Abstract
We implemented a chaotic Colpitts oscillator with an AT41486 bipolar junction transistor. Experimental results indicate that, chaotic signals can be obtained with wide bandwidths in the microwave band. A range resolution of about 5 cm is obtained for potential application. The microwave Colpitts oscillator exhibits rich nonlinear dynamical behaviours. But the correlation curve of the chaotic signal show many sidelobes which makes the unambiguous detection difficult. This is because the spectrum of the chaotic signal is not very flat and smooth. Therefore we try to get better characteristics of the chaotic signal through simulating two schemes. The first scheme uses a filter as the tranditional way. By simulating the filtered results of chaotic signal through the Chebyshev low pass filter, we find that the sidelobes of the correlation curve are not substantially lowered. The second scheme uses an optoelectronic feedback hybrid system. The optical output from a semiconductor laser is fed back to itself optoelectronically with a fixed DC bias current. By tuning the operating parameters, this system can be operated in different instable regions. For instances, regular pulsing (RP) region and chaotic pulsing (CP) region. Simultaneously, the modulation signal from Colpitts oscillator into optoelectronic feedback system, which will alter the carrier density of the semiconductor laser. There are four schemes used to generate the modified chaotic signals: (1) period-1 oscillation (P1) signal direct current modulating on regular pulsing region (RP) of the laser, (2) chaotic oscillation (CO) signal direct current modulating regular pulsing region (RP) of the laser, (3) period-1 oscillation (P1) signal direct current modulating on chaotic pulsing region (CP) of the laser, and (4) chaotic oscillation (CO) signal direct current modulating chaotic pulsing region (CP) of the laser. We find that the fourth scheme has the best correlation characteristic among them. Although the sidelobes of the correlation curve can not be eliminated completely, the second scheme has better correlation characteristic than the first. Therefore, it can increases noise tolerance of a radar system and reduces probability of making a detection mistake.