Photonic RF direct sampling with multiple electronic analog-to-digital converters (ADCs) for time-interleaved quantization is an effective technique for digitally receiving RF signals. Since band-pass sampling is inevitable for RF direct sampling, the required total sampling rate of a photonic ADC system could be remarkably higher than the low-pass Nyquist sampling rate given by twice the bandwidth of input signal. In order to reduce the sampling rate and thus boost the efficiency of the photonic ADC hardware, we propose a method of introducing time errors between time-interleaved channels with post correcting algorithm in frequency domain. Simulation is performed based on a photonic ADC consisting of multiple 2-GSa/s sub-ADCs. A wideband signal covering the frequency band from ~4 GHz to ~9 GHz is employed as the input RF signal for direct sampling in the simulation. Reduction of the total sampling rate from the band-pass Nyquist rate of 18 GSa/s to the low-pass Nyquist rate of 10 GSa/s is achieved, through which the advantage of the proposed method is verified.
We propose and experimentally demonstrate a scheme to generate wideband radar waveforms at W-band based on a photonic frequency multiplication module. Linear frequency modulated waveforms with an instantaneous bandwidth greater than 8 GHz and 800 us pulse width are generated, and high-resolution range profiles of corner reflectors are obtained experimentally. The results show the range resolution of the system is better than 2.5 cm, which could meet the requirements of high resolution imaging radars.
For the beamforming system of wideband radar, the use of phase shifters will bring obvious directional dispersion. In addition, the bandwidth and in-band performances of electrical delay lines and phase shifters are facing technical bottlenecks. In this paper, the theoretical analysis of beam scanning based on sub-wavelength stepped optical delay lines shows that the beam directional deviation is proportional to the minimum delay interval. The feasibility of the technique is verified by simulations and experiments. Under the circumstance of 8-12GHz microwave frequency range and ±60° azimuthal scanning range, the experimental results demonstrate that the squint of the broadband beam and the in-band directional dispersion can be reduced to less than 0.77° and 0.98°, respectively.
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