^{\tau} \frac{\eta}{B_s}(T_{load}+Trec)
\end{displaymath} (24)

Gildas manager 2018-03-23