# SDR-Enabled Multichannel Real-Time Measurement System for In Situ EMF Exposure Evaluation
## 3 Results
### Comparative Performances of the Measurement Instruments  
An important characteristic of the measurement chain is the noise level present at the receiver. This becomes important for applications that require an adequate signal-to-noise ratio (SNR) when the signal to be measured has a low amplitude. For the three instruments, the noise power was chosen as the indicator of the sensitivity of the instrument. The instrument noise power is composed of the thermal noise calculated according  
The lowest noise figure for the channel power measurements can be observed for the DO. This is because of the low oscilloscope resolution used (eight bits--maximum available) combined with the minimum vertical scale setting of the oscilloscope (1 mV/div). The increase in the vertical scale will automatically increase the CP level. Usually, the measured signals do not exceed 20 mV (maximum amplitude) which makes it possible to use a vertical scale of 4 mV/div (to optimally use the available vertical resolution) that increases the measured CP by 2-3 dB. Another possibility to increase the sensitivity of the DO is to use a lower filter bandwidth (below 1 GHz). The filter reduces the noise, thereby increasing the SNR. However, this option is not as feasible for high-frequency signals as those used in up-to-date appliances that require the use of the maximum available channel bandwidth (4 GHz). In Figure 11b we have represented the measured CP by the three measurement instruments over the frequency range 800-6000 GHz. For this experiment, a 20 MHz uplink LTE signal having a power of \(-40\) dBm was configured at the signal generator. The signal generator was directly connected by cable to the measurement instrument channel. We can observe that the SA measured CP presents the flattest and most accurate reading. For the SA, the deviation from the true value of \(-40\) dBm is due to cable loss, which increases with the frequency. The DO presents a higher deviation from the reference power level, but it is still capable to correctly indicate the CP. The deviation of the DO-measured CP from the SA-measured CP does not exceed 3 dB for frequencies below 4 GHz. Above 4 GHz, one can observe the rapid degradation of the DO performances, because of the maximum bandwidth limitation of 4 GHz that acts as a low pass filter for the measured signal. The SDR-indicated CP was calibrated for the 1.75 GHz frequency by setting a proper value of the multiplication constant in the Gnu Radio flowgraph. At the calibrated frequency, one can observe very good agreement between the SDR-measured CP and the SA-indicated CP. However, the SDR calibration does not remain valid for the entire frequency range and a new calibration must be performed if the operating frequency is changed [28]. In Figure 12, we have represented the results of the SDR calibration in two frequency bands: 3.3-3.7 GHz and 5-5.9 GHz. The two frequency bands were chosen as they are used for the 5G NR FR1 communication and the 5 GHz Wi-Fi standards. The top graphs present the comparative channel power values measured by the SA and the SDR across the investigated frequency bands, while in the bottom graphs, there are boxplots of CP variation. We can observe that we obtained very good agreement between the SDR and SA values. As expected, the values are more spread for the second frequency band (Figure 12 right) as in this case, the calibration bandwidth was of 1 GHz. Even so, we observe that most of the values in the entire frequency band (first to third quartile) exhibit a small variation from the median (<1 dB). The lower the calibration bandwidth the better the calibration result. To this extent we consider that this type of SDR demonstrates a satisfactory frequency response as the currently used RF channels are as highs as 100 MHz (5G NR FR1) or 160 MHz (IEEE 802.11 ax standard). Based on the measurement instrument performances, we have conducted a comparative analysis of the proposed multichannel measurement system.