Research On Ultra-wideband Short Range Impulse Imaging Radar System

Impulse radar, also can be called carrierless radar, baseband radar or time domain radar, is a kind of ultra-wideband radar. It has several advantages such as high resolution, great penetration capability, good anti-stealth capability, good EMI/EMC performance, low interception probability, low power and simple structure, etc.. It is widely used in the fields like see-through-wall imaging, ground penetration imaging, biomedical imaging and anti-stealth detection.In this dissertation, the key components of an ultra-wideband impulse imaging radar system are researched. An experiment platform is built and tested. The main contents of this dissertation are as follows:1. The performance and parameters of ultra-wideband time domain antenna are researched. The time domain performances of several kinds of ultra-wideband antenna are compared. An ultra-wideband Vivaldi antenna is researched in detail. A wideband microstrip-slotline balun is ultilized to realize the microstrip line to slotline transition and impedance matching. The performance of the antennas with different exponential curves is discussed. The radiation property of the exponentially tapered solt antenna in time domain and frequency domain is researched. Finally, an ultra-wideband antenna with bandwidth 0.48GHz-3.9GHz and gain 4dBi-10 dBi is designed and tested. The results obtained from simulation and experiment agreed very well.2. Applications of the defected ground structure(DGS) in microwave filters are researched. A novel DGS pattern with two transmission zeros is proposed. Compared with traditional dumbbell DGS, it has advantages such as high selectivity and wide stopband. Then a lowpass filter based on this novel DGS with excellent selectivity and ultra-wide stopband is designed and tested. The passband covers DC-3.25 GHz, insertion loss less than 0.6dB, the stopband with rejection level greater than 30 dB covers 3.75GHz-32 GHz. The square factor is less than 1.15.3. Several kinds of ultra-wideband pulse generator with different power and bandwidth are researched. Including picoseconds pulse generator based on rf transistor in avalanche mode; high power picoseconds marx bank pulse generator based on avalanche transistor and picoseconds pulse generator based on step recovery diode. The picoseconds pulse generator based on rf transistor could generate pulses with amplitude of 4.2V and pulse width 800 ps on 50? load, the highest repetition rate is 5MHz. The marx bank pulse generator based on avalanche transistors can generate pulses with amplitude of 1kV and pulse width 500 ps on 50? load, the highest repetition rate is 10 kHz. The pulse generator based on step recovery diode could generate pulses with amplitude of 12 V, pulse width 280 ps on 50? load, the highest repetition rate is 5MHz.4. The applications of equivalent time sampling method in impulse radar receiver are researched. The sample and hold circuit is a key component in the equivalent time sampling receiver. So the structure and design method of sample and hold circuit is discussed. A sample and hold circuit is consisted of a sampling gate, a strobe pulse generator and a timing control unit. The architecture, efficiency, merits and demerits of several typical sampling gates are analyzed in this dissertation; the influence of the strobe pulses on sample and hold circuit is discussed; several methods of generating high precision picoseconds stepping clock are researched. Then, a sample and hold circuit comprised of a balanced sampling gate based on quad-bridge shottky diodes, a bipolar sampling pulse generator based on step recovery diode and an equivalent sampling clock generator based on digital programmable delay line is designed. At last, based on this sample and hold circuit, an equivalent time sampling receiver with bandwidth 2.2GHz, equivalent sampling rate 20 Gsps and SNR 27.62 dB at-1dBFS is designed and tested.5. An experiment platform of impulse imaging radar system is built. This platform includes a pair of ultra-wideband Vivaldi antennas as transmitting/receiving antenn; a picoseconds pulse generator based on step recovery diode as transmitter; a lowpass filter with ultra-wide stopband and high selectivity based on a novel DGS resonator in the front-end of the receiver; HMC374 A from Hittite Microwave corporation as rf amplifier. An equivalent time sampling receiver is utilized to receive the amplified echoed pulse signal. Some experiments such as objects imaging in complex environments, hidden objects imaging and through-the-wall imaging are taken. And good experiment results are obtained.