\begin{table}%t13
\caption{\label{tab13}Principal requirements and design solutions in LFI.}
%\centering
\par
\begin{tabular}{ll}
\hline\hline\noalign{\smallskip}
 Requirement/ \\
Constraint & Design solution  \\
\hline
High sensitivity&Cryogenically cooled ($\sim$20~K) HEMT amplifiers. \\
Low residual $1/f$, immunity from receiver systematics&Pseudo-correlation differential design.  Cryogenic reference load ($\sim$4~K). Offset \\
  &  removal by gain modulation factor in post-processing.  Fast switching (4~KHz) \\
  &  of sky and reference signal to suppress backend $1/f$~noise. \\
Single telescope&``Internal'' reference load. \\
Modularity, cryo testing.&Phase switch in frontend modules. \\
Low power dissipation at the 20~K stage.&Two amplification stages (cold frontend, warm backend).  Low loss and thermal \\
  &  conductivity interconnecting waveguides. \\
Waveguide mechanical routing.&Phase switch and second hybrid in the frontend (avoids need of phase-matched \\
  & waveguides.) \\
\hline
\end{tabular}
\end{table}