## 2 Instrument Design
### Radiometer Assembly
#### 2.1.3 Back-End Stage  
The main purpose of the back-end stage is to measure the power level received at each one of the eight frequency bands, and each polarization of the antenna set. MERITXELL has been designed to perform these measurements using a common power detector, and hence, simplifying the overall design of the radiometer. Furthermore, this back-end stage allows to perform real-time processing, and post-processing of the radiometric signal in order to detect RFI signals, and mitigate them.  
The common power detector stage is completely performed by a Rohde & Schwarz (R&S(r))FSP40 Spectrum Analyzer (SA) with a frequency range up to 40 GHz (Rohde and Schwarz, Munich, Germany) [26], that allows to process the seven lower frequency bands, from L- to Ka-band. The rationale is that the SA functional block diagram is almost identical to the back-end stage of a passive Total-Power Radiometer (TPR). As can be seen in Figure 4, the signal coming from the front-end stage is first down-converted to IF using a frequency mixer, and a Local Oscillator (LO). Then, the IF signal is amplified and band-pass filtered with the so-called Resolution Bandwidth (RBW). After that, the envelope is detected and low-pass filtered with the so-called Video Bandwidth (VBW). Eventually, the measurement of the power level is displayed or stored for later processing. In addition, when the LO is a variable frequency oscillator, as it is in a SA, the back-end stage becomes a spectroradiometer.  
However, the real SA architecture is not identical to the functional block diagram. In fact, there are some differences even though the final performance is equivalent. The architecture of the R&S(r)FSP40 is based in a triple superheterodyne down-conversion followed to a digital IF stage as can be seen in Figure 5[26]. On the one hand, the triple down-conversion overcomes the difficulty of obtaining sufficient selectivity for the higher frequencies, as compared to the former single mixer. On the other hand, the digital IF stage replaces the analog envelope detector and the VBW, by adding versatility and functionality to the final display and storage of the data.  
Furthermore, the W-band is treated in a singular way because the R&S\({}^{\circledR}\)FSP40 main port can only reach up to 40 GHz. Therefore, the SA has been equipped with a R&S\({}^{\circledR}\)FSP-B21 external mixer port module which enables it with the capability to receive signals centered at a frequency higher than 40 GHz. In particular, W-band signals can be processed when the SA is combined with the harmonic mixer R&S\({}^{\circledR}\)FS-Z110. This harmonic mixer replaces the first two stages of the triple superheterodyne conversion, and therefore, it down-converts the W-band to a IF located at 404.4 MHz (see Figure 5). From thereafter, the W-band signal is processed in the same way than the other seven frequency bands.  
As above mentioned, the architecture of the R&S\({}^{\circledR}\)FSP40 enables the capability of performing real-time processing of the radiometric data in order to detect RFI signals because of the inherent features of a SA. With them, the signal may be divided into several sub-bands that can be weighted to equalize or mimic arbitrary frequency responses of different instruments, time intervals, and calculate histograms and statistic moments of the received signal for each sub-band.  
Moreover, the SA has the capability to store directly the In-phase and Quadrature (I/Q) components of the digital IF signal allowing a later post-processing of the radiometric data in the last stage. The post-processing stage is performed in the host computer that can run several RFI detection and mitigation algorithms such as time-frequency blanking, normality tests, or the Multiresolution Fourier Transform (MFT) detailed in [16]. Eventually, the radiometric data can be sent out to the host computer to perform an extra off-line processing.