Research On Oscillator And Generalized Phase-conjugating Circuit Of Retrodirective Antenna System At 2.45GHz

Retrodirective antenna array is able to reflect an incident wave toward the source direction with omni-directional receiving antenna patterns completely through the analog circuit, which can solve the contradiction between omnidirectivity and high gain. These unique features make retrodirective antenna array have a broad prospect in short-haul communication where wireless node only carrys limited energy. In the scheme of PON architecture, it requires oscillator to provide a zero initial phase for mixing. However, the initial phase of actual stable oscillator is a random constant, so the oscillator in this circuit is usually set up as an ideal oscillator. To solve this problem, this dissertation introduces the generalized phase-conjugating condition, and proves that the initial phase of the oscillator which provides a local oscillator signal can be any constant, that can meet the requirements of retrodirective antenna array. Accordingly, the scheme of negative resistance technique is applied in 4.9GHz microstrip oscillator for short-haul communication at 2.45 GHz. This dissertation uses S parameters of microwave network to design the FET DC bias circuit, negative resistance circuit, high frequency choke circuit and filter circuit. A feedback element is added in the gate of the DC bias FET to ensure that the active transistor is in a state of oscillation. Then in order to obtain the maximum oscillating power, the method of stability circle in Smith chart is used to design a suitable negative resistance and load matching network. At last, a LC bandpass filter is added to the oscillator to improve the spectral purity of the output signal. After optimization, the measured results show that the phase noise of the oscillator is-112.3d Bc/Hz@100k Hz,-133.6d Bc/Hz@1MHz, and the second harmonic rejection is-150 d Bc, which has met the requirements of the generalized phase-conjugating circuit. In the process of replacing the ideal oscillator with the real oscillator, the FET resistive mixer, 2.45 GHz and 4.9GHz microstrip hybrid ring, 2.45 GHz bandpass filter and matching circuit are designed to form the w hole generalized phase-conjugating circuit of balanced structure along with the 4.9GHz microstrip oscillator. After optimization, the circuit function is verified in ADS. With input phase of 0°, 40°, 80°, 120°, 160°, the output phase are-132.30°,-171.55°,-210.80°,-250.93°,-291.50° respectively, where the average error is 0.44°, which not only meets the requirements of retrodirective antenna array, but also has high accuracy of phase.So far, the real oscillator and generalized phase-conjugating circuit of the antenna system are completed, which will promote practical application of retrodirective antenna array.