# Low-Cost Sensor Based on SDR Platforms for TETRA Signals Monitoring
## 3 Results and Discussion  
### Results of the Laboratory Testing  
Noise power levels in the TETRA channel for the three types of SDR platforms and for the R&S FSV13 spectrum analyzer used as a reference are presented in Table 1 (the measurements were performed under the conditions of connecting an impedance of 50 ohms to the RF inputs). The number of FFT points was 2048 and the resolution bandwidth (RBW) of the spectrum analyzer was 3 kHz. In Figure 11, the noise floor can be observed for the entire TETRA Downlink band, between 390 and 395 MHz. In order to accurately measure the signal strength of the TETRA channel, we determined that a signal-to-noise (SNR) ratio of at least 5 dB is required. This means that the minimum power level that can be measured with the the SDR platforms is: \(-\)119.6 dBm for NI USRP 2930; \(-\)98.2 dBm for ADALM Pluto; \(-\)85.8 dBm for Hack RF One. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**R\&S FSV13** & **NI USRP 2930** & **ADALM Pluto** & **Hack RF One** \\ \hline \(-\)125.6 dBm & \(-\)124.6 dBm & \(-\)103.2 dBm & \(-\)90.8 dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Noise power level in a TETRA channel (channel 131, central frequency 393.2875 MHz). Figure 11: Display average noise level in the frequency band (390–395) MHz: (**a**) R&S FSV13; (**b**) HackRF One; (**c**) ADALM Pluto; (**d**) NI USRP 2930. The R&S SMBV 100 A generator set on a TETRA channel (channel 131, center frequency 393.2875 MHz) was used as a signal source to study the dynamic range of the amplitude. Its output level can vary between \(-112\) dBm and \(-32\) dBm. The measurement results are gathered in Table 2. The power levels displayed by the SDR NI USRP 2930 platform are very similar to those provided by the R&S FSV13 spectrum analyzer, considered as a reference. The ADALM Pluto SDR platform has a good response for an amplitude range of the input signal between \(-97\) dBm and \(-32\) dBm, while the SDR Hack RF One platform represents well only the amplitude range between \(-82\) dBm and \(-32\) dBm. These values represent the limits established by the different sensitivities of the two SDR platforms. Figure 12 provides a comparative observation of the frequency response in the range of interest. The signal was emitted by the R&S SM300 signal generator having the following settings: sweep operating mode in the frequency range (390-395) MHz; frequency step: 1 kHz; dwell time: 200 ms. It can be seen that, by comparison with the R&S FSV13 spectrum analyzer's response, the best behavior is provided by NI USRP 2930 platform, followed by the SDR ADALM Pluto platform. This result is explained by the good resolutions of the digital analog converter of the two platforms (12 bits for ADALM Pluto and 14 bits for Ni USRP 2930, respectively). The sensor has been designed to be able to acquire all 200 TETRA channels simultaneously, therefore an important parameter is the adjacent channel interference (ACI). Figure 13 shows the levels of the adjacent channels measured with the R&S FSV13 analyzer in the situation when a TETRA signal was applied to the input, respectively channel number 131, with a level of \(-62\) dBm. A comparison was made with the results provided by the other three SDR platforms. A synthetic view of attenuation of the adjacent channels is given in Table 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Output Power Signal** & \multirow{2}{*}{**Channel Power FSV13**} & \multirow{2}{*}{**Channel Power**} & \multirow{2}{*}{**Channel Power**} & \multirow{2}{*}{**Channel Power**} \\
**Generator SMBV** & & & & \\
**100 A [dBm]** & & & & \\ \hline \(-32\) & \(-33.2\) & \(-34.3\) & \(-33.2\) & \(-33.3\) \\ \hline \(-42\) & \(-43.6\) & \(-43.5\) & \(-43.3\) & \(-43.3\) \\ \hline \(-52\) & \(-53.5\) & \(-53.4\) & \(-53.2\) & \(-53.3\) \\ \hline \(-62\) & \(-63.6\) & \(-6 Figure 12: The frequency response, comparatively, in the frequency range (390–395) MHz: (**a**) R&S FSV13; (**b**) HackRF One; (**c**) ADALM Pluto; (**d**) NI USRP 2930.