# Low-Cost Sensor Based on SDR Platforms for TETRA Signals Monitoring
## 2 Material and Methods
### Sensor Testing Methodology in the Laboratory  
The TETRA signals' monitoring sensor was tested in laboratory conditions based on three SDR platform models: ADALM Pluto, Hack RF One and NI USRP 2930. The results of the measurements performed with our sensor were compared with those performed by a high-performance Rohde & Schwarz spectrum analyzer, model FSV13, which was used as reference. The TETRA signal source was provided by a Rohde & Schwarz SMBV 100 A vector signal generator. Additionally, in the sweep mode, the Rohde & Schwarz SM300 vector signal generator was used. Figure 8 shows the measurement set-up containing the three SDR platform models, the R&S SMV 100 A signal generators, the R&S SM300 and the R&S FSV13 spectrum analyzer.  
Three SDR platform models were chosen based on the following criteria: low-cost price (for this purpose, the SDR platforms ADALM Pluto and Hack RF One were chosen); real-time monitoring capabilities for a bandwidth of at least 5 MHz (for this purpose, the minimum acquisition band for each platform must be 5 MHz); small dimensions and the possibility of using a USB power supply in order to have a portable sensor (this is accomplished by the low cost of ADALM Pluto and Hack RF One platforms); high-quality  
Figure 8: Measurement set-up for laboratory testing.  
Figure 7: The Geo-Location module.  
monitoring capabilities (sensitivity, amplitude dynamic range, etc.) if precise measurement results are expected (for this purpose, the SDR NI USRP 2930 platform was chosen).  
The HackRF One platform has the following characteristics: frequency range: 1 MHz to 6 GHz; instantaneous bandwidth: up to 20 MHz; interface: USB 2.0; analog digital converter (ADC) resolution: 8 bits. The ADALM Pluto platform has the following characteristics: frequency range: 325 MHz to 3.8 GHz; up to 20 MHz of instantaneous bandwidth; 12-bit ADC resolution. The two platforms are low-cost and were chosen from the perspective of analyzing the influence of the resolution of the digital analog converter on the sensor performance. For the NI USRP 2930 platform: frequency range: 50 MHz to 2.2 GHz; 14-bit ADC resolution; maximum real-time instantaneous bandwidth: 20 MHz; noise figure: 5 to 7 dB.  
The laboratory tests of the sensor included: the frequency response in the range of TETRA signals (for Downlink, between 390 and 395 MHz); analysis of the amplitude dynamic range; adjacent channel interference attenuation analysis; determining the sensor's own noise level; analysis of the influence of the number of FFT points on the sensor's performance. The tests comprised all three SDR platforms, and a comparison was made with the reference values obtained by using the professional R&S FSV13 analyzer in the same measurement conditions.  
The R&S SM300 signal generator was used to study the frequency response. Its settings were: sweep operating mode in the frequency range from 390 to 395 MHz; frequency step: 1 kHz; dwell time: 200 ms.  
The study of the dynamic amplitude range was performed using the R&S SMBV 100 A generator as a signal source emitting a TETRA channel (channel 131, central frequency 393.2875 MHz) with an output level from \(-112\) to \(-32\) dBm.  
The attenuation of the adjacent channel was determined both for the 1 st order adjacent channels and for the 2 nd order adjacent channels. For this purpose, an R&S SMBV 100 A signal generator was used.  
The study of the noise of the SDR platforms and of the R&S analyzer FSVR13 was performed by measuring the noise power of the TETRA channel in the conditions in which the load impedance of 50 ohms was connected to the RF input.  
An analysis of the influence of the FFT number of points was performed taking into consideration three cases: 2048 points (frequency resolution of 2.5 kHz); 4096 points (frequency resolution of 1.25 kHz) and 8192 points (frequency resolution of 0.625 kHz).