THz Wave High Speed Switch And Matching Circuit Design

By the theoretical analysis and simulations, the impedance characteristics of the small-signal model of the GaN pHEMT transistor array and the large-signal model of the GaAs pHEMT transistor, as well as the dynamic impedance characteristics of the large-signal model of the diode have been analyzed. Meanwhile, comparison between the impedance characteristics of the power-dividing form transistor array and the single-branch form transistor array has been made. In this paper, the main work and innovations are as follows:1. According to the basic theory of frequency selective surface(FSS), rough dimensions of a 340 GHz I-shape FSS structure have been calculated. By HFSS software, the precise I-shape structure size have been obtained, and the transmission property of the double wire transmission line structure operating at high frequency has been investigated. The simulation results show that, at the frequency of 340 GHz, the I-shape structure has a good band-stop property. Moreover, by disconnecting the diode, we have found that this structure shows a good transmission characteristic when the structure center is discontinuous, and it has higher modulation depth if operating as a modulator. In addition, double wire transmission line can maintain good transmission characteristics at high frequency.2. Under the condition of small-signal model, simulations of the GaN pHEMT transistor array have been carried out to obtain the S-parameters and the impedance characteristics. Agreement between the experimental results and simulation results confirms the feasibility of analyzing the transistor array by ADS software. Experimental results indicate that the overall reflection coefficient of the transistor array is very small when the modulation velocity is low, and it rises with the increase of input frequency. In order to ensure that the modulation signal keeps a good transmission at high frequency, we have compared the impedance and S-parameters of transistor arrays in the form of power-dividing and single-branch, and have found that, with the increase of the operating frequency, the power-dividing transistor array is much easier to be matched.3. Based on the analysis of the small-signal model of GaN pHEMT transistor, the influence of the input power and the input frequency on the impedance characteristics of the large-signal model of GaAs pHEMT transistor has been studied. Experimental results show that, with the gradual increase of input power and the input frequency, the impedance decreases gradually, and the transistor become much difficult to be matched. In this chapter, we have designed a 0.1 GHz- 3 GHz broadband power divider. Afterwards, under the condition of large-signal model, we have analyzed the impedance characteristics of the power-dividing transistor array and the voltage of each branch. Results indicate that the transistor array in the form of power-dividing is much easier to be matched and its branch voltage is higher.4. The dynamic impedance of the diode has been analyzed in the last chapter of our paper. Simulation results show the harmonic components and the impedance characteristics and reveal that, compared with the transistor array in the form of single-branch, the power-dividing transistor array has better transmission characteristics and smaller reflection coefficient at the frequency of 500 MHz and 1 GHz. According to substantial simulations, we have found that the power-dividing transistor array is much easier to be matched at high frequency.