# Design and Implementation of a Multi-Band Active Radar Calibrator for SAR
## 3 Results
### Design of the ARC
#### 3.1.2 RF Subsystem Design  
The RF subsystem is very important, and many functions and performance aspects of ARC are realized based on the RF subsystem. The scheme of the RF subsystem is depicted in Figure 1. It consists of a receiver, optical fiber delay, a gain adjustor, an amplifier, an internal calibration module, an SFM, and so on. Based on the demands for the RCS of the ARC, the gains are about 54 dB for the L-band, 47 dB for the C-band, 50 dB for the X-band, and 59 dB for the Ka-band at maximum, and this can be reduced by 15 dB in 5 dB steps. The bandwidth is 200 MHz for the L-band, 240 MHz for the C-band, 1200 MHz for the X-band and 1200 MHz for the Ka-band. The multi-band ARC demands that the RF subsystem should be able to work from the L-band to the Ka-band. Furthermore, specifications such as gain stabilization and gain flatness are very high. Figure 3: Schematic illustration of the L/C-band common aperture antenna. OMT: ortho-mode transducer. Based on the value of RCS and the gain of the antenna, it is can be deduced from Equation (1) that the gain of the RF channel should be about 54 dB for the L-band, 47 dB for the C-band, 50 dB for the X-band, and 59 dB for the Ka-band at maximum. As is commonly known, many attenuating devices such as attenuators used for impedance matching, filters used to filter the useless signals and so on are situated in the RF channel, so the total gain of amplifiers will be far higher than the gain of the RF channel. Questions of stabilization and gain flatness will arise subsequently. Technology using a multistage and adjustable equalization filter was utilized to ensure the gain flatness. The cavity filter shown in Figure 4 with some tuning rods was used somewhere in the RF channel. The tuning rods were tuned until the gain flatness satisfied the requirement, and then they were fixed. Moreover, two SFMs were used to attenuate out-of-band spurious signals. For example, if the ARC was used to calibrate the SAR operating at C-band, the SFMs would be switched to the C-band filter. This can attenuate out-of-band spurious signals and reduce the thermal noise, and then the signal-to-noise ratio can be improved. * Fiber optic delay module  
There many methods that can be used to achieve tunable delay, such as optical fiber delay, bulk acoustic wave (BAW), surface acoustic wave (SAW) delay, digital delay, and so on. An optical fiber delay unit was chosen because of its advantages of no frequency translation being required, low loss, and a wide bandwidth. However, the optical fiber is temperature-sensitive, and its consequent amplitude instability should be paid sufficient attention. A temperature control measure was taken in the optical fiber delay unit, and a special design for the structure of the optical fiber was determined. The fiber optic delay module can move the point target response in the final image to a low-backscattering background. For example, when the ARC is placed near water surfaces, through fiber optic delay, the echo return can be delayed as if the ARC was located on the water where the background noise is low. This greatly reduces the demands for the calibration site. Furthermore, fiber optic delay can mean that the point target response in the final image is not at the location of the ARC, which reduces the re-radiation effects of the structure itself; therefore, a higher signal-to-clutter ratio and better calibration accuracy can be achieved [20]. Furthermore, distortion due to antenna leakage is non-coherent after fiber optic delay and can be suppressed with range compression [21]. Three optical fibers with the length of 0.5 us, 0.5 us and 1 us were used between laser and detector, and the delay was tuned through the optical switches. * Gain calibration circuit  
The gain stability is achieved by the gain calibration circuit, as depicted in Figure 5. A series of calibration pulses are fed to the circuit near its input through CP1.