## 2 Instrument Design
### Block Diagram
#### 2.1.1 Microwave Assembly  
The Microwave Assembly (MA) consists of the components of the front-end and the back-end (Figure 2). The mechanical input switch (SW) allows the selection of the noise source fed to the MA input, which could be either one of the radiometer inputs \(T_{\mathrm{RM},\mathrm{in}}\)H or \(T_{\mathrm{RM},\mathrm{in}}\)V to measure antenna brightness at horizontal or vertical polarization, or one of the tree-internal reference noise sources. The output of the switch is fed through an isolator (ISO1) tuned to the center of the radiometer band at 1,413.5 MHz to ensures a good match of the selected noise input to the receiver path. The microwave signal at the isolator output is directly fed into a 4-Section band-pass filter (BP1) before amplification. In this ELBARA II is unlike to other L-band radiometers currently used for the observation of terrestrial surfaces, e.g., the polarimetric radiometer EMIRAD [28] or the LEWIS radiometer [29]. The additional loss of \(<\)0.77 dB of the BP1 contributes less than 50 K to the total residual noise \(T_{\mathrm{RM},0}\) of the radiometer (see Section 2.2). This disadvantage is compensated for by the way RFI from outside the protected band is suppressed in the front-end before amplification, which avoids possible saturation of the first low-noise amplifier AMP1. To keep the residual noise \(T_{\mathrm{RM\,0}}\) of the radiometer as low as possible, the total transmission loss \(L_{\mathrm{front}}\) of the front-end must be minimized. This is due to the very low power level along the front-end, which is of the same order as the thermal noise caused by the losses of the front-end components. In accordance with (2), this is typically 10-13 W \(\approx\) -100 dBm for the frequency and bandwidth considered. In the back-end, the power levels are approximately 77 dB higher, which implies that the corresponding losses do not impact \(T_{\mathrm{RM\,0}}\) significantly. The front-end components are the input noise-power switch (SW), the isolator (ISO1), the band-pass filter (BP1), and three semi-rigid coaxial cables (Huber Suhner, \(\approx\)12 cm) with SMA connectors. Assuming the loss of one cable as 0.1 dB and using the specifications given in the Appendix results in \(L_{\mathrm{front}}\approx 0.15\)\(\mathrm{Db}+0.20\)\(\mathrm{d}\mathrm{B}+0.77\)\(\mathrm{d}\mathrm{B}+3\cdot 0.1\)\(\mathrm{d}\mathrm{B}=1.42\)\(\mathrm{d}\mathrm{B}\), corresponding to the power transmission factor \(t_{\mathrm{front}}=0.72\). Where there is perfect matching (no reflection), this corresponds to the front-end  
Figure 2: Block diagram of the ELBARA II radiometer. The abbreviations used and the specifications of the components are given in the Appendix. absorptivity 1 - \(t_{\rm front}\). Where the electronic components are in thermal equilibrium at the typical temperature \(T_{0}=313\) K (40 \({}^{\circ}\)C), the absorptivity equals the emissivity and the noise power caused by the front-end loss is \(T_{0}\)(1 - \(t_{\rm front}\)) \(\approx\) 87 K. Besides the losses in the front-end, the performance of the first low-noise amplifier (AMP1) determines \(T_{\rm RM\,0}\). In accordance with its specified noise figure \(NF\) = 0.5 dB, the noise temperature \(T_{\rm AMP1}\) of AMP1 is \(T_{\rm AMP1}\) = \(T_{0}\)(10\({}^{NF/10}\) - 1) \(\approx\) 38 K. These considerations yield the residual noise of the ELBARA II radiometer estimated from the component specifications as \(T_{\rm RM\,0}\) = \(T_{0}\)(1 - \(t_{\rm front}\)) + \(T_{\rm AMP1}\)\(\approx\) 125 K for \(T_{0}\) = s313 K.  
The output of AMP1 is attenuated by 3 dB and amplified a further 40 dB by AMP2. The output of AMP2 is filtered using a 6-Section band-pass filter. The band-pass filters (BP1/2) are both centered at 1,413.5 MHz and have a bandwidth of 22 MHz at -3 dB to be within the protected band allocation from 1,400 MHz to 1,427 MHz. The 3 dB attenuator between the amplifiers avoids amplifier instabilities. The output of the second amplifier (AMP2) is split into two channels using a symmetric power splitter.