# K/Ka-Band Very High Data-Rate Receivers: A Viable Solution for Future Moon Exploration Missions
## 4 Receiver Architecture
### Linearity and Signal Levels  
A preliminary system analysis can be obtained from the characteristics of the front-end building blocks, as they are reported in the corresponding data sheets. These characteristics are summarized in Table 4 and can be used to evaluate gain, noise and linearity of the receiver. The first row of the table quotes the short stage name used to identify each block in Figure 3.  
The overall gain of the RX chain is estimated, under the hypothesis of conjugate impedance matching among the stages, as the product (sum in dB) of the individual gains. The gain of both image and IF filters have been assumed to be \(-3\) and \(-4\) dB respectively. These values are compatible with the planar technologies (substrate integrated waveguides and microstrip) that will be adopted for the realization of these filters [17]. The overall gain of the RX front-end is about \(46\) dB. In Table 4, no variable gain amplifier (VGA) stages are considered here (there is an AGC also within the SDR). As an option, the first IF stage could be replaced by a VGA, or followed by a controlled attenuator. The noise figure of the receiver, \(F_{RX}\), is determined with the Friis' formula assuming a conjugate matching among the stages. The down-conversion chain is composed by F2, M1, F3, A1 and F4. Its noise figure is approximately equal to \(22\) dB, i.e., the sum in dB of the noise figures of F2, M1, F3 and A1 (this because F2, M1 and F3 are all passive devices). Using this value as an intermediate step, a total figure \(F_{RX}=3.01\) dB is estimated. By omitting F1, the noise figure can be as low as \(2.4\) dB and reduces to \(2\) dB if also the ferrite isolator is removed; this comes at the expense of a lower immunity of the receiver to out-of-band interferences and of a worse input matching. The \(1\) dB compression point of the proposed receiver is limited by that of the mixer. Dividing the \(1\) dB compression point of M1 (\(+3\) dBm at the input) by the gain of F1, LNA1, LNA2 and F2 (equal to \(46\) dB) one finds \(P_{1 dB}=-43\) dBm, which is the input-referred compression point of the receiver. Similarly, also the third-order intercept point is limited by the mixer. As a first approximation, dividing the \(IIP_{3}\) of M1 (\(+12\) dBm) by the gain of F1, LNA1, LNA2 and F2 (\(46\) dB), one obtains \(IIP_{3}\approx-34\) dBm. More accurate calculations, accounting for the intermodulation distortions of LNA1 and LNA2, gives an \(IIP_{3}\approx-34.4\) dBm. The receiver Spurious Free Dynamic Range (\(SFDR\)) is given by:  
\[SFDR=\frac{2}{3}\ (IIP_{3}+N_{i}) \tag{7}\] \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Par.**} & \multirow{2}{*}{**Unit**} & **I** & **F1** & **LNA 1** & **LNA 2** & **F2** & **M1** & **F3** & **A1** & \multirow{2}{*}{**F4**} & \multirow{2}{*}{**SDR**} \\  & & **Isol.** & **Pre.****BPF** & & **LNA Chain** & & **Img.****BPF** & & **Mixer** & **IF****BPF** & **IF****Amp.** & \\ \hline \(G\) & (dB) & \(-0.4\) & \(-0.6\) & \(25\) & \(25\) & \(-3\) & \(-12\) & \(-4\) & \(17\) & \(-1\) & \(0\)-\(65\) \\ \(F\) & (dB) & \(0.4\) & \(0.6\) & \(2.0\) & \(2.0\) & \(3.0\) & \(12.0\) & \(4.0\) & \(3.0\) & \(1\) & \(3.8\) \\ \(P_{1 dB}\) & (dBm) & – & – & \(-11\) & \(-11\) & – & \(3\) & – & \(1\) & – & n.a. \\ \(IIP_{3}\) & (dBm) & – & – & \(0\) & \(0\) & – & \(12\) & – & \(12\) & – & \(-17\) \\ Tech. & & RWG & RWG & MMIC & MMIC & SIW & MMIC & MSTR & MMIC & & IC \\ \hline \hline \end{tabular} Note that, in Table, RWG stands for Rectangular WaveGuide; SIW stands for Substrate Integrated Waveguide;  
MSTR stands for MicroSTRip; IC stands for Integrated Circuit (silicon based) and MMIC stands for Microwave  
Monolithic Integrated Circuit (typically GaAs based).  
\end{table} 
Table 4: Specification Breakdown of the Receiver Building Blocks. where all the quantities are expressed in dB and dBm. For a 50 MHz IF bandwidth (i.e., for the previously evaluated noise floor \(N_{l}\)) one gets \(SFDR\approx-39.7\) dB.