# Design of Wideband FMCW Radar Transceiver System Using Photonic Elements
## 3 Measurement and Analysis  
Figure 4 shows the signal generated in the transmission path; that is, the input signal to Tx. The center frequency and bandwidth of the initial IF signal are 2.5 GHz and 1.25 MHz, respectively. The transmit signal applied to the antennas is centered at 10 GHz with a bandwidth of 5 MHz that is quadruple the value of that of the IF signal, and the result demonstrates that the proposed transceiver can achieve wideband characteristics using photonic elements. Moreover, the measured full width at half maximum (FWHM) are 1.27 MHz at 2.5 GHz and 7 MHz at 10 GHz, respectively, which verify that the generated signal is designed close  
Figure 4: Measured center frequency and bandwidth of the (**a**) IF signal and (**b**) the quadrupled transmit signal applied to the antenna. to the rectangular waveform. The bandwidth of 5 MHz is used in the target detection mode in the proposed radar transceiver system, and the range resolution is 2.4 m when the pulse repetition interval is 200 us, and the number of the range bin is 4096 [16]. Figure 5 presents received signal powers according to the detection range, with the measured closing (approaching the radar system) and opening (away from the radar system) results specified as '\(\mathrm{o}\)' and '\(\mathrm{+}\)', respectively. The closing signal power is bigger than the opening signal power, and the curve fitting assuming the target's RCS is 0.1 m\({}^{2}\) is represented as the solid line. In the experiment, the minimum SNR is estimated to be 12.8 dB, which is in good agreement with the simulation result of 13 dB. In addition, the maximum target range is estimated to be more than 5 km. Therefore, these results demonstrate that the requirements of the maximum detection range are satisfied as listed in Table 1.  
The proposed radar transceiver with photonic elements has two ISAR image modes, and each mode consists of waveforms with different bandwidths to compare the resolution of ISAR images obtained from each bandwidth experimentally. Figure 6a presents the IF signal with 125 MHz at 2.5 GHz, and this signal is quadrupled to 500 MHz at 10 GHz through optical elements and radio frequency (RF) electronics as shown in Figure 6b. The wideband characteristics of the signal can be obtained by controlling the bias voltages of the DP-MZM and the output signal, such as optical power and RF signal. The optical transceiver module has been constructed to observe the output signal while adjusting the bias value in real time. Moreover, the temperature of optical components is a key factor to achieving the second-order modulation of DP-MZM; thus, the test port with thermoelectric cooling (TEC) was adopted for use in the optical transceiver module in order to continuously generate a stable signal. To obtain broad bandwidth for high-resolution ISAR images, the IF signal of another ISAR mode was designed with 500 MHz at 2.5 GHz as shown in Figure 6c. Figure 6d represents the transmit signal with a bandwidth of 2 GHz at 10 GHz to identify features of targets clearly in the cases of flights close to the radar transceiver system [12]. Figure 5: Received signal powers of closing and opening according to the detection range with curve fitting assuming a target RCS of 0.1 m\({}^{2}\). Figure 7 shows ISAR images for the drone target at 330 m distance using the proposed FMCW radar with photonic elements according to different bandwidths. Figure 7a shows the scenario for the field test with the measurement environment; the target (Matrice 600) has waypoints with waypoint 2 being 2.8 km away from the radar site with an altitude of 500 m. Figure 7b,c present ISAR images when the bandwidths of the proposed radar system are 500 MHz and 2 GHz, respectively. The range resolution of the proposed system can be improved from 2 m to 0.5 m when the bandwidth of the proposed radar system increases.